Download Chapter 19

Electric Forces and
Electric Fields
1. Two equal charges are separated by a distance r. At what point, between the two charges, would a third
test charge experience no net electrical force?
q1
a. 
r
4
r
2
3r
4
q2
r
2
r
4
r
c.
2
3
d. r
4
3
e. r
2
b.
ANS: c
2. What happens to the net electric flux through the surface of a sphere when the charge inside the sphere is
doubled?
a. It is doubled.
b. It is cut in half.
c. It increases by a factor of three.
d. It increases by a factor of four.
e. It remains the same.
ANS: a
3. What happens to the net electric flux through the surface of a sphere when its radius is doubled?
a. It is doubled.
b. It is cut in half.
c. It increases by a factor of three.
d. It increases by a factor of 4.
e. It remains the same.
345
346
Electric Forces and Electric Fields
ANS: e
4. A charge q is located inside a spherical surface of radius r. What happens to the electric flux when the
sphere is replaced by a cube of side r?
4
a. It changes by a factor of  .
3
b. It changes by a factor of .
c. It changes by a factor of 4.
3
d. It changes by a factor of  .
2
e. It remains the same.
ANS: e
5. The Coulomb force between two positive charges is
a. attractive.
b. repulsive.
c. a cross product.
d. a dot product.
e. linear.
ANS: b
6. The Coulomb force between two negative charges is
a. attractive.
b. repulsive.
c. a cross product.
d. a dot product.
e. linear.
ANS: b
7. The Coulomb force between a positive and a negative charge is
a. attractive.
b. repulsive.
c. a cross product.
d. a dot product.
e. linear.
ANS: a
8. Excess charge on a conductor
a. is evenly distributed.
b. is in the center.
c. is on the surface.
d. is fixed in space.
e. cannot be detected.
Chapter 19
ANS: c
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Electric Forces and Electric Fields
9. The force between two charges q1 and q 2 is proportional to
a.
q1 2 q2 2
.
r2
qq
b. 1 2 .
r
q1  q2
c.
.
r
q q
d. 1 2 2 .
r
q1 q2
e. 2 .
r
ANS: e
10. An electron and a proton are separated by 5.31  10 11 m . The magnitude of the charge on the proton and
the electron is 1.60  10 19 C , the mass of the proton is 1.67  10 27 kg , the mass of the electron is
9.11  10 31 kg , G  6.67  10 11 Nm 2 kg 2 , and k e  8.99  10 9 Nm 2 kg 2 . What electric force in N does the
electron exert on the proton?
a. 8.16  10 8
b. 4.91  10 4
c. 1.23  10 5
d. 10.65  10 5
e. 3.22  10 8
ANS: a
11. An electron and a proton are separated by 5.31  10 11 m . The magnitude of the charge on the proton and
the electron is 1.60  10 19 C , the mass of the proton is 1.67  10 27 kg , the mass of the electron is
9.11  10 31 kg , G  6.67  10 11 Nm 2 kg 2 , and k e  8.99  10 9 Nm 2 kg 2 . What is the gravitational force in N
that each exerts on the other?
a. 2.88  10 12
b. 2.35  10 22
c. 7.86  10 87
d. 3.60  10 47
e. 1.72  10 6
ANS: d
Chapter 19
349
12. An electron and a proton are separated by 5.31  10 11 m . The magnitude of the charge on the proton and
the electron is 1.60  10 19 C , the mass of the proton is 1.67  10 27 kg , the mass of the electron is
9.11  10 31 kg , G  6.67  10 11 Nm 2 kg 2 , and k e  8.99  10 9 Nm 2 kg 2 . What is the ratio of the electric force
to the gravitational force between them?
a. 3.930  10 4
b. 6.238  10 23
c. 2.27  10 39
d. 1.982  1017
e. 8.145  10 31
ANS: c
13. The electric field at a distance r from a charge q is
q2
a. k e
.
r
q
b. k e 2 .
r
q2
c. k e 2 .
r
q1 q2
d. k e 2 .
r
q
e. k e .
r
ANS: b
14. A 1.97  10 4 C charge is distributed in a spherical volume of radius 3.89 m. What is the volume charge
density in C m 3 ?
a. 6.22  10 5
b. 2.16  10 2
c. 7.99  10 7
d. 21.5  10 4
e. 2.53  10 8
ANS: c
15. A 2.83  10 6 C charge is distributed on the surface of a conducting sphere of radius 1.12 m. What is the
surface charge density in C m 2 ?
a. 1.32  10 7
b. 1.16  10 7
c. 1.49  10 7
d. 1.74  10 7
e. 1.80  10 7
ANS: e
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Electric Forces and Electric Fields
16. A 7.96  10 3 C charge is distributed uniformly on a wire 125 m long. What is the linear charge density in
C/m?
a. 3.14  10 5
b. 6.37  10 5
c. 5.69  10 5
d. 6.66  10 5
e. 6.43  10 5
ANS: b
17. What is the relationship between the electric field vector at a point and the electric field line going through
that point?
a. They are perpendicular to each other.

