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# Test Prep

### Multiple Choice

#### 18.1Electrical Charges, Conservation of Charge, and Transfer of Charge

1.

A neutral hydrogen atom has one proton and one electron. If you remove the electron, what will be the leftover sign of the charge?

1. negative
2. positive
3. zero
4. neutral
2.

What is the charge on a proton?

1. +8.99 × 10–9 C
2. −8.99 × 10–9 C
3. + 1.60 × 10–19 C
4. −1.60 × 10–19 C
3.

True or false—Carbon is more conductive than pure water.

1. true
2. false
4.

True or false—Two insulating objects are polarized. To cancel the polarization, it suffices to touch them together.

1. true
2. false
5.

How is the charge of the proton related to the charge of the electron?

1. The magnitudes of charge of the proton and the electron are equal, but the charge of the proton is positive, whereas the charge of the electron is negative.
2. The magnitudes of charge of the proton and the electron are unequal, but the charge of the proton is positive, whereas the charge of the electron is negative.
3. The magnitudes of charge of the proton and the electron are equal, but the charge of the proton is negative, whereas the charge of the electron is positive.
4. The magnitudes of charge of the proton and the electron are unequal, but the charge of the proton is negative, whereas the charge of the electron is positive.

#### 18.2Coulomb's law

6.

If you double the distance between two point charges, by which factor does the force between the particles change?

1. 1/2
2. 2
3. 4
4. 1/4
7.

The combined charge of all the electrons in a dime is hundreds of thousands of coulombs. Because like charges repel, what keeps the dime from exploding?

1. The dime has an equal number of protons, with positive charge.
2. The dime has more protons than electrons, with positive charge.
3. The dime has fewer protons than electrons, with positive charge.
4. The dime is polarized, with electrons on one side and protons on the other side.
8.
How can you modify the charges on two particles to quadruple the force between them without moving them?
1. Increase the distance between the charges by a factor of two.
2. Increase the distance between the charges by a factor of four.
3. Increase the product of the charges by a factor of two
4. Increase the product of the charges by a factor of four.

#### 18.3Electric Field

9.

What is the magnitude of the electric field 12 cm from a charge of 1.5 nC ?

1. 9.4 × 107 N/ C
2. 1.1 × 102 N/C
3. 9.4 × 102 N/C
4. 9.4 × 10–2 N/C
10.

A charge distribution has electric field lines pointing into it. What sign is the net charge?

1. positive
2. neutral
3. final
4. negative
11.

If five electric field lines come out of point charge q1 and 10 electric-field lines go into point charge q2, what is the ratio q1/q2?

1. –2
2. –1
3. –1/2
4. 0
12.

True or false—The electric-field lines from a positive point charge spread out radially and point outward.

1. false
2. true

#### 18.4Electric Potential

13.

What is the potential at 1.0 m from a point charge Q = − 25 nC?

1. 6.6 × 102 V
2. −2.3 × 102 V
3. −6.6 × 102 V
4. 2.3 × 102 V
14.
Increasing the distance by a factor of two from a point charge will change the potential by a factor of how much?
1. 2
2. 4
3. $1 2$
4. $1 4$
15.

True or false—Voltage is the common word for potential difference, because this term is more descriptive than potential difference.

1. false
2. true

#### 18.5Capacitors and Dielectrics

16.

Which magnitude of charge is stored on each plate of a 12 µF capacitor with 12 V applied across it?

1. –1.0 × 10–6 C
2. 1.0 × 10–6 C
3. –1.4 × 10–4 C
4. 1.4 × 10–4 C
17.

What is the capacitance of a parallel-plate capacitor with an area of 200 cm2, a distance of 0.20 mm between the plates, and polystyrene as a dielectric?

1. 2.3 nC
2. 0.89 nC
3. 23 nC
4. 8.9 nC
18.

Which factors determine the capacitance of a device?

1. Capacitance depends only on the materials that make up the device.
2. Capacitance depends on the electric field surrounding the device.
3. Capacitance depends on the geometric and material parameters of the device.
4. Capacitance depends only on the mass of the capacitor

#### 18.1Electrical Charges, Conservation of Charge, and Transfer of Charge

19.

