PHYSICS EXPERIMENTS — 1331-1

Experiment 1

Electrostatics

PHYSICS EXPERIMENTS — 1331-1

OBJECTIVE:

You are probably aware that nature contains only two kinds of charges. Electrons are negatively charged fundamental particles and atomic nuclei are positive. Objects can have either too many electrons to balance out the positive charge in the nuclei or too few. Everyone is also aware that like charges repel each other and unlike charges attract. Today, by direct investigation, we want to try to investigate this general knowledge, especially the fact that there are only two kinds of charge. We also will investigate where excess charge is located on a conductor, whether it is on the inside or on the outside.

APPARATUS:

• Van de Graaff (VdG) machine; several are around the edges of the room

• Electroscope (black box with windows, ball on top and metal bar and flexible gold foil inside)

• Two proof planes (shaped like a flat spoon with an insulating handle)

• Medium size metal sphere with base and support column and a hole in the top

• Rubber rod and fur, glass rod and silk, plastic rod and wool, plastic ruler and paper towel

• Wimshurst electrostatic machine on the front table

THEORY:

Your text has a good discussion of the underlying theory.

EXPERIMENT:

1. Warm Up Activities.

Our goal here is to get the electroscope into a state in which it can be used as a measuring tool to investigate the electrostatic behavior of other materials.

a)Charging by contact.

Please take the metal sphere to the VdG and touch it to the charged dome. Bring the sphere back and use the proof plane to take charge off the sphere and put charge onto the electroscope. Transfer enough charge so that the gold foil sticks out at a roughly 45 degree angle or so. It is known that the dome of the VdG is negative (electrons are carried to the dome on the moving belt) and so by this process we are charging our electroscope negatively. You will do this repeatedly today so be sure you can do it easily. The electroscope can be discharged (brought back to the electrically neutral state) merely by touching the ball on its top.

b)Charging by Induction.

It will be important to be able to take negative charge off the electroscope also, and that can be accomplished with a process called "charging by induction." We'll describe the process using a water pipe as a "ground" but you may have to improvise your own ground at your table. The procedure is to charge your metal sphere negatively by direct contact with the VdG. Then hold the proof plane touching the water pipe and bring the charged sphere near the proof plane but not touching it. Finally break the contact between the proof plane and the water pipe while the charged sphere remains in place. Ideally what has happened is that the negative sphere has repelled electrons from the proof plane into the ground and when you break the contact the proof plane has a deficit of electrons and so is charged positively. If you now touch the positive proof plane to the electroscope the electroscope gets a positive charge and the gold foil should stick out at some angle. You will do this repeatedly today so be sure you can do it easily.

2. Behavior of Like and Unlike Charges.

To a high degree of precision it is known that the universe is electrically neutral: there are is as much positive charge as there is negative. It is also impossible to change this net charge, a condition usually referred to as the law of conservation of electric charge. You should realize that the VdG does not make electric charge, it merely separates positive from negative charges. No mechanism is known that produces a net gain of either positive or negative charges; all that can be done is to change the distribution of charges.

Whatever charge is on an insulated electroscope remains there. Bringing another charged object near the electroscope will cause the electroscope's charges to distribute themselves in a new way, but the net charge remains conserved. Below are a series of short investigations on changing the distribution of charge on your electroscope.

a) Charge the electroscope negatively and then bring a negatively charged proof plane near the electroscope ball. What should happen is that negative electrons on the ball should be repelled downward to join the electrons already present on the gold foil and its support post, giving more repulsion to the gold foil. THEREFORE THE GOLD FOIL SHOULD STICK OUT FARTHER. Try it and be sure it works.

b) Charge the electroscope negatively and then bring a positively charged proof plane near the electroscope ball. What should happen is that negative electrons should be attracted up to the ball leaving fewer on the gold foil and its support post and resulting in less repulsion of the gold foil. THEREFORE THE GOLD FOIL SHOULD STICK OUT LESS FAR. Try it and be sure it works.

c) Charge the electroscope positively with a proof plane charged positive by induction. Now bring a positively charged proof plane near the electroscope ball. We expect that more negative electrons should be attracted upward to the ball leaving a larger net positive charge on the foil and support post. This gives more repulsion to the gold foil. THEREFORE THE GOLD FOIL SHOULD STICK OUT FARTHER. Try it and be sure it works.

d) Charge the electroscope positively with a proof plane charged positive by induction. Then bring a negatively charged proof plane near the electroscope ball. What should happen is that negative electrons on the ball should be repelled downward to at least partially replace the electrons removed from the electroscope initially. Hence the gold foil and its support post are closer to being neutral than they were initially. THEREFORE THE GOLD FOIL SHOULD STICK OUT LESS FAR. Try it and be sure it works.

3. Behavior of Other Charged Objects.

We know how to charge our electroscope either positively or negatively. We also know what positive/negative proof planes do to positive/negative electroscopes. IT WOULD BE INTERESTING TO INVESTIGATE WHETHER ALL CHARGED OBJECTS BEHAVE LIKE ONE OR THE OTHER OF THESE PROOF PLANES. It is conceptually possible that there is a third kind of charge that makes the foils of electroscopes behave differently from what we have so far encountered. Such an object would be a third kind of electric charge. The goal of the next section is to investigate this question with several different charged objects.

