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# Electric Current: Movement of Charges (page 2)

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Author: Janice VanCleave

### Try New Approaches

The potential energy of a battery varies with the difference in the volts (the potential energy per charge) between the terminals (the points at which connections are made to an electrical device). This difference, called voltage, is the potential difference measured in volts (V). The greater the potential difference, the greater the potential energy, and therefore the greater the voltage. As the potential difference increases, the amount of current that flows through a circuit increases. A 1.5-volt battery tells you that the potential difference between the terminals is 1.5 volts. A 6-volt battery has four times more potential difference than a 1.5- volt battery. When connected to electric circuits, the 6-volt battery gives an electric charge four times as much push as the 1.5-volt battery.

Although an increase in voltage does not increase tile speed of the electrical impulse, it does increase the speed of the electrons. As the speed of the electrons increases, more electrons move past a point in a given time period. Voltage can be thought of as a measure of the "push" on the free electrons. Demonstrate an increase in voltage by increasing the speed at which you move magnet A. Compare the distance that each magnet moves. Since magnet D doesn't have an opposing force, its movement is an indication of an increase in forward push.

### Design Your Own Experiment

1.

1. Electrica1 conduction is the movement of electric charges through a substance. Electrica1 conductivity is the measure of the ability of a substance to conduct an electric current. Substances with high electrical conductivity are those with a large concentration of free electrons, such as metals, and are called electrica1 conductors. Design an experiment to compare the electrical conductivity of materials. One way is to build an open electric circuit so that different materials can be used to close the circuit. Place a 1.5-volt battery in a battery holder. Screw a flashlight lamp with an E-10 screw-base into a lamp holder. Use wire cutters to cut three 6-inch (15-cm) pieces of 22-gauge single-strand insulated wire. Strip about 1 inch (2.5 cm) of insulation from both ends of each piece of wire. Use the wire to connect the terminals of the lamp base and battery, as shown in Figure 16.2. Holding the insulated part of the wires, touch the free metal ends of the wires together to make sure the lamp will glow. If it does, separate the free ends of the wires and touch them to opposite sides of a testing material, such as a penny. Note: Make sure that the wires touch only the testing material and not each other. You want to determine if the testing material can close the circuit by allowing electrons to move through it, causing the lamp to glow. CAUTION: The wires and the lamp can get hot enough to burn your skin if you leave the switch closed for longer than five seconds. Don't touch the lamp or the wires unless they have had time to cool after being disconnected.
2. Some conductors restrict the movement of electric charges more than others. The measure of the opposition to the flow of electric charges through a conductor is called resistance. The brightness of the lamp is an indication of the amount of electric current (measure of the amount of electrical charges moving through a current per unit of time) in the circuit. As the resistance increases, the current decreases, and so does the brightness of the lamp. Repeat the previous experiment comparing the brightness of the lamp with each material. To be sure that the electrons travel through the same amount of testing material for each sample, tape the wires to a craft stick so that the metal ends stick out and are 11/2 inch (1.25 cm) apart (see Figure 16.3).
3. Design a way to determine if the amount of testing material affects the resistance. Use a longer sample, such as a strip of aluminum foil or pencil lead, and touch the wires to the testing material at different distances apart. (For information on using a multimeter to measure electric current, see chapter 17, "Series Circuit: Sequential Path.")

### Get the Facts

1. A battery is a device that uses chemicals to produce an electric current. By 1800, Italian physicist Alessandro Volta (1745-1827) developed what was called the voltaic pile, which was a forerunner of a battery. The voltaic pile was able to produce a steady stream of electricity. How does the voltaic pile compare to modern batteries? For more information about Volta's battery, see the Franklin Institute Science Museum, The Ben Franklin Book of Easy and Incredible Experiments (New York: Wiley, 1995), p. 56. You can find more information on the voltaic pile and its use in creating an electric current in Albert Einstein and Leopold Infeld, The Evolution of Physics (New York: Touchstone, 1966), pp. 84–90.
2. Coulomb's law describes the force between two charged objects. How does distance affect this force? For more about Coulomb's law, see a physics text.
3. An electrical insulator has a low concentration of free electrons and is a poor electrical conductor. Use a physics text to find out more about electrical insulators.
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