Conceptual Physics

Paul G. Hewitt

Chapter 22 Electrostatics - all with Video Answers

Educators

Chapter Questions

00:27

Problem 1

What term is used for “electricity at rest”?

Karl Jacob

Numerade Educator

01:08

Problem 2

Why does the gravitational force between Earth and the Moon predominate over electrical forces?

Kara Merfeld

Numerade Educator

00:49

Problem 3

Which part of an atom is positively charged, and which part is negatively charged?

Karl Jacob

Numerade Educator

00:25

Problem 4

If a glass rod is rubbed with a piece of silk, what kind of charge is acquired by each?

Kara Merfeld

Numerade Educator

00:56

Problem 5

What is most commonly the net charge of an atom?

Karl Jacob

Numerade Educator

01:35

Problem 6

What is a positive ion? A negative ion?

Kara Merfeld

Numerade Educator

01:33

Problem 7

What is meant by conservation of charge?

Karl Jacob

Numerade Educator

03:02

Problem 8

What is meant by saying that charge is quantized?

Kara Merfeld

Numerade Educator

00:53

Problem 9

Name one particle that has exactly one quantum unit of charge.

Karl Jacob

Numerade Educator

01:13

Problem 10

How does one coulomb of charge compare with the charge of a single electron?

Kara Merfeld

Numerade Educator

01:50

Problem 11

How is Coulomb’s law similar to Newton’s law of gravitation? How is it different?

Karl Jacob

Numerade Educator

01:00

Problem 12

Why are metals good conductors both of heat and of electricity?

Kara Merfeld

Numerade Educator

01:01

Problem 13

Why are materials such as glass and rubber good insulators?

Karl Jacob

Numerade Educator

02:07

Problem 14

How does a semiconductor differ from a conductor or an insulator?

Kara Merfeld

Numerade Educator

02:24

Problem 15

What is a transistor composed of, and what are some of its functions?

Karl Jacob

Numerade Educator

01:01

Problem 16

How does the flow of current differ in a superconductor compared with the flow in ordinary conductors?

Kara Merfeld

Numerade Educator

01:07

Problem 17

What happens to electrons in any charging process?

Karl Jacob

Numerade Educator

00:41

Problem 18

What kind of charging occurs when you slide your body across a plastic surface?

Kara Merfeld

Numerade Educator

00:41

Problem 19

What kind of charging occurs during thunderstorms?

Kara Merfeld

Numerade Educator

01:00

Problem 20

What is the primary purpose of a lightning rod?

Kara Merfeld

Numerade Educator

01:07

Problem 21

In terms of net charge, how does an electrically polarized object differ from an electrically charged object?

Karl Jacob

Numerade Educator

00:27

Problem 22

What is an electric dipole?

Kara Merfeld

Numerade Educator

01:24

Problem 23

Give an example of a common electric dipole.

Karl Jacob

Numerade Educator

01:16

Problem 24

Give two examples of common force fields.

Kara Merfeld

Numerade Educator

00:59

Problem 25

How is the direction of an electric field defined?

Karl Jacob

Numerade Educator

01:14

Problem 27

Why is there no electric field at the center of a charged spherical conductor?

Katie Mcalpine

Numerade Educator

03:53

Problem 27

When charges mutually repel and distribute themselves on the surface of conductors, what becomes of the electric field inside the conductor?

Vishal Gupta

Numerade Educator

00:44

Problem 28

How much energy is given to each coulomb of charge that flows through a 1.5-V battery?

Kara Merfeld

Numerade Educator

01:29

Problem 29

A balloon may easily be charged to several thousand volts. Does that mean it has several thousand joules of energy? Explain.

Karl Jacob

Numerade Educator

00:44

Problem 30

Where is the energy stored in a capacitor?

Kara Merfeld

Numerade Educator

01:37

Demonstrate charging by friction and discharging from points with a friend who stands at the far end of a carpeted room. Scuff your way across the rug until your noses are close together. This can be a delightfully tingling experience, depending on how dry the air is and how pointed your noses are.

