Magnetism!and! Static!Electricity! Module! Studio4style!Class

Transcription

1 Next%Generation%Physical%Science% and%everyday%thinking%! Magnetism!and! Static!Electricity! Module! Studio4style!Class!

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3 Next%Generation%Physical%Science% and%everyday%thinking%!! Magnetism%and%Static% Electricity%Module% Unit!M! Developing!a!Model!! for!magnetism!!! Studio6style!Class!!!

4 Unit M: Developing a Model for Magnetism Table of Contents Activity # Activity (A) Title Page A1 Modeling and the Mystery Tube M-1 A2 Exploring Magnetic Effects M-11 Ext A 1 Exploring the Region Around a Magnet online A3 Developing a Model for Magnetism M-25 Ext B Evaluating Magnetism Models online A4 Better Model for Magnetism M-39 A5 Ext C A6 ED Explaining Phenomena involving Magnetism Explaining Another Magnetic Phenomenon Engineering Design: Is the US Losing Its Edge? M-51 online M-63 1 Extensions (Ext s) are online homework activities.

5 UNIT M Developing Ideas ACTIVITY 1: Modeling and the Mystery Tube Purpose When scientists try to explain what they see happening in the world around them, they construct models to help them understand why things happen as they do. In general, a scientific model is a set of connected ideas that can be used to explain phenomena that have already been observed and also guide the making of predictions about experiments that have yet to be performed. Models can be represented in various ways, such as physical objects, diagrams, written or verbal descriptions, and algebraic relationships (equations). In this course, you will encounter and use all of these types of models, but initially you will engage in one of the most important processes of science, that of developing your own models. When developing your own model or evaluating someone else s, it is important to be able to decide whether the model is good or not. The key question for this activity is: How can we decide if a model is good or not? Initial Ideas The Earth we live on is a member of the solar system, the major bodies of which are one star (the Sun) and eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). When asked to draw a diagram of how they think these bodies are arranged most people show the Sun at the center with the planets in orbit around it (in the order listed above). This is a common model for the solar system and is said to be heliocentric (sun-centered) Next Gen PET M-1

6 Unit M Do you think this heliocentric model is a good model for the solar system? If so, how do you know and what makes it good? However, for the majority of recorded history, the accepted scientific model was a geocentric (Earth-centered) one with the Sun and other planets orbiting around the Earth as shown here. This geocentric model was used for almost 2000 years, from around 400 BC until the 16 th century AD, to predict the future positions of the Sun and planets in the sky. These predictions were needed to construct calendars and for navigation. Why do you think this geocentric (Earth-centered) model was considered good and so accepted by scientists for such a long period of time? Starting in the 16 th century, the geocentric model began to be considered less and less good, and the heliocentric (Sun-centered) model was developed and eventually adopted as being better. Why do you think this was?! Participate in a whole class discussion about your answers to these questions. Make a note of any ideas that are different from those of your group. M-2

7 Activity 1: Modeling and the Mystery Tube Collecting and Interpreting Evidence Exploration #1: Developing a model for the mystery tube Your group will need: Mystery tube While it may be tempting to play with the tube, pulling strings at random, please do NOT do so. In order to illustrate the model building process, you should follow the specific steps below IN ORDER. STEP 1. The mystery tube has four strings emerging from four numbered holes. (The knots (or beads) attached to each string are simply to stop the ends from disappearing into the tube.) When you receive your tube, it should look like this with only string #1 pulled out. Your task in this activity is to develop a model of how these strings are arranged/connected inside the tube, based only on observations you can make from outside the tube. However, before you can propose an initial model, you need to have some evidence (actual observations) to base it on. So, choose only one of the shorter strings (#2, #3, or #4) and pull on it. (Do not pull on any of the other strings yet.) Which string did you pull and what happened (if anything) to each of the other strings when you did so? Now pull on string #1 and describe what happens when you do so. (Leave the tube with string #1 pulled out.) M-3

8 Unit M STEP 2. Now that you have some observations you should propose your first model for how the strings are arranged/connected inside the tube. Draw lines on the diagram to show how you think the strings are arranged/connected inside the tube. Briefly explain why you drew them as you did. First model for mystery tube Assuming your model can explain the outcomes of the simple tests you performed in STEP 1, you can consider it to be good, at least for now. However, to further test your model, you will now use it to guide you in making a specific prediction for the outcome of an experiment you have not yet performed. Making Predictions At many points in this course, you will be asked to make a prediction; that is to think about, and write down, what you think will happen before you perform an experiment. (Such questions will usually ask you to Imagine... or Suppose ) Note that a prediction in science is not just a guess, but should be supported by reasoning based on your current model for a particular situation. (This model will, in turn, usually be based on your prior experience or on your general ideas about how the world works.) When you are asked to make a prediction, it is very important that you take the time to explain the reasoning behind your response before you perform any associated experiments. Making (and explaining) predictions is an important step in developing and testing any model. Whenever you are being asked to make a prediction in this class, you will see this icon,, in front of the question. M-4

9 Activity 1: Modeling and the Mystery Tube Suppose you were now to pull on one of the remaining two strings you have not tested yet. (It is your choice which one, but not both!) DO NOT PULL THIS STRING YET! Look at your model drawing on the previous page and use it to predict what will happen (if anything). Record which string you have chosen to pull and what you think will happen to the other three strings when you do so. Briefly explain how your current model supports your prediction. You should now perform the experiment for which you have just made a prediction and record your observations. Recording Observations During this course, you will be performing lots of experiments and using computer simulations. You will usually be asked to record what you observe; for example, to write down what you see happening, or sketch a graph that you have recorded. It is important that you read carefully what it is you are supposed to record, as sometimes it may be only a small part of everything that actually happens. It is also important to be as precise as possible. Although it is difficult sometimes, try not to be influenced by what you think will happen and just record what you actually see. Also, be sure to discuss your observations with your group before you record them, as people sometimes see things differently. Whenever you are being asked to record your observations in this class, you will see this icon,, in front of the instruction or question. Now pull on your chosen string and describe what happens when you do so. Does your observation agree with your prediction or not? M-5

10 Unit M STEP 3. If your observation in STEP 2 agreed with your prediction, then your model is still good and may not need to be revised at this point. If not, then your model is no longer good and needs revising. However, before attempting any possible revisions, you should make some further observations. Spend a short time pulling on the three strings you have already tested, one at a time. DO NOT TOUCH THE LAST STRING YET! Describe the general behavior you observe when you pull on any of these three strings. If your first model (in STEP 2) can explain all the observations you have made so far, then it is still good. If it cannot then you should now try to make sense of these new observations and revise your model accordingly. Making Inferences After you have performed an experiment, you will often be asked to make some inferences; that is, try to make sense of what is happening, based on your ideas and the evidence provided by your observations. These inferences could be as simple as interpreting a graph to determine what the motion of an object was like in a small part of one experiment, or you could be asked to make a general statement about the way the world works, based on evidence from several experiments. You should be certain that any inferences you make are indeed supported by evidence, and you will often be asked to explain how this is so explicitly. Whenever you are being asked to make an inference you will see this icon,, in front of the question. If your model needs revising, discuss with your group how you could change it so that it explains all the observations you have made so far. M-6

11 Activity 1: Modeling and the Mystery Tube Draw lines on this diagram to show how you now think the strings are arranged/connected inside the tube. Briefly explain why you drew them as you did. (If your first model is still good then simply reproduce it here.) Revised model for mystery tube STEP 4. At this point you should again have a good model, because it can explain the outcomes of all the tests you have performed so far. Now you will test your revised model. Use your revised model above and predict what you think will happen (if anything) when you pull the one string you have not yet tested! What do you think will happen? Explain how your revised model supports your prediction. Now pull on this last string and describe what happens when you do so. Does this agree with your prediction or not? Finally, spend some time investigating the general behavior of the mystery tube by pulling on all the strings, one at a time. Describe the general behavior you observe when you pull on any of the strings. M-7

12 Unit M STEP 5. Having now determined the general behavior of the mystery tube, discuss with your group how you think the strings are arranged/connected inside the tube. Draw lines on this diagram to show how you now think the strings are arranged/connected inside the tube. This is your second revised model. Second revised model for mystery tube STEP 6. If the necessary the materials are available to you, now build a real 3- D model of the mystery tube based on your second revised model diagram in STEP 5. (Leave the ends open in case you need to make any changes.) If your 3-D model reproduces the behavior of the mystery tube then your current model is good. If not, then work with your group to revise it until you have a final model that is good. (It may not be perfect, but make sure it can reproduce at least some of the desired behavior.) Draw your final model for the mystery tube to the right. (If your second revised model was already good, you may omit this step.) Also, draw a diagram of your group s final model on a large presentation board. Final model for mystery tube M-8

13 Activity 1: Modeling and the Mystery Tube Summarizing Questions The questions in this section are intended to help you think about the key question for the activity, and issues related to it. In answering them you should review your initial ideas, and the predictions, observations, and inferences you made during the activity. Discuss these questions with your group and note your ideas. Leave space to add any different ideas that may emerge when the whole class discusses their thinking. S1. You probably had to revise your mystery tube model at least once during this activity. Why did you feel it necessary to do so? (If you did not revise it, why was it not necessary?) S2. Do you think your final model for the mystery tube is good or not? Why do you think so? S3. It is likely that not every group s final model is exactly the same. Is it possible that more than one model could be good? If so, why? (Do not check with other groups yet, you will see their models during the class discussion.) M-9

14 Unit M S4. How do you think what you did in this activity is similar to what scientists do when they investigate and try to develop models to explain real-world phenomena? How do you think what you did is not like what scientists do? S5. In general, in science, why is it important to make a prediction before performing a specific experiment? The scientific practice of modeling In the remainder of this module, you will be developing your own models to explain phenomena involving magnetism and static electricity. Just as you practiced in this lesson with the mystery tube, the scientific practice of modeling involves proposing an initial model based on some preliminary observations, testing it by making predictions, and revising it as necessary. This cycle of prediction and testing continues until your model can explain a wide range of phenomena of interest and also make accurate predictions for new situations where the model is still applicable. Engaging in this practice is essentially what scientists do most of the time. M-10

15 UNIT M Developing Ideas ACTIVITY 2: Exploring Magnetic Effects Purpose You are no doubt familiar with some magnetic phenomena, like using a magnet to hold paper on a refrigerator door, or using a compass to navigate. However, could you explain to someone else how these work? In this unit, you will first investigate some phenomena involving magnets to establish their basic properties, and then develop a model that can explain your observations and be used to make predictions for new experiments. What are some properties of magnetic interactions? Initial Ideas At some point in the past, you have probably played with a magnet and tried to use it to pick up various objects. You may also have played with a pair of magnets and observed how they interact with each other. Think back to those experiences and discuss with your group as you answer the following questions. What materials do you think a magnet could pick up; all materials, only metals, or only some specific materials? Why do you think so? 2016 Next Gen PET M-11

16 Unit M If you were to hold a magnet in each of your hands and bring them close together, what do you think you would feel? Would your answer depend on which parts of the magnets were close to each other? Draw diagrams to illustrate your thinking.! Sketch your group s diagrams on a presentation board and participate in a whole class discussion about the answers to these questions. Make a note of any ideas that are different from those of your group. Collecting and Interpreting Evidence Your group will need: 1 bar magnet 2 disk magnets Set of different materials three nails small float aluminum pie tin or deep Styrofoam plate container for collecting and pouring water (or easy access to faucet/sink) compass Please keep the three magnets far away from all the other materials until you are directed to use them. (On the floor would be a good place, but please don t drop them as they break easily) M-12

17 Activity 2: Exploring Magnetic Effects Exploration #1: How do magnets interact with other materials and with each other? In this exploration, you will test various materials to determine if they interact with a magnet and also test how two magnets interact with each other. STEP 1. Take one item from the set of materials, and record its name in Table I below. Bring each end of the bar magnet close to the material and record in the table whether the material is attracted to (A), repelled by (R), or shows no reaction to (O) each end of the bar magnet. Test all of the materials in your kit in the same way. You may also check other materials you are curious about. Record your results in the Table. Table I: Observations with Magnet and Materials Material Reaction to one end of magnet? Reaction to other end of magnet? M-13

18 Unit M STEP 2. Look over the data you recorded in Table I. Are all metals attracted to a magnet? If not, what materials do seem to be attracted to a magnet? Does this result surprise you? Scientists call materials that are attracted to a magnet, ferromagnetic materials. Iron (chemical symbol, Fe) is the most common ferromagnetic material, and objects or materials that include iron in them (like steel) are also ferromagnetic. (Nickel (Ni) and cobalt (Co) are also examples of ferromagnetic materials.) Magnets themselves are also made of ferromagnetic materials. STEP 3. Take the two small disk magnets in your hands and bring their faces together slowly, but try not to let them touch each other. Describe what you feel as they approach each other. Now turn one of the magnets over and bring them together again. Do they behave in the same way as before, or do you feel something different? If so, what? When scientists study the natural world, they focus their attention on different types of interactions between objects. When two objects interact, they act on or influence each other in some way. In this course, you will be studying many different types of interactions. The interactions you saw above, between two magnets, and also between a magnet and a ferromagnetic material, are examples of what we will call a magnetic interaction. M-14

19 Activity 2: Exploring Magnetic Effects How is the magnetic interaction between two magnets different from the magnetic interaction between a single magnet and a ferromagnetic material that is not itself a magnet? In the rest of this activity, you will use iron (or steel) nails to explore some important properties of the magnetic interaction. To start, you will check whether a nail can itself be turned into a magnet. You will not need the two disk magnets or the sample materials for the rest of this activity, so it is best to put them away now. Exploration #2: What happens when a nail is rubbed with a magnet? In this experiment, you will distinguish between two types of nails: those that are rubbed with a magnet (for now we will call them rubbed nails), and those that are not rubbed with a magnet (unrubbed). Initially, all your nails should be unrubbed. Please keep the bar magnet far away from the iron nails until you are directed to use it. Once you rub a nail, it is no longer unrubbed. Please do not rub the nails until you are asked to do so. STEP 1. To make a sensitive detector, place the dish on the table with enough water in it to fill it to a depth of about one-half inch. Place the small float in the water and put one of the unrubbed nails on it. The nail may stick out more than the one shown here in the figure. (If it does not float freely, you may need to add a little more water to the dish.) STEP 2. Now make a rubbed nail as follows. Far away from the floating nail, pick up a second unrubbed nail and hold it horizontally at its head end. (The end you would hit with a hammer.) Pick up the bar magnet, hold it at right angles to the M-15

20 Unit M nail and slide one end of the magnet (either end is OK) all the way from the head to the point end of the nail. Then lift up the bar magnet and repeat this a few more times, always sliding it in the same direction (not back and forth). (Your instructor may demonstrate this technique for the class. If you are still not sure about how to do this, ask for help.) After you have rubbed the nail, be sure to place the magnet back in its safe location. STEP 3. You will now investigate how this rubbed nail interacts with the floating unrubbed nail by doing the following. Hold the rubbed nail horizontally in your hand, and bring its tip close to (but not touching) the floating unrubbed nail. See picture to the right showing that the held nail should be horizontal (just above and parallel to the surface of the water) and at right angles to the floating nail. Always test held and floating nails this way. Do not bring the held nail downward from above (picture below to the left), and do not bring it parallel to the floating nail (see picture below to the right). Do NOT do it this way (from above) Do NOT do it this way (parallel) What, if anything, happens to the tip of the floating unrubbed nail? Is it attracted (A), repelled (R), or does it show no reaction (O)? Record your observation in the appropriate box in Table II on the next page. M-16

21 Activity 2: Exploring Magnetic Effects Next, bring the point end of the rubbed nail near the head end of the unrubbed nail and again record your observation in Table II. Table II: Interactions between Rubbed and Unrubbed Nails (A, R or O) Point end of (held) magnetrubbed nail Head end of (held) magnetrubbed nail Point end of unrubbed nail Head end of unrubbed nail Finally, bring the head end of the rubbed nail near the point end of the unrubbed nail, and then bring it near the head end of the unrubbed nail. Again, record both observations in Table II. Do the two ends of the unrubbed nail behave the same way or differently when each end of the magnet-rubbed nail is brought nearby? STEP 4. Lay the rubbed nail aside for a moment. Imagine that you removed the floating nail, rubbed it with the magnet in exactly the same way that you rubbed the other nail, and then floated it again. (DON T DO IT YET!) You would then have two rubbed nails one held and one floating. Predict what you think would happen if you were to bring the tip of the held rubbed nail near the tip of the floating rubbed nail. Predict what you think would happen if you were to bring the tip of the held rubbed nail near the head of the floating rubbed nail. M-17