radians with the field line.
3

c. The electric field vector makes an angle of
radians with the field line.
2
b. The electric field vector makes an angle of
d. The electric field vector is tangent to the field line.
1
e. The electric field vector has a value 2 times the value of the field line.
r
ANS: d
18. What is the relationship between the number of field lines per unit area and the field strength?
a. Directly proportional
b. Inversely proportional
c. Directly proportional to the square
d. Directly proportional to the cube
e. Inversely proportional to the square
ANS: a
19. Electric field lines start
a. at positive charges or at infinity.
b. at negative charges or infinity.
c. at infinity and end at positive charges.
d. midway between positive charges and negative charges.
e. midway between positive charges and infinity.
ANS: a
20. The electric flux through a surface of fixed area is maximum when the surface is
a. parallel to the electric field.
b. antiparallel to the electric field.
c. perpendicular to the electric field.
d. at an angle of

radians to the electric field.
4
e. closed, but does not contain the charge.
Chapter 19
ANS: c
21. For a closed surface, Gauss’s law states that  e is equal to
q
a.
.
4 o
o
.
q
q2
c.
.
o
b.
q
.
o

e. 2o .
q
d.
ANS: d
22. When a conductor is in electrostatic equilibrium, the electric field inside the conductor is
q
a. k e 2 .
r
b. zero.
q
c.
.
4 o
q
d. 22 .
r
e.
o
.
q
ANS: b
23. The electric field just outside a charged conductor is

.
o

b.
.
4 o
a.
c.
q
.
o
o
.

