Physics for Scientists Engineers

Physics for Scientists & Engineers with Modern Physics (Volume 2) 5e (Global Edition) By Douglas C.Giancoli (Solutions Manual All Chapters, 100% Original Verified, A+ Grade) (Chapters 21-35) All Chapters Solutions Manual Supplement files download link at the end of this file.(Chapter Number 32 Is Missing) 1 / 4

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CHAPTER 21: Electric Charge and Electric Field

Responses to Questions 1.Suspend a plastic ruler by a thread and then rub it with a cloth. As shown in Fig. 21–2a, the ruler is negatively charged. Now bring the charged comb close to the ruler. If the ruler is repelled by the comb, then the comb is negatively charged. If the ruler is attracted by the comb, then the comb is positively charged.

2.The shirt or blouse becomes charged as a result of being tossed about in the dryer and rubbing against the dryer sides and other clothes. When you put on the charged object (shirt), it causes charge separation within the molecules of your skin (see Fig. 21–9), which results in attraction between the shirt and your skin.

3.Fog or rain droplets tend to form around ions because water is a polar molecule, with a positive region and a negative region. The charge centers on the water molecule will be attracted to the ions or electrons, since opposite charges attract.

4.See also Fig. 21–9 in the text. The negatively charged electrons in the paper are attracted to the positively charged rod and move towards it within their molecules. The attraction occurs because the negative charges in the paper are closer to the positive rod than are the positive charges in the paper, and therefore the attraction between the unlike charges is greater than the repulsion between the like charges.

5.A plastic ruler that has been rubbed with a cloth is charged. When brought near small pieces of paper, it will cause separation of charge (polarization) in the bits of paper, which will cause the paper to be attracted to the ruler. A small amount of charge is able to create enough electric force to be stronger than gravity. Thus the paper can be lifted.On a humid day this is more difficult because the water molecules in the air are polar. Those polar water molecules will be attracted to the ruler and to the separated charge on the bits of paper, partially neutralizing the charges and thus reducing the attraction.

6.The net charge on a conductor is the sum of all of the positive and negative charges in the conductor.If a neutral conductor has extra electrons added to it, then the net charge is negative. If a neutral conductor has electrons removed from it, then the net charge is positive. If a neutral conductor has the same amount of positive and negative charge, then the net charge is zero.The “free charges” in a conductor are electrons that can move about freely within the material because they are only loosely bound to their atoms. The “free electrons” are also referred to as “conduction electrons.” A conductor may have a zero net charge but still have substantial free charges. 2 / 4

Chapter 21 Electric Charge and Electric Field

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• Most of the electrons are strongly bound to nuclei in the metal ions. Only a few electrons per atom

(usually one or two) are free to move about throughout the metal. These are called the “conduction electrons.” The rest are bound more tightly to the nucleus and are not free to move. Furthermore, in the cases shown in Figs. 21–7 and 21–8, not all of the conduction electrons will move. In Fig. 21–7, electrons will move until the attractive force on the remaining conduction electrons due to the incoming charged rod is balanced by the repulsive force from electrons that have already gathered at the left end of the neutral rod. In Fig. 21–8, conduction electrons will be repelled by the incoming rod and will leave the stationary rod through the ground connection until the repulsive force on the remaining conduction electrons due to the incoming charged rod is balanced by the attractive force from the net positive charge on the stationary rod.

• The electroscope leaves are connected together at the top. That connection can be

modeled as a tension force. The horizontal component of this tension force balances the electric force of repulsion. The vertical component of the tension force balances the weight of the leaves.

• The balloon has been charged. The excess charge on the balloon is able to polarize the water

molecules in the stream of water, similar to Fig. 21–9. This polarization results in a net attraction of the water towards the balloon, so the water stream curves towards the balloon.

