Name Section Question Sheet for Laboratory 3: E-1: Electrostatics PART I. CHARGE OBJECTIVE: To build a qualitative model for charge by observing forces between charged objects. APPARATUS: 1. Tape, hard rubber (ebonite) rod, fur, acrylic rod, silk cloth. INTRODUCTION: In this part of the lab you will explore and build a model for charge that explains the interactions of charged objects. These objects are all insulators as opposed to the conductors you will use in PART II. You will mechanically charge these insulators as instructed below, and that charge will not be free to move around on the insulator whereas charge is free to move around on a conductor. WARNING: Static charges can be influenced by factors beyond your control. Charge transfer through the triboelectric effect can depend on details such as the cleanliness of the insulating materials, and the mechanical motion used to transfer the charge. Water vapor in the air can drain charge from, or change the type of charge, on an insulator (particularly on humid days). If you are getting confusing or contradictory results in this lab, get help from your TA. EXPERIMENTS: 1. Press a piece of scotch tape, about 10 cm in length, firmly onto a smooth unpainted surface such as a notebook or lab bench. (For ease of handling, make a handle by folding a short section of the tape over on itself.) Then very quickly and authoritatively peel the tape off the surface and hang it from the edge of the lab table (if it is attracted to and sticks to the metal frame of the table, try hanging it instead from the back of a lab chair). Make sure it doesn t touch anything, such as your hand, or you might effect the charge. Describe the behavior of the tape as you bring nominally uncharged objects toward it (e.g., your hand, a pen ).
2. Make another 10 cm piece of tape with handles. Take off your first piece from the edge of the lab table and stick both down to the top of the lab table. Peel both off very quickly, one with each hand. Bring them towards each other in the air and describe the behavior. Now stick one to the edge of the lab table (or lab chair), and bring the second tape close to it. Describe the behavior. It is important to keep your hands and other objects away from the tapes during this experiment. Explain why this is necessary. How does the distance between the tapes affect the interaction between them? 3. Discard the tape from parts 1) & 2). Each member of your group should now press new tape onto the surface (but don t peel it off yet) and write B (for bottom) on it. Then press another new tape on top of each B tape and label it T (for top). (Handles are particularly useful here.) Pull one of the pairs off as a unit, then pull apart the top and bottom tapes. Hang one Bottom tape and one Top tape from your lab bench. Then Pull off one of the other pairs, and separate them as above. Bring one of these Bottom or Top tapes near each of the hanging tapes and record the interaction (as repelled or attracted) between tapes in the table below. Peel off the last pair from the bench and double check your results. Do all this quickly so the charge doesn t drain off into the air. Free Tape Free Tape Top Bottom Top Bottom Top Hanging Tape Bottom 2
Do bottom tapes or top tapes have more net charge on them? Or are the about the same? How can you tell? 4. Charge the rubber (ebonite) rod with fur and bring it near each tape (Top and Bottom) in turn and record the interactions (as repelled or attracted) in the table above T Hanging Tape B Rubber Rod Compare the interactions of the rod and tapes to the interactions between the tapes in part 3. Describe any similarities or differences. 3
5. Base your answers to the following questions on your observations so far. a. Is it possible that there is only one type of charge? If so, explain. If not what is the minimum number of different types of charge needed to account for your observations so far? Explain your reasoning. b.. Which tape, Top or Bottom, has the same type charge as the rubber rod? Explain your reasoning. Call this charge the rubber charge. c. Which tape, Top or Bottom, has a different type of charge as the rubber rod? Explain. Call this the anti-rubber charge. d. What is the general rule for the interaction between charges? Your general rule should work for all combinations of rubber and anti-rubber charge. e. In class we said that electrons are transferred from one object to another when rubbing. What type of experiment would you need do to tell which of your charges, rubber or anti-rubber, corresponds to extra electrons? 4
PART II. THE ELECTROSCOPE OBJECTIVE: To use the electroscope as a measuring device to explore charge motion in conductors. APPARATUS: 1. Electroscope, three conducting spheres on insulated stands, black rubber (ebonite) rod, fur, acrylic rod, silk cloth. INTRODUCTION: Here you will extend your model of charge from Part I to conductors. We said that charge is free to move around on conductors but it is only the electrons that can move. You will use what you observed about forces in Part I and the fact that only electrons move to explain why the electroscope behaves as it does as well as how charge behaves on conductors. The electroscope, pictured at left, consists of a conducting case and two aluminum leaves hanging from a conducting rod with a ball on top. The rod does not make electrical contact with the case where is passes through so electrons (or charge) cannot flow between the rod and the case. The leaves are like the tape in Part I except that they are conductors, not insulators. WARNING: The electroscope is fairly sensitive, so unknown charges anywhere in the vicinity can influence your results. Make sure charged rods are far away when you don t want them to influence your system. Inadvertently touching a wire or conducting object can discharge them. Having your hand or other conducting object near any part of the system can influence the results. Water vapor in the air can drain charge from materials (particularly on humid days). The insulating stands of the conducting spheres can drain charge from the sphere when they get dirty. Your TA can clean them with alcohol for you. EXPERIMENTS: 1. Touch your hand to the connection at the top of the electroscope to neutralize (remove any charge from) the leaves. Rub the rubber rod with fur and bring it near the ball (not touching) of the electroscope then move it away, observing the leaves during the process. a. Record your observations and your thoughts on why the leaves behave as they do. Share your thoughts with your lab partners and try to come to a consensus. 5
