You’re probably familiar with some of the effects of static electricity: It makes sparks when you comb your hair on a cold day, and it makes balloons stick to the wall at a birthday party. In this Snack, static electricity makes electric “fleas” jump up and down.
A large sheet of paper, 11 × 17 inches (28 × 43 cm or A3)
Four supports, each about 1 to 2 inches (2.5 to 5 cm) high
A sheet of acrylic plastic or other clear plastic (about 1 foot [30 cm] square and 1/8 inch [3 mm] thick)
Tiny bits of “stuff”: aluminized ceiling glitter works well, as do grains of rice, puffed rice cereal, spices (dried dill weed or basil, ground cloves, ground nutmeg), or bits of Styrofoam
A piece of wool cloth or fur
Put the piece of paper on the table.
Place the supports on the paper beneath the four corners of the plastic.
Scatter the tiny bits of Styrofoam, spices, ceiling glitter, or rice under the plastic. (You can set up this assembly on any tabletop.)
To Do and Notice
Charge the plastic by rubbing it vigorously with the piece of wool cloth or fur. Watch the “fleas” dance!
Try different types of material for charging the plastic, including your hand, and experiment with other materials for fleas. Also, try the plastic at different heights.
What's Going On?
Both the plastic and the “fleas” start out electrically neutral: That is, they have an equal number of positive and negative charges. When you rub the plastic with the wool cloth, the cloth transfers negative charges to the plastic. These negative charges polarize the fleas, attracting the positive charges to the tops of the fleas and pushing the negative charges to the bottoms of the fleas. The attraction between the negative plastic and the positive charge concentrated on the tops of the fleas makes the fleas jump up to the underside of the plastic.
When a flea actually touches the plastic, some of the plastic’s negative charge flows to the flea. The top of the flea becomes electrically neutral. But since the whole flea was originally neutral, the flea now has some excess negative charge. The negatively charged flea and the negatively charged plastic repel each other strongly, which causes the flea to jump quickly back to the table. As the flea’s excess negative charge slowly drains away to the tabletop, or to the air, the flea again becomes neutral and is ready to jump up to the plastic once more.
While the “fleas” are dancing, put your ear near or on the piece of plastic. Listen to the tapping of the fleas as they hit the plastic. The tapping rate slowly decreases as the charge on the plastic is depleted. The dance of the fleas sounds like the clicking of a Geiger counter measuring a radioactive source that is decaying.