Compare the mass of the electron with the mass of the proton.

1. The mass of the electron is about 1,000 times that of the proton.
2. The mass of the proton is about 1,000 times that of the electron.
3. The mass of the electron is about 1,836 times that of the proton.
4. The mass of the proton is about 1,836 times that of the electron.
20.

The positive terminal of a battery is connected to one connection of a lightbulb, and the other connection of the lightbulb is connected to the negative terminal of the battery. The battery pushes charge through the circuit but does not become charged itself. Does this violate the law of conservation of charge? Explain.

1. No, because this is a closed circuit.
2. No, because this is an open circuit.
3. Yes, because this is a closed circuit.
4. Yes, because this is an open circuit.
21.
Two flat pieces of aluminum foil lay one on top of the other. What happens if you add charge to the top piece of aluminum foil?
1. The charge will distribute over the top of the top piece.
2. The charge will distribute to the bottom of the bottom piece.
3. The inner surfaces will have excess charge of the opposite sign.
4. The inner surfaces will have excess charge of the same sign.
22.

The students in your class count off consecutively so each student has a number. The odd-numbered students are told to act as negative charge, and the even-numbered students are told to act as positive charge. How would you organize them to represent a polarized material?

1. The even-numbered and odd-numbered students will be arranged one after the other.
2. Two even-numbered will be followed by two odd-numbered, and so on.
3. Even-numbered students will be asked to come to the front, whereas odd-numbered students will be asked to go to the back of the class.
4. Half even-numbered and odd-numbered will come to the front, whereas half even-numbered and odd-numbered will go to the back.
23.

An ion of iron contains 56 protons. How many electrons must it contain if its net charge is +5e?

1. five electrons
2. 51 electrons
3. 56 electrons
4. 61 electrons
24.
An insulating rod carries $+ 2.0 nC$ of charge. After rubbing it with a material, you find it carries $− 3 nC$ of charge. How much charge was transferred to it?
1. $− 5 nC$
2. $− 3 nC$
3. $− 1 nC$
4. $+ 2.0 nC$
25.

A solid cube carries a charge of +8e. You measure the charge on each face of the cube and find that each face carries +0.5e of charge. Is the cube made of conducting or insulating material? Explain.

1. The cube is made of insulating material, because all the charges are on the surface of the cube.
2. The cube is made of conducting material, because some of the charges are inside the cube.
3. The cube is made of insulating material, because all the charges are on the surface of the cube.
4. The cube is made of insulating material, because some of the charges are inside the cube.
26.

You have four neutral conducting spheres and a charging device that allows you to place charge q on any neutral object. You want to charge one sphere with a charge q/2 and the other three with a charge q6 . How do you proceed?

1. Charge one sphere with charge q. Touch it simultaneously to the three remaining neutral spheres.
2. Charge one sphere with charge q. Touch it to one other sphere to produce two spheres with charge $q 2 q 2$. Touch one of these spheres to one other neutral sphere.
3. Charge one sphere with charge q. Touch it to one other sphere to produce two spheres with charge $q 2 q 2$. Touch one of these spheres simultaneously to the two remaining neutral spheres.
4. Charge one sphere with charge q. Touch it simultaneously to two other neutral spheres to produce three spheres with charge q/3. Touch one of these spheres to one other neutral sphere.

#### 18.2Coulomb's Law

27.
Why does dust stick to the computer screen?
1. The dust is neutral.
2. The dust is polarized.
3. The dust is positively charged.
4. The dust is negatively charged.
28.

The force between two charges is 4 × 10–9 N . If the magnitude of one charge is reduced by a factor of two and the distance between the charges is reduced by a factor of two, what is the new force between the charges?

1. 2 × 10–9 N
2. 4 × 10–9 N
3. 6 × 10–9 N
4. 8 × 10–9 N
29.

True or false—Coulomb’s constant is k = 8.99 × 109 N · m2 / C2. Newton’s gravitational constant is G = 6.67 × 10−11 m3 / kg ⋅ s2. This tells you about the relative strength of the electrostatic force versus that of gravity.

