Movers: How Does Electricity Produce Movement?

Problem

How does electricity produce movement?

Materials

• modeling clay
• flexible plastic drinking straw
• ruler
• scissors
• tissue paper
• cellophane
• tape
• transparent plastic report folder
• sheet of typing paper

Procedure

NOTE: This activity works best when the air is cold and dry.

1. Press a walnut-sized piece of clay onto a table top.
2. Insert the inflexible end of the straw into the clay.
3. Bend the flexible end of the straw to form a horizontal arm.
4. Measure and cut a I-inch × 8-inch (2.5-cm × 20-cm) strip from the tissue paper.
5. Bend the paper strip in half and hang it on the horizontal arm of the straw. The hanging ends of the paper will be called "leaves."
6. Use a small piece of tape to secure the top of the paper strip to the straw's arm.
7. Cut a 4-inch (10-cm) square from the plastic folder.
8. Lay the sheet of typing paper on the table, and rub the square of plastic back and forth across the paper five or six times.
9. Hold the plastic square close to one of the paper leaves, and touch the plastic to the leaf.



10. Repeat the procedure, touching the plastic to the second leaf.
11. Rub the plastic on the paper again. Holding it perpendicular to the table, insert it between the two paper leaves.

Results

At first the paper leaves move toward the plastic sheet, but after being touched by the plastic they move away from each other and from the plastic sheet.

Why?

Electricity is a form of energy associated with the presence and movement of electrical charges. The two known types of electrical charges are positive and negative. Atoms, such as those in paper and plastic, contain positively-charged protons in their nucleus and negatively-charged electrons outside the nucleus. The plastic will become negatively charged if the electrons are rubbed off the atoms of paper and collect on the plastic. The paper that forms the leaves is electrically neutral until the charged plastic is moved close to them. The law of electric charges states that unlike charges attract each other and like charges repel (push apart from) each other. The negatively charged electrons in the electrically neutral paper are repelled by the negatively charged plastic. This results in an excess of positive charges on the paper's surface closest to the plastic. The attraction between the unlike charges—the positive charges on the paper and the negative charges on the plastic—is great enough to overcome the downward pull of gravity and move the paper toward the plastic.

Touching the leaves with the plastic allows electrons to be transferred to the leaves, giving the leaves a negative charge like that of the plastic sheet. Again, the law for electric charges states that like charges repel each other. The negative charge on the leaves results in the leaves pushing away from each other as well as from the negatively-charged plastic sheet. The force of attraction or repulsion between electrical charges is called an electrostatic force.

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