Lab 1: Electric Interactions I

Transcription

1 Page 1 of 6 LEARNING GOALS OF THE LAB: 1. Learn how to explain the interactions of charged and uncharged objects. 2. Learn how to test a 3. Find how the strength of the electric interaction depends on the distance between the interacting charged objects. I. TESTING EXPERIMENT: ARE ELECTRIC AND MAGNETIC INTERACTIONS THE SAME? Your friend Gaurang says that the electric interaction is actually the same as the magnetic interaction, that they are just two different ways of describing the same phenomena. Design an experiment to test Gaurang s You remember that one way to cause objects to become electrically charged is to rub them together. You begin thinking about what this means if Gaurang is right. Remember, the goal of a testing experiment is to disprove the hypothesis that is being tested. It is much more productive to disprove an idea than it is to just find another piece of supporting evidence for it. Just one experiment that disproves an idea disproves it for good, but even a thousand experiments that support the idea won t prove it. Experiment one thousand and one might disprove it! C2 C3 C4 RUBRIC C: Ability to design and conduct a testing experiment (testing an idea/hypothesis/explanation or mathematical relation) Scientific Ability Is able to design a reliable experiment that tests the hypothesis Is able to distinguish between a hypothesis and a prediction Is able to make a reasonable prediction based on a hypothesis Missing The experiment does not test the No prediction is made. The experiment is not treated as a testing experiment. No attempt to make a prediction is made. Inadequate The experiment tests the hypothesis, but due to th7e nature of the design it is likely the data will lead to an incorrect judgment. A prediction is made but it is identical to the A prediction is made that is distinct from the hypothesis but is not based on it. Needs some improvement The experiment tests the hypothesis, but due to the nature of the design there is a moderate chance the data will lead to an inconclusive judgment. A prediction is made and is distinct from the hypothesis but does not describe the outcome of the designed experiment. A prediction is made that follows from the hypothesis but does not incorporate assumptions Adequate The experiment tests the hypothesis and has a high likelihood of producing data that will lead to a conclusive judgment. A prediction is made, is distinct from the hypothesis, and describes the outcome of the designed experiment A correct prediction is made that follows from the hypothesis and incorporates assumptions.

2 Page 2 of 6 C5 C8 Is able to identify the assumptions made in making the prediction Is able to make a reasonable judgment about the hypothesis No attempt is made to identify any assumptions. No judgment is made about the An attempt is made to identify assumptions, but the assumptions are irrelevant or are confused with the A judgment is made but is not consistent with the outcome of the experiment. Available equipment: Plastic and foam rods, rubbing materials, bar magnet, swivel. Include the following in your lab write up: a) List and briefly describe the properties of magnets. Relevant assumptions are identified but are not significant for making the prediction. A judgment is made and is consistent with the outcome of the experiment but assumptions are not taken into account. b) Describe your experimental procedure. Include a labeled diagram of your setup. c) What does Gaurang s hypothesis predict will happen when you perform the experiment? All assumptions are correctly identified. A reasonable judgment is made and assumptions are taken into account. d) Describe any assumptions that you made in making your prediction. How does each of these assumptions affect the prediction? How might each assumption hinder your ability to make a judgment about the hypothesis? e) Perform the experiment and describe the outcome in words and with a picture. f) What is your judgment about Gaurang s hypothesis? II. OBSERVATION EXPERIMENT: HOW DOES THE ELECTRIC FORCE BETWEEN TWO CHARGED OBJECTS DEPEND ON THE DISTANCE BETWEEN THEM? (QUALITATIVE) Design an experiment to investigate qualitatively how the strength of the electric interaction between two objects depends on the distance between them. Available equipment: Plastic and foam rods, rubbing materials, swivel. a) Describe your experimental procedure. Include a labeled sketch. b) What assumptions are you making in your experimental design? What will you do to make these assumptions as reasonable as possible? c) Perform your experiment. Record the results. d) Use your observations to determine qualitatively how the strength of the electric interaction between charged objects depends on the distance between them. Carefully explain your reasoning using words and force diagrams. Hint: This is trickier than it might seem at first. e) Design an experiment that would allow you to investigate the same phenomena quantitatively. In other words, design an experiment that would allow you to determine how the magnitude of the electric force exerted by one charged object on another depends on the distance between those two objects. You may use equipment other than what is listed above. Hint: What ways do you know of measuring forces?