1. true
2. false
30.

An atomic nucleus contains 56 protons, for iron. Which force would this nucleus apply on an electron at a distance of 10×10–12 m?

1. 0.65 × 10–4 N
2. 0.02 × 10–4 N
3. 1.3 × 10–4 N
4. 72.8 × 10–4 N

#### 18.3Electric Field

31.

The electric field a distance of 10 km from a storm cloud is 1,000 N/C . What is the approximate charge in the cloud?

1. 0.0011 C
2. 11 C
3. 110 C
4. 1,100 C
32.

Which electric field would produce a 10 N force in the +x- direction on a charge of – 10 nC ?

1. − 1.0 × 109 N/C
2. 1.0 × 109 N/C
3. 1.0 × 1010 N/C
4. 1.0 × 1011 N/C
33.

A positive charge is located at x = 0 . When a negative charge is placed at x = 10 cm , what happens to the electric field lines between the charges?

1. The electric field lines become denser between the charges.
2. The electric field lines become denser between the charges.
3. The electric field lines remains same between the charges.
4. The electric field lines will be zero between the charges.

#### 18.4Electric Potential

34.

The energy required to bring a charge q = − 8.8 nC from far away to 5.5 cm from a point charge Q is 13 mJ. What is the potential at the final position of q?

1. −112 MV
2. −1.5 MV
3. −0.66 MV
4. +1.5 MV
35.

How is electric potential related to electric potential energy?

1. Electric potential is the electric potential energy per unit mass at a given position in space.
2. Electric potential is the electric potential energy per unit length at a given position in space. This relation is not dimensionally correct.
3. Electric potential is the electric potential energy per unit area in space.
4. Electric potential is the electric potential energy per unit charge at a given position in space.
36.

If it takes 10 mJ to move a charge q from xi = 25 cm to xf = − 25 cm in an electric field of $(–20 N/C) x ^ , (–20 N/C) x ^ ,$ what is the charge q?

1. −1.0 mC
2. +0.25 mC
3. + 1.0 mC
4. +400 mC
37.
Given the potential difference between two points and the distance between the points, explain how to obtain the electric field between the points.
1. Add the electric potential to the distance to obtain the electric field.
2. Divide the electric potential by the distance to obtain the electric field.
3. Multiply the electric potential and the distance to obtain the electric field.
4. Subtract the electric potential from the distance to obtain the electric field.

#### 18.5Capacitors and Dielectrics

38.
If you double the voltage across the plates of a capacitor, how is the stored energy affected?
1. Stored energy will decrease two times.
2. Stored energy will decrease four times.
3. Stored energy will increase two times.
4. Stored energy will increase four times.
39.

A capacitor with neoprene rubber as the dielectric stores 0.185 mJ of energy with a voltage of 50 V across the plates. If the area of the plates is 500 cm2, what is the plate separation?

1. 20 µm
2. 20 m
3. 80 µm
4. 80 m
40.

Explain why a storm cloud before a lightning strike is like a giant capacitor.

1. The storm cloud acts as a giant charged capacitor, as it can store a large amount of charge.
2. The storm cloud acts as a giant charged capacitor, as it contains a high amount of excess charges.
3. The storm cloud acts as a giant charged capacitor, as it splits in two capacitor plates with equal and opposite charge.
4. The storm cloud acts as a giant charged capacitor, as it splits in two capacitor plates with unequal and opposite charges.
41.

A storm cloud is 2 km above the surface of Earth. The lower surface of the cloud is approximately 2 km2 in area. What is the approximate capacitance of this storm cloud-Earth system?

1. 9 × 10–15 F
2. 9 × 10-9 F
3. 17.7 × 10-15 F
4. 17.7 × 10-9 F

### Extended Response

#### 18.1Electrical Charges, Conservation of Charge, and Transfer of Charge

42.
Imagine that the magnitude of the charge on the electron differed very slightly from that of the proton. How would this affect life on Earth and physics in general?
1. Many macroscopic objects would be charged, so we would experience the enormous force of electricity on a daily basis.
2. Many macroscopic objects would be charged, so we would experience the small force of electricity on a daily basis.
3. Many macroscopic objects would be charged, but it would not affect life on Earth and physics in general.
4. Macroscopic objects would remain neutral, so it would not affect life on Earth and physics in general.
43.