Dominador Tan

Numerade Educator

01:06

Problem 32

Write a letter to Grandpa and tell him why he’d be safe in a lightning storm if he was inside an automobile.

Kara Merfeld

Numerade Educator

02:13

Problem 33

Briskly rub a comb through your hair or on a woolen garment and bring it near a small but smooth stream of running water. Is the stream of water deflected?

Karl Jacob

Numerade Educator

02:16

Problem 34

Coulomb's law: $F=k \frac{q_{1} q_{2}}{d^{2}}$
Two point charges, each with 0.1 $\mathrm{C}$ of charge, are
0.1 $\mathrm{m}$ apart. Given that $k=9.0 \times 10^{9} \mathrm{N} \cdot \mathrm{m}^{2} / \mathrm{C}^{2}$ (the proportionality constant for Coulomb's law), show that
the force between the charges is $9.0 \times 10^{\circ} \mathrm{N} .$

Kara Merfeld

Numerade Educator

00:53

Problem 35

Solve for the force in the preceding problem when the charges are twice as far apart.

Karl Jacob

Numerade Educator

02:08

Problem 36

Two point charges are separated by 6 cm. The attractive force between them is 20 N. Find the force between them when they are separated by 12 cm. (Why can you solve this problem without knowing the magnitudes of the charges?)

Kara Merfeld

Numerade Educator

04:07

Problem 37

A metallic sphere of radius a, carrying charge Q, is brought in contact with an uncharged sphere B. What
will be the remaining charge on sphere B?

Vishal Gupta

Numerade Educator

03:11

Problem 38

Two pellets, each with a charge of 1 microcoulomb $\left(10^{-6} {C}\right),$ are located 3 ${cm}(0.03 \mathrm{m})$ apart. Show that the electrical force between them is 10 ${N}$ . What would be the mass of an object that experiences this same force in Earth's gravitational field?

Kara Merfeld

Numerade Educator

02:50

Problem 39

Electronic types neglect the force of gravity on electrons. To see why, compute the force of Earth’s gravity on an electron and compare it with the force exerted on the electron by an electric field of magnitude 10,000 V/m (a relatively small field). The mass and charge of an electron are given on the inside back cover.

Karl Jacob

Numerade Educator

02:48

Problem 40

Atomic physicists ignore the effect of gravity within an atom. To see why, calculate and compare the gravitational and electrical forces between an electron and a proton separated by $10^{-10} {m}$ . The charges and masses are given on the inside back cover.

Kara Merfeld

Numerade Educator

01:08

Problem 41

A droplet of ink in an industrial ink-jet printer carries a charge of $1.6 \times 10^{-10} {C}$ and is deflected onto paper by a force of $3.2 \times 10^{-4} {N}$ . Show that the strength of the electric field to produce this force is 2 million ${N} / {C}$ .

Karl Jacob

Numerade Educator

View

Problem 42

The potential difference between a storm cloud and the ground is 100 million V. If a charge of 2 C flashes in a bolt from the cloud to Earth, what is the change in the potential energy of the charge?

Nick Johnson

Numerade Educator

01:14

Problem 43

Energy of 0.1 ${J}$ is stored in the metal sphere on top of a Van de Graaff generator. A spark carrying 1 micro- coulomb $\left(10^{-6} {C}\right)$ discharges the sphere. Show that the
sphere's potential relative to ground is $100,000 {V}$ .

Karl Jacob

Numerade Educator

02:27

Problem 44

Find the voltage change when (a) an electric field does 12 J of work on a 0.0001-C charge and (b) the same electric field does 24 J of work on a 0.0002-C charge.

Kara Merfeld

Numerade Educator

02:23

Problem 45

In 1909, Robert Millikan was the first to find the charge of an electron in his now-famous oil-drop experiment. In that experiment tiny oil drops were sprayed into a uniform electric field between a horizontal pair of oppositely charged plates. The drops were observed with a magnifying eyepiece, and the electric field was adjusted so that the upward force on some negatively charged oil drops
was just sufficient to balance the downward force of gravity; that is, when suspended, the upward force qE just equaled mg. Millikan accurately measured the charges on many oil drops and found the values to be whole-number

Karl Jacob

Numerade Educator

01:23

Problem 46

The three pairs of metal, same-size spheres have different charges on their surfaces, as indicated. Each pair is brought together, allowed to touch, and then separated Rank, from greatest to least, the total amounts of charge on the pairs of spheres after separation.

