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 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. Hanging Tape Top Bottom Top Free Tape Bottom Top Free Tape Bottom3 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 Hanging Tape T 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.4 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?5 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
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