Physics for Scientists & Engineers
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with Modern Physics (Volume 2) 5e
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(Global Edition) By Douglas C.
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Giancoli (Solutions Manual All
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Chapters, 100% Original Verified,
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A+Grade) l l
(Chapters 21-35) l
All Chapters Solutions Manual
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Supplementfilesdownloadlinkat
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theendofthisfile.
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(Chapter Number 32 Is Missing) l l l l
,CHAPTER 21: Electric Charge and Electric Field l l l l l l
Responses to Questions l l
1. Suspend a plastic ruler by a thread and then rub it with a cloth. As shown in Fig. 21–2a, the ruler is
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negatively charged. Now bring the charged comb close to the ruler. If the ruler is repelled by the comb,
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then the comb is negatively charged. If the ruler is attracted by the comb, then the comb is positively
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charged.
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2. The shirt or blouse becomes charged as a result of being tossed about in the dryer and rubbing
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against the dryer sides and other clothes. When you put on the charged object (shirt), it causes
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charge separation within the molecules of your skin (see Fig. 21–9), which results in attraction
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between the shirt and your skin.
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3. Fog or rain droplets tend to form around ions because water is a polar molecule, with a positive region
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and a negative region. The charge centers on the water molecule will be attracted to the ionsor
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electrons, since opposite charges attract.
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4. See also Fig. 21–9 in the text. The negatively
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charged electrons in the paper are attracted to
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the positively charged rod and move towards it
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within their molecules. The attraction occurs
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because the negative charges in the paper are
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closer to the positive rod than are the positive
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charges in the paper, and therefore the attraction
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between the unlike charges is
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greater than the repulsion between the like charges.
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5. A plastic ruler that has been rubbed with a cloth is charged. When brought near small pieces of paper, it
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will cause separation of charge (polarization) in the bits of paper, which will cause the paper to be
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attracted to the ruler. A small amount of charge is able to create enough electric force tobe stronger than
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gravity. Thus the paper can be lifted.
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On a humid day this is more difficult because the water molecules in the air are polar. Those polarwater
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molecules will be attracted to the ruler and to the separated charge on the bits of paper, partially
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neutralizing the charges and thus reducing the attraction.
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6. The net charge on a conductor is the sum of all of the positive and negative charges in the conductor.If a
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neutral conductor has extra electrons added to it, then the net charge is negative. If a neutral conductor
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has electrons removed from it, then the net charge is positive. If a neutral conductor has the same amount
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of positive and negative charge, then the net charge is zero.
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The “free charges” in a conductor are electrons that can move about freely within the material
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because they are only loosely bound to their atoms. The “free electrons” are also referred to as
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“conduction electrons.” A conductor may have a zero net charge but still have substantial free
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charges.
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,Chapter 21 Electric Charge and Electric Field
7. Most of the electrons are strongly bound to nuclei in the metal ions. Only a few electrons per atom
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(usually one or two) are free to move about throughout the metal. These are called the “conduction
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electrons.” The rest are bound more tightly to the nucleus and are not free to move. Furthermore, inthe
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cases shown in Figs. 21–7 and 21–8, not all of the conduction electrons will move. In Fig. 21–7,
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electrons will move until the attractive force on the remaining conduction electrons due to the incoming
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charged rod is balanced by the repulsive force from electrons that have already gathered atthe left end of
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the neutral rod. In Fig. 21–8, conduction electrons will be repelled by the incoming rod and will leave
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the stationary rod through the ground connection until the repulsive force on the remaining conduction
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electrons due to the incoming charged rod is balanced by the attractive force from the net positive
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charge on the stationary rod.
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8. The electroscope leaves are connected together at the top. That connection can be
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modeled as a tension force. The horizontal component of this tension force balances
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the electric force of repulsion. The vertical component of the tension force balances
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the weight of the leaves.
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9. The balloon has been charged. The excess charge on the balloon is able to polarize the water molecules
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in the stream of water, similar to Fig. 21–9. This polarization results in a net attraction ofthe water
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towards the balloon, so the water stream curves towards the balloon.
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10. (a) When the leaves are charged by induction, no additional charge is added to the leaves. If the
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charged rod is near the top of the electroscope it repels charge onto the leaves causing them to
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separate as in Fig. 21–11(a). When the rod is removed the charge returns to its initial equilibrium
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position and the leaves come back together.