Kara Merfeld

Numerade Educator

02:29

Problem 47

Shown are three separate pairs of point charges. Assume the pairs interact only with each other.
Rank the magnitudes of the forces between the pairs, from largest to smallest.

Karl Jacob

Numerade Educator

00:49

Problem 48

At the atomic level, what is meant by saying that something is electrically charged?

Kara Merfeld

Numerade Educator

01:27

Problem 49

Why is charge usually transferred by electrons rather than by protons?

Karl Jacob

Numerade Educator

00:28

Problem 50

Why aren’t objects with vast numbers of electrons normally electrically charged?

Kara Merfeld

Numerade Educator

01:14

Problem 51

Why do clothes often cling together after tumbling in a clothes dryer?

Karl Jacob

Numerade Educator

02:28

Problem 52

Why is dust attracted to a DVD wiped with a dry cloth?

Kara Merfeld

Numerade Educator

00:54

Problem 53

When combing your hair, you scuff electrons from your hair onto the comb. Is your hair then positively or negatively charged? How about the comb?

Karl Jacob

Numerade Educator

00:48

Problem 54

At some automobile toll booths, a thin metal wire protrudes from the road, making contact with cars before they reach the toll collector. What is the purpose of this wire?

Kara Merfeld

Numerade Educator

01:21

Problem 55

Why are the tires for trucks carrying gasoline and other flammable fluids manufactured to be electrically
conducting?

Karl Jacob

Numerade Educator

01:20

Problem 56

An electroscope is a simple device consisting of a metal ball that is attached by a conductor to two thin leaves of metal foil protected from air disturbances in a jar, as shown in the sketch. When the ball is touched by a charged body, the leaves that normally hang straight down spread apart. Why (Electroscopes are useful not only as charge detectors but also for measuring the quantity of charge: The more charge transferred to the ball, the more the leaves diverge.)

Kara Merfeld

Numerade Educator

01:45

Problem 57

The leaves of a charged electroscope collapse in time. At higher altitudes, they collapse more rapidly. Why is this true? (Hint: The existence of cosmic rays was first indicated by this observation.)

Karl Jacob

Numerade Educator

00:58

Problem 58

Is it necessary for a charged body actually to touch the ball of the electroscope for the leaves to diverge? Defend your answer.

Kara Merfeld

Numerade Educator

01:42

Problem 59

A crystal of salt consists of negative and positive ions. How does the net charge of the negative ions compare with that of the positive ions? Explain.

Karl Jacob

Numerade Educator

01:02

Problem 60

How can you charge an object negatively with only the help of a positively charged object?

Kara Merfeld

Numerade Educator

01:31

Problem 61

When one material is rubbed against another, electrons jump readily from one to the other but protons do not. Why is this? (Think in atomic terms.)

Karl Jacob

Numerade Educator

00:59

Problem 62

What does the inverse-square law tell you about the relationship between force and distance?

Kara Merfeld

Numerade Educator

00:31

Problem 63

The $50,000$ billion billion $(5 \times 10^{22})$ freely moving electrons in a penny repel one another. Why don't they fly out of the penny?

Karl Jacob

Numerade Educator

01:31

Problem 64

How does the magnitude of the electrical force between a pair of charged particles change when the particles are moved half as far apart? One-third as far apart?

Kara Merfeld

Numerade Educator

01:56

Problem 65

How does the magnitude of the electrical force compare between a pair of charged particles when they are brought to half their original distance of separation? To one-quarter their original distance? To four times their original distance? (What law guides your answers?)

Karl Jacob

Numerade Educator

00:48

Problem 66

When you double the distance between a pair of charged particles, what happens to the force between them? Does it depend on the sign of the charges? What law defends your answer?