3 Page 3 of 6 III. OBSERVATION EXPERIMENT: HOW DOES ELECTRIC FORCE DEPEND ON THE DISTANCE AND THE MAGNITUDE OF CHARGES (QUANTITATIVE)? Charles Coulomb ( ) is most famous for his investigation into the electric interaction. His goal was to find how the electric force exerted by one charged object on another depends on the magnitudes of the charges and on their separation. He used the setup shown to measure the electric force that one small charged metal ball exerts on another similar ball. The setup includes a wood rod suspended at its middle by a thin thread. Attached at each end of the rod is a small metal ball. One of the balls is charged, the other is not. When a second small charged ball is brought near the charged ball attached to the rod, the suspended rod rotates, twisting the thread. The magnitude of the electric force can be determined from the angle by which the string twists. At the time Coulomb did his experiment it was not possible to measure the actual value of the electric charge on the metal balls but he did know that if you touched a charged metal ball to an identical uncharged metal ball, the charge would be evenly split between them. Coulomb used this to vary the charge on the balls in a controlled way, then measure how the electric force between the balls depended on that. The table provides data that resembles what Coulomb might have collected. Charges (q 1, q 2 ) Distance Force 1, 1(unit) 1(unit) 1 (unit) 1/2, 1 1 1/2 1/4, 1 1 1/4 1, 1/2 1 1/2 1, 1/4 1 1/4 1/2, 1/2 1 1/4 1/4, 1/4 1 1/16 1, 1 2 1/4 1, 1 3 1/9 1, 1 4 1/16 a) Find patterns in the data. For each pattern you find construct a mathematical relation between the relevant quantities that describes the pattern quantitatively. Also, decide what the independent variable in the relation is, and what the dependent variable is. b) Is the table of data that you analyzed in Experiment 3 a real data table or a table of fabricated data? Explain your reasoning. c) In this experiment, what effects would cause random uncertainties in the measured values? What effects would cause systematic uncertainties? A systematic uncertainty is one that causes the measured value to be consistently less than the actual value, or consistently greater than the actual value. For each effect that you describe, decide whether it causes the measured value to be less than, or greater than the actual value. IV. OBSERVATION EXPERIMENT: INTERACTIONS OF A CHARGED OBJECT WITH AN UNCHARGED OBJECT The goal of this experiment is to determine if charged objects interact with neutral objects. If they do, investigate the interaction between them. Available equipment: Plastic rods, metal rod, a swivel, rubbing materials, foam board, aluminum foil ball attached to a string, paper.

4 Page 4 of 6 Part 1: Rub one end of a plastic rod (rod 1) with felt or fur and bring it close (but not touching) to an unrubbed plastic rod (rod 2) on a swivel. Repeat, but with a different rod that has been rubbed with a different material. a) Record your observations in words and with a picture. b) Devise an explanation for how the interaction between the rubbed rod and the unrubbed rod works. Hint: Think about the internal microscopic structure of the rod. Use charge diagrams to illustrate your explanation. A charge diagram is a sketch of the situation with plusses and minuses added to show regions with a lack of or excess of electrons. Part 2: Rub one end of rod 1 with felt or fur and bring it close to the end of an unrubbed metal rod (rod 3) on a swivel. Repeat, but with a different rod that has been rubbed with a different material. c) Record your observations in words and with a picture. Did the rubbed rod exert a weaker, stronger, or similar strength force on the metal rod as compared to the plastic rod in part 1? d) Devise an explanation involving a possible internal structure of the metal rod that might explain why the rubbed rods attract it in the way that they do. Use charge diagrams to illustrate your explanation. Part 3: Go to Click on Electricity and magnetism, and then on Structure of materials. You will perform the first three experiments virtually using the videos. Follow instructions on the website, discuss the questions with your group partners, and record your answers. For reference, the questions are also shown below. In addition, use charge diagrams to illustrate your explanations. If the video doesn t load inside the web browser, right- click on HERE below the video window, select Save Target As, and save the video to the desktop, then double- click the file on the desktop. Experiment 1: Rubbed Rod and Aluminum Soda Can: Qualitative Observation Experiment Record your observations and devise an explanation for the observed phenomenon. What do you need to assume about the internal structure of the can to account for the observations? Remember, there is NO CONTACT between the objects. Experiment 2: Rubbed Rod and Plastic Water Bottle: Qualitative Observation Experiment 1. Record your observations and compare them to the observations in a similar experiment where instead of the plastic bottle you had a metal soda can (Experiment 1). 2. What do you need to assume about the internal structure of plastic to account for the observations. Remember, there is NO contact between the objects. 3. How is this explanation similar to the explanation of the experiment with a metal soda can? How is it different? 4. Suggest some possible differences between the internal structure of metals and plastic that can explain the first two experiments? Are they consistent with your knowledge of chemistry? Experiment 3: Conductors and dielectrics: Testing Experiment Make sure you read the description of the experiment at the top of the page and do what it says. Make sure you make a prediction. 1. Why did the aluminum disk start moving away from the can even before the charged ball of the generator touched it? 2. Why did the aluminum disk fly away from the soda can after the ball touched the can? Explain your reasoning. 3. What do you need to assume about the internal structure of the soda can in order to explain this? 4. If the aluminum disk became charged, what type of charge did it receive: a. The same as the ball of the Wimshurst generator which was brought close to the can

