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# Static Electricity: Stationary Charges (page 2)

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Author: Janice VanCleave

### Try New Approaches

1. Does the material from which the bobs are made affect the time they stay charged after the balloon is removed? Repeat the experiment using different materials, such as Styrofoam, cotton, and aluminum foil, to make the bobs.
2. How would using a positively charged object affect the results? Since the balloon attracted the electrons from the wool scarf, the scarf becomes positively charged. Repeat the original experiment, again rubbing the balloon with the scarf, but this time using the scarf instead of the balloon to charge the bobs.

1.
1. Charging by conduction is the process of electrically charging a neutral body by touching it with a charged body. Design a way to charge a body by conduction. One way would be to use the procedure from the original experiment, but this time instead of just holding the charged balloon near the bobs, touch the charged balloon to the two bobs for 1 to 2 seconds. Determine the time it takes for the bobs to lose their static charge, which is called static discharge.
2. Does the type of material being charged affect the results of the conduction experiment? Repeat the previous experiment using different materials for the bob such as paper and/or cotton balls.
2.
1. Design an experiment to determine how distance between charged materials affects the repulsive force of like electrical charges. One way is to use charged transparent tape. First lay a ruler next to the edge of a table. Tear off one piece of tape about 5 inches (12.5 cm) long. Press 4 inches (10 cm) of the tape to the table, leaving the last inch of the tape to hang over the table's edge. Repeat, pressing a second piece of 5-inch tape to the table near the first piece of tape. Wrap the free end of one piece of tape clockwise around the pointed end of a pencil. Repeat, wrapping the other piece of tape counterclockwise around a pencil as shown in Figure 15.3. Then charge two pieces of tape by quickly raising the pencils, ripping both pieces of tape from the table. Immediately hold the pencils parallel to the table with the 4-inch (10-cm) piece of tape hanging down. Move the pencils so that one piece of tape is above each end of the ruler and the sticky sides of the tape are face to face. Keep the pencil with the tape above the zero end of the ruler stationary, then slowly move the second pencil closer to the first one. Stop when there is any motion of the tape pieces, indicating repulsion of one piece to the other. Record the distance between the pencils in an Electrical Charge/Distance Data table like Table 15.l. Repeat this procedure four times. Average the results. Repeat, using different lengths of tape, such as 2 inches (5 cm) and 8 inches (20 cm). For more information about the effect of distance on electrical forces, see Larry Gonick and Art Huffman, The Cartoon Guide to Physics (New York: HarperPerennial, 1991), p.108.
2. Repeat the experiment to determine if distance affects the attractive force between unlike charges. To produce unlike charges on the tape, first tear off one piece of transparent tape about 5 inches (12.5 cm) long. Wrap about 1 inch (2.5 cm) around the pointed end of a pencil. Lay the taped pencil on a table, with the smooth side of the tape against the surface. Wrap the end of a second 5-inch (12.5-cm)-long piece of tape around the pointed end of a second pencil. Place the sticky side of one of the tape strips against the smooth side of the other piece of tape. Holding the pencils, pull the tape apart. Immediately repeat the experiment, measuring the distance at which the attractive force between the tape is first observed.

### Get the Facts

1. In 1785, French physicist Charles Coulomb (1736–1806) used a type of balance to measure the force between two charged spheres. Use a physics text to find out more about Coulomb's experiment and the equation he used to describe the relationships among electric forces, charge, and distance. This equation is called Coulomb's law. Another source of information is Karl F. Kuhn, Basic Physics: A Self-Teaching Guide (New York: Wiley, 1996), p. 139.
2. The triboelectric series ranks materials according to amount of energy needed to remove their electrons. For information about the triboelectric series, see Karl F. Kuhn, Basic Physics: A Self-Teaching Guide (New York: Wiley, 1996), p. 143.
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