Chapter 19: Electric Charges, Forces, and FieldsAnswers to Even-Numbered Conceptual Questions2. No, the basic physics of electric charges would not have been affected at all by an opposite assignment ofpositive and negative labels. The use of + and – signs, as opposed to labels such as A and B, has the distinctadvantage that it gives zero net charge to an object that contains equal amounts of positive and negativecharge.4. 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, itacquires charge from the comb. Now the piece of paper and the comb have charge of the same sign, andhence there is a repulsive force between them.6. No. Even uncharged objects are attracted to a charged rod, due to polarization effects. See Figure 19-5 andthe accompanying discussion.8. No. If the ball is displaced slightly upward from the equilibrium position, the attractive electrostatic forcewill be larger than the gravitational force, which will displace the ball farther upward. Similarly, if the ball isdisplaced slightly downward, the gravitational force is now stronger than the electrostatic force, and the ballwill move farther downward. Therefore, the equilibrium is unstable.10. The proton can be moving in any direction at all relative to the direction of the electric field. On the otherhand, the direction of the proton’s acceleration must be in the same direction as the electric field.12. No. The electric field of this system is nonzero, unless the separation d vanishes.14. Electric fields can exist in a vacuum, just as light can propagate through a vacuum. In fact, we shall see inChapter 25 that light is simply a wave of oscillating electric and magnetic fields. Therefore, it also followsthat magnetic fields can exist in a vacuum as well.16. The electric flux through the Gaussian surface depends on q1; in general, the electric flux through a surfacedepends on the charge that is enclosed by the surface. Because charge q2 is outside the Gaussian surface,however, it has no effect whatsoever on the total electric flux through the surface.18. No. The electric flux through a surface depends on the total charge enclosed by the surface, but is completelyindependent of the location of the enclosed charges.Solutions to Problems and Conceptual Exercises1. Picture the Problem: An electrically neutral object is given a positive charge.Strategy: Each electron carries both charge and mass, so adding or subtracting electrons will change the object’s mass.Solution: 1. (a) Giving the object a positive charge requires the removal of electrons, which have mass. We conclude that the object’s mass will decrease as a result of being charged. 2. (b) The best explanation is I. To give the object a positive charge we must remove some of its electrons; this will reduce its mass. Statement II is false, and statement III fails to distinguish between charge and mass conservation.Insight: Likewise, giving an object a negative charge requires the addition of electrons, increasing the object’s mass.Copyright © 2010 Pearson Education, Inc. All rights reserved. This material is protected under all copyright laws as they currently exist. Noportion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.19 – 1Chapter 19: Electric Charges, Forces, and Fields James S. Walker, Physics, 4th Edition2. Picture the Problem: An electrically neutral object is given a negative charge.Strategy: Each electron carries both charge and mass, so adding or subtracting electrons will change the object’s mass.Solution: 1. (a) Giving the object a negative charge requires the addition of electrons, which have mass. We conclude that the object’s mass will increase as a result of being charged. 2. (b) The best explanation is I. To give the object a negative charge we must give it more electrons, and this will increase its mass. Statement II is false, and statement III fails to distinguish between charge and mass conservation.Insight: Likewise, giving an object a positive charge requires the removal of electrons, decreasing the object’s mass.3. Picture the Problem: A balloon is charged by rubbing and held near a stream of water (see p. 655).Strategy: Table 19-1 indicates the relative charging due to rubbing for a variety of materials. The more minus signs for a material, the more readily it acquires electrons. Use the table to answer the conceptual question.Solution: 1. (a) Rubber is low on the list and is presented with four minus signs, indicating it readily acquires electrons. We conclude that its charge is more likely to be negative. 2. (b) If the charge on the balloon were reversed so that it is positively charged, the stream of water will still deflect toward the balloon. The stream of water is electrically neutral, but the water molecules orient themselves so that their negative ends point toward the balloon. This alignment ensures that the electrical attraction between the balloon and the negative part of each molecule exceeds the repulsion between the balloon and the positive part of each molecule.Insight: The stream of water will deflect toward the balloon regardless of whether its charge is positive or negative, justas bits of paper are attracted equally well to a charged amber (−) rod or a charged glass (+) rod.4. Picture the Problem: Several objects undergo triboelectric charging when they are rubbed together.Strategy: Table 19-1 indicates the relative charging due to rubbing for a variety of materials. The more plus signs associated with a material, the more readily it gives up electrons and becomes positively charged. Similarly, the more minus signs for a material, the more readily it acquires electrons. Use the table to answer the conceptual questions.Solution: 1. (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. 2. (b) For glass and rubber, we see that the rubber acquires a negative charge and the glass acquires a positive charge. 3. (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.Insight: Note that in part (a) the glass acquired a