True or false—Conservation of charge is like balancing a budget.

1. true
2. false
44.

True or false—Although wood is an insulator, lightning can travel through a tree to reach Earth.

1. true
2. false
45.

True or false—An eccentric inventor attempts to levitate by first placing a large negative charge on himself and then putting a large positive charge on the ceiling of his workshop. Instead, while he attempts to place a large negative charge on himself, his clothes fly off.

1. true
2. false

#### 18.2Coulomb's Law

46.

Electrostatic forces are enormous compared to gravitational force. Why do you not notice electrostatic forces in everyday life, whereas you do notice the force due to gravity?

1. Because there are two types of charge, but only one type of mass exists.
2. Because there is only one type of charge, but two types of mass exist.
3. Because opposite charges cancel each other, while gravity does not cancel out.
4. Because opposite charges do not cancel each other, while gravity cancels out.
47.

A small metal sphere with a net charge of 3.0 nC is touched to a second small metal sphere that is initially neutral. The spheres are then placed 20 cm apart. What is the force between the spheres?

1. 1.02 × 10−7 N
2. 2.55 × 10−7 N
3. 5.1 × 10−7 N
4. 20.4 × 10−7 N

#### 18.3Electric Field

48.

Point charges are located at each corner of a square with sides of 5.0 cm . The top-left charge is q1 = 8.0 nC The top right charge is q2 = 4.0 nC. The bottom-right charge is q3 = 4.0 nC. The bottom-left charge is q4 = 8.0 nC. What is the electric field at the point midway between charges q2 and q3?

49.

A long straight wire carries a uniform positive charge distribution. Draw the electric field lines in a plane containing the wire at a location far from the ends of the wire. Do not worry about the magnitude of the charge on the wire.

1. Take the wire on the x-axis, and draw electric-field lines perpendicular to it.
2. Take the wire on the x-axis, and draw electric-field lines parallel to it.
3. Take the wire on the y-axis, and draw electric-field lines along it.
4. Take the wire on the z-axis, and draw electric-field lines along it.

#### 18.4Electric Potential

50.

A square grid has charges of Q = 10 nC are each corner. The sides of the square at 10 cm . How much energy does it require to bring a q = 1.0 nC charge from very far away to the point at the center of this square?

1. 1.3 × 10−6 J
2. 2.5 × 10−6 J
3. 3.8 × 10−6 J
4. 5.1 × 10−6 J
51.

How are potential difference and electric-field strength related for a constant electric field?

1. The magnitude of electric-field strength is equivalent to the potential divided by the distance.
2. The magnitude of electric-field strength is equivalent to the product of the electric potential and the distance.
3. The magnitude of electric-field strength is equivalent to the difference between magnitude of the electric potential and the distance.
4. The magnitude of electric-field strength is equivalent to the sum of the magnitude of the electric potential and the distance.

#### 18.5Capacitors and Dielectrics

52.

A 12 μF air-filled capacitor has 12 V across it. If the surface charge on each capacitor plate is σ = 7.2 mC / m2, what is the attractive force of one capacitor plate toward the other?

1. 0.81 × 105 N
2. 0.81 × 106 N
3. 1.2 × 105 N
4. 1.2 × 106 N
53.

Explain why capacitance should be inversely proportional to the separation between the plates of a capacitor.

1. Capacitance is directly proportional to the electric field, which is inversely proportional to the distance between the capacitor plates.
2. Capacitance is inversely proportional to the electric field, which is inversely proportional to the distance between the capacitor plates.
3. Capacitance is inversely proportional to the electric field, which is directly proportional to the distance between the capacitor plates.
4. Capacitance is directly proportional to the electric field, which is directly proportional to the distance between the capacitor plates.