Kara Merfeld

Numerade Educator

01:14

Problem 67

When you double the charge on only one of a pair of particles, what effect does this have on the force
between them? Does the effect depend on the sign of the charge?

Karl Jacob

Numerade Educator

01:00

Problem 68

When you double the charge on both particles in a pair, what effect does this have on the force between them? Does it depend on the sign of the charge?

Kara Merfeld

Numerade Educator

01:21

Problem 69

The proportionality constant k in Coulomb’s law is huge in ordinary units, whereas the proportionality
constant G in Newton’s law of gravitation is tiny. What does this indicate about the relative strengths of these two forces?

Karl Jacob

Numerade Educator

01:46

Problem 70

How do electric field lines indicate the strength of an electric field?

Kara Merfeld

Numerade Educator

01:31

Problem 71

How is the direction of an electric field indicated with electric field lines?

Karl Jacob

Numerade Educator

01:02

Problem 72

Suppose that the strength of the electric field about an isolated point charge has a certain value at a distance of 1 m. How will the electric field strength compare at a distance of 2 m from the point charge? What law guides your answer?

Kara Merfeld

Numerade Educator

01:14

Problem 73

In the phenomenon of superconductivity, what happens to electrical resistance at low temperatures?

Karl Jacob

Numerade Educator

01:22

Problem 74

Measurements show that there is an electric field surrounding Earth. Its magnitude is about 100 N/C
at Earth’s surface, and it points inward toward Earth’s center. From this information, can you state whether Earth is negatively or positively charged?

Kara Merfeld

Numerade Educator

01:55

Problem 75

Why are lightning rods normally at a higher elevation than the buildings they protect?

Karl Jacob

Numerade Educator

01:27

Problem 76

Why aren’t metal-spiked shoes a good idea for golfers on a stormy day?

Kara Merfeld

Numerade Educator

03:27

Problem 77

If a large enough electric field is applied, even an insulator will conduct an electric current, as is evident in lightning discharges through the air. Explain how this happens, taking into account the opposite charges in an atom and how ionization occurs.

Karl Jacob

Numerade Educator

00:56

Problem 78

If you rub an inflated balloon against your hair and place the balloon against a door, what mechanism causes it to stick? Explain.

Kara Merfeld

Numerade Educator

01:21

Problem 79

When a car is moved into a painting chamber, a mist of paint is sprayed around its body. When the body if given a sudden electric charge and mist is attracted to it—presto—the car is quickly and uniformly painted. What does the phenomenon of polarization have to do with this?

Karl Jacob

Numerade Educator

01:49

Problem 80

How can a charged atom (an ion) attract a neutral atom?

Kara Merfeld

Numerade Educator

02:08

Problem 81

If you place a free electron and a free proton in the same electric field, how will the forces acting on them
compare?

Karl Jacob

Numerade Educator

00:43

Problem 82

How will the accelerations of the proton and the electron in the preceding problem compare?

Kara Merfeld

Numerade Educator

01:48

Problem 83

How will the directions of travel compare for the electron and the proton in the preceding problem?

Karl Jacob

Numerade Educator

01:23

Problem 84

Why is the magnitude of the electric field zero midway between identical point charges?

Kara Merfeld

Numerade Educator

03:25

Problem 85

Imagine a proton at rest a certain distance from a negatively charged plate. It is released and collides with the plate. Then imagine the similar case of an electron at rest the same distance away from a similarly charged but posi- tive plate. In which case will the moving particle greater speed when the collision occurs? Why?

Karl Jacob

Numerade Educator

00:46

Problem 86

A gravitational field vector points toward Earth; an electric field vector points toward an electron. Why do electric field vectors point away from protons?

Kara Merfeld

Numerade Educator

02:04

Problem 87

When an electric field is applied to a shallow bath of vegetable oil, why do tiny bits of thread floating in the oil align with the field like compasses in a magnetic field?