2
e.
.
o
d.
ANS: a
351
352
Electric Forces and Electric Fields
24. Each of two small spheres is charged positively, the combined charge being 40 C . Each sphere is repelled
from the other by a force of magnitude 2.0 N when the two spheres are 50 cm apart. Determine the charge in
C on the sphere having the smaller charge. ( k e  8.99  10 9 Nm 2 C 2 )
a. 1.1
b. 1.4
c. 2.0
d. 3.3
e. 17
ANS: b
25. A particle (charge  40 C ) is located on the x-axis at the point x  20 cm , and a second particle (charge
 50 C ) is placed on the x-axis at x  30 cm . What is the magnitude of the total electrostatic force in N on a
third particle (charge  4.0 C ) placed at the origin ( x  0 )? ( k e  8.99  10 9 Nm 2 C 2 )
a. 41
b. 16
c. 56
d. 35
e. 72
ANS: c
26. If Q  30 C , q  5.0 C , and d  30 cm , what is the magnitude of the electrostatic force on q in N?
( k e  8.99  10 9 Nm 2 C 2 )
d
Q
2d
q
2Q
a. 15
b. 23
c. zero
d. 7.5
e. 38
ANS: d
27. A charge of 80 C is placed on the x-axis at x  0 . A second charge of 50 C is placed on the x-axis at
x  50 cm . What is the magnitude of the electrostatic force in N on a third charge of 4.0 C placed on the xaxis at x  30 cm ? ( k e  8.99  10 9 Nm 2 C 2 )
a. 13
b. 77
c. 39
d. 25
e. 45
ANS: b
Chapter 19
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Electric Forces and Electric Fields
28. Three point charges are positioned on the x-axis: 32 C at x  0 , 20 C at x  40 cm and 60 C at
x  60 cm . What is the magnitude of the electrostatic force on the 32 C charge? ( k e  9.0  10 9 Nm 2 C 2 )
a. 84
b. 12
c. 36
d. 50
e. 48
ANS: b
29. A particle ( m  50 g , q  5.0 C ) is released from rest when it is 50 cm from a second particle ( Q  20 C ).
Determine the magnitude of the initial acceleration in m s 2 of the 50 g particle. ( k e  8.99  10 9 Nm 2 C 2 )
a. 54
b. 90
c. 72
d. 65
e. 36
ANS: c
30. A 50 C point charge is placed at the origin, and an identical charge is placed on the x-axis at x  4.0 m .
What is the magnitude of the electrostatic force in N on a 20 C charge placed on the x-axis at x  3.0 m ?
( k e  8.99  10 9 Nm 2 C 2 )
a. 4.8
b. 6.4
c. 9.6
d. 8.0
e. 0.5
ANS: d
31. A point charge Q is placed on the x-axis at x  2.0 m . A second point charge –Q is placed at x  3.0 m . If
Q  40 C , what is the magnitude of the electrostatic force in N on a 30 C charge placed at the origin?
( k e  8.99  10 9 Nm 2 C 2 )
a. 7.2
b. 3.9
c. 1.5
d. 14
e. 8.1
ANS: c
Chapter 19
355
32. A point charge Q is placed on the x-axis at x  2.0 m . A second point charge, –Q is placed at x  1.0 m . If
Q  60 C , what is the magnitude of the electrostatic force in N on a 40 C charge placed at the origin?
( k e  8.99  10 9 Nm 2 C 2 )
a. 16
b. 27
c. 32
d. 11
e. 3.0
ANS: b
33. A point charge Q is placed on the x-axis at the origin. An identical point charge is placed on the x-axis at
x  1.0 m and another at x  1.0 m . If Q  40 C , what is the magnitude of the electrostatic force in N on the
charge at x  1.0 m ? ( k e  8.99  10 9 Nm 2 C 2 )
a. 29
b. 14
c. 11
d. 18
e. 7.0
ANS: d
34. A uniform surface charge density of 5.0 nC m 2 is distributed over the entire xy plane. Consider a
spherical (radius  5.0 cm ) surface centered on the origin. Determine the electric flux in Nm 2 C for this
surface. (  o  8.85  10 12 C 2 Nm 2 )
a. 7.1
b. 5.3
c. 6.2
d. 4.4
e. 1.4
ANS: d
35. A uniform surface charge density of 500 nC m 3 is distributed throughout a spherical volume (radius
 16 cm ). Consider a cubical (4.0 cm along the edge) surface completely inside the sphere. Determine the
electric flux in Nm 2 C for this surface. (  o  8.85  10 12 C 2 Nm 2 )
a. 7.1
b. 3.6
c. 12
d. 19
e. 970
ANS: b
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Electric Forces and Electric Fields
36. A point charge +Q is located on the x-axis at x  a , and a second point charge –Q is located on the x-axis at