• (a) When the leaves are charged by induction, no additional charge is added to the leaves. If the

charged rod is near the top of the electroscope it repels charge onto the leaves causing them to separate as in Fig. 21–11(a). When the rod is removed the charge returns to its initial equilibrium position and the leaves come back together.(b) When the leaves are charged by conduction, positive charge is placed onto the electroscope from the rod causing the leaves to separate. When the rod is removed, the charge remains on the electroscope and the leaves remain separated. If not all of the excess charge leaves the rod, then when the rod is removed, the leaves might come back together slightly from their maximum deflection.(c) Yes. The electroscope has a negative charge on the top sphere and on the leaves. Therefore the electroscope has a total net negative charge, so it must have been charged by conduction.

• Coulomb’s law and Newton’s law are very similar in form. When expressed in SI units, the

magnitude of the constant in Newton’s law is very small, while the magnitude of the constant in Coulomb’s law is quite large. Newton’s law says the gravitational force is proportional to the product of the two masses, while Coulomb’s law says the electrical force is proportional to the product of the two charges. Newton’s law only produces attractive forces, since there is only one kind of gravitational mass. Coulomb’s law produces both attractive and repulsive forces, since there are two kinds of electrical charge.

• The gravitational force between everyday objects on the surface of the Earth is extremely small.

(Recall the value of G: 11 2 2

6.67 10 N m kg.−  ) Consider two objects sitting on the floor near each other. They are attracted to each other, but the maximum force of static friction for each is much greater than the gravitational force each experiences from the other, and so they don’t move. Even in an absolutely frictionless environment, the acceleration resulting from the gravitational force would be so small that it would not be noticeable in a short time frame. We are aware of the gravitational force between objects if at least one of them is very massive, as in the case of the Earth and satellites or the Earth and you.

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Physics for Scientists & Engineers with Modern Physics, 5e, Global Edition Instructor Solutions Manual

© 2023 Pearson Education, Ltd. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.724 The electric force between two objects is typically zero or very close to zero because ordinary objects are typically neutral or very close to neutral. We are aware of electric forces between objects when the objects are charged. An example is the electrostatic force (static cling) between pieces of clothing when you pull the clothes out of the dryer.

• Coulomb observed experimentally that the force between two charged objects is directly

proportional to the charge on each one. For example, if the charge on either object is tripled, then the force is tripled. This is not in agreement with a force that is proportional to the sum of the charges instead of to the product of the charges. Secondly, if two equal but opposite charges are placed near to each other, they produce an attractive force. But the “sum” version would say the net force is 0 in such a case. Also, a charged object is not attracted to or repelled from a neutral insulating object. If the numerator in Coulomb’s law were proportional to the sum of the charges, then there would be a force between a neutral object and a charged object, because the their total charge would not be 0.

• Assume that the charged plastic ruler has a negative charge residing on its surface. That charge

polarizes the charge in the neutral paper, producing a net attractive force. When the piece of paper then touches the ruler, the paper can get charged by contact with the ruler, gaining a net negative charge. Then, since like charges repel, the paper is repelled by the comb.

• The test charge creates its own electric field. The measured electric field is the sum of the original

electric field plus the field of the test charge. By making the test charge small, the field that it causes is small. Therefore the actual measured electric field is not much different than the original field.Also, if the test charges are large, their fields might significantly re-distribute the charges causing the original field, and then the measurement would not represent the field of the original configuration of charges.

• When determining an electric field, it is best, but not required, to use a positive test charge. A

negative test charge would be fine for determining the magnitude of the field. But the direction of the electrostatic force on a negative test charge will be opposite to the direction of the electric field. The electrostatic force on a positive test charge will be in the same direction as the electric field. In order to avoid confusion, it is better to use a positive test charge. If we used a negative test charge but wanted to have the same result for the electric field, we would have to define , 0.qq= − EF

• See Fig. 21–35(b). A diagram of the electric field lines around two negative charges would be just

like this diagram except that the arrows on the field lines would point towards the charges instead of away from them. The distance between the charges is l.

• The electric field will be strongest to the right of the positive charge (between the two charges) and

weakest to the left of the positive charge. To the right of the positive charge, the contributions to the field from the two charges point in exactly the same direction, and therefore add. To the left of the positive charge, the contributions to the field from the two charges point in exactly opposite directions, and therefore subtract. Note that this should be confirmed by the density of field lines in Fig. 21–35a. l

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