22 Unit M Now remove the floating nail, rub it with the magnet in exactly the same way as you did before, and replace it on the floater. Then test your predictions. Repeat the same set of four tests that you did in STEP 3, but now with the two rubbed nails. Record your observations in Table III below. Table III: Interactions between Two Rubbed Nails (A, R or O) Point end of (held) magnetrubbed nail Head end of (held) magnetrubbed nail Point end of magnet-rubbed nail Head end of magnet-rubbed nail Do the two ends of the magnet-rubbed floating nail behave the same way or differently when each end of the magnet-rubbed nail is brought nearby? Based on your observations, would you claim that rubbing a nail in the way you did turned it into a magnet, or does it still behave like a ferromagnetic material that is not itself a magnet? What evidence supports your answer? When a nail (or any other object made of a ferromagnetic material) is rubbed with a magnet and behaves in the same way as you observed above, we say it is magnetized. Therefore, from now on, we will refer to a rubbed nail as a magnetized nail, and an unrubbed nail as an unmagnetized nail. M-18

23 Activity 2: Exploring Magnetic Effects Some magnetized objects retain their magnetism for very long periods of time, and we call them permanent magnets. The bar magnet you are using is probably made from alnico, (an alloy that consists of iron mixed with aluminum, nickel and cobalt) that is a good permanent magnet. Other ferromagnetic materials, which tend to lose their magnetism easily after being magnetized are sometimes called temporary magnets. STEP 5. Suppose you were to touch a magnetized nail all over with your fingers. Do you think the nail would still be magnetized after you did this, or would it act more like an unmagnetized nail now? Why do you think so? Now test your thinking by touching one of your magnetized nails all over with your fingers. Place it on the float and test its ends with your second magnetized nail. Was the nail still magnetized after you touched it all over or not? Now suppose you dropped a magnetized nail in water. Do you think the nail would still be magnetized after you did this, or not? Why do you think so? Again, test your thinking by dropping one of your magnetized nails in your pan of water. Then place it on the float and test its ends with another magnetized nail. Was the nail still magnetized after it was immersed in water, or not? M-19

24 Unit M Exploration #3: Does a magnetized nail interact with anything when there is no other magnet or nail nearby? In the previous experiment, you discovered that a magnet-rubbed nail itself becomes magnetized. Consider floating such a magnetized nail. If you do not bring another nail or magnet nearby, does anything interesting happen to the floating magnetized nail? You will answer that question in this experiment. STEP 1. Place a magnetized nail on the floater, making sure the other rubbed nail and the bar magnet are far away. Now do the following several times. (You may have to wait up to a minute each time for the nail to settle into a stable position.) Aim the floating nail in different directions in the middle of the pan, then release it and wait until it settles into a stable position. (Make sure the floater does not get stuck against the side of the pan while this is happening.) Spin the floating rubbed nail gently, and again wait until it settles into a stable position. Your instructor will point out the approximate directions for north, south, east and west. Does the floating magnetized nail end up pointing in a different direction each time, or does it always seem to end up pointing in the same direction? If so, in which direction does the pointed end of the nail seem to want to point? Compare your observations with that of several other groups. What is the same about how other groups nails behave (if anything)? What is different (if anything)? M-20

25 Activity 2: Exploring Magnetic Effects Whenever a rubbed nail, or any magnet, is allowed to rotate freely, without another magnet nearby, one end will always end up pointing (approximately) towards the geographical North Pole of the Earth. By mutual agreement, scientists define this end of the magnet as the north-seeking pole (or N-pole for short) of the magnet. The opposite end of the magnet, by definition, is called the south-seeking pole (S-pole). (Your bar magnet may already have its ends labeled as N and S to signify this.) Thus, when you rub your nail, you turn it into a magnet with a N-pole and a S-pole. Is the tip (pointed) end of your group s floating magnetized nail a N-Pole or a S-Pole? What about the head end? STEP 2: Place your compass on the table, far away from the bar magnet and magnetized nails. The needle in the compass is made of a special ferromagnetic material that has been magnetized and retains its properties for a long time; i.e. it is a small permanent magnet. The compass needle is free to pivot, and so one end of the needle will always point towards geographic north and by definition, that is the N-pole of the compass needle. (Notice that, in effect, your floating magnetized nail is also a compass needle.) Which end of your compass needle is a N-pole, the colored tip or the uncolored tip? (Do not rely on the labels on the compass itself. Instead, use the directions indicated by your instructor in STEP 1 to help you.) Exploration #4: How do the poles of two magnets interact with each other? STEP 1: Lay your compass on the table and rotate it so that the N-pole end of the compass needle is aligned with the N marking (for the North direction) on the casing of the compass (as in the picture above). M-21

26 Unit M Suppose you were to lay one of your magnetized nails on the table with its N- pole pointing towards the E (for East) label on the compass and then slide the nail toward the compass, as shown in the picture below. [Note: Your magnetized nail may have its N-pole at its head end, in which case you would slide its head end toward the compass.] N S Predict what you think would happen to the compass needle when you do this. Will the N-pole of the compass needle rotate toward the nail, away from the nail, or not move at all? Why do you think so? STEP 2. Now test your prediction by sliding the N-pole of your magnetized nail towards the East label on the compass needle. What happens to the N-pole of the compass needle? Is it attracted to or repelled by the N-pole of the nail? Move your magnetized nail away from the compass and turn it around so that its S-pole faces the E-label of the compass. Now slide it towards the compass again. What happens to the N-pole of the compass needle now? What about the S-pole of the compass needle? Is this what you expected? M-22

27 Activity 2: Exploring Magnetic Effects Do like poles (N-N or S-S) of the magnetized nail and compass needle attract or repel each other? Do unlike poles (N-S, or S-N) attract or repel each other? Check your conclusions with at least one other group to make sure you all agree. If not, repeat the observations. Your statement about how like and unlike poles interact with each other is known as the Law of Magnetic Poles. Summarizing Questions Discuss these questions with your group and note your ideas. Leave space to add any different ideas that may emerge when the whole class discusses their thinking. S1: An elementary school student asks you for advice about a science project she is doing on recycling. She suggests that a large permanent magnet could be used to separate metals from non-metals in the trash passing through a recycling station. What do you think of this idea and why? S2. Because of the way its ends interact with a magnetized nail, scientists sometimes say that an unmagnetized nail is one-ended, whereas a magnetized nail is said to be two-ended. What do you think they mean by these terms? M-23

28 Unit M S3. In this activity, you magnetized a nail by rubbing its surface with a magnet. Do you think that whatever causes a nail to be magnetic also lies on its surface, or inside the nail? What evidence supports your thinking? M-24

29 UNIT M Developing Ideas ACTIVITY 3: Developing a Model for Magnetism Purpose In the previous activity, you investigated some magnetic phenomena and discovered that magnet-rubbed (magnetized) nails behave differently from unrubbed (unmagnetized) nails. Thus, rubbing the nail with a magnet must change the nail in some way. But, how does it change the nail? To answer this question, you need to develop a model; a picture and description of what you think is going on in the nail when it is rubbed. A good model can do two important things: (1) it can be used to explain observations from experiments already done; and (2) it can guide the making of predictions about experiments that have not yet been done. After scientists make their predictions based on their initial model, they (or other scientists) perform the experiments. If the predictions are confirmed through the new experiments, the scientists retain their model because it can explain their new observations. However, if the results of the new experiments differ from the predictions, scientists use the new evidence to revise their model so it can explain the new set of observations (as well as the previous observations). Then they use their revised model to make new predictions. They develop confidence in their model only after it can be used repeatedly to make predictions that are confirmed by new experiments. Thus, a critically important activity of scientists is to develop, test, and revise models. In Activity 1 of this unit, you followed this procedure in developing a model for the Mystery Tube. (Note that, while you do not know if your model corresponds to what was actually in the tube, if it worked to explain all your observations, then it was a good model.) In this activity, you will begin the process of developing, testing, and revising your own model for magnetism. How can you develop a model of magnetism? 2016 Next Gen PET M-25

30 Unit M Initial Ideas To begin this activity, you should spend a few minutes individually considering your own initial model for what happens when a nail is magnetized. You should represent this model using both diagrams and a written description. Imagine that a nail is rubbed with a magnet in such a way that its pointed end becomes a north pole. Below are two drawings of the nail, representing its state before and after rubbing. Sketch what you think might be different about the nail in these two conditions (unmagnetized and magnetized). Think about what entities (small particles) might be inside the nail, and what might happen to them in the process of rubbing with a magnet, that causes the nail to become magnetized. Your individual model: Unmagnetized (before rubbing) Magnetized (after rubbing) Describe your initial model in words, in particular how the Magnetized picture differs from the Unmagnetized picture and how rubbing with a magnet causes this difference. If you are showing some type of entities inside the nails, describe what you imagine these entities represent and how they might get rearranged. M-26

31 Activity 3: Developing a Model for Magnetism Share your model with other members of your group and also listen to them describe their models. Here are some things to consider about communicating your model so that others can understand and evaluate it. Assumptions: You should clearly state any assumptions that are being made. In your model for magnetism this might be the nature of any entities involved and how they can move (if at all). Reasoning: You should explain any changes shown in your diagrams (and/or described in your written/verbal description) in terms of why they occur. In your model for magnetism this might be how AND why any entities involved get rearranged (if at all). Clarity and Consistency: Your diagrams and written/verbal descriptions should be clear, understandable, and consistent with each other. Of course, your model should also be explanatory. That is, you should be able to use it to explain all (or at least most) of the observations you have already made. After each member of your group has shared their own model discuss and decide on a single model that your group thinks is best. Your instructor may give you a separate worksheet to help you work together on this. If the group s best model is different from your own initial model, draw a representation of the group s model and briefly describe it in words. Your group s first model: Unmagnetized Magnetized Description: M-27

32 Unit M How does this model explain why rubbing an unmagnetized nail with a magnet results in it becoming magnetized? How does this model represent that an unmagnetized nail is one-ended (both ends behave the same way), but a magnetized nail is two-ended (the two ends behave differently)? Prepare a presentation board showing your group s best model and participate in a whole class discussion. After listening to other groups present their models discuss with your group what you now consider to be the best model. Draw it below and briefly describe it. Current Best Model of nails: Unmagnetized Magnetized Description: Why you think this model is best? M-28

33 Activity 3: Developing a Model for Magnetism Using consistent symbols Before testing what you now consider to be the best model (by using it to make a prediction) we need to consider the symbols being used by the class in their models to represent any entities in the nails. Up to now some groups may have used + and symbols to represent two different types of entities, and other groups may have used N and S symbols in the same way. However, for consistency in comparing models, it would be a good idea to agree on one set of symbols. You probably realize that plus (+) and minus ( ) charges have to do with electric charges and that batteries have + and labels at their ends, whereas magnets have N and S labels at their ends (poles). Since we are focusing on magnetic effects here, not electric effects, to keep things simple, we suggest everyone uses N and S symbols in their model (if appropriate). If necessary, redraw your current best model for an unmagnetized and magnetized nail below (the one you decided was best after the class discussion). Remember, if you previously used + and symbols simply translate then to N and S symbols. Unmagnetized Magnetized Collecting and Interpreting Evidence Exploration #1: Using your model to make a prediction Important: When you make predictions you must base them on your current model. Do not change your model as a result of just thinking about the situation, because then you are not testing your model. If the outcome of the experiment turns out to be exactly what you had predicted, then don t modify your model. On the other hand, if the outcome is different from your prediction, even in small ways, then you need to consider how to revise your model. Finally, for this process to be useful, the predictions you make should be precise, not vague and general. Only then will the experiment really test your model appropriately. M-29

34 Unit M STEP 1. To help your group make a prediction based on your current model (rather than on some other intuition), you should use the following procedure. On a separate blank piece of paper draw a large version of your current model for a magnetized nail. Next, draw a thick vertical line through the exact middle of your model drawing, and then tear your drawing in half, exactly along that line. You should end up with two drawings, each representing half of the magnetized nail. Separate these two halves on your table. Copy your drawings of the two halves of the model below, showing your model s representation of the two halves of the nail. Head half of magnetized nail Point half of magnetized nail Now look at each half piece and answer these questions based on your model drawing. Does your model of the head half piece (on the left) suggest that it, by itself, is one-ended, two-ended, or something different? (If different, try to describe it in words.) How does it indicate this? Does your model of the point half piece (on the right) suggest that it, by itself, is one-ended, two-ended or something different? (If different, try to describe it in words.) How does it indicate this? Your drawings above represent what your model suggests would be in each piece of a magnetized nail that is cut in half. You will now use these to make some predictions about what you would find if you actually did this and tested each piece of the cut nail separately. M-30

35 Activity 3: Developing a Model for Magnetism Using your diagrams on the previous page, indicate on the picture below whether each end of the two cut pieces should be a N-pole (N), a S-pole (S) or have no pole (NoP). You should label all four ends the original head and point, and the two cut ends. Briefly explain why you labeled all four ends as you did. (Remember: do not change your model at this time.) Prediction for poles on two halves of cut magnetized nail Now, suppose you brought each end of both halves of the cut magnetized nail toward the E-label of a north-pointing compass. In each case, according to your model, do you predict that the N-pole end of the compass needle would be attracted (rotate toward) or be repelled (rotate away), or would nothing happen? Write your prediction (attract, repel, or nothing) next to each picture below and briefly state why your current model would predict that. (Again, do not change your model when using it to make a prediction.) Head piece head end Point piece point end Head piece cut end Point piece cut end M-31

36 Unit M STEP 2. Discuss with your group what your model drawings indicate about how the magnetic strength of each half of a cut magnetized nail would compare with the magnetic strength of a whole (uncut) magnetic nail. Do you think the magnetic strength of each cut half would be weaker than, the same as, or stronger than that of the whole nail? Describe how your model drawings support your thinking. Now that you have made some definite predictions based on your current model, you will perform the corresponding experiment to see whether your model is good as it is, or whether you need to consider modifying it. Exploration #2: What happens when a magnetized nail is cut in half? You will need: bar magnet three unrubbed nails compass small piece of tape STEP 1. Lay your compass on the table and rotate it so that the N-pole end of the compass needle is aligned with the N marking (for the North direction) on the casing of the compass (as in the picture above). You should leave the compass in this position for the rest of this activity. Rub one of your nails with the magnet so that the pointed end becomes a N- pole. A convenient way to do this is to rub from the head to the pointed end using the S-pole of the bar magnet. Remember to put your magnet far away after rubbing the nail and compass after you are done. M-32

37 Activity 3: Developing a Model for Magnetism To check that your nail is magnetized as expected, slide each end of it toward the E-label on the compass. Is the point end of the magnetized nail a N-pole and the head end a S- pole? How does the behavior of the compass needle indicate this? STEP 2. Now ask your instructor to cut your magnetized nail in half. Before your instructor cuts the nail, however, you must show him or her how you used your current model to guide your predictions After the nail is cut, make sure you keep both halves away from the magnet. STEP 3. Slide each end of the head piece of the cut nail toward the E-label on the compass. Repeat this with the point piece of the cut nail. Use the pictures below to record your observations. Write attract, repel, or nothing, to indicate the observed behavior of the N-pole end of the compass needle. Head piece head end Point piece point end Head piece cut end Point piece cut end M-33

38 Unit M Do the two halves of the cut nail appear to be one-ended, two-ended, or something different? How do you know? Mark the four ends in the diagram below according to whether each was a N-pole, S-pole, or no pole. Check your observations with those of at least two other groups to make sure you all agree. If necessary, repeat the experiment and re-record the observations. How did your observations compare with your predictions? STEP 4. Magnetize a second nail in exactly the same way as before, but leave it uncut. Now use the compass to test the magnetic strength of each half of your cut nail as compared to the magnetic strength of the uncut nail. (To ensure it is a fair test, make sure you do all tests with the N-pole end of each piece/nail held the same distance from the compass.) How did the magnetic strengths of the cut pieces compare to that of the whole nail? How do you know? STEP 5. If your observations were exactly the same as your predictions, and if your predictions were truly based on your current model, you should not change your model at this time. However, if your observations were different from your predictions, then your group needs to discuss how you might change your model so it can explain both your new observations and your previous observations. M-34