For each of the investigations (a) through (f) below we have to do two measurements, one with a negative electroscope and one with a positive electroscope. If the object being studied does different things to the two electroscopes then it is either positive or negative according to what we knew above. And we can easily tell whether it is positive or negative. However, if the object does the same thing to the two different electroscopes then perhaps you have discovered a new kind of charge, neither positive nor negative.

a) First charge the electroscope negatively with the VdG, sphere, and proof plane as usual. Rub a rubber rod on a piece of fur, bring the rod near the electroscope ball, and note how the gold foil behaves. Then charge the electroscope positively by induction with the VdG, sphere, and proof plane. Rub a rubber rod on a piece of fur, bring the rod near the electroscope ball, and note how the gold foil behaves. State whether the rubber rod is positively or negatively charged or whether it behaves differently from a charged proof plane as you saw in #2 above.

b) Do this again with the glass rod rubbed on silk. State whether the glass is positively or negatively charged or whether it behaves differently from a charged proof plane as you saw in #2 above.

c) Do this again with the plastic rod rubbed on wool. State whether the plastic is positively or negatively charged or whether it behaves differently from a charged proof plane as you saw in #2 above.

d) Do this again with the plastic ruler rubbed on a paper towel. State whether the ruler is positively or negatively charged or whether it behaves differently from a charged proof plane as you saw in #2 above.

e) Go up to the front table and turn the crank on the Wimshurst for a few seconds. Take charge off one of the metal balls with a proof plane and, using the same methods as described above, determine and state whether the charge taken off that ball is positive or negative or whether it behaves differently from a charged proof plane as you saw in #2 above.

f) Go up to the front table and turn the crank on the Wimshurst for a few seconds. Take charge off the other one of the metal balls with a proof plane and, using the same methods as described above, determine and state whether the charge taken off that ball is positive or negative or whether it behaves differently from a charged proof plane as you saw in #2 above.

4. Result.

What we expect of the above investigations is that we should be able to find only two kinds of charge in nature: the charge we take directly off the dome of the VdG machine (called negative) and the charge we get "by induction" (and called positive) using the charge from the VdG. DO YOUR EXPERIMENTS AGREE WITH WHAT WE EXPECT OR DID YOU FIND A CHARGE THAT DOES NOT ACT LIKE EITHER ONE OF THESE AND WHICH THEREFORE MUST BE A THIRD KIND OF ELECTRIC CHARGE?

5. Where Charge Normally Resides on a Conductor.

It is common knowledge that if electrons are placed on a metal conductor they will move around and through the metal itself until they stop on the outside surface of the conductor. No matter whether you charge the metal by contact on its inside or outside surface, the charge ends up on the outside surface. Please try to verify this, as outlined below.

(Sometimes proof plane experiments fail because the handle becomes conducting with time. If this happens you might try cleaning the handle with some alcohol and drying it off with a paper towel.)

(a)• Charge the electroscope either positive or negative as you did above, whichever is convenient.

• Charge a proof plane by direct contact with the VdG dome.

• Touch the proof plane to the outside of the metal sphere.

• Use the other proof plane to try to take charge off the sphere by directly touching it to its outside. Then move it near the charged electroscope to check its charge, if any.

DID YOU REMOVE ANY CHARGE AND IF SO OF WHAT SIGN?

(b)• Charge the electroscope either positive or negative as you did above.

• Charge a proof plane by contact with the VdG dome.

• Touch the proof plane to the inside of the metal sphere.

• Use the other proof plane to try to take charge off the sphere by directly touching it to its outside. Then move it near the charged electroscope to check its charge, if any.

DID YOU REMOVE ANY CHARGE AND IF SO OF WHAT SIGN?

(c)• Charge the electroscope either positive or negative as you did above.

• Charge a proof plane by contact with the VdG dome.

• Touch the proof plane to the outside of the metal sphere.

• Use the other proof plane to try to take charge off the sphere by directly touching it to its inside. Then move it near the charged electroscope to check its charge, if any.

DID YOU REMOVE ANY CHARGE AND IF SO OF WHAT SIGN?

(d)• Charge the electroscope either positive or negative as you did above.

• Charge a proof plane by contact with the VdG dome.

• Touch the proof plane to the inside of the metal sphere.

• Use the other proof plane to try to take charge off the sphere by directly touching it to its inside. Then move it near the charged electroscope to check its charge, if any.

DID YOU REMOVE ANY CHARGE AND IF SO OF WHAT SIGN?

6. A Possible Way to Take Charge Off the Inside of a Conductor.

This investigation may or may not work for you. Success depends on both your experimental skills and the state of your equipment. If you fail at first you may want to try again with equipment swapped temporarily from an adjacent station. Or often cleaning the proof plane (or metal sphere) "handles" with alcohol may make them better insulators and it might be a good idea to clean at least the proof 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.

Be sure there is no direct contact between proof planes or with the sphere, except for the touch of the uncharged proof plane to the sphere's inside. DID YOU GET ANY CHARGE? IF SO WAS IT THE SAME SIGN OR THE OPPOSITE OF THE FIRST PROOF PLANE'S CHARGE?

7. Result.

Were you able to show that charge normally resides on the outside of a conductor? Were you able to show that under certain circumstances it is possible to take charge off the inside of a conductor?

Be able to describe the latter case in words, saying what you did, paying special attention to + & - signs.

rev. 8/05