x   a . A Gaussian surface with radius r  2 a is centered at the origin. The flux through this Gaussian surface
is (  o  8.85  10 12 C 2 Nm 2 )
a. zero because the negative flux over one hemisphere is equal to the positive flux over the other.
b. greater than zero.
c. zero because at every point on the surface the electric field has no component perpendicular to the surface.
d. zero because the electric field is zero at every point on the surface.
e. none of the above
ANS: a
37. The xy plane is “painted” with a uniform surface charge density equal to 40 nC m 2 . Consider a spherical
surface with a 4.0 cm radius that has a point in the xy plane as its center. What is the electric flux in Nm2/C for
that part of the spherical surface for which z  0 ? (  o  8.85  10 12 C 2 Nm 2 )
a. 14
b. 11
c. 17
d. 20
e. 23
ANS: b
38. A long cylinder (radius  3.0 cm ) is filled with a nonconducting material that carries a uniform charge
density of 1.3 C m 3 . Determine the electric flux in Nm 2 C for a spherical surface (radius  2.0 cm ) that has
a point on the axis of the cylinder as its center. (  o  8.85  10 12 C 2 Nm 2 )
a. 5.7
b. 4.9
c. 6.4
d. 7.2
e. 15
ANS: b
39. Two charges of 15 pC and –40 pC are inside a cube with sides that are of a 0.40 m length. Determine the
net electric flux in Nm 2 C for the surface of the cube. (  o  8.85  10 12 C 2 Nm 2 )
a. +2.8
b. –1.1
c. +1.1
d. –2.8
e. –0.47
ANS: d
Chapter 19
357
40. The total electric flux through a closed cylindrical (length  1.2 m , diameter  0.20 m ) surface is equal to
5.0 Nm 2 C . Determine the net charge within the cylinder in pC. (  o  8.85  10 12 C 2 Nm 2 )
a. –62
b. –53
c. –44
d. –71
e. –16
ANS: c
41. Charges q and Q are placed on the x-axis at x  0 and x  2.0 m , respectively. If q  40 pC and
Q  30 pC , determine the net flux in Nm 2 C through a spherical surface (radius  1.0 m ) centered on the
origin. (  o  8.85  10 12 C 2 Nm 2 )
a. –9.6
b. –6.8
c. –8.5
d. –4.5
e. –1.1
ANS: d
42. Charge of uniform density ( 6.0 nC m 3 ) fills the hollow cylindrical region having an inner radius of 2.0 cm
and an outer radius of 3.0 cm. Consider a 2.0 m long cylindrical (radius  3.0 cm ) surface that is coaxial with
the charged cylinder. Determine the electric flux in Nm 2 C through this surface. (  o  8.85  10 12 C 2 Nm 2 )
a. 3.0
b. 2.5
c. 2.1
d. 3.4
e. 3.9
ANS: c
43. A uniform linear charge density of 4.0 nC/m is distributed along the entire x-axis. Consider a spherical
(radius  5.0 cm ) surface centered on the origin. Determine the electric flux in Nm 2/C for this surface.
(  o  8.85  10 12 C 2 Nm 2 )
a. 68
b. 62
c. 45
d. 79
e. 23
ANS: c
358
Electric Forces and Electric Fields
44. When a glass rod is rubbed by silk and is brought near a rubber rod that has been rubbed by fur, they
a. attract.
b. repel.
c. align with the earth’s north pole.
d. align with the earth’s south pole.
e. have no effect on each other.
ANS: a
45. When fur that rubbed a rubber rod is brought near silk that rubbed a glass rod, they
a. attract.
b. repel.
c. align with the earth’s north pole.
d. align with the earth’s south pole.
e. have no effect on each other.
ANS: a
46. The ancient Greeks found that ___________, when rubbed, became electrified and attracted pieces of straw
or feathers.
a. glass
b. amber
c. rubber
d. polycarbonate
e. styrofoam
ANS: b
47. Which of the following confirmed the inverse-square force law for electricity?
a. Coulomb
b. Ampere
c. Oerstead
d. Gilbert
e. Henry
ANS: a
48. A proton and an electron generate an electric field
a. that starts at the proton and ends at the electron.
b. that starts at the electron and ends at the proton.
c. that starts midway between the electron and the proton.
d. that ends midway between the electron and the proton.
e. but there is no electric field associated with the proton.
ANS: a
Chapter 19
359
49. What is the magnitude (in N/C) of the electric field 50.0 m from the center of a spherical charge
