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Unformatted text preview: CHAPTER 18 ELECTRIC FORCES AND ELECTRIC FIELDS ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. 1.9 10 13 2. (b) Suppose that A is positive and B is negative. Since C and A also attract each other, C must be negative. Thus, B and C repel each other, because they have like charges (both negative). Suppose, however, that A is negative and B is positive. Since C and A also attract each other, C must be positive. Again we conclude that B and C repeal each other, because they have like charges (both positive). 3. (a) The ball is electrically neutral (net charge equals zero). However, it is made from a conducting material, so it contains electrons that are free to move. The rod attracts some of these (negative) electrons to the side of the ball nearest the rod, leaving the opposite side of the ball positively charged. Since the negative side of the ball is closer to the positive rod than the positive side, a net attractive force arises. 4. (d) The fact that the positive rod repels one object indicates that that object carries a net positive charge. The fact that the rod repels the other object indicates that that object carries a net negative charge. Since both objects are identical and made from conducting material, they share the combined net charges equally after they are touched together. Since the rod repels each object after they are touched, each object must then carry a net positive charge. But the net electric charge of any isolated system is conserved, so the total net charge initially must also have been positive. This means that the initial positive charge had the greater magnitude. 5. (c) This distribution is not possible because of the law of conservation of electric charge. The total charge on the three objects here is 9 8 q , whereas only q was present initially. 6. (c) This is an example of charging by induction. The negatively charged rod repels free electrons in the metal. These electrons move through the point of contact and into the sphere farthest away from the rod, giving it an induced charge of q . The sphere nearest the rod acquires an induced charge of + q . As long as the rod is kept in place while the spheres are separated, these induced charges cannot recombine and remain on the spheres. 7. (b) Coulombs law states that the magnitude of the force is given by 1 2 2 q q F k r = . Doubling the magnitude of each charge as in A would increase the numerator by a factor of four, but this is offset by the change in separation, which increases the denominator by a factor of 175 ELECTRIC FORCES AND ELECTRIC FIELDS 2 2 = 4. Doubling the magnitude of only one charge as in D would increase the numerator by a factor of two, but this is offset by the change in separation, which increases the denominator by a factor of ( 29 2 2 2 = ....
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