Electrostatics. Experiment NC. Objective. Introduction. Procedure

Transcription

1 Electrostatics Experiment NC Objective In this experiment you will explore various aspects of electrostatic charging and electrostatic forces. Introduction You are probably aware of various phenomena associated with static electricity or static charge. Some of the more familiar examples include: rubbing a balloon in your hair and sticking it to a wall, getting a shock when you walk on a carpet and touch a doorknob, or the sticking together of clothes taken from a dryer. Charge is the name given to the property of matter responsible for electromagnetic interactions. As you may know, ordinary matter is composed of atoms of the various elements, and all atoms are made from neutrons, protons, and electrons. Neutrons and protons reside in the atomic nucleus around which the electrons are in continual motion. Neutrons have no charge while the protons and electrons have equal but opposite charge, with the proton charged positively and the electron charged negatively. Normally, matter is uncharged with equal numbers of protons and electrons. However, electrons are fairly mobile and quite a few can be removed from or added to a material by simple rubbing. As the number of electrons added or removed increases, further charging becomes more difficult. Because the force between charges is so strong, effects such as attraction and repulsion can be quite noticeable even if only a very small fraction of the atoms in a material gain or lose electrons. The forces arising between charged objects can be explained quantitatively by Coulomb s law. The Coulomb force arises between any two charged particles. It is attractive if the particles have opposite charge and repulsive if their charges are of the same sign. In either case, the force acts along the line joining the two particles. The strength of the force increases as the particles are brought nearer one another or if the size of either charge increases. The magnitude of the Coulomb force F is proportional to the magnitude of the charges q 0 and q 1 and is inversely proportional to the square of the distance r between them. This dependence is expressed by Coulomb s law. F = k q 0 q 1 r 2 (1) The experimentally determined proportionality constant k = Nm 2 /C 2. Procedure Use the sheets at the end of this writeup to record your observations and answers to the questions posed. Precede any such observations and/or answers with the associated procedure step number, and write in complete sentences. NC 1

2 NC 2 Introductory Physics Laboratory Charged Plastic Tape In the first part of this experiment you will make charged strips of ordinary sticky tape and observe their behavior under various conditions. The charge on these tapes will be distributed all along the tape and your observations can not be explained precisely by the Coulomb force between two charged particles. Nonetheless, aspects of Coulomb s law should become apparent as you make your observations and where an explanation is requested, you should relate the observation to the appropriate dependence in Coulomb s law. Each strip of tape should be about 20 cm (or 8 inches, about the width of this page). One end should be folded over about 5 mm to make a non-sticky handle. The first step is to smoothly lay down one such piece of tape, called the base tape, (sticky side down) on the lab table. This one will not be moved during the experiment. First run your finger along the base tape stuck to the lab table. This removes any charge that may be on the base tape. Then smooth another tape strip, sticky side down, over the one on the table, running your finger along the entire length. A smooth, wrinklefree contact is important. We call this a U- tape. (U stands for upper, as you will see later in this experiment.) Mark the tape with a U near the handle. Holding down the base tape, grab the handle of the U-tape and very quickly peel it off the base tape. If the U-tape flies up and sticks to your hand (due to static cling or the tape glue), free it with your other hand with minimal touching of the tape. Each time you need to make or remake a U- tape, remember to first run your finger along the base tape. Attraction or Repulsion 1. Stick a freshly-made U-tape to the edge of the lab table and slowly bring a finger near it. Describe what happens. Check whether it matters if you bring your finger in towards the sticky or smooth side of the tape and record the results. Hopefully, you noticed the U-tape is attracted to your finger. If it was not, try recharging the U-tape according to the instructions. If a second try also fails, check your technique with the instructor. The explanation for the attraction of the tape to your finger will be discussed later in this experiment. The attraction does serve as a good check on whether you have a well-charged tape. The tapes will lose their charge over time. When in doubt, do the bring-your-finger-close-andcheck-for-attraction test. If the attraction is too weak, recharge the tape. To discharge or neutralize a charged tape, hang it from the table edge and hold down the hanging end with one hand while you run a finger completely up and down the smooth side of the tape. To partially neutralize a tape, run a finger along only a part of the tape. You might touch only half the width of the tape and run down its entire length to partially neutralize the whole tape, or you might touch the whole width of the tape but only run your finger down half the length of the tape to totally neutralize half the tape. 2. Neutralize the hanging tape, and while it is hanging freely, check what happens as you now bring your finger near it. Describe. 3. Prepare two labeled U-tapes. Hang one U-tape from the table edge. Holding both ends of the other U-tape, bring it parallel to and close by the hanging one. Do Rev. 1.2

