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Unformatted text preview: CHAPTER 18 ELECTRIC FORCES AND ELECTRIC FIELDS CONCEPTUAL QUESTIONS ____________________________________________________________________________________________ 1. REASONING AND SOLUTION In Figure 18.8, the grounding wire is removed first, followed by the rod, and the sphere is left with a positive charge. If the rod were removed first, followed by the grounding wire, the sphere would not be left with a charge. Once the rod is removed, the repulsive force caused by the presence of the rubber rod is no longer present. Since the wire is still attached, free electrons will enter the sphere from the ground until the sphere is once again neutral. ____________________________________________________________________________________________ 2. REASONING AND SOLUTION A metallic rod is given a positive charge by the process of induction as illustrated in Figure 18.8. a. The metallic object becomes positive because, during the induction process, electrons are forced from the object to the earth. The mass of the object will decrease by an amount equal to the mass of the electrons that left the metallic object. b. The metallic object becomes negative because, during the induction process, electrons are pulled onto the object from the earth. The mass of the object will increase by an amount that is equal to the mass of the "excess" electrons that are pulled onto the object. ____________________________________________________________________________________________ 3. REASONING AND SOLUTION When the charged insulating rod is brought near to (but not touching) the sphere, the free electrons in the sphere will move. If the rod is negatively charged, the free electrons will move to the side of the sphere that is opposite to the side where the rod is; if the rod is positively charged, the free electrons will migrate to the side of the sphere where the rod is. In either case, the region of the sphere near the vicinity of the rod will acquire a charge that has the opposite sign as the charge on the rod. a. Since oppositely charged objects always attract each other, the rod and sphere will always experience a mutual attraction. b. Since the side of the sphere in the vicinity of the rod will always have charge that is opposite in sign to the charge on the rod, the rod and the sphere will always attract each other. They never repel each other. ____________________________________________________________________________________________ 4. REASONING AND SOLUTION On a dry day, just after washing your hair to remove natural oils and drying it thoroughly, you run a plastic comb through it. As the surface of the comb rubs against your hair, the comb becomes electrically charged. If the comb is brought near small bits of paper, the charge on the comb causes a separation of charge on the bits of paper. Since the paper is neutral, it contains equal amounts of positive and negative charge. The charge on the comb causes the regions of the bits of paper that are 146 ELECTRIC FORCES AND ELECTRIC FIELDS...
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