39 Activity 3: Developing a Model for Magnetism If you have no need to change your model at this time, simply redraw it below. However, if your model needs to be modified in light of these results, after discussing with your group, draw your new model below. Model of nail after observations of cutting magnetized nail in half Before Rubbing After Rubbing Briefly describe in words how the model you drew above can account for the observations you have just made about the two halves of a cut magnetized nail. Exploration #3: What happens when a magnetized nail is cut in unequal length pieces? STEP 1. To help you think further about your model, you should now consider the following. Suppose a full length magnetized nail (with its point end again a N-pole) were cut into two pieces of unequal length (say a 1/4- length piece and a 3/4-length piece, or a 1/3-length piece and a 2/3-length piece, or some other division). On the diagram below, sketch how your current model would represent the two unequal pieces after cutting. Label the four ends as either N, S, or NoP (no pole) and briefly explain how your model shows this. Longer piece Shorter piece M-35

40 Unit M Now suppose you were to cut just the longer piece of the nail in two again, so you have a shorter head piece and a piece taken from the middle of the nail. On the diagram below sketch how your current model would represent these two pieces after cutting. Label the four ends as either N, S, or NoP (no pole) and briefly explain how your model indicates these choices. Head piece Middle piece STEP 2. Now test your predictions by magnetizing another nail, having your instructor cut it in two pieces according to your directions, and then testing the ends of the two pieces with your compass as before. Do these results suggest that the longer piece is one-ended, two-ended, or something else? What about the shorter piece? Mark the four ends in the diagram below according to whether each was a N-pole, S-pole, or no pole. Longer piece Shorter piece M-36

41 Activity 3: Developing a Model for Magnetism STEP 3. Next stick a small piece of tape (or otherwise mark) on the longer piece of the cut nail, near the cut end. (This is so you will be able to tell the two ends apart after cutting.) Have your instructor cut the longer piece of your magnetized nail according to your directions, and then testing the ends of the two pieces with your compass as before. Do these results suggest that the head piece is one-ended, two-ended, or something else? What about the middle piece? Mark the four ends in the diagram below according to whether each was a N-pole, S-pole, or no pole. Check with some other groups. Do their observations seem to suggest that the results of both experiments you did in this exploration depend on exactly where the magnetized nail was cut or are every group s results the same? Summarizing Questions S1. When a magnetized nail is cut into pieces, is each piece one-ended, twoended, or something different, or does the result depend on exactly where the nail is cut? What evidence supports your answer? S2. Work with your group on revising your model (if necessary) so that it can explain all the observations you made in this activity. Remember it should also still explain those observations made previously. Again, your instructor may give you a separate worksheet to help as you discuss this. M-37

42 Unit M a) Sketch your revised model below, showing an unmagnetized nail, a magnetized nail, and a magnetized nail that has been cut in an arbitrary location. Model of nail after observations of cutting magnetized nail in two pieces Unmagnetized Magnetized After Cutting b) How does your revised model represent that both pieces of a cut magnetized nail are two-ended? b) How does your revised model represent that each piece of a cut nail has a weaker magnetic strength than the whole nail before it was cut? c) How does your revised model account for the observation that cutting a magnetized nail anywhere along its length would still give two pieces that are both two-ended? (If it cannot account for this, say why not.) Prepare a presentation board showing your model ready for the class discussion. Participate in a whole class discussion. After listening to other groups present their models and explanations draw what you consider to be the best model below and describe why you think it is best. New Model of nails Before Magnetizing After Magnetizing After Cutting M-38

43 UNIT M Developing Ideas ACTIVITY 4: Better Model for Magnetism Purpose In the previous activity you tested your model by performing some experiments cutting magnetized nails. Most students probably realized that they needed to modify their initial models. However, you may have found it challenging to modify your model to account for of all the new evidence. When this happens in science, it is often the case that scientists need to radically reconceptualize (think very differently about) their model. For example, in the case of the model for magnetism, perhaps you need to think differently about what kinds of entities are inside the nail, because thinking in terms of separate N and S entities might not be fruitful. In this activity you will work with both physical analogies and a computer simulator to help you consider additional possible ways to revise your model. How can you develop a better model for magnetism? Collecting and Interpreting Evidence Note: You should not regard the outcomes of these investigations as phenomena to be explained by your model, but instead as analogies for what might be happening when you magnetize a single nail. That is, use them as the basis for alternative ideas you might use in rethinking your model. Exploration #1: How do collections of magnets behave? You know that a magnetized nail (which is a small magnet) produces a magnetic effect that makes a compass needle move when the nail is near. In this experiment you will use a compass to investigate the magnetic effect of a collection of magnetized nails. You will need: bar magnet 4 (unrubbed) nails magnetic compass piece of masking tape access to computer with internet connection 2016 Next Gen PET M-39

44 Unit M STEP 1. Lay the magnetic compass on the table and rotate it so the colored tip of the needle points towards the north marking, N. Slide the point end of an unmagnetized nail across the table toward the E label of the compass. (See picture to right.) How does the compass needle behave as you slide the nail closer and closer? Why does this behavior make sense? Now rub the nail with the magnet so that its point end becomes a N-pole and slide its pointed end towards the E label on the compass. How does the compass needle behave as you slide the nail closer? Does this behavior confirm that point end of the nail is indeed a N-pole? (If not, you should rub it in a different way, and check again.) STEP 2. Having seen how a single unmagnetized and magnetized nail affect a compass needle, you will now investigate the magnetic effect of a combination of magnetized nails. First, take your magnetized nail and slide its point toward the E label on the compass until the compass needle points in the NW direction (half way between N and W). Use two small pieces of tape to mark the position of the compass and the point end of the nail (represented by dashed lines in the diagram). Leave both the compass and the single magnetized nail with its point on these marks. M-40

45 Activity 4: Better Model for Magnetism Suppose you were to place three more identically magnetized nails close to the one that is already there, all with their point ends (N-poles) on the same mark, as shown here. Do you think the compass would still point in a NW direction (as it did with one nail) or do you think it would move further toward the W direction, or back toward the N direction? Why do you think so? Magnetize three more nails so their point ends are a N-pole and place them beside the first one as in the diagram. (Keep them as close together as possible.) Describe what happened (if anything) to the compass needle when three more magnetized nails were added. Is this what you expected? Each magnetized nail is a small magnet. Do several small magnets (nails), oriented the same way (all N-Poles pointing in the same direction) seem to produce a stronger magnetic effect, a weaker effect, or the same effect as a single small magnet? How do you know? STEP 3. Now suppose you were to flip two of the magnetized nails so that their head ends (S-poles) were on the mark, as shown here. Do you think the compass would now point in the NW direction (as it did with one nail) or do you think it would still point closer to the W direction (as it did with four identically aligned nails), or would it point more toward the N direction? Why do you think so? M-41

46 Unit M Turn two of the magnetized nails so their head ends (S-poles) are on the mark. Leave the other two nails with their point ends (N-poles) on the mark. In which direction does the compass needle point now? If several small magnets are oriented so half of them have their N-poles facing one way, and half facing the opposite way, is the magnetic effect they produce more like a magnetized or an unmagnetized nail? Why do you think this is? STEP 4. You can further check your thinking by watching a movie of a simulator that models what happens when small magnets are oriented in different ways. Go to the Next Gen PET Student Resources web page and open up UM-A4 Movie 1. It shows a small bar magnet with a meter that measures magnetic influence 1. (See diagram below.) In effect, the reading on the meter is an indication of how strongly any combination of magnets would influence a nearby compass needle (or another magnet) placed where the meter is located. First, only a single magnet is present. Then, three additional small bar magnets are added to the simulation and placed as shown here. Record the value of the meter on the pictures of both arrangements Which arrangement has the higher reading on the meter and why do you think this is? 1 The T on the meter stands for Tesla, the units in which scientists measure the strength of magnetic influence. The unit is named in honor of Nikola Tesla ( ), the Croatianborn American electrical engineer, who did pioneering work with magnetism and alternating current (AC) electricity. M-42

47 Activity 4: Better Model for Magnetism Now imagine the four small magnets were turned so they were all oriented in different directions, as shown here. Do you think the meter reading for this arrangement would be less than, equal to, or greater than it was when the magnets were all oriented in the same way? Why do you think so? To check your thinking, watch UM-A4 Movie 2, in which the four magnets are rearranged as shown above. What is the meter reading in this arrangement and does this agree with your prediction above? How does the reading compare to that from one single magnet? Why do you think it is so low? STEP 5. It seems that a collection of small magnets can produce different magnetic effects, depending on how they are oriented with respect to each other. Suppose you had a large number of small magnets. How would you arrange them to produce the strongest possible magnetic effect? How would you arrange them to produce little or no magnetic effect? M-43

48 Unit M Exploration #2: How can the orientation of small magnets be changed? You have now seen how the magnetic influence of a collection of small magnets can be changed by arranging/orienting them differently. Perhaps this has given you an idea for how to change your model for what happens when a nail is magnetized. However, before considering how to revise your model (if at all) let us think about how some small randomly oriented magnets could all be made to align in the same direction. In Exploration #1 you did this by physically manipulating the small magnets (magnetized nails) with your hands, but is there a different way to do it? You will need: access to computer with internet connection bar magnet test tube partially filled with iron filings and taped shut magnetic compass hammer and scrap wood STEP 1. Imagine you have a small magnet that is fixed in position, but is free to pivot (rotate) around its center (like a compass needle.) How could you use a second magnet to make the pivoting magnet rotate so its N-pole points in a particular direction? Go to the PET Student Resources web page and open up UMA4 Movie 3. It shows a simulation of bar magnet, the south pole of which is just above and to the left of another magnet that cannot move, but can rotate around a pivot through its center Play the movie and watch the behavior of the pivoting magnet as the bar magnet is dragged across the screen from left to right above it. Describe what happens to the magnet on the pivot as the bar magnet is dragged from left to right. M-44

49 Activity 4: Better Model for Magnetism Which end of the magnet on the pivot is its N-pole: the end with the small dot, or the end without the small dot? How do you know? STEP 2. You have now seen how a magnet could be used to change the orientation of a single small magnet that is fixed in place, but free to pivot around its center. But how could this help to align a large number of randomly oriented small magnets? In thinking about this consider how it might help you think about your model for magnetism. You should have a test tube that is partially filled with iron filings, each of which is like a very tiny magnet. Shake the test tube a few times and then hold it horizontally so the filings are deposited all along the test tube, as shown here. (Your tube might not be this full.) If it is not already, set up your compass so that the needle is aligned with the N label on the case. Lay the test tube on the table and slide the rounded end slowly towards the end of the E label of the compass, from the side. See picture below. Describe what happens, if anything, to the compass needle as the test tube approaches the compass. Try sliding the other end of the tube toward the compass also. Do your observations suggest that the test tube currently behaves more like a magnetized nail or more like an unmagnetized nail? M-45

50 Unit M STEP 3. Imagine that the rounded end of the test tube is like the pointed end of a nail, and the taped end of the test tube is like the head of the nail. Rub the test tube with your bar magnet in the same way that you would rub a nail when trying to make the pointed end a north pole. As you slide the end of the magnet slowly along the test tube as shown here (Npole from tip to head), carefully observe what happens to the iron filings. (However, do not do it so slowly that the filings rise up and follow the magnet all the way to the other end of the test tube.) Repeat this process a few times as you answer the following questions. When you are finished, remember to place the magnet far away. Describe the behavior of the iron filings near the top of the layer as the magnet is dragged across the test tube. (Assuming they are laying down before the magnet approaches, what happens as the magnet passes over them? What about after the magnet has passed?) Draw a series of pictures of the behavior of a few filings as the magnet passes over them to help your description. How is the behavior of at least some of the iron filings similar to that of the pivoting magnet in the simulator in STEP 1? M-46

51 Activity 4: Better Model for Magnetism STEP 4. Without shaking it, very carefully lay the test tube on the table and slide its rounded end towards the E-label of the compass, from the side. (Same procedure as in STEP 2 above, but do not shake the tube.) Then carefully turn the tube around and slide the other end toward the compass. Describe what happens to the compass needle in the two cases. Does the rubbed test tube behave more like a magnetized nail or more like an unmagnetized nail? If it behaves more like a rubbed nail, which end of the test tube behaves like a N-pole, and which end behaves like a S-pole? STEP 5. Shake the test tube vigorously. Lay it on the table and repeat STEP 4. Describe what happens to the compass needle now as the test tube approaches it. Does the shaken test tube now behave more like a magnetized nail or an unmagnetized nail? Why do you think this is? STEP 6. You have seen that after rubbing the test tube of iron filings with a magnet it behaved like a magnetized nail. However, after shaking it again, it behaved more like an unmagnetized nail. (We say it was demagnetized.) Using the test tube of iron filings as an analogy for the magnetized nail, suggest a way you could demagnetize a rubbed nail (or at least greatly weaken its magnetic strength ). Explain why you think this would work. M-47

52 Unit M Take one of your magnetized nails (or rub an unrubbed nail) and check its magnetic strength using the compass. Then try out your idea for demagnetizing it and test it again. (If your idea does not work well, try dropping the nail on the floor a few times, or hitting it with a hard object such as a hammer or a block of wood.) Did you succeed in demagnetizing the nail, or at least greatly diminishing its magnetic strength? Revising your model again To remind you, the observations you made in this activity were NOT intended as phenomena you need to explain with your model. Rather they were intended to promote thinking about possible alternative ideas on which to base your model. In particular, since the test tube with iron filings behaved very much like a nail, it can also serve as an analogy for the nail. Consider all the observations you made in this activity. How might your observations of what happens inside the test tube when you rub it with a magnet suggest what the entities inside the nail might be like and what might be happening inside the nail when you rub it with a magnet? Discuss with your group how you might revise your model (if necessary) to incorporate these ideas and so better account for all the evidence you have seen in this unit. Again, your instructor may give you a separate worksheet to help with this. When your group has decided on a new best model move on to answer the summarizing questions. M-48

53 Activity 4: Better Model for Magnetism Summarizing Questions S1. a) Sketch and briefly describe your revised model for an unmagnetized and a magnetized nail. (If you think your model from the end of the previous activity is still good, just redraw that.) Unmagnetized Magnetized Description: b) How does your model explain that an unmagnetized nail is one-ended but a magnetized nail is two-ended? c) How does your model explain that when a magnetized nail is cut into two or more pieces, each piece is two-ended, no matter where the cuts are made? Participate in a whole class discussion as groups present their latest model and the class tries to come to consensus on a best model. M-49

54 Unit M If the class consensus model is different from your group s final model, sketch and describe it below. Unmagnetized Magnetized Description: How does this model explain that an unmagnetized nail is one-ended but a magnetized nail is two-ended? How does this model explain that when a magnetized nail is cut into two or more pieces, each piece is two-ended, no matter where the cuts are made? M-50

55 UNIT M Applying Ideas ACTIVITY 5: Explaining Magnetic Phenomena By now the class has likely reached consensus on a model of magnetism that should explain all the observations that have been made thus far. This model is likely to be fairly closely aligned with models that scientists have developed, because it is based on the same evidence. The class model is likely based on the alignment of entities in a ferromagnetic material that behave like tiny magnets. Scientist s models are also based on this idea and they refer to these tiny magnets as domains. Thus, we will refer to the class consensus model as either the alignment of tiny magnets model or the alignment of domains model, and in this activity you will apply it to make some predictions and explain some other phenomena. Using your model In this activity you should use the class consensus model (alignment of tiny magnets model) to help guide your thinking as you make predictions and explain new observations. You will need: bar magnet nails (unrubbed) magnetic compass other materials as necessary Exploration #1: How can you rub a nail to ensure a particular end becomes a north pole? STEP 1. You know that you can magnetize a nail by rubbing it with one pole of a bar magnet, but how can you use your model to predict which end of the nail will become while pole (N or S)? Discuss with your group and, based on the class consensus model, predict two different ways that you could rub a nail with a bar magnet so that the point end becomes a South Pole. [Note: You can use either end of the bar magnet to do the rubbing, and you can rub the nail either direction, from head to point or the other way.] 2016 Next Gen PET M-51

56 Unit M Below, draw pictures showing which direction you propose to rub the nail, and which end of the bar magnet you will use. Also briefly explain your reasoning. STEP 2. After making your two predictions, try them out! Use your compass and apply the Law of Magnetic Poles to check if your tip is indeed a South Pole. Did both your predicted methods work to make the tip of the nail a South Pole? If not, experiment with some other methods until you have two that do work and draw and explain them below. What are the characteristics of a good scientific explanation? In many activities throughout this course, after you have developed some ideas about how the world works (a model) you will be asked to either write or evaluate an explanation in terms of those ideas. For example, after Exploration #1, suppose you were now asked to write a scientific explanation for why rubbing an unmagnetized nail in a particular way results in the tip of the nail becoming a S-pole. While you have already been writing some informal explanations in this unit we now introduce a more formal structure for writing a particular type of explanation that is common in science. M-52

57 Activity 5: Explaining Magnetic Phenomena Though there are many different types of explanation in science, people generally understand a scientific explanation to be a logical argument as to why a real-world event (that has already been observed) happened as it did. Such an explanation is essentially a claim that the event can be explained in a particular way, with a supporting argument constructed using a particular model. In this unit the model used would be the class consensus model (alignment of tiny magnets model). In general a good explanation should include one or more diagrams and a written part (narrative). I. Represent the model using diagrams Scientists find it useful to represent their ideas using diagrams. In this unit the class developed a model of magnetism that can be represented visually by showing how some magnetic entities are arranged inside a ferromagnetic object. Therefore, it will usually be appropriate to start your explanation by drawing one or more such diagrams. (In later units you will be introduced to different types of diagrams that will be appropriate for the models developed in those topics.) However, since diagrams are static in nature the model-based reasoning that explains how and why things occurred as they did is given in a written narrative. II. Write the Explanation Narrative When you are trying to explain why a certain initial action in the real world leads to a particular observable outcome, your written narrative should use your model to clearly connect the two. To do this you should describe what effect this initial action would have in terms of the model, including reasoning as to why this would happen. The effect on the model should also be connected explicitly to the observable outcome that was to be explained, again including reasoning as to why this is so. One way to visualize the structure of such a narrative is shown to the right. M-53

58 Unit M For example, as you probably discovered in Exploration #1, you can magnetize a nail so that its tip becomes a south pole by rubbing it from tip to head with the S-pole of a bar magnet. The initial action in this case is therefore the rubbing of the nail in this particular way and the observable outcome is that the tip of the nail is now a S- pole. As you can probably already imagine, this can be explained using the alignment of tiny magnets model by saying that rubbing the nail in this way aligns all the domains (tiny magnets) in such a direction as to magnetize the nail with the tip end becoming a S-pole. Therefore the general structure of a formal scientific explanation for why this occurs should follow that shown to the right. As you draw diagrams and write an explanation narrative, you should keep the following three criteria in mind. Your explanation should be: Well-Constructed: Your diagrams should be clear and easy to read and your narrative should be well written and easy to follow. Also, the diagrams and narrative should be consistent with each other. Your explanation should be relevant and focus on the phenomenon that is to be explained. (Other things that happened either before or after, or are not important for the particular phenomenon to be explained, may be briefly mentioned, but should not be a major part.) Accurate: All ideas used in the explanation should correspond to the class consensus model, and other agreed-upon ideas (such as the Law of Magnetic Poles). Well-Reasoned: In your narrative, connections between different steps should be supported using reasoning that is both logical and consistent with established ideas. (For example, plausible reasons should be given for why the magnetic entities involved become rearranged.) These criteria should also be used to evaluate the explanations (diagrams and narratives) written by others. If these criteria are met, the explanation can be considered good. If at least one aspect of these criteria is not met the explanation should be considered problematic and in need of revision. M-54

59 Activity 5: Explaining Magnetic Phenomena Let us now look at an example of such an explanation. Explanation: Why does rubbing a nail from tip to head with the S-pole of a bar magnet result in the tip of the nail becoming a S-pole? Represent the model using diagrams: Before Rubbing After Rubbing Write the narrative: Before the nail is rubbed, the magnetic domains (tiny magnets) inside it are aligned randomly, which means the nail is unmagnetized. When the S-pole of a magnet is rubbed from the tip to head of the nail it attracts the north poles of all the tiny magnets and rotates them so they all line up with their north poles facing the head of the nail and their south poles facing the tip. Because the south poles of the tiny magnets are all facing the tip of the nail, the nail is now magnetized with this end being the S-pole. Examine this explanation and answer the following questions about it. Is the explanation well-constructed? (Is it easy to read and are the diagram and narrative consistent with each other? Does it focus mainly on explaining why the tip of the nail becomes a S-pole?) Is the explanation accurate? (Does it use the alignment of tiny magnets model and apply the Law of Magnetic Poles correctly?) Is the explanation well-reasoned in terms of: a) Is a plausible reason given as to why the tiny magnets (domains) in the nail align as shown in the diagram? If so, highlight the part of the narrative where this is done. M-55

60 Unit M b) Is a reason given as to why the particular alignment of tiny magnets shown means that the tip end of the nail is a S-pole? If so, use a different color to highlight where in the narrative this is done. Do you think this explanation is good or problematic? Why do you think so? Exploration #2: Can you magnetize a nail without touching it? Up to now, in order to magnetize a nail, you have physically rubbed it with a magnet, but could a nail be magnetized without touching? STEP 1. Imagine that you hold the North Pole of the bar magnet about 0.5 cm away from the tip of an unmagnetized nail for about 5 seconds, as shown. The magnet and nail do not touch. Then you remove the magnet and put it away. Do you think the nail would remain unmagnetized, or would it now be magnetized? If the latter, would its tip end be a North Pole or a South Pole? Explain your prediction in terms of the class model. STEP 2. To test your prediction, first get an unrubbed nail. Check that the nail is unmagnetized by sliding each end toward the E-label on an appropriately oriented compass. [If it is already magnetized, then you need to try another nail.] M-56

61 Activity 5: Explaining Magnetic Phenomena Next, hold the tip of the nail close to (but not touching) the N-Pole of a magnet for a few seconds. Be sure to hold both the nail and the magnet so they don t move towards each other and touch. If they do touch, start over with another unrubbed nail. Remove the magnet and place it far away. Then slide each end of the nail toward the E-label on your compass again. What happens to the compass needle as each end approaches it? Is the nail magnetized or not? If magnetized, which end is its North Pole and which end its South Pole? Does this agree with your prediction? If not, briefly explain, in terms of the class consensus model, what you think happened when the nail was held close to the magnet. Throughout this unit you have been told several times to be very careful to keep your magnet far away from any unrubbed nails. Given the result you have just seen, why do you think this is? M-57

62 Unit M STEP 3. Now imagine that you took the same nail you just used (already magnetized), and held its tip about 0.5 cm away from the South Pole of the bar magnet for 5 seconds. After removing the magnet, would you expect the nail to be magnetized or not? If you think it would be magnetized, which end (tip or head) would be the North Pole? Again, explain your prediction in terms of the class model. Try it, and then test the nail with the compass in the usual way. Describe what happens. Which end of the nail is a north pole now? Is this result consistent with your prediction? If not, briefly explain, in terms of the class consensus model, what you think happened when the south pole (tip end) of the already magnetized nail was held close to the south pole of the magnet. Recall that some ferromagnetic materials (e.g. steel, which is an alloy containing iron) can be easily magnetized and can easily have their poles reversed (as you saw above with the nails). It is also relatively easy to demagnetize them. Such materials, when magnetized, are known as temporary M-58

63 Activity 5: Explaining Magnetic Phenomena magnets. A magnetized nail is an example of a temporary magnet. Certain other kinds of ferromagnetic materials (e.g. Alnico) can be made into permanent magnets. These materials are difficult to magnetize, but once magnetized, they are difficult to demagnetize and they retain their Pole orientation for a very long time. Bar magnets and compass needles are examples of permanent magnets. STEP 4. One observation you made earlier in this unit is that unmagnetized ferromagnetic materials are attracted toward both ends of a magnet. You should now be in position to use the class model to construct a scientific explanation for this phenomenon. Explanation: Why do both ends of a magnet attract an unmagnetized ferromagnetic object (such as a steel paper clip). (Hint: You can think of the paperclip as being similar to an unmagnetized nail.) Represent the model using diagrams: (We suggest three diagrams of the paper clip. One before the magnet is near, one with the N-pole of a magnet near it and one with the S-pole of a magnet near it.) (Space to write your narrative is on the next page.) M-59

64 Unit M Write the narrative: (Use the suggested structure shown to the right to help you. Be sure to explain why both poles of the magnet attract the paper clip.) Note that in a similar way you could also explain why refrigerator magnets stick to steel refrigerators, or how Etch-A-Sketch -type toys work (a magnet attracts tiny particles of iron). Exploration #3: How do the strengths of the magnetic influence of different parts of a bar magnet compare? STEP 1. You know that the ends of a bar magnet have a strong magnetic influence, but what about the middle? Do you think the magnetic influence of the middle of a bar magnet is weaker or stronger than that of the ends, or do you think the middle and ends have about the same strength? Use the class consensus model, including a diagram, to support your prediction. M-60

65 Activity 5: Explaining Magnetic Phenomena STEP 2. Design an experiment to find out whether the middle of the magnet is stronger, weaker, or equally as strong as the ends. Describe what you did and also your results. STEP 3. A group of students did their own experiment, in which they saw that the ends of bar magnet could pick up many more paper clips than the middle. They then wrote the following explanation for why this is. In the magnet there are lots of N particles and S particles. Because the magnet is magnetized, at one end there are lots of N particles so it is the N-pole of the magnet. At the other end there are lots of S particles, so it is the S-pole. The magnetic effects of all the particles in each end add together to give each end of the magnet a strong magnetic influence so they are able to pick up lots of paper clips. However, in the middle there is a mix of N and S particles so their magnetic effects cancel out to give a very weak magnetic influence, so it is not able to pick up many paper clips. You should now evaluate this explanation according to the suggested criteria. You should also briefly explain your answer in each case. Is the explanation well-constructed? M-61

66 Unit M Is the explanation accurate? Is the explanation well-reasoned? Do you think this explanation is good or problematic? Why do you think so? STEP 4. If you found this explanation to be problematic, construct your own explanation below. Represent the model using a diagram: Write the narrative:! Participate in a class discussion to go over the explanations in this activity. M-62

67 UNIT M Engineering Design ACTIVITY 6: The Maglev System Designing a model maglev train system Maglev train in Japan (Image Courtesy of Yosemite, GNU Free Documentation License.) The Maglev Train The maglev (short for magnetic levitation) train has long been the holy grail of ground transportation. By using magnetic effects to levitate above their track, maglev trains need no wheels and have no friction with the track, resulting in an ultra-fast and ultra-quiet ride. Go to the PET Student Resources web page and open up UMA6 Movie 1 to watch a short video of a Japanese maglev train being tested in So far maglev trains are also very expensive. Counting a planned Tokyo-to- Osaka leg, the Japanese maglev project is expected to cost upwards of $100 billion. If that sounds prohibitive, consider that the United States spends significantly more than that on highways in a single year. And while driving on the highway might get you from Los Angeles to San Francisco in six hours (if you're lucky), a maglev train like the one Japan's building could theoretically do it in an hour and 15 minutes. In fact, California has been trying to build a Los Angeles-to-San Francisco high-speed rail line for some 30 years, but the fight for funding has been tooth-and-nail. The state is now slated to have a 220-mph train up and running by 2028 but that's just a conventional bullet train, the kind Japan has had for decades. There were once plans for a California-Nevada maglev train, but that never left the station, and the money for planning it ended up being reallocated to a highway project. (Future Tense, November 30, ) 1 By Will Oremus (2012). Why can't we have a 300-MPH floating train like Japan? Future Tense: The Citizens Guide to the Future. Retrieved from: /11/30/japan_s_300_mph_maglev_train_why_can_t_the_us_build_high_speed_rail.html 2016 Next Gen PET UM-63

68 Unit M Although it could be argued that the US is falling behind other countries in technological accomplishments, a renewed focus on engineering as a part of science education could make a difference, at least in the next generation. Thus, engineering is being included in new science standards being introduced by many states (such as the Next Generation Science Standards 2 ) and involves more than robotics competitions and building model bridges. Students of all ages are expected to be able to apply the science they have learned to understand how technologies work and use their knowledge to design devices or procedures that address practical problems. Engineering design The details of the engineering design process can look very different depending on the context in which it is applied. However all engineering projects follow the same basic steps: 1. A problem is defined, consisting of an identified need, the goals to be achieved (criteria for success), and the constraints (limitations) that will be imposed. 2. Solutions to the problem are developed through a process of brainstorming possible approaches, and then considering how well each is likely to meet the goals (within the constraints). Promising solutions are then tested to check their feasibility. 3. The most promising solution is then optimized so it best meets the goals, while staying within the constraints. (Often this involves compromise between and among these goals and constraints.) At various points in the Next Gen PET materials, after you have developed some ideas about the science involved, you will apply your ideas in a short engineering design project. Your Maglev project In this activity you will use the ideas you have developed about magnetic interactions to design and build a simple prototype maglev system that can carry a small load over a short distance. Before you begin the design process your 2 NGSS Lead States (2013). Next Generation Science Standards: Practices, Core Ideas, and Crosscutting Concepts. Washington, DC: National Academy Press. UM-64

69 Activity 6: The Maglev System instructor will show you the materials that will be available to you and explain what your prototype maglev should be able to do. The materials you will use are: Sturdy material such as foam board or cardboard. You will use this to make a platform that will represent a maglev train car. A box or tray in which you will lay your magnetic track and within which your platform will run. Strips of magnetic tape that can be used to levitate the platform. Tape (masking, transparent or double-sided) Scissors (for cutting the card and tape) Small object (your choice) that can be carried by your platform You will now start the design process. Since this is probably your first experience of engineering design we have completed the initial steps for you. Read through these carefully as they will help you in the brainstorming part of the process that comes next. Define the problem, goals, and constraints Problem: To transport a small object from one end of the box/tray to the other on a platform that is levitated above a magnetic track at the bottom of the box/tray. Goals: Your prototype maglev system is successful if: The platform levitates, and is stable, at all places in the box/tray. The platform can be moved from one end of the box/tray to the other and remain levitated at all points. The levitated platform can carry a small object as it is moved from one end of the box/tray to the other. Constraints: The limitations you will place on your design are: Instead of magnetic propulsion, you may use your hand to move the platform along in the box/tray. To minimize cost you should use the smallest amount of magnetic strip possible. UM-65

70 Unit M Develop solutions Now you should brainstorm possible solutions. First think about what type of magnets you think would be best to use to levitate your platform above the track. Suppose you go into a store that sells a wide variety of magnets. You are looking for strips of magnetic material or magnets that you can lay along the bottom of the box/tray and also fix to the platform. The sales person gives you two choices (A and B). Given the goals for your project, which type of magnetic strip do you choose and why? A Flat strips that are magnetized so that the top surface is one pole and the bottom surface is the other pole. B N (Top) S (Bottom) Flat strips that are magnetized so that one end is one pole and the other pole is on the other end. S N Now sketch how you propose to arrange the magnetic strips to construct your magnetic track and platform for your model maglev train system. Label the sketch and briefly explain how your arrangement works to produce levitation. UM-66

71 Activity 6: The Maglev System Optimize the most promising solution Now obtain the materials from your instructor and build a prototype of your design. If you have to make any modifications to your original design, make note of them below and explain why the modification was needed. After you have a working version, show your instructor your model. Sketch your final design for both the track and train car together. Explain how it works. UM-67

72 Unit M Elementary children and engineering design You may be asking yourself whether elementary school children can really engage in the engineering design process. Go to the PET Student Resources web page and open up UMA6 Movie 2 to watch a video of fourth graders in Medford, MA using their knowledge of the properties of magnets to design a maglev transportation system 3. You should start watching from a time code of 04:20. After you have watched the video answer the following questions. Were the children successful in designing a simple prototype maglev system? How did the children s solution compare to yours? Was it similar or very different? What evidence did you see that they were engaging in any of the steps of the engineering design process? 3 The curriculum this class is using is called Engineering is Elementary (EiE), which was developed at the Museum of Science in Boston, MA. To find out more about these materials go to: UM-68

73 Activity 6: The Maglev System Challenge questions It is also important for engineers to consider what might go wrong in a situation (such as natural disasters) involving their designs. Use the model of magnetism you developed in this unit to consider these questions about a real maglev train system. 1. Imagine that you ve gone to Japan and you ve been invited to ride in the control room of a maglev train. You re traveling at 300 miles per hour and one of the engineers reads a gauge that says one of the magnets under the train has cracked. Should you worry? Why or why not? (Apply ideas from Unit M Activity 3: Developing a Model for Magnetism.) 2. A radio call reports that catastrophe was narrowly averted when a fire on the line 50 miles ahead was put out. The crew laughs about it and assures you not to worry, that you will make it to Osaka on time. What do you say to them? (Apply ideas from Unit M Extension C: Explaining Another Magnetic Phenomenon.) UM-69

74 Unit M UM-70

75 Next%Generation%Physical%Science% and%everyday%thinking%! Magnetism%and%Static% Electricity%Module% Unit!SE! Developing!a!Model!! for!static!electricity!!!!! Studio7style!Class!!!