3 Electrostatics NC 3 they repel, attract, or not interact. Give a reason for this behavior. 4. Bring the tape held between your hands up to the hanging tape from different directions the front, the back, and one side or the other, say. What do you notice about the direction of the deflection? Does it move backward, forward, left or right? What do you think might be a rule for the direction of the electrostatic force between two charges relative to the direction of the line from one to the other? 5. How does the size of the deflection change as the tapes are brought closer and closer together? What feature of the electrostatic force might be the cause? 6. Compare the deflection for two freshlyprepared U-tapes with the deflection after one of them has been partially neutralized by touching only half the width of the tape and running your finger down its entire length. How does the size of the deflection change? What feature of the electrostatic force might be the cause? The L-Tape Now you will make two different kinds of tape a U-tape and an L-tape. To do so, smooth down a new strip of tape (with a handle and labeled L) on top of the base tape. Then smooth down a U-tape on top of this. Holding down the base tape, slowly remove, in one piece, both the L-tape and U-tape, without letting them separate. Hang the double tape to the desk and check if there is an attraction when you bring your finger close. Get rid of any charge by running your finger along the smooth side of the U-tape, and recheck for attraction to your finger. If it appears neutralized, quickly pull the tapes apart. 7. Stick one tape to the edge of the table. Holding both ends of the other, bring it near the hanging tape. Describe the interaction as repulsive, attractive, or noninteracting. What rule for interacting charges do you suppose is responsible for the observed behavior? 8. Stick both tapes to the table edge about 10 cm apart. Rub the Plexiglas sheet with the felt cloth. Rub quickly in one direction. Slowly bring the Plexiglas sheet near the two hanging tapes. Describe what is observed. It is known from other experiments that the Plexiglas will be positively charged. What does this imply is the sign of the charge on the U- and L-tapes? 9. Repeat the previous step using the foam sheet instead of the Plexiglas. Describe the tape deflections and use them to determine the charge on the foam sheet. The physics of how charge is transferred when dissimilar materials are rubbed together is complex. There are no simple rules to predict the signs of the charge. 10. Predict what will happen if you bring your finger near each tape, giving some real reasoning. Then try it. Was your prediction upheld by observation? The reasoning behind the actual behavior is definitely not obvious to most people. It is described next. If your reasoning was not correct, read the next section and correct it. Some Puzzles The first puzzle is why pulling tapes apart charges them. The details of this puzzle are not completely understood, involving topics in Rev. 1.2

4 NC 4 Introductory Physics Laboratory surface physics and chemistry. We will not even attempt to resolve this puzzle. The second puzzle is why both positively or negatively charged tapes are attracted to your finger. If like charges repel and unlike charges attract, shouldn t your hand, no matter which charge it has, attract one tape and repel the other. The answer to this puzzle has to do with the ability of charge to move around. The charge on the tapes is not very free to move around. It stays put. (However, it is slowly neutralized by charges floating around in the air and attracted to the tape.) However, in some materials, called electrical conductors, charge can move freely throughout the material. Metals are good conductors in which negative charges (electrons) are free to move. Salt water is a good conductor in which positive and negative ions (charged nuclei or molecules) are free to move. Your body is also a good conductor. Hopefully, you have demonstrated that like charges repel and unlike charges attract and that the force of attraction or repulsion is larger for charges that are close together and decreases as charges move farther apart. When you bring your finger near a positively charged object, electrons in your body are attracted toward it creating a negative charge on your finger. Assuming you are electrically isolated, other parts of your body must acquire a positive charge as the electrons leave those regions and go to your finger, but these positive charges will be farther from the external positive charge that caused the change. Consequently, the attractive force from the negative charge in your fingertip is greater than the repulsive force from the positive charge elsewhere on your body and the external positive charge is attracted to your finger. When you bring your finger near a negatively charged object, electrons are repelled from your finger and move to places on your body farther from the charged object. Since your finger normally has equal positive and negative charge, this charge movement leaves your finger with a net positive charge. Your positively charged finger being closer to the negatively charged object, once again causes a net attractive force between your finger and the object. The third puzzle is why you can neutralize a (positive) U-tape by running your finger along the smooth side. (Recall that on a U-tape the sticky side is against the other tape. Thus, the charge is located on the sticky side since pulling apart the two surfaces is what separates the charge.) This logic is correct. The reason it can be neutralized from the smooth side is because running your finger along this side drops off a nearly identical amount of negative charge on the smooth side as there was positive charge on the sticky side. Since the charges are so close together, separated by only the thickness of the tape, they cancel each other s effects. The Pie Plate Demonstrator The pie plate demonstrator is illustrated in Fig. 1. The first thing needed to get the demonstrator to show anything is a plate of electrostatic charge. This is created by vigorously rubbing a Plexiglas sheet (not shown) with a piece of felt. Place the Plexiglas flat on the lab table, grab the felt in the middle and bunch it up so that lots of edges will rub against the plate. Holding down the Plexiglas in one corner, rub with quick, hard strokes in one direction. If you are having trouble getting it charged, try turning it over and rubbing the other side or ask the instructor to clean it. As with the tape, the Plexiglas may need to be recharged from time to time. Rev. 1.2