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(b) When the leaves are charged by conduction, positive charge is placed onto the electroscope from
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the rod causing the leaves to separate. When the rod is removed, the charge remains on the
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electroscope and the leaves remain separated. If not all of the excess charge leaves the rod,then
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when the rod is removed, the leaves might come back together slightly from their maximum
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deflection. l
(c) Yes. The electroscope has a negative charge on the top sphere and on the leaves. Therefore the
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electroscope has a total net negative charge, so it must have been charged by conduction.
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11. Coulomb’s law and Newton’s law are very similar in form. When expressed in SI units, the magnitude
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of the constant in Newton’s law is very small, while the magnitude of the constant in Coulomb’s law is
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quite large. Newton’s law says the gravitational force is proportional to the product of the two masses,
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while Coulomb’s law says the electrical force is proportional to the product of the two charges.
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Newton’s law only produces attractive forces, since there is only one kind of gravitational mass.
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Coulomb’s law produces both attractive and repulsive forces, since thereare two kinds of electrical
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charge.
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12. The gravitational force between everyday objects on the surface of the Earth is extremely small.
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(Recall the value of G: 6.6710−11 N m2 kg2 .) Consider two objects sitting on the floor near each
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other. They are attracted to each other, but the maximum force of static friction for each is much greater
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than the gravitational force each experiences from the other, and so they don’t move. Even inan
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absolutely frictionless environment, the acceleration resulting from the gravitational force would be so
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small that it would not be noticeable in a short time frame. We are aware of the gravitational force
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between objects if at least one of them is very massive, as in the case of the Earth and satellitesor the
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Earth and you.
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, Physics for Scientists & Engineers with Modern Physics, 5e, Global Edition Instructor Solutions Manual
The electric force between two objects is typically zero or very close to zero because ordinary objects
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are typically neutral or very close to neutral. We are aware of electric forces between objectswhen the
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objects are charged. An example is the electrostatic force (static cling) between pieces of clothing when
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you pull the clothes out of the dryer.
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13. Coulomb observed experimentally that the force between two charged objects is directly proportional to
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the charge on each one. For example, if the charge on either object is tripled, then theforce is tripled. This
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is not in agreement with a force that is proportional to the sum of the charges instead of to the product of
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the charges. Secondly, if two equal but opposite charges are placed near to each other, they produce an
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attractive force. But the “sum” version would say the net force is 0 insuch a case. Also, a charged object
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is not attracted to or repelled from a neutral insulating object. If the numerator in Coulomb’s law were
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proportional to the sum of the charges, then there would be a force between a neutral object and a
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charged object, because the their total charge would not be 0.
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14. Assume that the charged plastic ruler has a negative charge residing on its surface. That charge
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polarizes the charge in the neutral paper, producing a net attractive force. When the piece of paperthen
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touches the ruler, the paper can get charged by contact with the ruler, gaining a net negative charge.
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Then, since like charges repel, the paper is repelled by the comb.
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15. The test charge creates its own electric field. The measured electric field is the sum of the original electric
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field plus the field of the test charge. By making the test charge small, the field that it causes is small.
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Therefore the actual measured electric field is not much different than the original field. Also, if the test
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charges are large, their fields might significantly re-distribute the charges causing theoriginal field, and
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then the measurement would not represent the field of the original configuration ofcharges.
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16. When determining an electric field, it is best, but not required, to use a positive test charge. A negative
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test charge would be fine for determining the magnitude of the field. But the direction of theelectrostatic
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force on a negative test charge will be opposite to the direction of the electric field. The electrostatic
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force on a positive test charge will be in the same direction as the electric field. In order to avoid
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confusion, it is better to use a positive test charge. If we used a negative test charge but
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wanted to have the same result for the electric field, we would have to define E = −F q, q 0.
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17. See Fig. 21–35(b). A diagram of the electric field lines around two negative charges would be just like
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this diagram except that the arrows on the field lines would point towards the charges instead ofaway
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from them. The distance between the charges is l.
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18. The electric field will be strongest to the right of the positive charge (between the two charges) and
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weakest to the left of the positive charge. To the right of the positive charge, the contributions to thefield
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from the two charges point in exactly the same direction, and therefore add. To the left of the positive
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charge, the contributions to the field from the two charges point in exactly opposite directions, and
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therefore subtract. Note that this should be confirmed by the density of field lines in Fig. 21–35a.
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