Rub the rubber rod with fur and bring it near the ball again and this time do touch the rod to the ball, then move it away, observing the leaves during the process. b. Record your observations and your thoughts on why the leaves behave as they do. Share your thoughts with your lab partners and try to come to a consensus. 6
4. In this section you use an electroscope to make measurements of a test object. Take the longest banana-plug cable from the wall. Tape one end to the top of the electroscope so that it makes electrical contact, and plug the other end into one of the conducting spheres on insulating supports. Make sure the cable is suspended so it does not touch the lab table (charge will drain to the lab table if it touches), and keep your hands and charged objects away from the cable. A conducting cable now connects the electroscope and conducting sphere.. Charge the ebonite rod with the fur, and touch it to the conducting sphere, and remove the rod far away from the sphere and electroscope. Explain what the electroscope leaves do, and describe the charge distribution. On the last page of this packet there are drawings where you can sketch out charge distributions if you find it helpful. b. Charge the ebonite rod again, and transfer some more charge to the sphere. Remove the rod far away from the sphere and electroscope. What happened to the leaves? c. Use the information from parts a and b to describe what physical quantity the deflection of the electroscope leaves measures here. You will see in the next section that your conclusion is valid only when the sphere is isolated, and not interacting with other charged objects. 7
5. Now you use the same setup as in Part 4 above to figure out why touching your hand to charged conductors neutralizes them. a. Continue to charge up the conducting sphere with the ebonite rod until the electroscope leaves each deflect about 45. Take the ebonite rod far away. If you re not sure if it is far enough away, move it around and see if the electroscope reading changes. If you re not able to charge to system to ~ a 45 angle, or if the leaves slowly come back together, ask your TA some of the insulators may be dirty and charge is leaking off. Bring your hand close to the conducting sphere without touching it. Describe the deflection of the leaves, and explain how the charge distribution in various parts of the system has changed. b. Touch the 2 nd conducting sphere with your had to make sure it is discharged. You want to see what happens when this neutral object is brought close to the conducting sphere. Slide it along the table slowly right up next to the 1 st sphere, without touching them. What has happened to the electroscope leaves? If you can t tell, quickly move the 2 nd sphere away from the first while watching the electroscope leaves. Compare the behavior with that of your hand in part a. c. Again bring the 2 nd (neutral) conducting sphere up to the 1 st sphere but this time touch them together. What happens to the electroscope leaves? Remove the 2 nd sphere and describe any changes. Explain how the charge distribution differs before and after touching the spheres. d. Now take the 3 rd sphere and connect it to the 2 nd with the shortest banana-plug cable you can find. Make sure the cable connecting them is far from the table and far from your body. Touch your finger to one of the connected spheres to neutralize them. Slide one of the connected spheres toward the 1 st sphere, making sure your hand stays away from the banana-plug cable. Watch the electroscope as you touch the 1 st sphere with the connected spheres. Explain what happened to the charge distribution. 8
e. If there are extra spheres not being used, and if you are interested, you can connect more and more spheres together and repeat d above (making sure to charge the 1 st sphere to 45 on the electroscope). But you probably see the trend. Describe the trend below. f. Again charge the 1 st conducting sphere to 45. Now touch the 1 st conducting sphere with your hand. What happens to the electroscope leaves? Remove your hand and describe any changes. Explain how the charge distribution differs before and after you touched the sphere. g. Explain why touching a charged object with your hand neutralizes it. h. What is the moral to the story told in part 5? 9
6. In this section you figure out how a nearby charge can distort the charge density on a conducting object. You do this using a technique called charging by induction. Set up as below. Touching 3 2 1 a. Charge the ebonite rod with the fur. Bring the rod close to sphere 1 on the side opposite its contact point with sphere 2, but do not touch. If you hear a spark jump (small crack) you have transferred some charge, and you need to neutralize the spheres and start over. Without moving the ebonite rod, pull sphere 2 away from sphere 1. Put the ebonite rod far enough away that it doesn t influence sphere 1. Move spheres 2 and 3 far enough away that they don t influence sphere 1 (be careful not to touch the cable connecting spheres 2 and 3. Describe what happened, and describe the final charge distribution on your system. b. Move spheres 2 and 3 back towards sphere 1, and touch spheres 1 and 2. Careful again not to touch anything inappropriate. Move spheres 2 and 3 away from 1. What happened? Describe the charge distribution now. 10
c. Neutralize all spheres. Repeat the procedure of part a. After you finish, charge the ebonite rod again and bring it toward sphere 1. Describe what happens to the electroscope leaves, starting from when the rod is very far away. Explain how this is consistent or inconsistent with the charge distribution you determined in part a. If inconsistent, revise your charge distribution. d. Repeat the procedure of part a, but use your hand instead of spheres 2 and 3. Is the effect bigger or smaller? Explain why. e. What is the moral to the story told in part 6? 11
7. If there is time, you can complete this section. Ask your TA if they want you to do this. Estimate (very roughly) the number of electrons on both leaves when they are separated. Your answer should just be a power of 10 (order of magnitude estimation). Make any reasonable simplifying assumptions you feel necessary, for instance how many Newtons of force are required to hold a leaf to the side. Ask your TA if your group can t decide if a particular assumption is reasonable. Show your reasoning and state all your assumptions. 12
Use these pictures to sketch out charge distributions if you find it helpful. 3 2 1 3 2 1 1 1 13