76 Unit SE: Developing a Model for Static Electricity Table of Contents Activity # Activity (A) Title Page A1 Exploring Static Electric Effects SE-1 Ext A 1 Which Charge is Which? online Ext B The Law of Electric Charges online A2 Developing a Model for Static Electricity SE-11 A3 Ext C Representing Uncharged Objects in Your Model Electroscope and Negatively ( ) Charged Object SE-25 online Ext D What do the Charged Entities Represent? online A4 Ext E A5 A6 Refining Your Model for Different Materials What Happens When a Charged Object is Discharged? Interactions Between Charged and Uncharged Objects Explaining Phenomena Involving Static Electricity SE-39 online SE-47 SE-59 A7 ED Engineering Design: Refueling Safety SE-67 1 Extensions (Ext s) are online homework activities.

77 UNIT SE Developing Ideas ACTIVITY 1: Exploring Static Electric Effects Purpose In the previous unit you explored some magnetic effects and then went on to develop a model that explains these effects in terms of tiny entities within magnetic materials. You are also likely familiar with some other phenomena, usually associated with static electricity, like the static cling by which clothes stick together when you remove them from a dryer, or the shock you receive when you walk across a carpet and then touch a metal door handle. In this unit you will develop another model to explain some effects associated with static electricity. To start, in this activity you will observe some static electric effects and look for some patterns on which to base your initial model 1. What are some properties of interactions involving electrified objects? Initial Ideas In the previous unit you found that only certain materials interact with a magnet. Will it be only these same materials that interact with electrified objects, or will different materials show static electric effects? What kinds of materials do you think can be involved in static electric effects; all materials (metals and non-metals), all metals but not nonmetals, or only certain specific materials? 1 Because static electric effects are sometimes difficult to observe in humid conditions, your instructor may direct you to watch movies of some, or all, of these experiments on the Next Gen PET Student Resources website Next Gen PET SE-1

78 Unit SE In the previous unit you also found that two particular magnetized objects can both attract and repel each other, depending on which parts of them (poles) are brought near to each other? Do you think the same is true of electrified objects? If you were to bring two electrified objects close together, how do you think they would react to each other? Would your answer depend on which parts of the electrified objects were close to each other?! Participate in a whole class discussion about the answers to these questions. Make a note of any ideas that are different from those of your group. Collecting and Interpreting Evidence Your group will need: Roll of sticky tape Pen, or other permanent marker Support stand from which to hang tape. (This could be a meter stick projecting beyond the edge of a table.) Various materials to test Balloon Exploration #1: What types of objects show static electric effects? STEP 1. You are no doubt aware that some objects can be electrified by rubbing them, but for these experiments you will use a different technique to electrify two pieces of sticky tape. SE-2

79 Activity 1: Exploring Static Electric Effects Read through the following steps first, and then go through them quickly, but carefully. Static electricity effects sometimes wear off quickly, so if you don t observe any types of interactions you might consider re-electrifying the tapes. (If your instructor has directed you to watch a movie instead of trying this yourself, you should watch USE-A1 Movie 1.) Prepare two pieces of sticky tape, each about 5 inches long. Fold over about 1 /2 inch of both ends of both pieces of sticky tape. These ends will serve as handles that will allow you to work with the tape without touching the sticky surfaces. Place one of the pieces of tape on the desk in front of you, sticky side down. Using a pen, or other permanent marker, label one of the handles on this piece B (for Bottom). Now place a second piece of tape directly on top of the first, again sticky side down. Label this piece T (for Top). Press your finger over the two pieces to make sure they are firmly stuck together. (The bottom piece will also be stuck to the table, but that is not important.) Now slowly peel both pieces of tape, still stuck together, from the table, (If the two pieces of tape become separated press them firmly together again.) Holding a handle on each piece of tape in each hand, quickly rip them apart. After separating the tapes, keep them far enough from each other so they don t touch again. Attach them to the support stand so they hang straight down below it, (or have one of your group hold one tape in each hand.) The act of ripping the two tapes apart should have electrified both pieces. Later you will try to explain why this happens, but for now you will just look for how these electrified tapes interact with non-electrified objects, if at all. SE-3

80 Unit SE STEP 2. To find out how the various materials in your envelope interact with the electrified tapes, you should slowly bring each one close to the bottom of each of the two tapes in turn. As soon as you see any reaction from the tape, pull the object away again. Try not to let the tape touch any of the objects. For each item, record in the table below whether the tape is attracted (A) to it, repelled (R) from it, or there is no effect (O). Add two other items of your own choice to the table and test them. Finally, bring the tip of your finger close to each tape to see if there is any reaction. Table I: Observations of Electrified Tapes near Objects (A, R or O) Wooden Iron Plastic Aluminum Copper Nickel Finger strip nail pen/ruler foil strip wire strip T-tape B-tape Check your observations with at least two other groups and try to resolve any differences. What do your observations seem to show about what types of materials can interact with electrified objects? Is this the same as, or different from, the types of materials that interact with magnets? When Benjamin Franklin experimented with electrified objects he imagined them as containing some type of electrical fluid and so said they were charged (as in charge [fill] your glasses for a toast ) when describing them. While Franklin s use of charged is probably different from the sense in which most people think of it today, we still use his terminology. Thus, from now on we will refer to electrified objects as being charged with static electricity. SE-4

81 Activity 1: Exploring Static Electric Effects Exploration #2: How do electrically charged objects interact with each other? STEP 1. You have now seen what happens when an uncharged object is brought near a charged object. But what would happen if two charged objects were brought near each other? Do you think they would behave like two magnets (which attract or repel depending on which ends/faces are brought close) or would they behave in a different manner? Explain your thinking. STEP 2. To check your thinking, prepare a new pair of charged B and T tapes, just as you did in Exploration #1. Now, slowly bring the non-sticky surfaces toward each other. (If not doing this yourself, watch USE-A1 - Movie 2.) As soon as you see any reaction, move them apart again. It is important to try not to let the tapes touch each other! (If they do, you may have to go through the whole charging process again!) What happens as the B and T tapes approach each other? Do they attract, repel, or is there no reaction? Now turn one of the tapes upside-down, hold it by its other end and repeat. Next, turn one of the tapes around so its sticky surface side faces the nonsticky side of the other tape, and bring them together again. Finally, bring both sticky sides together. Do the results depend which ends/faces are tested, or does the same thing always happen? SE-5

82 Unit SE Now go to the PET Student Resources website and watch USE-A1 - Movie 3, that shows an experiment involving two plastic coffee stirrers being tested in the same way that you tested nails in Unit M. To distinguish the ends, one end of each stirrer will have a small piece of tape attached to it. One of the stirrers will be electrically charged by rubbing it all over with wool, and then placed on a floating disk. The second stirrer will be charged in the same manner and then both ends of it will be brought close to both ends of the floating charged stirrer. Does what happens depend on which ends of the stirrers are tested, or does the same thing always happen regardless of the ends used? Do charged objects seem to be one-ended (all parts of a charged object behave in the same way), two-ended (the two ends or faces of a single charged object behave differently), or something else? How do your observations with both the charged tapes and the charged stirrers support your answer? What do your observations imply about the electric charge on a charged object? Does such an object have only one type of charge all over it, or different types of charge in different places? STEP 3. You have seen that during rubbing with wool, and the peeling apart of two tapes, objects involved become charged with static electricity. But is there only one type of charge, or is there more than one and if so, how many are there? SE-6 Suppose you prepared two pairs of charged tapes (call them T1/B1 and T2/B2) and brought tapes T1 and T2 together. What do you think would happen and why?

83 Activity 1: Exploring Static Electric Effects To check your thinking prepare two pairs of charged tapes, labeling them B1, T1, B2, and T2, and hang them from your support stand (or watch USE-A1 - Movie 4). Alternatively, just have two group members hold the tapes in their hands. Now bring the tapes toward each other in the various combinations corresponding to the cells in Table II below. Remember to work quickly, but carefully. Electric charge effects sometimes wear off quickly, so if you don t observe any types of interactions you might consider re-charging the tapes. If you have difficulty making these observations a video of the experiments is available (USE-A1 Movie 4). Record the results of all the tests in Table II below. (Enter A for attract, R for repel, or O for no reaction.) Table II: Observations with Charged Tapes B2 T2 B1 T1 Check your observations with at least two other groups and try to resolve any differences. What do the results from these experiments with charged tapes suggest about the number of types of charge involved and how they interact with each other? SE-7

84 Unit SE STEP 4: Now you will check whether the ideas you have developed about charges using the pairs of tapes also apply to objects charged by rubbing them together. Blow up the balloon. Rub one side vigorously against your hair. (It s best to use a member of your group who has long, straight and dry hair.) (Alternatively, watch USE-A1 - Movie 5.) After moving the balloon away from your hair, quickly bring the rubbed part of the balloon close to the rubbed part of your hair again. Does anything happen to your hair? If so, what? Why do you think this happens? Prepare a new pair of charged B and T tapes and hang them from the support. Rub the balloon on your hair again and quickly bring the rubbed part of the balloon close to the B tape and then the T tape. What happens to the B tape when the balloon is brought near? Is it attracted, repelled, or does nothing happen? What happens to the T tape when the balloon is brought near? Is it attracted, repelled, or does nothing happen? Do you think the rubbed part of the balloon has the same type of charge as the T tape, the B tape, or a different type than both? Why do you think so? SE-8

85 Activity 1: Exploring Static Electric Effects Finally, watch USE-A1 - Movie 6, in which a Styrofoam TM plate and an acrylic sheet (a type of clear plastic) are rubbed together and each brought toward a pair of charged B and T tapes. Describe how both tapes behave when the rubbed Styrofoam TM plate is brought near. Describe how both tapes behave when the rubbed acrylic sheet is brought near. Do these results suggest that the rubbed plate has the same type of charge as the B tape, the T tape, or some different type of charge? What about the rubbed acrylic sheet? Summarizing Questions Discuss the following questions with your group and note your answers. Be sure to support all of your answers with evidence from this activity. S1: How many types of electric charge are there and how do they interact with each other? S2: When an object becomes charged (either by rubbing or peeling) is it oneended (same type of charge all over), two-ended (two different types of charge at different locations), or something different? SE-9

86 Unit SE S3: When two objects are charged, either by rubbing together or peeling apart, do they both have the same type of charge or does each have a different type of charge? S4: Suppose you and your neighbors both rubbed a Styrofoam TM plate with an acrylic sheet and then brought the two Styrofoam TM plates together. What do you think would happen and why? How do you know? S5: If you rub a balloon on your hair, it will pick up some small pieces of paper when held a short distance above them? Given all the evidence you have seen in this activity, what can you say about whether the paper is charged with the same or opposite type of charge to the balloon, or could it be uncharged? S6: How are the interactions involving charged objects similar to those involving magnets? How are they different? Does this suggest that static electric interactions and magnetic interactions are really the same thing, or that they should be treated as two separate interactions? Participate in a class discussion. SE-10

87 UNIT SE Developing Ideas ACTIVITY 2: Your Initial Model for Static Electricity Purpose The goal of the previous unit was to develop and test a model for magnetism that can account for what happens when a nail is rubbed with a magnet. In this activity you will begin the process of developing a model for static electricity that can account for how objects become charged with static electricity when they are rubbed together (or tapes pulled apart) and why they behave as they do when interacting with other objects, both charged and uncharged. You know from the previous activity that there seem to be two types of electric charge, just as there are two types of magnetic poles. In models for magnetism we use north (N) and south (S) symbols. However, because the static electric interactions are different from magnetic interactions in at least some ways, we should use different symbols when representing charged materials. To make this distinction we suggest you use positive (+) and negative ( ) symbols in your models. (The origin of these terms is discussed in a homework assignment you may already have completed.) Initial Ideas How can you construct a model of static electricity and use it to explain your observations? You have seen that when an uncharged Styrofoam TM plate is rubbed with an uncharged acrylic sheet, they both become charged. According to our naming convention (discussed in a homework assignment after the previous activity) the acrylic becomes positively (+) charged and the Styrofoam TM becomes negatively ( ) charged. Discuss with your group members what might be different about the Styrofoam TM and acrylic before and after they were rubbed together and how you can use this to explain some of the observations you made in the previous activity Next Gen PET SE-11

88 Unit SE In particular, think about what entities might be inside and/or on the surface of the plate and sheet before and after rubbing and use + and symbols to represent them. Your instructor may give you a worksheet to help you develop your initial model. Use the two drawings of the plate and sheet below to show your group s thinking, representing their states before and after they were charged. (The drawings represent a cross-section through the plate and sheet.) Your group s initial model: Before rubbing together After rubbing together Briefly describe your initial model. Be sure to include your assumptions about any entities involved. (What do they represent? Where they are located? Can they move or not and, if so, how?) Use your model to briefly explain the observation that the Styrofoam and acrylic become oppositely charged when rubbed together, and that each of them is one-ended. SE-12

89 Activity 2: Initial Model for Static Electricity Prepare a presentation board showing your explanation (diagrams and narrative) for the class discussion. Participate in a whole class discussion. After listening to other groups present their models and explanations draw and describe what you consider to be the best model below. Your group s best model: Before rubbing together After rubbing together Description: Why do you think this is the best model? SE-13

90 Unit SE Collecting and Interpreting Evidence Note: While it would be good to obtain your evidence from experiments you do yourself, it is notoriously difficult to obtain reliable and consistent results with static electricity experiments in humid conditions, such as a classroom full of people. (You will consider why this might be later in this activity.) Therefore, in this activity, you will mostly be making observations of experiments from videos made under controlled conditions. These can all be accessed from the Next Gen PET Student Resources website. Exploration #1: What happens when a charged object touches an uncharged object? In the Initial Ideas section you proposed a model that you believe best explains some observations you have already made. As stated before, an important criterion of a good model is that it can also help you make predictions about new experiments. Important: In the remainder of this lesson you will use your model to make predictions, and then test them. In each case, you must base your prediction on your current model. Do not change your model as a result of just thinking about the experiment. If the outcome of the experiment turns out to be exactly what you had predicted, then there is no need to revise your model. On the other hand, if the outcome is different from your prediction, even in small ways, then you need to consider how to revise your model. STEP 1. The experiments you will consider shortly use a device called an electroscope, made from an empty metal soda can taped to an upturned Styrofoam cup. Some loose strands of tinsel (which is effectively thin strips of metal) are hung from the ring-pull tab of the can so that the ends are free to move. Go to the Next Gen PET Student Resources website and watch USE-A2 - Movie 1. It first shows a Styrofoam TM plate and an acrylic sheet being charged by rubbing them together. The positively (+) charged acrylic sheet is then brought close to and allowed to touch the tinsel on the electroscope. (Note that they are NOT rubbed together.) SE-14

