Investigation 11 - Electric Charge

In this exercise we will examine the interaction between charged objects in order to develop a model for electric charge.

Materials:roll of magic tapePVC pipe

aluminum coated pith ballpiece of wool cloth

tissue paper

I.Electrical Interactions

A.Cut off a piece of magic tape, about 10-15 cm in length. (For ease in handling, make a “handle” by folding over the end of tape to form a portion that is not sticky.) Hang the piece of tape from the edge of the table.

Describe the behavior of the tape as you bring objects toward it (e.g., a hand, a pen, etc.). Later on, we will be able to explain why this occurs.

B.Make another piece of tape as described above. Bring the second tape toward the first. Describe your observations.

How does the distance between the tapes affect the interaction (force) between them? How so?

C.Have two members of your group preparepieces of tape as instructed above (section I, part A). Have them lightly press their tape onto the lab table and write “B” (for bottom) on them. Then lightly press a second tape on top of each B tape and label them “T” (for top).

Remove each pair of tapes from the table as a unit. After they are off the table, quickly separate the T and B tapes.

Describe the interaction between the following pairs of tape when they are brought near one another.

two T tapes
(T-T Interaction) / two B tapes
(B-B Interaction) / one T and one B tape
(T-B or B-T Interaction)

Notice that the tapes exert a force on each other. We know this force is not a gravitational force, but some “other” force. Give a reason why we know this force is not a gravitational force:

This “other” force is referred to as an “electrostatic force.” Just as mass is the property responsible for the gravitational force, the property responsible for the electrostatic force is called “charge.” We say that the tapes are electrically charged when they interact as you have observed.

The two T tapes must have the same kind of charge because they were charged in the same way, and likewise the two B tapes must have the same kind of charge. But the T and B tapes must have different kinds of charge since the T-B interaction is different from the T-T and B-B interactions.

How many different kinds of charge do there appear to be?

They are referred to as “positive charge” and “negative charge.”

D.Obtain a PVC pipe and a piece of wool. Rub the pipe with the wool, and then hold the pipe near newly made T and B tapes.

1.Compare the interactions between the pipeand the tapes.

pipe and the T tape / pipe and the B tape

We say that the pipe is also electrically charged because the pipe interacts with the tapes in the same way the tapes interacted with each other.

2.By convention, the PVC pipe is said to be negatively charged when rubbed with wool. We know that two T tapes repel, two B tapes repel, but a T tape and B tape attract. The plastic rod repels the

______tape and attracts the ______tape. Which tape, T or B, is positively charged? ______

3.Write a brief statement that summarizes your observation of the forces of attraction and repulsion in terms of “like charges” and “unlike charges.”

4.How does the strength of the electrostatic force vary as the charged objects are moved closer and further away from each other?

Are your observations consistent with Coulomb’s law? (Do you remember Coulomb’s Law?)

Please remove all tape from the tabletop before continuing.

II.A model for electric charge

A.A small ball with zero net charge (electrically neutral) is positively charged on one side and negatively charged on the other side. The ball is placed near a positive point charge, as shown.

Will the ball be attracted toward, repelled from, or unaffected by the positive point charge? Explain.

B.Hang theuncharged(zero net charge) metal-coated pith ball from the insulating string. Now charge a piece of tape as in of section I,part A and bring the tape toward the ball.

Describe what you observe.

Through careful observations of physical phenomena, scientists develop models, or mental pictures, to account for observations. These scientific models can also be used to predict physical behavior. From observations of electrical phenomena we can develop a model for electric charge.

C.Use your model for electric charge (section II, part A above) to account for the electrical attraction between the charged tape and an uncharged metal-coated pith ball. As part of your answer, draw a sketch of the charge distribution on the tape and on the ball both before and after they are brought near one another.

before (very, very far apart)after (close together)

Rhetorical question: Do you know why the charge redistributed itself on the ball?

III.Charge Polarization

A.Suppose a PVC pipe is charged with the piece of wool.

1.What kind of charge should the PVC pipe have? (Check Section I, part D2.) ______

Describe what must happen when the PVC pipe is brought close to the metal-coated pith ball.

This situation is exactly the same as the tape being brought close to the metal-coated pith ball in section II, parts B and C, except that the PVC pipe has a negative charge. In this situation, draw how the charges are distributed on the PVC pipe and the metal-coated pith ball.

2.Tear off little pieces of tissue paper and place them on the table. Remember that paper is unlike the metal-coated pith ball -- the paper is a nonconductor (insulator).

Now charge the PVC pipe and bring it close to the pieces of paper. What happened to the pieces of paper?

Because the paper is attracted to the pipe, the charge distribution on the paper must be such as to cause the attraction. In this situation, draw how the charges are distributed on the PVC pipe and the piece of paper.

The phenomenon taking place in the paper is called charge “polarization.” Remember polarization?

B.Go to one of the sinks in the laboratory and turn on the water until a thin stream of water is flowing. Now charge the PVC pipe with the wool (try not to get the wool or the pipe wet), and move the pipe toward the stream of water.

What happens to the stream of water?

Draw how the charges must be distributed on the pipe and in the stream of water in order for the stream to be attracted to the pipe.