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Chapter 18 | Electric Charge and Electric Field 789
Figure 18.9 (a) When enough energy is present, it can be converted into matter. Here the matter created is an electron–antielectron pair. ( is the electron's mass.) The total charge before and after this event is zero. (b) When matter and antimatter collide, they annihilate each other; the total
charge is conserved at zero before and after the annihilation.
The law of conservation of charge is absolute—it has never been observed to be violated. Charge, then, is a special physical quantity, joining a very short list of other quantities in nature that are always conserved. Other conserved quantities include energy, momentum, and angular momentum.
18.2 Conductors and Insulators
PhET Explorations: Balloons and Static Electricity
Why does a balloon stick to your sweater? Rub a balloon on a sweater, then let go of the balloon and it flies over and sticks to the sweater. View the charges in the sweater, balloons, and the wall.
Figure 18.10 Balloons and Static Electricity (http://cnx.org/content/m55300/1.2/balloons_en.jar)
Applying the Science Practices: Electrical Charging
Design an experiment to demonstrate the electrical charging of objects, by using a glass rod, a balloon, small bits of paper, and different pieces of cloth (like silk, wool, or nylon). Also show that like charges repel each other whereas unlike charges attract each other.
Learning Objectives
By the end of this section, you will be able to:
• Define conductor and insulator, explain the difference, and give examples of each.
• Describe three methods for charging an object.
• Explain what happens to an electric force as you move farther from the source.
• Define polarization.
The information presented in this section supports the following AP® learning objectives and science practices: