Building an Electroscope

An electroscope is a scientific instrument which is used to detect the presence and magnitude of electric charge on a body. In this experiment you will build a simple electroscope, and study static electricity.

There are no major safety precautions for this experiment.

1. 4 plastic drinking straws with flexible ends.
2. 2 plastic 35 mm film cans.
3. Enough modeling clay to fill the film cans halfway.
4. A roll of 3-M Scotch Magic™ Tape, 3/4 inch (2 cm) width.

1. A plastic comb and hair or a piece of wool cloth.

Assembly:

1. Press enough modeling clay into both film cans to fill them halfway to the top.

1. Press the inflexible ends of two drinking straws into the clay in each can, and bend the flexible ends to form horizontal arms that extend in opposite directions. The heights of the straws should be the same.

Experiment:

1. Tear off two, 4 inch (10 cm) pieces of tape.
2. Press each piece firmly to a tabletop or other flat surface, leaving one end of each tape sticking up as a handle.
3. Quickly pull the tapes from the table and stick one piece on an arm of a straw in one film can, and the other piece on an arm of a straw in the other film can.
4. Move the cans so that the two tapes are face to face, about 6 inches (15 cm) apart. Then move the cans closer together. Notice that the two tapes repel each other.
5. Tear off two more pieces of tape and press the sticky side of one against the smooth side of the other, leaving one end of each tape sticking out as a handle.
6. Quickly pull the tapes apart and stick them to the two remaining arms. Bring the arms close together. Notice that these two tapes attract each other.
7. Run the comb through your hair, or rub the comb with the wool cloth. Then hold the comb near the dangling tapes. Notice that the comb repels the piece of tape whose smooth side was in the middle of the “sandwich” and attracts the tape whose sticky side was in the middle. When you hold the comb near the tapes pulled from the flat surface, the comb will repel both tapes if they were pulled from a Formica™ surface; the comb may attract tapes pulled from other surfaces.

1. Try pulling other kinds of tape from various surfaces, or rubbing various objects together, and then bringing the tape or objects near the tapes on the arms. Bring your hand near the tapes and notice what happens.

Q: What does this tell us?

A: Opposite charges attract and like charges repel.

Q: Why might your hand attract both positive and negative charges?

A: Your hand is (usually) uncharged and therefore can be attracted by either negative or positive charges.

When you rip the two pieces of tape off the table, there is a tug-of-war for electric charges between each tape and the table. The tape either steals negative charges (electrons) from the table or leaves some of its own negative charges behind, depending on what the table is made of (a positive charge doesn’t move in this situation). In any case, both pieces of tape end up with the same kind of charge, either positive or negative. Since like charges repel, the pieces of tape repel each other.
When the tape sandwich is pulled apart, one piece rips negative charges from the other. One piece of tape therefore has extra negative charges. The other piece, which has lost some negative charges, now has an overall positive charge. Since opposite charges attract, the two tapes attract each other.

When you run a plastic comb through your hair, the comb becomes negatively charged. Tapes repelled by the comb have net negative charge, and tapes attracted by the comb either have net positive charge or are uncharged.

You may have found that your hand attracts both positively and negatively charged tapes. Your body is usually uncharged, unless you have acquired a charge — by walking across a carpet, for example. An uncharged object attracts charged objects. When you hold your hand near a positively charged tape, the tape attracts electrons in your body. The part of your body nearest the tape becomes negatively charged, while a positive charge remains behind on the rest of your body. The positive tape is attracted to the nearby negative charges more strongly than it is repelled by the more distant positive charges, and the tape moves toward your hand.

Since some table surfaces will not charge the tape, be sure to test your surfaces before trying this Snack with an audience.

Charge leaks slowly off the tape into the air or along the surface of the tape, so you may have to recharge your tapes after a few minutes of use.

You can use your electroscope to test whether an object is electrically charged. First use the comb to determine the charge on a piece of tape, and then see whether an object whose charge is unknown repels the tape. If the tape is negatively charged and an object repels it, then the object is negatively charged. Don’t use attraction to judge whether an object is charged: A charged object may attract an uncharged one. If tape is attracted to an object, the tape and the object may have opposite charges, or the tape may be charged and the object uncharged, or the object may be charged and the tape uncharged. But if the tape is repelled by the object, the tape and the object must have the same charge. The only way that tape and an object will neither repel nor attract is if both are uncharged.

You can keep the electroscope for further use.