PHYS 1402 – General Physics II

Charge

Leader: ___________________ Recorder: __________________________

Skeptic: ___________________ Encourager: ________________________

Materials

Scotch Tape

Ebonite rod

Fur

Introduction

Electric charge is one of the fundamental properties possessed by matter. We can learn about charge because of the forces that charges exert on each other. These forces can be observed simply with everyday objects. In this procedure we will make some simple qualitative observations and deduce some of the properties of charge.

Procedure

In this lab we will conduct several qualitative experiments to study the properties of electric charges and the force between them.

Take two pieces of scotch tape about three inches long and fold a ½" piece over on the end of each to make a small handle. Stick each piece of the scotch tape firmly to a hard, non-metallic surface such as a lab table. Pull up the tapes rapidly from the table and bring them near to each other. Be sure to keep the tapes form sticking to yourself.

1) How do the tapes respond when you bring them near to each other?

2) Does the effect remain if you pull the tapes away from each other again?

Press one of the pieces of tape onto the hard surface again and press the other directly on top of the first. Rapidly pull up the two pieces together then separate them. Bring the two pieces of tape close together.

3) How do the tapes respond when you bring them near to each other?

4) Does the effect remain if you pull the tapes away from each other again?

5) Is the effect noticeable when the tapes are far apart?

6) Does the effect seem to depend on the distance the tapes are apart? Explain.

7) Does the effect seem to depend directly or inversely on the distance? Explain.

At this point we have seen two types of behavior. In the first case, we saw the tapes repel each other. In the second case, we saw them attract.

For this step you will need two people. Press two tapes onto the hard surface and label them B for bottom. Press a second tape onto each of the first two and label them T for top. Pull up the tapes rapidly and then separate them. Of the four tapes you have made, find which ones attract each other and which ones repel.

8) Describe your results

When we pull up the tapes rapidly, they acquire a very small net charge. If the tapes are prepared in the same way, then they acquire the same type of charge. We learn about charge because of the forces exerted by the charges on each other.

9) Explain, giving specific evidence, how the series of observations you have made show that there are two types of charge.

10) Explain, giving specific evidence, how your observations demonstrate that like charges always repel and that opposite charges attract.

We typically describe charges as + and -. This convention was made in the 18^th century by Benjamin Franklin. We can determine the sign of the charge on the tapes if we test them against something of known charge. By convention, if we vigorously rub an ebonite rod with a piece of fur, the ebonite rod will become negatively charged.

Press one tape on the table and press a second on top of the first. Pull up both tapes together and then separate them. Have a second person charge the ebonite rod by rubbing it vigorously. Test the ebonite rod against each of the tapes.

11) Describe your results.

12) What is the sign of the charge on the bottom tape? Explain.

13) What is the sign of the charge on the top tape? Explain.

14) If you pull up a tape by itself it gets a negative charge. From where does the tape obtain the excess electrons? Explain.

15) After charging the tape, what is the sign of the object from which the tape received the excess electrons? Explain.

16) Summarize what you have learned about charged objects and the force between charges from this lab activity.