ism_ch19

# ism_ch19 - Chapter 19 Electric Charges, Forces, and Fields...

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1 Chapter 19 Electric Charges, Forces, and Fields Answers to Even-numbered Conceptual Questions 2. Giving an object a negative charge means transferring electrons to the object. This, in turn, increases its mass. 4. No, the basic physics of electric charges would not have been affected at all by an opposite assignment of positive and negative labels. The use of + and – signs, as opposed to labels such as A and B, has the distinct advantage that it gives zero net charge to an object that contains equal amounts of positive and negative charge. 6. (a) Referring to Table 19-1, we see that rubbing rabbit fur against glass will result in a positive charge for the rabbit fur, and a negative charge for the glass. (b) For glass and rubber, we see that the rubber acquires a negative charge. Note that in this case, the charge on the glass is positive; hence, the charge acquired by a material depends not only on the material itself, but also on the material that it is rubbed against. (c) Noting that rabbit fur and glass are adjacent in Table 19-1, whereas glass and rubber are widely separated, we conclude that the magnitude of triboelectric charge is greater in the glass- rubber case. 8. Initially, the bits of paper are uncharged and are attracted to the comb by polarization effects. (See Figure 19-5 and the accompanying discussion.) When one of the bits of paper comes into contact with the comb, it acquires charge from the comb. Now the piece of paper and the comb have charge of the same sign, and hence we expect a repulsive force between them. 10. No. Even uncharged objects will be attracted to a charged rod, due to polarization effects. See Figure 19-5 and the accompanying discussion. 12. Both force laws depend on the product of specific properties of the objects involved; in the case of gravity it is the mass that is relevant, in the case of electrostatics it is the electric charge. In addition, both forces decrease with increasing distance as 1/ r 2 . The extremely important difference between the forces, however, is that gravity is always attractive, whereas electrostatic forces can be attractive or repulsive. 14. The answer is (b), the center of the square. At the center of the square, the forces exerted by the charges + q and – q are in the same direction – regardless of the sign of the third charge – giving a large net force. At an empty corner of the square, the third charge is farther from the other charges, and the two forces acting on it are in different directions. Both of these effects tend to decrease the magnitude of the net force. 16. No. If the ball is displaced slightly upward from the equilibrium position, the attractive electrostatic force will be larger than the gravitational force, which will displace the ball farther upward. Similarly, if the ball is displaced slightly downward, the gravitational force is now stronger than the electrostatic force, and the ball will move farther downward. Therefore, the equilibrium is unstable.

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## This homework help was uploaded on 04/07/2008 for the course PHYS 105 taught by Professor Klie during the Spring '08 term at Ill. Chicago.

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ism_ch19 - Chapter 19 Electric Charges, Forces, and Fields...

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