91 Activity 2: Initial Model for Static Electricity The acrylic sheet is then gently pulled away again. After this is done the strands of tinsel are hanging in a more spread out arrangement than before, as seen in the still frames from the movie shown below. Before: Tinsel strands are close together After: Tinsel strands are more spread out After it has touched the positively (+) charged acrylic sheet, do you think the tinsel is itself charged or not and, if so, what type of charge does it have: the same as the acrylic sheet or the opposite? Why do you think so? After it has touched the tinsel, do you think the acrylic sheet is still charged or not and, if so, what type of charge does it have: + or? Why do you think so? SE-15

92 Unit SE Use the diagrams below to show what +/ charged entities (if any) you think are inside or on the surface of 1) the tinsel, 2) the end of the soda can, and 3) the acrylic sheet before and after they have been in contact. Explain your thinking. Before contact: After contact: End of Tinsel Charged End of Tinsel Charged Can (close together) Acrylic Can (spread out) Acrylic How do your diagrams account for the observation that the strands of tinsel are hanging in a more spread out arrangement after contact than before? (Assume that the charged acrylic is too far away to directly affect the tinsel at each of these points in time.) STEP 2. Suppose that, with the tinsel still in this spread-out arrangement, a positively (+) charged acrylic sheet and a negatively ( ) charged Styrofoam TM plate were each brought close to it (separately), but not allowed to touch it. Consider some different possibilities, depending on whether the tinsel might be charged or not. If the spread-out tinsel strands were uncharged, how would they react to the positively (+) charged acrylic sheet? What about the negatively ( ) charged Styrofoam TM plate? (Recall, you saw how charged and uncharged objects interact in the previous activity.) SE-16

93 Activity 2: Initial Model for Static Electricity If the spread-out tinsel strands were positively (+) charged, how would they react to the positively (+) charged acrylic sheet and the negatively ( ) charged Styrofoam TM plate? Why is this? If the spread-out tinsel strands were negatively ( ) charged, how would they react to the positively (+) charged acrylic sheet and the negatively ( ) charged Styrofoam TM plate? Why is this? Now consider which of these three possibilities corresponds to your model, as represented by your diagrams in STEP 1. After the positively (+) charged tinsel has touched the charged acrylic sheet, what does your model predict for how it should react to each of a positively (+) charged acrylic sheet and negatively ( ) charged Styrofoam TM plate? STEP 3. Now watch the movie of the experiment being performed (USE-A2 - Movie 2). The pictures below are still frames taken from the movie. (The first part of this movie is simply a repeat of the acrylic touching the tinsel.) Charged acrylic near tinsel Charged Styrofoam near tinsel SE-17

94 Unit SE Were the spread-out tinsel strands attracted or repelled by the positively (+) charged acrylic sheet? What about the negatively ( ) charged Styrofoam TM plate? From these results what can you infer about the tinsel strands after they had touched the charged acrylic sheet? Did they have a positive (+) charge, a negative ( ) charge, or no charge? How do you know? STEP 4. Open USE-A2 - Movie 3, which shows a simulation of the experiment in STEP 1 being performed. The setup shows an object on the left with black strips hanging on each side. This represents the soda-can electroscope with the tinsel strands hanging from it. On the right are two objects representing the acrylic sheet and Styrofoam plate. At the beginning of the movie none of the objects are charged, but as it plays charged areas will be represented by red (+) or blue ( ) colors. If you have not already done so, play the movie now. First, the acrylic and Styrofoam TM are charged by rubbing them together. Next, the charged acrylic is turned around and the charged surface is touched to the tinsel on the electroscope three times. Answer the following questions about what the simulator shows when this is done. When the positively (+) charged acrylic is touched to the tinsel, does the electroscope (including the tinsel) become positively (+) or negatively ( ) charged? Does this agree with your conclusion from the observations in STEP 2? SE-18

95 Activity 2: Initial Model for Static Electricity After the charged acrylic has touched the electroscope, does the acrylic remain positively (+) charged, become negatively ( ) charged, or is it no longer charged? Does this agree with your prediction in STEP 1? The thickness of the red and blue coloring in the simulation indicates how strongly a particular area of an object is charged. Immediately after the acrylic and Styrofoam TM were rubbed together, how did the strength of the charge on each one compare? Were they about the same, or were they very different? How can you tell? Each time the charged acrylic was touched to the tinsel what happened to the strength of the charge on the electroscope? What about the strength of the charge on the acrylic? In each case, how do you know? What do these simulator results suggest is happening when the positively (+) charged acrylic touches the tinsel on the electroscope? STEP 5. If your model diagrams in STEP 1 are not consistent with these observations, then your thinking needs to be revised. Talk with your group members about how you might revise your thinking, if at all. On the next page, draw your group s revised model representation for the +/ charged entities (if any) inside or on the surface of the tinsel, end of the soda can, and acrylic sheet before and after they have been in contact. SE-19

96 Unit SE Before contact: After contact: End of Tinsel Charged End of Tinsel Charged Can (close together) Acrylic Can (spread out) Acrylic Explain, in terms of the charged entitites involved in your model, what you think happened when the tinsel strands touched the positively (+) charged acrylic sheet. When the tinsel strands on an electroscope are hanging in a more spread out pattern, what does this indicate about the tinsel and why? Note: The behavior of the tinsel on an electroscope will serve as a tool in further experiments. If the tinsel is itself uncharged it will not react to an uncharged object, but it will be attracted toward any charged object, regardless of whether it is positively (+) or negatively ( ) charged. (You know that there is always an attraction between charged and uncharged objects.) Thus we can use it to check if an object is charged or not. If the tinsel becomes spread out, we know it is now charged and we can test what type of charge it has by using charged acrylic and Styrofoam TM. SE-20

97 Activity 2: Initial Model for Static Electricity Exploration #2: What happens when a charged object is touched all over with an uncharged object? STEP 1. Watch USE-A2 - Movie 4, in which an experimenter rubs a plastic coffee stirrer with wool and then touches it all over with his fingers. To check whether the stirrer is charged or not at various points in the process it will be brought close to the uncharged tinsel strands on a soda can electroscope. Was the stirrer charged before it was rubbed with wool? What about after? How do you know? Explain, in terms of the +/ charged entitites involved in your model, what you think happened when the plastic stirrer was rubbed with the wool. Now consider what happened when the experimenter touched the stirrer all over with his fingers. Was the stirrer still charged after this was done? How do you know? Explain, in terms of the +/ charged entitites involved in your model, what you think happened when the experimenter touched the plastic stirrer with his fingers. SE-21

98 Unit SE STEP 2. Now suppose the stirrer was charged again, but instead of touching the charged stirrer all over with his fingers, the experimenter immersed it in some water and immediately removed it. Do you think the stirrer would still be charged after it was immersed in water, or not? Explain your thinking in terms of the charged entities in your model. To see the experiment being performed, watch USE-A2 - Movie 5. Was the stirrer charged after it was immersed in water? How do you know? Is this what you predicted? If this result is not in agreement with your prediction, describe how your model might explain it in terms of the charged entities involved. How might this result help explain why it is difficult to do static electricity experiments when there is a lot of humidity in the atmosphere? Summarizing Questions S1. You already know that two uncharged objects can be charged by rubbing them together (or peeling tapes apart). What do your observations in this activity suggest happens when an uncharged object touches a second object that is already charged? SE-22

99 Activity 2: Initial Model for Static Electricity S2. Do your observations in this activity suggest that your model should incorporate the idea that charged entitites always stay with a particular object or that they can be transferred between objects under some circumstances? What evidence supports your answer? S3. Do your observations suggest that in your model you should regard the charged entities responsible for giving an object its overall charge as being inside the body of the object or on its surface? Again, what evidence supports your answer? S4. Discuss with your group how you might revise your model (if necessary) to be consistent with the evidence you have seen in this activity. a) Use the diagrams below to represent your latest ideas about the +/- charged entities associated with the Styrofoam TM plate and acrylic sheet both before and after they are rubbed together. Before rubbing together After rubbing together SE-23

100 Unit SE b) Briefly describe how your model accounts for the observation that the acrylic sheet and Styrofoam TM plate are uncharged before rubbing and oppositely charged after rubbing. S5. Use the diagrams below to represent your current ideas about the +/- charged entities involved when a positively (+) charged acrylic sheet is touched to the tinsel on an uncharged soda can electroscope. Before contact: After contact: End of Tinsel Charged End of Tinsel Charged Can (close together) Acrylic Can (spread out) Acrylic Briefly describe what you think happens in this case, in terms of the +/- charged entities.! Participate in a class discussion about these questions. After listening to other groups present their models and explanations make a note of any ideas that seem useful. SE-24

101 UNIT SE Developing Ideas ACTIVITY 3: Representing Uncharged Objects in your Model Purpose In the previous activity you developed your initial model to account for static electric phenomena. It is common for most groups to have similar models for charged objects, associating arrangements of positively (+) and negatively ( ) charged entities with them. However, there is often more variation in the model representations of uncharged objects (such as the acrylic sheet and Styrofoam TM plate before they were rubbed together), with some showing no entities, some showing neutral (uncharged) entities, and some showing equal numbers of + and charged entities. The key question for this activity is: What is an appropriate way to represent uncharged objects in your model for static electricity? Initial Ideas In the previous activity you were introduced to a device called an electroscope, made from a soda-can and some tinsel. (thin metal strips). Suppose you had such a sodacan electroscope that was uncharged. According to your current model how would you represent this uncharged eleclectroscope? Use this diagram to show your thinking, drawing whatever entities (if any) you think necessary on both the can and the tinsel Next Gen PET SE-25

102 Unit SE How does your diagram indicate that the soda-can and tinsel are both uncharged? What evidence (if any) do you have to support this representation? Participate in a whole class discussion about your ideas on how to represent uncharged objects. Make a note of any ideas that are different from those of your group. Collecting and Interpreting Evidence Exploration #1: What happens when a positively (+) charged object is brought close to an uncharged electroscope? STEP 1. Suppose you rubbed an acrylic sheet and a Styrofoam TM plate together to charge them, and then brought the positively (+) charged acrylic sheet close to (but not touching) the base (non-tinsel end) of the uncharged electroscope you represented in Initial Ideas section. Do you think doing this this would have any effect on the electroscope? Do you think either the soda-can or the tinsel would become electrically charged when this is done? If so, in the case of the tinsel, how would you know? To check your thinking, watch USE-A3 - Movie 1. In this movie an acrylic sheet will be charged by rubbing it with a Styrofoam TM plate. After this the charged acrylic will be brought near to the base end of an uncharged sodacan electroscope, but not touch it, and then be moved further away again. This will be done several times. SE-26

103 Activity 3: Representing Uncharged Objects When the charged acryclic sheet is brought close to the other end of the soda can does the tinsel become more spread out, more clumped together, or does it not show any reaction. What happens when the acrylic sheet is removed again? What does this indicate about whether the tinsel became charged or not when the acrylic sheet was near the other end of the electroscope? What about when the acrylic sheet was removed? How do you know? STEP 2. Work with your group to try and explain this behavior using your model, revising it if necessary. Use the diagrams below to show how your current model now represents the objects involved (soda can, tinsel, and acrylic) before the charged acrylic is brought close, while it is close (with the tinsel spread out), and after it has been removed again. Before Positively (+) charged acrylic close to base end After SE-27

104 Unit SE Explain how your model accounts for the behavior of the tinsel in terms of any entities involved. STEP 3. You have now seen that when the positively (+) charged acrylic sheet is held close to the base end of the soda-can electroscope, the tinsel at the other end becomes more spread out. From this you can infer that while the acryclic sheet is held in this position the tinsel becomes charged, but was it positive (+) or negative ( )? Also, did the base end of the can become charged or not and what happen to the charge on the acrylic? Use your model diagrams from STEP 2 and current thinking to make some predictions about these questions While the positively (+) charged acrylic is held close to the other end, does your model suggest that the spread-out tinsel becomes positively (+) or negatively ( ) charged? In this case would you expect the spread out tinsel to be attracted, repelled, or show no reaction, if a second positively (+) charged acrylic sheet were brought close to it? What about if a negatively ( ) charged Styrofoam TM plate were brought close? (With the first charged acrylic sheet still being held close to the other end.) Briefly explain your reasoning. While the positively (+) charged acrylic is held close to the base end of the electroscope, does your model suggest that the base end of the can itself (closest to the acrylic) becomes positively (+) or negatively ( ) charged, or does it remain uncharged? After it is moved away from the base end of the electroscope, what do you think the strength of the positive (+) charge on the acrylic will be SE-28

105 Activity 3: Representing Uncharged Objects like? Will it be the same as before it was brought close, weaker than it was, or will it no longer be charged? Why do you think so? STEP 4. To check your thinking, first watch USE-A3 - Movie 2. It shows the experimenter preparing two sets of charged acrylic and Styrofoam TM. He first holds one of the positively (+) acrylic sheets close to the base end of the uncharged electroscope and the tinsel spreads out (as it did in the previous movie). Keeping the first positively (+) charged acrylic sheet close to the base end of the electroscope, he then brings the other positively (+) charged acrylic sheet close to the tinsel, followed by one of negatively ( ) charged Styrofoam TM plates. Describe how the tinsel reacts to the presence of the positively (+) charged acrylic. Does it attract, repel, or is there no reaction? What about its reaction to the negatively ( ) charged Styrofoam TM? Do these results agree with your prediction on the previous page or not? When the positively (+) charged acrylic was held close to the base end of the uncharged electroscope did the tinsel have a positive (+) or negative ( ) charge? How do you know? Now watch USE-A3 Movie 3, which shows a simulation of the experiment being performed. base end tinsel end In the simulator, when the positively (+) charged acrylic is brought close, does the type of charge on the tinsel (+ or ) agree with what you conluded from the previous movie? How do you know? SE-29

106 Unit SE While the positively (+) charged acrylic is held close to one side of the simulator electroscope, does that side of the electroscope (closest to the acrylic) become positively (+) or negatively ( ) charged, or does it remain uncharged? How do you know? Finally, after it has been charged initially, does the strength of the positive (+) charge on the acrylic change at all? How do you know? STEP 5. If these results are not consistent with your diagrams in STEP 2, think about how you could revise your model of +/ charged entitites to explain them. Draw your new model representation for this situation here. Before Positively (+) charged acrylic close to base end Briefly explain how the proximity of positively (+) charged acrylic at the base end results in the tinsel end of the electroscope acquiring an an overall positive (+) charge and the base end an overall negative ( ) charge. The simulator showed that when the positively (+) charged acrylic sheet is moved further away, both ends of the electroscope lose their overall charge. How can you explain this with your model? SE-30

107 Activity 3: Representing Uncharged Objects Exploration #2: What happens when a negatively ( ) charged object is brought close to an uncharged electroscope? STEP 1. Suppose you again started with an uncharged soda-can electroscope. Now imagine you brought a negatively ( ) charged Styrofoam TM plate close to the base end of this uncharged electroscope and then removed it again. Consider how you would now use your model to represent the electroscope (soda can and tinsel) and Styrofoam TM plate before, during, and after this is done. Use the diagrams below to show your thinking. Before Negatively ( ) charged Styrofoam TM close to base end After According to your model diagram above, when the negatively ( ) charged Styrofoam TM is held close to the base end of the soda can, would the tinsel end acquire an overall positive (+) charge, an overall negative ( ) charge, or would it remain uncharged? What about the base end itself? SE-31

108 Unit SE As a result of this, would you expect the tinsel to spread out as it did when the positively (+) charged acrylic was held close, or would something else happen? If so, what? STEP 2. To check your thinking watch USE-A3 - Movie 4. In this movie the experimenter first brings a negatively ( ) charged Styrofoam TM plate close to the base end of an uncharged soda-can electroscope. What happens to the tinsel when the negatively ( ) charged Styrofoam TM plate is brought close to the base end of the electroscope (but before the second plate is brought close to the tinsel). Is this what you predicted? Next, while the first negatively ( ) charged Styrofoam TM plate is kept close to the base end of the electroscope, the experimenter brings a second negatively ( ) charged Styrofoam TM plate close to the tinsel end. How does the tinsel react (attract, repel, or no reaction) to this second negatively ( ) charged Styrofoam TM plate? Does this indicate that the tinsel currently has an overall positive (+) or negative ( ) charge? Now watch USE-A3 - Movie 5, which shows a simulation of this experiment being performed. In the simulator, while the negatively ( ) charged Styrofoam TM is held close to the electroscope, what type of charge (+ or ) does the tinsel end have? What about the side closest to the Styrofoam TM? Does this agree with your predictions? SE-32