Amna Khalid

Numerade Educator

00:49

Problem 88

Suppose that a metal file cabinet is charged. How does the charge concentration at the corners of the cabinet compare with the charge concentration on the flat parts of the cabinet?

Kara Merfeld

Numerade Educator

00:41

Problem 89

If you expend 10 J of work to push a 1-C charge against an electric field, what is its change of voltage?

Karl Jacob

Numerade Educator

00:42

Problem 90

When released, what is the kinetic energy of the 1-C charge of the preceding problem if it flies past its starting position?

Kara Merfeld

Numerade Educator

01:25

Problem 91

What is the voltage at the location of a 0.0001-C charge that has an electric potential energy of 0.5 J (both measured relative to the same reference point)?

Karl Jacob

Numerade Educator

01:04

Problem 92

Why is it safe to remain inside a car during a lightning storm?

Kara Merfeld

Numerade Educator

01:18

Problem 93

How do the charges on opposing plates of a capacitor compare?

Karl Jacob

Numerade Educator

01:06

Problem 94

In order to store more energy in a parallel-plate capacitor whose plates differ by a fixed voltage, what change should you make in the plates?

Kara Merfeld

Numerade Educator

01:10

Problem 95

Why is it dangerous to touch the terminals of a high-voltage capacitor even after the charging circuit is turned off?

Karl Jacob

Numerade Educator

00:56

Problem 96

An electron volt (eV) is a unit of energy. Which is larger: a GeV or a MeV?

Kara Merfeld

Numerade Educator

01:06

Problem 97

What is the magnitude of the electric field inside the dome of a charged Van de Graaff generator?

Karl Jacob

Numerade Educator

01:25

Problem 98

Would you feel any electrical effects if you were inside the charged sphere of a Van de Graaff generator? Why or why not?

Kara Merfeld

Numerade Educator

01:36

Problem 99

A friend says that the reason one’s hair stands out while touching a charged Van de Graaff generator is simply that the hair strands become charged and are light enough so that the repulsion between strands is visible. Do you agree or disagree?

Karl Jacob

Numerade Educator

00:58

Problem 100

When you remove your wool suit from the dry cleaner’s garment bag, the bag becomes positively charged. Discuss how this occurs.

Kara Merfeld

Numerade Educator

01:48

Problem 101

Plastic wrap becomes electrically charged when pulled from its box. As a result, it is attracted to objects such as food containers. Does the wrap stick better to plastic containers or to metal containers? Discuss.

Prashant Bana

Numerade Educator

01:08

Problem 102

Strictly speaking, when an object acquires a positive charge by the transfer of electrons, what happens to its mass? Discuss what happens to its mass when it acquires a negative charge. (Think small!)

Kara Merfeld

Numerade Educator

01:17

Problem 103

Strictly speaking, will a penny be slightly more massive if it has a negative charge or a positive charge? Discuss.

Karl Jacob

Numerade Educator

01:37

Problem 104

It is relatively easy to strip the outer electrons from a heavy atom like that of uranium (which then becomes a uranium ion), but it is very difficult to remove the inner electrons. Discuss why this is so.

Kara Merfeld

Numerade Educator

01:16

Problem 105

If electrons were positive and protons were negative, discuss whether Coulomb’s law would be written the same or differently.

Karl Jacob

Numerade Educator

03:02

Problem 106

If you are caught outdoors in a thunderstorm, why shouldn’t you stand under a tree? Can you think of a reason why you should not stand with your legs far apart? Or why lying down can be dangerous? (Hint: Consider the electric potential difference.)

Kara Merfeld

Numerade Educator

03:03

Problem 107

Two pieces of plastic, a full ring and a half ring, have the same radius and charge density. Which electric field at the center has the greater magnitude? Discuss and defend your answer.

Karl Jacob

Numerade Educator

01:21

Problem 108

You are not harmed by contact with a charged metal ball, even though its voltage may be very high. Is the reason similar to why you are not harmed by the hotter- than-1000°C sparks from a fireworks sparkler? Discuss and defend your answer in terms of the energies that are involved.

Kara Merfeld

Numerade Educator

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