distribution of 2.00 m radius that contains a 5.00 C charge? ( k e  9  10 9 Nm 2 C 2 )
a. 2.82  10 7
b. 1.80  10 7
c. 9.97  10 6
d. 1.53  10 7
e. 3.87  10 7
ANS: b
50. A 0.100 kg mass is placed in an electric field of 500 N/C at the Earth’s surface. The mass is supported
motionless by the field. What is the charge in C on the mass?
a. 2.43  10 3
b. 9.98  10 2
c. 1.96  10 3
d. 1.54  10 3
e. 3.01  10 3
ANS: c
51. An electron of mass 9.11  10 31 kg and charge 1.60  10 19 C is located 10 10 m from a proton of mass
1.672  10 27 kg and charge 1.60  10 19 C . What acceleration in m s 2 does the electron experience due to the
electric force if k e  8.99  10 9 Nm 2 C 2 ?
a. 2.53  10 22
b. 3.72  1012
c. 9.93  10 21
d. 1.27  10 22
e. 2.49  10 22
ANS: a
52. An electron and a proton are placed in the same uniform electric field. The ratio of the acceleration of the
electron to that of the proton is approximately
a. 4000:1.
b. 2000:1.
c. 1000:1.
d. 500:1.
e. 1:1.
ANS: b
360
Electric Forces and Electric Fields
53. An electron is placed in a uniform electric field directed toward the right side of the page. What is the
direction of the force the field exerts on the electron?
a. Toward the right
b. Toward the left
c. Toward the top of the page
d. Toward the bottom of the page
e. Perpendicular to the plane of the page
ANS: b
54. An electrostatic generator is used to accelerate electrons from rest to 6.0  10 6 m/s in a distance of 0.030 m.
If the electron has a mass of 9.11  10 31 kg and a charge of 1.60  10 19 C , what uniform electric field in N/C
is required?
a. 8.98  10 3
b. 3.42  10 3
c. 2.84  10 3
d. 4.45  10 3
e. 2.23  10 3
ANS: b
55. A proton of mass 1.67  10 27 kg is accelerated by a field of 600 N/C. How fast in m/s is it traveling after
20.0 nanoseconds if it has a charge of 1.60  10 19 C ?
a. 1.72  10 3
b. 1.11  103
c. 1.15  10 3
d. 1.27  10 3
e. 2.80  10 3
ANS: c
56. A proton of mass 1.67  10 27 kg and charge 1.60  10 19 C is accelerated from rest to 2.00  10 6 m s by a
field of 860 N/C. What is its acceleration in m s 2 ?
a. 8.37  1010
b. 8.43  1010
c. 7.26  1010
d. 8.24  1010
e. 9.78  1010
ANS: d
Chapter 19
361
57. A proton of mass 1.67  10 27 kg and charge 1.60  10 19 C with speed of 5.00 m/s moving horizontally
enters a vertical uniform electric field of 1.50 N/C. What is the vertical component of the proton’s velocity in
m/s after it has traveled 0.0100 m in a horizontal direction?
a. 2.87  10 5
b. 2.01  10 5
c. 1.76  10 5
d. 1.58  10 5
e. 1.55  10 5
ANS: a
58. Three balls of very small mass coated with a conducting material hang side by side without touching.
When a positive charge is placed on ball A,
A
B
C
a. B and C do not move.
b. B moves toward A. C does not move.
c. B moves toward A. C moves a smaller distance toward B.
d. B moves away from A. C does not move.
e. B moves away from A. C moves a smaller distance in the same direction.
ANS: c
59. Three balls of very small mass coated with a conducting material hang side by side without touching.
When a positive charge is placed on ball A, B is repelled by A, but C is attracted to B. We can conclude that
A
B
C
a. B is positively charged and C is negatively charged.
b. B is positively charged and C has no excess charge.
c. B is negatively charged and C is positively charged.
d. B is negatively charged and C has no excess charge.
e. B is positively charged and C is either negatively charged or has no excess charge.
ANS: e
362
Electric Forces and Electric Fields
60. Robin gives a glass rod a positive charge by rubbing it with silk. The glass rod is brought near a can of
soup sitting on an insulating plate. Robin’s friend Rhoda touches the opposite side of the metal can and
breaks contact with it before the glass rod is taken away. The can
a. remains uncharged.
b. is negatively charged.
c. is positively charged.
