Physics Experiments 133 1-Xxx
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PHYSICS EXPERIMENTS — 133 1-1
Experiment 1 Electrostatics
OBJECTIVE: charge so that the gold foil sticks out at a roughly You are probably aware that nature contains only 45 degree angle or so. It is known that the dome of two kinds of charges. Electrons are negatively the VdG is negative (electrons are carried to the charged fundamental particles and atomic nuclei are dome on the moving belt) and so by this process we positive. Objects can have either too many are charging our electroscope negatively. You will electrons to balance out the positive charge in the do this repeatedly today so be sure you can do it nuclei or too few. Everyone is also aware that like easily. The electroscope can be discharged (brought charges repel each other and unlike charges attract. back to the electrically neutral state) merely by Today, by direct investigation, we want to try to touching the ball on its top. investigate this general knowledge, especially the fact that there are only two kinds of charge. We b) Charging by Induction. also will investigate where excess charge is located It will be important to be able to take negative on a conductor, whether it is on the inside or on the charge off the electroscope also, and that can be outside. accomplished with a process called "charging by induction." We'll describe the process using a water APPARATUS: pipe as a "ground" but you may have to improvise • Van de Graaff (VdG) machine; several are your own ground at your table. The procedure is to around the edges of the room charge your metal sphere negatively by direct • Electroscope (black box with windows, ball on contact with the VdG. Then hold the proof plane top and metal bar and flexible gold foil inside) touching the water pipe and bring the charged • Two proof planes (shaped like a flat spoon with sphere near the proof plane but not touching it. an insulating handle) Finally break the contact between the proof plane • Medium size metal sphere with base and support and the water pipe while the charged sphere remains column and a hole in the top in place. Ideally what has happened is that the • Rubber rod and fur, glass rod and silk, plastic rod negative sphere has repelled electrons from the and wool, plastic ruler and paper towel proof plane into the ground and when you break the • Wimshurst electrostatic machine on the front contact the proof plane has a deficit of electrons and table so is charged positively. If you now touch the positive proof plane to the electroscope the THEORY: electroscope gets a positive charge and the gold foil Your text has a good discussion of the underlying should stick out at some angle. You will do this theory. repeatedly today so be sure you can do it easily.
EXPERIMENT: 2. Behavior of Like and Unlike Charges. 1. Warm Up Activities. To a high degree of precision it is known that the Our goal here is to get the electroscope into a universe is electrically neutral: there are is as much state in which it can be used as a measuring tool to positive charge as there is negative. It is also investigate the electrostatic behavior of other impossible to change this net charge, a condition materials. usually referred to as the law of conservation of electric charge. You should realize that the VdG a) Charging by contact. does not make electric charge, it merely separates Please take the metal sphere to the VdG and touch it positive from negative charges. No mechanism is to the charged dome. Bring the sphere back and use known that produces a net gain of either positive or the proof plane to take charge off the sphere and put negative charges; all that can be done is to change charge onto the electroscope. Transfer enough the distribution of charges. 1-2 PHYSICS EXPERIMENTS — 133
Whatever charge is on an insulated electroscope 3. Behavior of Other Charged Objects. remains there. Bringing another charged object We know how to charge our electroscope either near the electroscope will cause the electroscope's positively or negatively. We also know what charges to distribute themselves in a new way, but positive/negative proof planes do to the net charge remains conserved. Below are a positive/negative electroscopes. IT WOULD BE series of short investigations on changing the INTERESTING TO INVESTIGATE WHETHER distribution of charge on your electroscope. ALL CHARGED OBJECTS BEHAVE LIKE ONE OR THE OTHER OF THESE PROOF PLANES. It a) Charge the electroscope negatively and then is conceptually possible that there is a third kind of bring a negatively charged proof