5 Page 5 of 6 b. The opposite charge? What part of the video helps you answer this question? 5. What can you say about the internal composition of the can that explains both parts of the experiment: before the ball touched it and after it touched it. 6. Why do you think nothing happened to the disk when the experiment was repeated with a plastic bottle? What do you need to assume about the internal structure of the bottle to explain what happened? 7. Why do we call metals electric conductors and plastic materials electric insulators? V. WHY DID WE DO THIS LAB? 1. List two reasons for why a non- physicist might be interested in electrostatic interactions. 2. What is the main difference between an observational experiment and a testing experiment? 3. Explain briefly why it is that neutral objects are attracted to charged objects. VI. HOMEWORK Use any additional resources you want (class notes, Internet, textbook, etc.) to answer the following questions: 1. What is the difference between an electric insulator and an electric conductor? Give examples of materials that are electric insulators and examples that are electric conductors. 2. Explain what happens inside a neutral conductor when a charged object is brought nearby. Explain what happens inside a neutral insulator when a charged object is brought nearby. Use charge diagrams to illustrate your explanation. 3. Complete the charge diagrams shown below by adding plus and minus signs to the metal rod in the appropriate places as needed. a) A positively charged object is brought near one end of a metal rod. b) A positively charged object is placed at each end of a metal rod. c) A positively charged object is placed at each end of a metal rod but one has a greater charge than the other. 4. In 1891 a man named William Von Osten began showing his horse in public exhibitions. Clever Hans could apparently perform mathematical calculations and would give the answers by tapping his hoof. For example, if he was asked to give the sum of 3 plus 2 he would tap his hoof five times. More than a dozen scientists observed Hans and were convinced there was no signaling or trickery. They were impressed that Hans performed almost as well without Von Osten as with him.

6 Page 6 of 6 Admirers of Clever Hans' explained his behavior saying that Hans was capable of understanding human speech and performing mathematical calculations. Countless experiments supported this conclusion! However, in 1904 by Oskar Pfungst proposed a different explanation. He suggested that the horse was responding to subtle physical cues in the posture of the questioner. Oskar Pfungst hypothesized that Hans could perceive very subtle postural cues from his listeners that would tell him when to start and stop tapping. He tested his hypothesis in the following experiment. He asked Hans a question to which nobody in the audience knew the answer. If Oskar's hypothesis was correct, then Hans should not be able to see the cues and consequently would not respond correctly. Indeed, when the experiment was carried out Hans did not answer correctly. Pfungst also tested what happened when the horse couldn't see the person who did know the answer. In this case also, the horse didn't respond correctly. Clever Hans was indeed clever not because he could count and understand English but because he was extraordinarily sensitive to the expectations of people around him. Von Osten and Pfungst had different explanations of Hans' behavior. What did they do differently in testing their ideas of how Hans could count?