d. is negatively charged on the side that was near the glass rod, positively charged on the opposite side.
e. is positively charged on the side that was near the glass rod, negatively charged on the opposite side.
ANS: b
61. A student walks into a lab on a dry day and finds two pieces of aluminum foil hanging as shown. She can
safely conclude that
a. they both have like charges.
b. they have unlike charges.
c. one may be positively charged, the other neutral.
d. one may be negatively charged, the other neutral.
e. (b), (c) and (d) are all possible.
ANS: e
62. Two balls of very low mass coated with a conducting material hang side by side and touch each other. A
glass rod with a positive charge is brought near ball A, but does not touch it. A and B are moved apart and
then the glass rod is taken away. We conclude that
a. A and B are both positively charged.
b. A and B are both negatively charged.
c. A is positively charged, B negatively charged.
d. A is negatively charged, B positively charged.
e. we have no way of telling which ball has what charge, if any.
ANS: d
Chapter 19
363
63. A very large plane has a uniform positive surface charge on both sides. A particle with a 7 nC charge on it
is moved straight out from a point 20 cm in front of the plane to a point 40 cm from the plane. At the 40 cm
position, the force on the charge is
a. 1/4 the force at the 20 cm position.
b. 1/2 the force at the 20 cm position.
c. the same as the force at the 20 cm position.
d. 2 times as large as the force at the 20 cm position.
e. 4 times as large as the force at the 20 cm position.
ANS: c
64. A 3 mg particle with a 3 nC charge is 40 cm away from a 5 mg particle with a 5 nC charge. The force the
5 nC charge exerts on the 3 nC charge is
a. 5/3 the force the 3 nC charge exerts on the 5 nC charge.
b. equal to the force the 3 nC charge exerts on the 5 nC charge.
c. 3/5 the force the 3 nC charge exerts on the 5 nC charge.
d. 25/9 the force the 3 nC charge exerts on the 5 nC charge.
e. 9/25 the force the 3 nC charge exerts on the 5 nC charge.
ANS: b
65. A line of charge of length 2a with linear charge density  lies along the x-axis, as shown. To find the field
at x  b , we must use
x
–a
a. E 
b. E 
c. E 
d. E 
0
a
b
a
kdx
.
2
 a( b  x )
z
z
z
z
z
a
kdx
.
2
 a( b  x )
a
kdx
.
2
 a( b  x )
a
kdx
.
2
(
b
a  x)
e. E  2
a
kdx
.
2
0 (b  x )
ANS: a
66. The electric field at a point on the surface of a conductor can never be
a. perpendicular to the surface.
b. tangent to the surface.
c. zero.
d. proportional to the charge density on the surface.
e. greatest where the radius of curvature of the surface is greatest.
364
Electric Forces and Electric Fields
ANS: b
67. A plane of charge of surface charge density  lies in the y-z plane. The x-axis has its origin at the center of
the plane. The correct graph of the electric field E x versus distance x from the plane is
Ex
Ex
Ex
x
x
a.
x
b.
Ex
c.
Ex
x
x
d.
e.
ANS: c
68. A radial line is drawn outward from the center of a solid conducting sphere. Charge +Q is placed on the
sphere. The correct graph of E r versus r is
Er
Er
Er
r
r
a.
b.
Er
Er
r
d.
ANS: e
r
e.
r
c.
Chapter 19
365
69. A solid uncharged conducting sphere of radius a is supported by insulators at the center of a conducting
spherical shell of inner radius b and outer radius c. Charge +Q is placed on the spherical shell. The value of
the electric field at r such that a  r  b is
a. 0.
k eQ
b.
.
r2
k eQ
c.
.
r2
 k eQ a2
d.
 .
r 2 b2
 k eQ a2
e.
 .
r 2 b2
ANS: a
70. A very large plane of charge with   4.0  10 9 C m 2 lies in the y-z plane. A single charge Q  3.0  10 7 C
lies on the +x axis at x  2.0 m . The electric field in N/C at x  5.0 m has the value
a. 104.
b. 226.
c. 300.
d. 526.
e. 752.
ANS: d
71. A very large plane of charge with   4.0  10 9 C m 2 lies in the y-z plane. A single charge Q  3.0  10 7 C