plane near the charge that makes the foils of electroscopes behave electroscope ball. What should happen is that differently from what we have so far encountered. negative electrons on the ball should be repelled Such an object would be a third kind of electric downward to join the electrons already present on charge. The goal of the next section is to the gold foil and its support post, giving more investigate this question with several different repulsion to the gold foil. THEREFORE THE charged objects. GOLD FOIL SHOULD STICK OUT FARTHER. Try it and be sure it works. For each of the investigations (a) through (f) below we have to do two measurements, one with a b) Charge the electroscope negatively and then negative electroscope and one with a positive bring a positively charged proof plane near the electroscope. If the object being studied does electroscope ball. What should happen is that different things to the two electroscopes then it is negative electrons should be attracted up to the ball either positive or negative according to what we leaving fewer on the gold foil and its support post knew above. And we can easily tell whether it is and resulting in less repulsion of the gold foil. positive or negative. However, if the object does the THEREFORE THE GOLD FOIL SHOULD STICK same thing to the two different electroscopes then OUT LESS FAR. Try it and be sure it works. perhaps you have discovered a new kind of charge, neither positive nor negative. c) Charge the electroscope positively with a proof plane charged positive by induction. Now bring a a) First charge the electroscope negatively with the positively charged proof plane near the electroscope VdG, sphere, and proof plane as usual. Rub a ball. We expect that more negative electrons should rubber rod on a piece of fur, bring the rod near the be attracted upward to the ball leaving a larger net electroscope ball, and note how the gold foil positive charge on the foil and support post. This behaves. Then charge the electroscope positively gives more repulsion to the gold foil. by induction with the VdG, sphere, and proof plane. THEREFORE THE GOLD FOIL SHOULD STICK Rub a rubber rod on a piece of fur, bring the rod OUT FARTHER. Try it and be sure it works. near the electroscope ball, and note how the gold foil behaves. State whether the rubber rod is d) Charge the electroscope positively with a proof positively or negatively charged or whether it plane charged positive by induction. Then bring a behaves differently from a charged proof plane as negatively charged proof plane near the you saw in #2 above. electroscope ball. What should happen is that negative electrons on the ball should be repelled b) Do this again with the glass rod rubbed on silk. downward to at least partially replace the electrons State whether the glass is positively or negatively removed from the electroscope initially. Hence the charged or whether it behaves differently from a gold foil and its support post are closer to being charged proof plane as you saw in #2 above. neutral than they were initially. THEREFORE THE GOLD FOIL SHOULD STICK OUT LESS FAR. c) Do this again with the plastic rod rubbed on Try it and be sure it works. wool. State whether the plastic is positively or PHYSICS EXPERIMENTS — 133 1-3 negatively charged or whether it behaves differently from a charged proof plane as you saw in #2 above. (Sometimes proof plane experiments fail because the handle becomes conducting with time. If this d) Do this again with the plastic ruler rubbed on a happens you might try cleaning the handle with paper towel. State whether the ruler is positively or some alcohol and drying it off with a paper towel.) negatively charged or whether it behaves differently from a charged proof plane as you saw in #2 above. (a) • Charge the electroscope either positive or negative as you did above, whichever is e) Go up to the front table and turn the crank on the convenient. Wimshurst for a few seconds. Take charge off one • Charge a proof plane by direct contact with of the metal balls with a proof plane and, using the the VdG dome. same methods as described above, determine and • Touch the proof plane to the outside of the state whether the charge taken off that ball is metal sphere. positive or negative or whether it behaves • Use the other proof plane to try to take charge differently from a charged proof plane as you saw off the sphere by directly touching it to its in #2 above. outside. Then move it near the charged electroscope to check its charge, if any. f) Go up to the front table and turn the crank on the DID YOU REMOVE ANY