Physics 2 Electrostatics Lab

Purpose

The goal of this exercise is for you to observe the two different types of charge and demonstrate two means of charging a proof plane – by conduction and by induction.

Materials

Ring stand

Red clamp

Nonmetallic pen or pencil  (you supply)

Scotch tape

Plastic pipe (CPVC)

Wool pad

Proof plane

Procedure

The goal is to charge the proof plane by two different methods, achieving two different types of charge on it.  To be able to confirm this you will use two oppositely charged pieces of tape.  It is very important that you can confirm and easily tell that the two pieces of tape have opposite charge.  Follow the steps outlined below…

1. Form a horizontal “support” bar for hanging the tape by clamping a wooden pencil or plastic pen to a ring stand as shown in the diagram.

2. Obtain a piece of “scotch” tape about 10 cm long (4 in).  Fold over about 1 cm at one end of the tape to form a non-sticky “handle”.  Press the tape firmly on the surface.  Write the letter “B” (for bottom) on the “handle” part.

3. Prepare a second piece of tape as you did in step #1 but this time tape it directly on top of the “B” piece with the handle at the same end.  Label this one “T” (for top).  You now have two pieces of tape stuck together, one on top of the other, with sticky sides down – sort of like two pieces of bread in a sandwich.  Grasping both handles, remove the pair as one unit and then separate the two pieces and hang on the support side by side. 

4. If you were successful then these two pieces of tape have opposite charges.  Rub the CPVC pipe with the wool pad and then bring it close to the two pieces of tape.  You should see one piece attracted to the pipe and the other piece repelled by the pipe.  If this is not the case start over and try again.  You must have two pieces of tape that are clearly opposite charges.
Note: the electric charge on the tape should remain in place for a while but it will dissipate over time (more so in humid conditions).  So, you may have to “recharge” the tape pair or create a new pair after a certain period of time.

5. Take the metal disk of the proof plane and bring it near but do not touch the pieces of tape.  The metal disk at this point is presumably neutral and yet you should see that it attracts both pieces of tape, unlike the pipe which attracts one and repels the other.  Can you explain what you see?

6. Now hold the proof plane by the insulating handle and then use the charged CPVC pipe to charge the metal disk by conduction.  Simply charge the pipe and then touch the metal disk.  Because the pipe is an insulator you may need to touch different parts of it to the metal disk in order to transfer a significant amount of charge.  Pretend as if there is some invisible paint on the pipe and you are trying to “wipe it off” on the metal plate.

7. Test the charge of the metal disk by bringing it near the tape pair.  If it is charged you should see it attract one piece of tape and repel the other piece of tape.  Compare the behavior of the metal disk and the CPVC pipe.  Does the metal disk have the same type of charge as the pipe?  It should if there has been charge by conduction.  If both pieces of tape are attracted to the metal disk then it is probably neutral (or nearly so) and you have failed to achieve your goal.  If you fail the first time then keep trying until you are successful!

8. Discharge the metal plate of the proof plane by rubbing it with your hand and/or touching it to a metal chair or the metal faucet at the sink.

9. Now you will try to use the CPVC pipe to produce a net opposite charge on the metal disk by the process of induction.  This is kind of tricky.  Charge up the CPVC pipe.  Bring it close but not touching the surface of the disk.  Do not let a spark occur or let it touch – if you do then start over.  While you are holding the pipe close to the metal disk, allow the end of the metal on the proof plane handle to briefly touch the top of the ring stand.  Then remove and separate the objects.  At this point the metal disk should have a net charge!

10. Test the charge on the disk using the tape pair.  Compare and contrast how it affects the pair with the way that the CPVC pipe affects the pair.  Is the metal disk oppositely charged from the pipe?  It should be.  If not – start over and keep trying.  It can be done!

Questions – write responses on separate paper.

1. How does the creation of the oppositely charged tape pair illustrate the concept of conservation of charge?

2. Consider the process of charging the metal disk by conduction.  (a) How and why does charge transfer from the pipe to the metal disk – what is happening to electrons?  Explain.  (b) By what observations is it absolutely clear that the metal disk attains the same type of charge as the pipe?  Explain.

3. Consider the process of charging the metal disk by induction.  (a) How and why does the metal disk obtain a net charge when it is briefly touched to the ring stand – what is happening to electrons?  Explain.  (b) By what observations is it absolutely clear that the metal disk attains the opposite type of charge as the pipe?  Explain.