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23.2 Charging Objects By Induction It is convenient to classify materials in terms of the ability of electrons to move through S E C T I O N 2 3 . 2 • Charging Objects By Induction 709 Quick Quiz 23.1 If you rub an inflated balloon against your hair, the two materials attract each other, as shown in Figure 23.3. Is the amount of charge present Quick Quiz 23.2 Three objects are brought close to each other, two at a time. When objects A and B are brought together, they repel. When objects B and C Properties of electric charge • There are two kinds of charges in nature; charges of opposite sign attract one another and charges of the same sign repel one another. • Total charge in an isolated system is conserved. Electrical conductors are materials in which some of the electrons are free electrons 1 that are not bound to atoms and can move relatively freely through the material; electrical insulators are materials in which all electrons are bound to atoms and cannot move freely through the material. Figure 23.3 (Quick Quiz 23.1) Rubbing a balloon against your hair on a dry day causes the balloon and your hair to become charged. Charles D. Winters 1 A metal atom contains one or more outer electrons, which are weakly bound to the nucleus. When many atoms combine to form a metal, the so-called free electrons are these outer electrons, which are not Materials such as glass, rubber, and wood fall into the category of electrical insulators. In contrast, materials such as copper, aluminum, and silver are good electrical con- ductors. When such materials are charged in some small region, the charge readily dis- Semiconductors are a third class of materials, and their electrical properties are somewhere between those of insulators and those of conductors. Silicon and |