CHARGE AND IF SO Wimshurst for a few seconds. Take charge off the OF WHAT SIGN? other one of the metal balls with a proof plane and, using the same methods as described above, (b) • Charge the electroscope either positive or determine and state whether the charge taken off negative as you did above. that ball is positive or negative or whether it • Charge a proof plane by contact with the VdG behaves differently from a charged proof plane as dome. you saw in #2 above. • Touch the proof plane to the inside of the metal sphere. • Use the other proof plane to try to take charge 4. Result. off the sphere by directly touching it to its What we expect of the above investigations is outside. Then move it near the charged that we should be able to find only two kinds of electroscope to check its charge, if any. charge in nature: the charge we take directly off the DID YOU REMOVE ANY CHARGE AND IF SO dome of the VdG machine (called negative) and the OF WHAT SIGN? charge we get "by induction" (and called positive) using the charge from the VdG. DO YOUR (c) • Charge the electroscope either positive or EXPERIMENTS AGREE WITH WHAT WE negative as you did above. EXPECT OR DID YOU FIND A CHARGE THAT • Charge a proof plane by contact with the VdG DOES NOT ACT LIKE EITHER ONE OF THESE dome. AND WHICH THEREFORE MUST BE A THIRD • Touch the proof plane to the outside of the KIND OF ELECTRIC CHARGE? metal sphere. • Use the other proof plane to try to take charge 5. Where Charge Normally Resides on a off the sphere by directly touching it to its Conductor. inside. Then move it near the charged It is common knowledge that if electrons are electroscope to check its charge, if any. placed on a metal conductor they will move around DID YOU REMOVE ANY CHARGE AND IF SO and through the metal itself until they stop on the OF WHAT SIGN? outside surface of the conductor. No matter whether you charge the metal by contact on its (d) • Charge the electroscope either positive or inside or outside surface, the charge ends up on the negative as you did above. outside surface. Please try to verify this, as outlined • Charge a proof plane by contact with the VdG below. dome. 1-4 PHYSICS EXPERIMENTS — 133
• Touch the proof plane to the inside of the Be sure there is no direct contact between proof metal sphere. planes or with the sphere, except for the touch of • Use the other proof plane to try to take charge the uncharged proof plane to the sphere's inside. off the sphere by directly touching it to its DID YOU GET ANY CHARGE? IF SO WAS IT inside. Then move it near the charged THE SAME SIGN OR THE OPPOSITE OF THE electroscope to check its charge, if any. FIRST PROOF PLANE'S CHARGE? DID YOU REMOVE ANY CHARGE AND IF SO OF WHAT SIGN? 7. Result. Were you able to show that charge normally 6. A Possible Way to Take Charge Off the Inside resides on the outside of a conductor? Were you of a Conductor. able to show that under certain circumstances it is This investigation may or may not work for you. possible to take charge off the inside of a Success depends on both your experimental skills conductor? and the state of your equipment. If you fail at first you may want to try again with equipment swapped Be able to describe the latter case in words, temporarily from an adjacent station. Or often saying what you did, paying special attention to + & cleaning the proof plane (or metal sphere) "handles" - signs. with alcohol may make them better insulators and it might be a good idea to clean at least the proof rev. 8/05 plane handles right away. The theory is not obvious at this point in time but will be clear in our discussion of Gauss's Law (Chapter 23). Let's just try to get some experimental results here and worry about the theory later.
(a) • Charge your electroscope, either sign. • Discharge your metal sphere. • Charge one proof plane by direct contact with (a strong) VdG. • Hold this charged proof plane steady inside the metal sphere but not touching it. • Use the second, uncharged, proof plane to try to take charge off the outside of the sphere. Then move it near the charged electroscope to check its charge, if any. DID YOU GET ANY CHARGE? IF SO WAS IT THE SAME SIGN OR THE OPPOSITE OF THE FIRST PROOF PLANE'S CHARGE?
(b) • Charge your electroscope, either sign. • Discharge your metal sphere. • Charge one proof plane by direct contact with (a strong) VdG. • Hold this charged proof plane steady inside the sphere but not touching it. • Use the second, uncharged, proof plane to try to take charge off the inside of the sphere. Then move it near the charged electroscope to check its charge, if any.