109 Activity 3: Representing Uncharged Objects STEP 3. If these results are not consistent with your diagrams in STEP 1, think about how you could revise your model of +/ charged entitites to explain them. Draw your new model representation for this situation here. Before Negatively ( ) charged Styrofoam TM close to base end Briefly explain how the proximity of the negatively ( ) charged Styrofoam TM at the base end results in the tinsel end of the electroscope acquiring an an overall negative ( ) charge and the base end an overall positive (+) charge. Summarizing Questions S1. Which of the following ideas about uncharged objects do you think would be most appropriate to include in your model? What evidence from this activity supports your choice? a) They have no electric entities associated with them. When such objects are involved in static electric interactions + and charged entities are created. b) They have neutral (uncharged) electric entities associate with them. When such objects are involved in static electric interactions these neutral entities are changed into either + or charged entities. c) They have equal numbers of + and charged entities associated with them. When such objects are involved in static electric interactions at least some of these entities move around. SE-33

110 Unit SE S2. Complete the diagrams below to show your current thinking about how to represent the entities associated with an uncharged electroscope under the indicated circumstances. No charged objects nearby Positively (+) charged acrylic close to base end Negatively ( ) charged Styrofoam TM close to base end. S3. Use your model to explain why the tinsel on an uncharged electroscope goes back to hanging normally when the charged objects are moved away from the other end. SE-34

111 UNIT SE Developing Ideas ACTIVITY 4: Refining your Model for Different Materials Purpose You are currently developing a model for static electricity that involves the movement of +/ charged entities within and/or between objects. In this lesson you will continue this development to account for how different materials behave when involved in static electricity effects. How can you refine your model to account for the behavior of metals and non-metals? Initial Ideas In a previous lesson you saw that when some tinsel strands on a soda-can electroscope come into direct contact with a charged object, the tinsel itself becomes charged. In this lesson we will use two electroscopes, one made with a metal soda can as you saw in the previous activities, and the second made with a plastic water bottle. Both are initially uncharged. Suppose the base end of both the soda can and water bottle on the uncharged electroscopes (the ends opposite the tinsel) was touched for a few seconds with a positively (+) charged acrylic sheet, and then the acrylic was removed. What do you think would happen? In terms of the +/ charged entities involved in your model, what do you think will happen when the positively (+) charged acrylic sheet is touched to the base of both electroscopes. Use the diagrams on the next page to illustrate your thinking. (Assume that, in all cases, the acrylic is too far from the electroscope to affect it at these points in time.) 2016 Next Gen PET SE-35

112 Unit SE Soda-can electroscope Before contact: After contact: Water-bottle electroscope Before contact: After contact: Briefly explain your thinking. SE-36

113 Activity 4: Materials and Static Electricity Initially the tinsel on the untouched end of both electroscopes is uncharged. After contact with the charged acrylic at the base end do you think that the tinsel at the other end of either electroscope (or both) would be charged or not. How does your model show this? What evidence would you look for to indicate whether the tinsel is charged or not? Why would this evidence support your prediction? Prepare a presentation board showing your diagrams for the class discussion. Be prepared to describe your thinking. Participate in a whole class discussion. After listening to other groups present their diagrams and explanations make a note of any ideas that are different from those of your group. Collecting and Interpreting Evidence Note: Again, while it would be good to obtain your evidence from experiments you do yourself, you will mostly be making observations of experiments from videos made under controlled conditions. Exploration #1: Can you charge the tinsel on an electroscope without touching it directly? STEP 1. In the Initial Ideas section you made predictions for what you think will happen to the tinsel on the end of two different electroscopes when the other end is touched with a charged acrylic sheet. To check your thinking watch USE-A4 - Movie 1 to see what actually happens when these experiments are performed. Some still frames from the movie are show on the next page. SE-37

114 Unit SE Soda-can electroscope Water-bottle electroscope Before contact After contact Before contact After contact When the positively (+) charged acrylic sheet was touched to the metal soda can electroscope, did the tinsel at the other end become charged? What about the plastic water bottle electroscope? How do you know? STEP 2. Now consider how you could test what type of charge (+,, or uncharged) the tinsel on each electroscope has. If the tinsel on an electroscope was positively (+) charged how would it behave when a positively (+) charged acrylic sheet and a negatively ( ) charged Styrofoam TM plate were each brought near to (but not touching) it? What about if the tinsel was negatively ( ) charged? What if it was uncharged? Briefly explain your reasoning. The second movie for this activity follows directly after the first, in that the base end of the soda can and water bottle electroscopes have already been touched by the positively (+) charged acrylic sheet. In this movie a positively (+) charged acrylic sheet and negatively ( ) charged Styrofoam TM plate are then brought close to (but not touching) the tinsel strands on both electroscopes. SE-38

115 Activity 4: Materials and Static Electricity Watch USE-A4 - Movie 2 now and record your observations (Attract, Repel or nothing) in Table 1 below. Table 1. Reaction of tinsel to charged objects. Positively (+) charged acrylic Negatively ( ) charged Styrofoam TM Tinsel on soda-can electroscope Tinsel on water-bottle electroscope Were the tinsel strands on the metal soda-can electroscope positively (+), or negatively ( ) charged, or were they still uncharged? How do you know? Were the tinsel strands on the plastic watter bottle electroscope positively (+), or negatively ( ) charged, or were they still uncharged? How do you know? STEP 3. You have now seen that touching the positively (+) charged acryclic to the base end of the uncharged soda-can electroscope caused the tinsel at the other end to become charged. Do you think the soda-can itself also became charged, or not? If so, do you think it was positively (+) or negatively ( ) charged all over, or did different parts have different types of charge? Why do you think so? SE-39

116 Unit SE After it was touched to the soda-can, do you think the acrylic was still positively (+) charged to the same degree, or was it now less positively (+) charged, negatively ( ) charged, or even uncharged? Why do you think so? Watch USE-A4 Movie 3, which shows a simulation of the positively (+) charged acrylic being touched to the base end of a metal (like the soda-can) electroscope. After being touched in this way, was the tinsel on the electroscope positively or negatively ( ) charged, or was it still uncharged? Does this agree with your conclusion from watching the experiments in STEP 2? Was the rest of the metal electroscope charged after this was done? If so, what type of charge was it (+ or ) and how was it distributed (all over, or only in particular places)? How do you know? Was the acrylic still positively (+) charged to the same degree, or was it now less positively (+) charged, negatively ( ) charged, or even uncharged? How do you know? STEP 4. If all these results do not agree with your predictions, discuss with your group how you could modify your model to account for them, particularly in terms of the mobility of the charged entities in different types of material. SE-40

117 Activity 4: Materials and Static Electricity Describe any change in your thinking that can account for why the metal soda can and the plastic soda bottle produced different results in these experiments. Briefly explain, in terms of the +/ charged entities involved in your model, what you now think happened when the positively (+) charged acrylic sheet was touched to the base of both electroscopes and why the tinsel behaved as it did in both cases. Exploration #2: How can charged materials be discharged? You have seen how some uncharged objects can become charged, either by rubbing them together (or peeling apart, in the case of the tapes), or by touching them with another object that is already charged. Now you will consider how a charged object can be turned into an uncharged object, a process called discharging. STEP 1. Imagine you have a soda-can electroscope and a water-bottle electroscope. You charge the tinsel strands on both by touching them directly with a positively (+) charged acrylic sheet. Yin Activity 2 of this unit you already saw that this would cause the tinsel strands to themselves become positively (+) charged and hang in a more spread out pattern than before they were touched. Suppose you now touched the non-tinsel end of each electroscope (the base of the can or bottle) with your finger (being careful not to touch the tinsel itself). SE-41

118 Unit SE Do you think the tinsel on the soda-can electroscope would remain charged or not? What about the tinsel on the water-bottle electroscope? Explain your answers in terms of your model and your ideas about the behavior of the +/ charged entities involved in the two different materials. According to the answers you gave above, predict how you expect the positively (+) charged (spread out) tinsel to behave when each electroscope is touched? Briefly explain why. STEP 2. Watch USE-A4 - Movie 4, in which the tinsel on both electroscopes will first be charged by allowing the strands to touch a positively (+) charged acrylic sheet. The experimenter will then touch a finger to the non-tinsel end on both electroscopes. Watch the behavior of the tinsel when this is done. At the end, the experimenter will allow the tinsel to touch a finger directly. Could the tinsel on the soda-can electroscope be discharged by touching the other end of the can? How do you know? Could the tinsel on the water-bottle electroscope be discharged by touching the other end of the bottle? How do you know? How can your model explain these results in terms of the behavior of the +/ charged entities in the tinsel, the metal soda can, and the plastic water bottle. (Recall that tinsel strands are metal.) SE-42

119 Activity 4: Materials and Static Electricity How would you use your model to explain that the tinsel on the waterbottle electroscope could not be discharged by touching the other end of the plastic bottle, but it could be discharged by touching it directly with a finger? STEP 3. Now let s consider how we could discharge an object that has no metal anywhere on it. You know that when rubbed together the rubbed surface of an acrylic sheet becomes positively (+) charged and the rubbed surface of a Styrofoam TM plate becomes negatively ( ) charged. If you wanted to discharge these two objects again do you think it would be sufficient to touch each of them at only one point on the charged surface, or would it be better to touch as much of the rubbed surface as possible? Explain your thinking in terms of your ideas about the behavior of the +/ charged entities in these non-metallic materials. Now watch USE-A4 - Movie 5, which shows both of these ideas being tested. Whether the materials remain charged or not will be checked by seeing whether they attract the tinsel on an uncharged soda-can electroscope. (Recall that there is always an attraction between a charged an an uncharged object.) Does the result agree with your prediction? If not, discuss with your group how you could change your model (in terms of the behavior of charged entities in non-metals) to account for it. SE-43

120 Unit SE STEP 4. In a previous activity you saw a plastic stirrer being charged by being rubbed with wool and then dropped in some water and immediately removed. After it was immersed in water the stirrer was no longer charged. (If necessary, you can watch USE-A4 - Movie 6 to remind you.) Assuming the rubbed plastic stirrer was negatively ( ) charged, how can your model explain this result in terms of the behavior of the +/ charged entities involved 1? Summarizing Questions S1: As a result of the evidence you have seen in this activity, you may have felt it necessary to revise your model so that it can account for the difference between how metals and non-metals behave when involved in static electric effects. How have you revised your model to account for these differences? S2: Suppose you were to wear thick rubber gloves, then hold a steel rod in one hand and rub the rod with nylon. After dong this you would find that the steel rod was negatively charged. However if you did this while holding the steel rod in your bare hand, you would find that it did not become charged. Briefly explain why this is. SE-44 1 Although water is a non-metal, the particular structure of individual water molecules means that they interact very readily with the charged entities involved in static electricity.

121 Activity 4: Materials and Static Electricity S3: At this stage your model probably accounts for an object being charged by assuming that it has more of one type of charge (+ or ) than the other. Do the observations you have made so far in this unit suggest that it would be better to regard the excess of one type of charged entity as lying on the surface of an object, or deep within the body of the object? Why do you think so? S4: In this activity you saw that when the base end of an uncharged soda-can electroscope was touched with a positively (+) charged acrylic sheet, the whole electroscope, including the tinsel at the other end, became positively (+) charged. Use the diagrams below to show how your current model can account for this in terms of the +/ charged entities involved. (Again, assume that in these diagrams the charged acrylic is too far from the electroscope to affect it.) Before contact: After contact: Briefly explain what you think happens and why. SE-45

122 Unit SE S5: Watch USE-A4 - Movie 7. The simulator in this movie shows that if the base end of an uncharged soda-can electroscope is touched with a negatively ( ) charged Styrofoam TM plate the whole electroscope, including the tinsel at the other end, becomes negatively ( ) charged. Draw diagrams and write a short narrative to show how your current model could account for this. SE-46! Participate in a class discussion about these questions. After listening to other groups present their models and explanations make a note of any ideas that seem useful. Conductors and insulators Scientists classify materials in which at least some charged entities are able to move around relatively freely as conductors. Materials for which none of the charged entities can move around freely are called insulators. Under normal circumstances, all metals are conductors and most non-metals are insulators. However, there are a few non-metals, such as carbon and water (especially if there are impurities in it) that are also reasonably good conductors.

123 UNIT SE Developing Ideas ACTIVITY 5: Interactions between charged and uncharged objects Purpose In the last few activities you have been developing a model to explain phenomena associated with static electricity in terms of the behavior of +/ charged entities (representing protons and electrons respectively) that the model assumes to be within all materials. At the beginning of this unit you saw that when an uncharged object is brought close to either a positively (+) or negatively ( ) charged object, there is always an attraction between them. In this activity you will try to use (and possibly revise) your model to account for this phenomenon. How can your model explain why a charged and an uncharged object attract each other? Initial Ideas Your current model probably represents uncharged objects as having equal numbers of positive (+) and negative ( ) charged entities, so making them neutral overall. However, is there a way you could arrange an equal number of + and charges so that they would attract a charged object? Recall the Electric Field Hockey simulator you first saw in an earlier homework assignment. Suppose you had a single + charged puck in front of the goal as shown below. Do you think you could arrange a single + and a single charge behind the goal so that the puck enters the goal when the Start button is clicked? If so, sketch an arrangement above that you think would work and briefly explain why? If not, why not? 2016 Next Gen PET SE-47

124 Unit SE What if the puck in front of the goal were charged? Do you think the same single + and single charge behind the goal could be arranged to make this puck enter the goal? If so, sketch an arrangement below that you think would work and briefly explain why? If not, why not? Participate in a class discussion. Make a note of any ideas that are different from those of your group but seem to make sense. Collecting and Interpreting Evidence Exploration #1: How can you arrange + and charges to attract a charged object? STEP 1. Open USE-A5 Sim 1. When the simulator opens, a single positively (+) charged puck is shown on the left side of the window, with the goal on the right. Drag a single + and a single charge from the boxes at the top right, arrange them as you suggested in the Initial Ideas section, and click the Start button? Does your predicted arrangement work to make the puck enter the goal? If not, find one that does work, sketch it below, and try to explain why it does. SE-48

125 Activity 5: Charged and Uncharged Objects In the Initial Ideas section you also likely suggested an arrangement that you thought would make a negatively ( ) charged puck enter the goal. Check your thinking by giving the puck a negative ( ) charge. Do this by first clicking on the Reset button, then uncheck the Puck is Positive box at the bottom of the window. Finally, rearrange the charges behind the goal to match your prediction, and run it again. Does your predicted arrangement work to make the charged puck enter the goal? If not, find one that does work, sketch it below, and try to explain why it does. Do not close the simulator yet as you will need it again later in this activity. STEP 2. Now imagine you had large group of charged entities comprised of an equal number of + and charges. How could you arrange the members of this group such that a separate positively (+) charged object was attracted toward the group as a whole? Why would this work? How could you arrange the same group of equal numbers of + and charges such that the group as a whole attracted a negatively (-) charged object? Again, why would this work? SE-49

126 Unit SE Exploration #2: How can uncharged metals and charged objects attract each other? In the first activity of this unit you saw that there was an attraction between all the metal samples you used and both the B ( ) and T (+) tapes you prepared. You will now use the Electric Field Hockey simulator to help you think about how your model can account for the attraction between uncharged metals and both positively (+) and negatively ( ) charged objects (such as the two tapes). STEP 1. Return to the simulator, click the Clear button, and also re-check the Puck is Positive box. Imagine the area behind the goal is where an uncharged metal object (such as a soda can) is located. Drag 6 to 8 positive (+) charges into this area to represent the positively (+) charged entities in your model of this metal object. (The dashed box is not shown on the simulator, so just put them in the general area behind the goal.) Since the metal object is uncharged, your model needs the same number of negative ( ) charges. To simulate the neutral state of the atoms in an uncharged metal object, drag a negative ( ) charge directly on top of each + charge. (Though the + charges will be hidden they are still there underneath.) STEP 2. Now imagine a positively (+) charged puck was placed to the left of the goal. What effect would this have on the +/ charges in this uncharged metal object. (This is like bringing the positively (+) charged acrylic close to the soda can electroscope in Exploration #1 of Activity 3.) Assuming that only the negative ( ) charged entities can move, sketch how you think the negative (-) charges would now be arranged within the metal object. (Note: They should still stay within the object, so do not move them out of the dashed box. Since the positive (+) charges do not move, they are already shown in the same locations as they were before.) SE-50

127 Activity 5: Charged and Uncharged Objects Briefly explain why you placed the negative ( ) charges where you did on the diagram. Unfortunately, the simulator will not move the charges in the area behind the goal so you will have to do it yourself! Rearrange the negative ( ) charges now according to your prediction. (Remember to keep them in the general area indicated by the dashed box.) What effect do you think this new arrangement of charges in the object behind the goal will have on the + charged puck when it is released, attract, repel, or no effect? Why do you think so? To check your thinking, click on the Start button and describe what happens to the positively (+) charged puck. Recall that you are using your model to try to explain why there is an attraction between a positively (+) charged object (the puck) and an uncharged object. If your arrangement of negative ( ) charges did not result in the positively (+) charged puck being attracted toward the object behind the goal, find an arrangement that does work and sketch it. Once you have an arrangement of charges that attracts the puck, consider the following question about it. SE-51

128 Unit SE In the simulator you had to move the negatively ( ) charged entities around yourself to create this rearrangement. In reality, why would a nearby positively (+) charged object (the puck in the simulator) cause them to become arranged in this way naturally? STEP 3. Now suppose the puck were negatively ( ) charged. What influence would it have on the charged entities in the uncharged metal object now? First, reset the simulator and return the negative ( ) charges in the object to their original positions on top of the positive (+) charges (which should not have moved). Now imagine a negatively ( ) charged puck was placed to the left of the goal. What effect would this have on the +/ charges in this uncharged metal object. Assuming that only the negative ( ) charged entities can move, sketch how you think the negative (-) charges would now be arranged within the metal object. (As before, they should still stay within the object, and the positive (+) charges are again shown in the same locations as they were before.) Briefly explain why you placed the negative (-) charges where you did. To check your thinking, return to the simulator and rearrange the charges according to your prediction above. Also be sure to change the charge on the puck to be negative. Click on the Start button and describe what happens to the negatively ( ) charged puck. SE-52

129 Activity 5: Charged and Uncharged Objects Recall that you are using your model to try to explain why there is an attraction between a negatively ( ) charged object (the puck) and an uncharged object. If your arrangement of negative ( ) charges did not result in the negatively ( ) charged puck being attracted toward the object behind the goal, find an arrangement that does work and sketch it. Once you have an arrangement of charges that attracts the puck, consider this question about it. In the simulator you had to move the negatively ( ) charged entities around to create this rearrangement yourself. In reality, why would a nearby negatively ( ) charged object (the puck in the simulator) cause them to become arranged in this way naturally? In this exploration you thought about how your model could explain the attraction between an uncharged metal object and a charged object. You did this by using your ideas about how the charged object influenced the negatively ( ) charged entities in the uncharged object. Because the object was metal these negative ( ) charges were able to move through the object due to their attraction to, or repulsion from, a nearby charged object. When the charged entities in an uncharged object rearrange such that one part has an overall + charge while another part has an overall charge we say that the object is polarized. (Note that, overall, the object is still uncharged because it has an equal number of + and charged entities.) SE-53

130 Unit SE Exploration #3: How can uncharged insulators and charged objects attract each other? In the first activity of this unit you also saw that there was also an attraction between all the non-metal samples you used and both the B ( ) and T (+) tapes you prepared. However, in the previous activity you likely modified your model to explain the differences between conductors and insulators by saying that that the negative ( ) charges (electrons) cannot move through a non-metal like they can in a metal, so how can this be? STEP 1. Reset the simulator and return the +/ charges in the object to their original positions. (In pairs on top of each other.) This area behind the goal will now represent an uncharged insulator. Keep the puck as a negative charge for now. Now move each of the negative ( ) charges very slightly to the right, further away from the charged puck. (Most of each charge should still be on top of its corresponding + charge.) If you were to click the Start button now, how do you think the negatively ( ) charged puck would behave? Why do you think so? To check your thinking, click on the Start button. Is there an attraction between the negatively ( ) charged puck and the uncharged object behind the goal? How do you know and why do you think this is? What do you think might be happening to the atoms in an uncharged insulator material for the +/ charges to be arranged in this way? What could cause this to happen? SE-54

131 Activity 5: Charged and Uncharged Objects STEP 2. In order to explain the results you saw with a plastic electroscope in the previous activity you probably revised your model to include the idea that the negatively ( ) charged entities (which can be taken to represent electrons) were not able to move through the plastic material itself like they could through the metal soda can. (This is in addition to the idea that the + charged entities (which can represent protons) are fixed in place.) However, in an insulator material it is possible for the average position of the electrons to be shifted very slightly while they still remain attached to the same atom. When this happens, one side of each atom acts like it has a very small positive (+) charge while the other side acts like it has a very small negative ( ) charge. In other words, the individual atoms themselves become polarized. Why would the presence of the nearby negatively ( ) charged puck cause the atoms in the insulator object in the simulator to become polarized as you arranged them in STEP 1 (and shown here in a way that is probably more like your model representation)? STEP 3. Now suppose the puck in the simulator were positively (+) charged. How would the atoms in the non-metal become polarized now? To show your thinking insert appropriately arranged pairs of + and signs on the diagram above. Explain your arrangement below. With all the atoms in the uncharged insulator object polarized as you suggest, would there be an attraction or a repulsion between the object and the positively (+) charged puck? Explain why. SE-55

132 Unit SE STEP 4. Reset the simulator and make the puck positively (+) charged. Then rearrange the negative ( ) charges in the uncharged insulator object to make each of the atoms polarized in the way you predicted in STEP 3. Describe how you are arranging the negative (-) charges to simulate the polarization you predicted. Now click on the Start button. Is there an attraction or repulsion between the + charged puck and the uncharged object behind the goal? Summarizing Questions S1. A positively (+) charged puck is brought close to an uncharged metal object and the negative ( ) charges in the metal object rearrange so that they are closer to the left hand end (as shown in the diagram). If the charged puck is prevented from moving (by holding it), but the metal object is allowed to move, what will happen and why? What if both were free to move? S2. Rub one side of an inflated balloon on a sweater or small piece of wool (or someone s hair) to charge it. Hold the rubbed part of the balloon against a wall and release it. Providing the conditions are not too humid, the balloon should stay attached to the wall, at least for a short time. SE-56

133 Activity 5: Charged and Uncharged Objects To help you think about how to explain this behavior, open USE-A5 - Sim 2 and see how the simulator model behaves in this case. However, you should note that if the wall is made of an insulating material, then the simulator model greatly exaggerates the movement of the negative ( ) charges (assuming they represent electrons). (Note also that, in reality, you had to rotate the balloon to place the charged side against the wall, which the simulator does not do!) a) Draw diagrams showing the charged balloon and the wall when they are both far apart and close together. (We suggest you use appropriately oriented pairs of + and signs, rather than the exaggerated movement shown by the simulator.) b) Explain how the influence of the charged balloon affects the wall such that there is an attraction between them and hence why the balloon sticks to the wall. SE-57

134 Unit SE S3. You have seen that when a rubber balloon is rubbed with wool it becomes negatively ( ) charged and you can stick it to the wall. However, because of their relative positions in the triboelectric series (given in an earlier homework assignment), if you rub a rubber balloon with cellophane, the balloon will actually become positively (+) charged. Would such a positively (+) charged balloon stick to the wall or not? If you think so, explain in terms of your model (including diagrams), why this would happen. If not, why not?! Participate in a class discussion about these questions. After listening to other groups present their models and explanations make a note of any ideas that seem useful. SE-58

135 UNIT SE Applying Ideas ACTIVITY 6: Explaining Phenomena Involving Static Electricity Comparing the Class Ideas and Scientists' Ideas By now the class has likely reached consensus on a model of static electricity that should explain all the observations that have been made thus far. Your teacher may give you a summary of the ideas the class has developed based on the evidence you have seen. This model is likely to be fairly closely aligned with models that scientists have developed, because it is based on the same evidence. We will refer to the class consensus model as the charges in materials model, and in this activity you will apply it to make some predictions and explain some other phenomena. Explaining Static Electric Phenomena Recall that a good explanation should be well-constructed. Also, now that the class has come to a consensus on a good model, any explanation should be accurate; that is - all ideas used should correspond to the class consensus model. This means that the diagrams should include pictures of +/ charged entities in objects and further, that they should behave according to the assumptions of the model: i) Only negatively ( ) charged entities should move (or transfer between objects. ii) In conducting materials (mostly metals) the negatively ( ) charged entities can move through the material, but in insulating materials (most non-metals) they should remain attached to a particular positive (+) charge, but can move to one side or the other of it. Also, the written narrative should be well-reasoned. This means it should include a description of what happens to the negatively ( ) charged entities involved in the given situation and give plausible reasons for why this happens. This will usually be because of the attractive or repulsive influence of some other charged object Next Gen PET SE-59

136 Unit SE Exploration #1: Can a charged object pick up small uncharged objects? Your group will need the following:! Balloon! Small pieces of aluminum foil and paper STEP 1. You have seen that when a rubber balloon is rubbed on wool (or hair) it becomes negatively ( ) charged. Suppose you rubbed a balloon on your hair and then held it above a mixture of small pieces of aluminum (metal) and paper (non-metal). Do you think the balloon would attract any of the pieces of the aluminum and/or paper and so pick them up? Why do you think so? To test your prediction, first spread out your aluminum and paper pieces in a small area on the table. Next inflate your balloon and have one of your group rub one side of it on their hair. Quickly lower the rubbed area of the balloon toward the small pieces on the desk and note carefully what happens. Which pieces does the balloon pick up; neither, only one type, or both? Also check with other groups to see what they observe. In Activity 1 of this unit you saw some charged tapes move toward uncharged objects when they were held close. Why was it that the charged objects (the tapes) moved in that case, but in this situation it was the uncharged objects (aluminum and paper pieces) that moved? SE-60

137 Activity 6: Explaining Phenomena STEP 2. Now use the model of charges in materials to explain this result. First, use the diagrams below (which are not to scale) to show the +/ charged entities on the negatively ( ) charged balloon and one of the uncharged aluminum pieces both when the balloon is far away and then very close (but not touching). Represent the model using diagrams: (Remember that aluminum is a metal.) Balloon Balloon Aluminum Aluminum Write the narrative: (Use the flow diagram shown here to help you structure your narrative.) SE-61

138 Unit SE STEP 3. Here is one student s explanation for why the negatively ( ) charged balloon attracted the paper pieces. Balloon Balloon Paper Paper The balloon was rubbed on the hair, negative charges were transferred from the hair to the balloon, so it has more negative charges than positive charges, so it is negatively charged. The hair now has fewer negative charges than positive charges so it is positively charged. When the balloon is brought close to the paper the positive charges in the paper are attracted move to the top and the negative charges move to the bottom. Because the positive charges in the paper are closer to the negatively charged balloon there is an attraction between them and so the paper is attracted toward the balloon. Do you think explanation is well-constructed? (Is it clear and easy to follow? Are the diagram and narrative consistent with each other? Is it relevant?) If not, why not? Is this explanation accurate? (Do all the ideas used correspond to the class consensus model?) If not, what is inaccurate?) SE-62

139 Activity 6: Explaining Phenomena Is this explanation well-reasoned? (Are plausible reasons given for why the charged entities rearrange as described?) If not, why not? Over all, is this explanation good or problematic? Why do you think so? STEP 3. In a particular type of air purifier the air in a building is passed through a device in which are large charged metal plates. As the air moves past these plates dust particles and pollen grains are removed from the flowing air. Assuming these particles are insulating materials, write a scientific explanation for how such a device works. Represent the model using diagrams: (Draw diagrams showing a single dust grain and a charged plate both when they are far apart, and then close together.) Write the narrative: (Explain what happens when the dust grain comes close to the plate and why this results in it being removed from the air stream.) SE-63

140 Unit SE Exploration #2: How can you explain why clothes stick together in a dryer? STEP 1. A student doing her laundry takes wet clothes from a washing machine and puts them in the dryer. When they are dry she takes them out and finds a pair of cotton socks stuck to a polyester shirt because of static cling. Wondering what has happened to cause this she does a quick experiment using a balloon hanging from string (left over from the previous night s party!) She rubs the balloon on her hair and then lets it hang freely from the string. When she brings the clothes near the balloon she finds the balloon is attracted to the cotton socks but repelled by the polyester shirt. However, she finds that the cotton socks repel each other. What type of charge (+ or ) does the polyester shirt have, or is it uncharged? How do you know? What type of charge (+ or ) do the cotton socks have, or are they uncharged? How do you know? (Be careful, remember there is always an attraction between charged and uncharged objects!) STEP 2. On the next page, write a scientific explanation for what happened in the dryer to give the shirt and socks the type of charges they have. SE-64

141 Activity 6: Explaining Phenomena Represent the model using diagrams: Before drying After drying Write the narrative: SE-65

142 Unit SE STEP 3. Answer these additional questions about this situation. When the student checked on her laundry halfway through the drying cycle it was still slightly damp, but there was also no static cling evident. Why do you think this was? To reduce static cling you can put a dryer sheet in the dryer with your laundry. These sheets coat the fibers of all your clothes with the same waxy substance as it tumbles with them. How might this substance work to reduce the static cling. SE-66

143 UNIT SE Engineering Design ACTIVITY 7: Refueling Safety Refueling Safety: An Engineering Design Challenge Any situation in which volatile fuels are involved can be hazardous. One particularly dangerous situation is when vehicles such as cars and aircraft are refueled when there is a chance that the objects involved (people, pumps, vehicles) may have acquired a static charge. In this case any static discharge may create a spark that can ignite the fuel vapor. Go to the Next Gen PET Student Resources web site and watch USE-A7 Movie 1. It shows what can happen if a person is not careful about static electric effects when pumping gas into a car. An important aspect of the engineering design process is to anticipate potentially dangerous situations, such as that shown in the movie, and build in safeguards to minimize risks. Sometimes these safeguards take the form of safety instructions that operators of equipment (such as gas pumps) should follow. The illustration at right was pasted to a gas pump at a filling station. The two bullet points state: BEFORE fueling, discharge any static electricity build up by touching your bare hand to a metal surface away from the nozzle. Do not re-enter your vehicle while gasoline is pumping. Re-entry could cause static electricity build up. Image by Cary Sneider When considering the questions in this activity you should base your thinking on the model of static electricity your class has just developed Next Gen PET SE-67

144 Unit SE Explain each of the two bullet points of the safety instructions on the previous page. Why are they important and why would heeding them minimize any risks? One of the reasons why many people disregard such safety instructions, and so make catastrophic errors, is that they have no mental models to help them understand why safety precautions are necessary. Use your class consensus model of static electricity to make drawings that could help someone develop a mental model to understand i) how a person could become electrically charged in these circumstances, and ii) why not following the safety instructions could lead to a dangerous static discharge. SE-68

145 Engineering Design: Refueling Safety Aircraft refueling Serving as an airport safety officer is a tough job. Many of the personnel who have mission critical jobs have limited educational backgrounds, so training them how to work safely around the planes sometimes means teaching them some science. There is no better example than when refueling planes. As illustrated below, airplanes are refueled from underground tanks by trucks with large powerful pumps. British Airways plane being refueled. GNU Free Documentation License, Creative commons As an airplane flies, friction with the air often causes a buildup of static electrical charge on the outer metal skin of the airplane. If a discharge spark occurs during refueling this could ignite the very flammable aviation fuel and so there should be safety precautions to minimize this risk 1. Your initial challenge is to develop some safety precautions that employees should follow while refueling a plane. You should also think about how you would train workers to make sure they are followed. You can use the same engineering design process that you would when designing a device. Problem: To ensure any possible static electric charge is removed from a pane before refueling. Goals: Your safety precautions are successful if: Following them completely discharges a plane. They are followed precisely by all relevant workers. 1 See Wikipedia: Aviation Fuel: Safety Precautions SE-69