lies on the +x axis at x  2.0 m . The electric field in N/C at x  1.0 m has the value
a. +226.
b. +2400.
c. –2400.
d. –2700.
e. +2700.
ANS: c
72. Three small metal foil spheres hang next to each other in a row. Sphere B attracts spheres A and C. A
metal wire is brought into contact with A and C and is then removed. After the wire is taken away, we can
conclude that
A
B
C
a. A and C have negative charges.
b. A and C have positive charges.
c. A and C are both uncharged.
d. A and C are both uncharged or have charges of the same sign.
366
Electric Forces and Electric Fields
e. only that A and C are charged and have like charges.
ANS: d
Chapter 19
367
73. Object A has a charge of 2 C , object B has a charge of 6 C , and object C has a charge of 4 C . All
three objects are equidistant from one another. Which of the following statements about the magnitudes of
the electric forces the objects exert on one another is true? ( FAB means the force A exerts on B, etc.)
1
a. FAC  FAB
2
1
b. FAC  FCB
2
1
c. FAB  FBC
2
d. FBC  FCB
e. FAC  FCA
ANS: c
74. Five charges of 1.00 C magnitude are placed in a row, each charge being 1.00 m distant from its
neighboring charges. If charges 1, 2 and 3 are positive, while charges 4 and 5 are negative, what is the
magnitude, in N/C, of the electric field (from the other charges) at the position of charge 3?
1
2
3
4
5
a. 0
b. 2.25  10 2
c. 4.50  10 2
d. 2.25  10 4
e. 4.50  10 4
ANS: d
75. Five charges of 1.00 C magnitude are placed in a row, each charge being 1.00 m distant from its
neighboring charges. If all five charges are positive, what is the magnitude, in N/C, of the electric field (from
the other charges) at the position of charge 3?
1
2
a. 0
b. 2.25  10 2
c. 4.50  10 2
d. 2.25  10 4
e. 4.50  10 4
ANS: a
3
4
5
368
Electric Forces and Electric Fields
76. Five charges of 1.00 C magnitude are placed in a row, each charge being 1.00 m distant from its
neighboring charges. If all charges are positive, what is the magnitude, in N/C, of the electric field (from the
other charges) at the position of charge 4?
1
2
3
4
5
a. 0
b. 3.25  10 3
c. 2.13  10 2
d. 3.25  10 3
e. 2.13  10 4
ANS: d
77. An electron ( m  9.11  10 31 kg, q  1.60  10 19 C) arrives at point A with velocity v  2.0  103 m/s to the
right. What magnitude in N/C and direction are needed for a uniform electric field in the region between A
and B, if the electron is to arrive at point B, 2.0 m to the right of A, with a velocity of v  4.0  103 m/s?
a. 5.70  10 6 N/C, to the left
b. 5.70  10 6 N/C, to the right
c. 1.7  10 5 N/C, to the left
d. 1.7  10 5 N/C, to the right
e. 2.3  10 5 N/C, to the left
ANS: c
78. The electric flux through the sides of a cube is zero. The flux through the top is +20 N/C, while the flux
through the bottom is zero. Each plane of charge shown has   17.7  10 11 C m 2 . Which diagram shows the
planes of charge which give rise to the flux in this situation?




b.
 =0

d.
ANS: d
c.




a.
e.
Chapter 19
369
79. Each plane of charge has charge density . In which case is the electric flux through the top and bottom of
the cube, seen here in side view, not equal in magnitude?






a.


b.





d.

c.

e.
ANS: d
80. All but one of the statements below describe a property of a conductor in electrostatic equilibrium. Which
statement is not correct?
a. The electrostatic field is zero everywhere inside the conductor.
b. A net charge on an isolated conductor resides entirely on its surface.
c. The electric field just outside any point on the surface of a charged conductor is perpendicular to the
surface of the conductor.
d. The surface charge density is highest at locations where the radius of curvature is large, e.g., where the
surface is essentially flat.
e. The magnitude of the electric field at a point just outside a charged conductor is
charge density at that point.
ANS: d

, where  is the surface
0