By suspending pieces of tape from a straw, you can build an electroscope—a device that detects electrical charge. A commonly available brand of plastic tape can gain or lose negatively charged electrons when you stick it to a surface and then rip it off. A plastic comb will enable you to identify whether the pieces of tape are positively or negatively charged.
- Two plastic 35-mm film canisters or small paper cups
- Enough modeling clay to fill the cans or cups halfway
- Four plastic drinking straws with flexible ends
- A roll of 3M Scotch® Magic™ Tape, 3/4-inch (2-cm) width (don’t substitute other brands of tape the first time you try this activity; once you know what to expect, you can experiment with other tapes)
- A plastic comb (not shown)
- Someone with a full head of hair (alternatively, you can use a piece of wool cloth)
- Press enough modeling clay into each film canister or paper cup to fill them both halfway to the top.
- Press the inflexible ends of two drinking straws into the clay in each canister, and bend the flexible ends to form horizontal arms that extend in opposite directions. The heights of the straws should be the same.
Tear off two pieces of tape, each 4 inches (10 cm) long. Press each piece firmly to a tabletop or other flat surface, leaving one end of each tape piece sticking up (or out over the table’s edge) as a handle. Quickly pull the tapes from the table and stick one piece on the arm of a straw in one film canister, and the other piece on the arm of a straw in the other film canister.
Move the canisters so the two pieces of tape are face to face, about 6 in (15 cm) apart. Then move the canisters closer together. Notice that the two tapes repel each other.
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 piece sticking out as a handle. Quickly pull the stuck-together tape apart and stick the pieces to the two remaining arms. Bring the arms close together. Notice that these two tapes attract each other.
Run the comb through your hair, or rub the comb with the wool cloth. Then hold the comb near the dangling tape pieces. Notice that the comb repels the piece of tape with its smooth side in the middle of the “sandwich” and attracts the tape with its sticky side 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 tape pulled from other surfaces.
Note that some table surfaces will not charge the tape, so be sure to test yours before trying this activity. You can also put a few pieces of tape onto a piece of cardboard, leave the tape there, and put a second strip of tape on top of the first. Then, when you pull off the top piece of tape, it will always be charged.
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 straw arms. Bring your hand near the tapes and notice what happens.
When you rip the two pieces of tape off the table, there is a tug-of-war for electric charges between tape and 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 charge, now has an overall positive charge. Because opposite charges attract, the two pieces of tape attract each other.
When you run a plastic comb through your hair, the comb becomes negatively charged. Tapes repelled by the comb have a net negative charge, and tapes attracted by the comb either have a net positive charge or are uncharged.
You may have found that your hand attracts both positively and negatively charged tape. 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 piece of 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.
Note that charge leaks slowly off the tape into the air or along the surface of the tape, so you may have to recharge your tape 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 with an unknown charge 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.
Charge a tape, and hang it from a straw. Charge a comb by running it through your hair or rubbing it with wool. Bring the charged comb near the charged tape, and watch the tape move. Wiggle the comb side-to-side near the tape, and watch the tape wiggle side-to-side. Scientists model this interaction by saying that the comb creates an electric field that moves across space and exerts a force on the charges on the tape. When you wiggle the comb side-to-side, the electric field radiating from the comb also oscillates side to side. The oscillations of the electric field create magnetic oscillations, which, when combined, are also known as electromagnetic waves. These waves travel outward at the speed of light and will cross space even through a vacuum.