5 Electrostatics NC 5 neon bulb cassette tape aluminum covered straw Figure 1: The electrostatic pie plate demonstrator. A Plexiglas sheet, charged by rubbing with felt, is also needed. The aluminum pie plate is a conductor and should be handled by the plastic cylinder so that it does not become neutralized. In the instructions to follow, always hold, place, lift, or lower the pie plate by the plastic cylinder. Placing an uncharged pie plate on the positively charged Plexiglas does not put much net charge on the plate. Thus, charge transfer between the Plexiglas and the pie plate will not be an explanation for any of the effects you will see. Very rarely, the Plexiglas may become too charged up (by vigorous rubbing in a cool, dry room), and the first time the pie plate is put on top, a spark may jump between them. This would transfer a significant net charge to the pie plate with a corresponding reduction of charge on the Plexiglas. If this happens, touch the pie plate to get rid of this charge. Do not rerub the Plexiglas; the remaining charge will still be enough to run the experiments, but the sparking should not occur again. 11. Charge the Plexiglas sheet and place it on top of the large-diameter plastic cylinder. The table top is a poor electrical conductor, but it is a good enough to prevent the apparatus from working properly if the charged sheet is placed directly on the table. Then, holding the pie plate by its plastic cylinder and away from the Plexiglas, make sure it is uncharged by touching it with your finger. Place the pie plate on top of the Plexiglas. Observe that the cassette tape rises. The cassette tape is a flexible conductor and the glued end remains in contact with the pie plate. Thus, when the rim of the plate becomes charged (positively or negatively), the tape also gets a like charge and the repulsion between them makes the tape go up. Note that the tape going up only indicates there is charge on the rim of the pie plate; it does not provide the sign of that charge or tell whether or not there is charge on the base of the pie plate. 12. Lift the plate off the Plexiglas. The tape may go up, but don t worry about that now. We will explore that a bit later. (a) For now, simply touch the plate while it is off the Plexiglas to neutralize it and make the tape drop. (b) Place the pie plate back on the Plexiglas. The tape should rise again. (c) With the tape still up, lift the pie plate off the Plexiglas. The tape should fall. (d) Repeat the lowering and raising of the pie plate to see that the tape rises when the pie plate is on the Plexiglas and falls when it is off the Plexiglas. 13. At each step (a)-(c), give an explanation for what happens. In particular, describe whether any charge moves on or off the pie plate, whether charge rearranges itself within the pie plate, whether the rim of the pie plate or its base or both have charge, whether there is a net charge on Rev. 1.2

6 NC 6 Introductory Physics Laboratory the plate overall, and give the signs of the (rim, base, and net) charges. Hint: think about what happens to the electrons in the pie plate in the presence of the positively charged Plexiglas. Charging by Induction 14. Charge the Plexiglas and holding the pie plate away from the Plexiglas, neutralize the plate by touching it. (a) Place the pie plate on the Plexiglas. The tape should go up. (b) With the pie plate on the Plexiglas and the tape still up, touch the pie plate with your finger. The tape should fall. (c) Lift the plate and the tape should now rise up. (d) With the plate up in the air and the tape indicating a rim charge, lower the plate onto the Plexiglas. The tape should fall. (e) Repeat the raising and lowering of the pie plate to see that on the Plexiglas the tape goes down, and off the Plexiglas the tape goes up. 15. At each step (a)-(d), give an explanation for what happens. In particular, describe whether any charge moves on or off the pie plate, whether charge rearranges itself within the pie plate, whether the rim of the pie plate or its base or both have charge, whether there is a net charge on the plate overall, and give the signs of the (rim, base, and net) charges. Some Neat Stuff 16. If necessary, adjust the aluminum-covered straw to the height of the rim and about five millimeters from it. Neutralize the plate and then place it on the charged Plexiglas. Slowly bring a finger closer and closer to the hanging straw and describe what happens. Provide an explanation. What is the charge on the rim after the straw stops oscillating? Where does it go? 17. Observe the neon light bulb at the front of the room. Like the one on your pie plate, it has two rod-like electrodes inside, one connected to each wire coming out of the bulb. Think of the power supply as a source of charge positive charge from the positive (red) terminal, negative charge from the negative (black) terminal. The light from the bulb emanates from around only one of the electrodes. Is the lit electrode connected to the positive or the negative side of the power supply? Swap the connections to the bulb. Which side lights now? 18. Dim the room lights. Neutralize the plate and put it back on the charged Plexiglas. With the tape up and indicating a charged rim, touch the free wire of the light bulb while watching the electrodes inside the bulb. The light produced may be weak so you may have to shadow the bulb to see it. Then touch the rim to completely discharge it. Lift the plate and again touch the free light bulb wire while observing the light bulb. Without recharging the Plexiglas, repeat placing the pie plate on and then off the Plexiglas, each time touching the free wire while observing which electrode lights. For each case (on or off the Plexiglas), state whether the light emanates from the electrode connected to the free wire or from the electrode attached to the rim. Based on the results of the previous step, explain how your results are consistent (or inconsistent) with the sign of the charge on the rim as determined in Steps 12 and 14. Rev. 1.2

7 Experiment NC Electrostatics Title Sheet Name (Seating No.): Partner (Seating No.): Course: Date: UFID: Section: Instructor: Comments on the experiment or writeup:

8 Experiment NC Electrostatics Observations/Explanations

9 Experiment NC Electrostatics Observations/Explanations

10 Experiment NC Electrostatics Observations/Explanations