Physics 124B
Practice exam #2
Chapters 20-21
Your 25 problems will be selected from the following 60 problems.


1.
Which one of the following situations results in a conventional electric current that flows westward?
A.
a beam of protons moves eastward
B.
an electric dipole moves westward
C.
a beam of electrons moves westward
D.
a beam of electrons moves eastward
E.
a beam of neutral atoms moves westward


2.
How many electrons flow through a battery that delivers a current of 3.0 A for 12 s?
A.
4
B.
36
C.
4.8 × 1015
D.
6.4 × 1018
E.
2.2 × 1020


3.
A 10-A current is maintained in a simple circuit with a total resistance of 200 ?. What net charge passes through any point in the circuit during a 1-minute interval?
A.
200 C
B.
400 C
C.
500 C
D.
600 C
E.
1200 C


4.
Which one of the following combinations of units is equivalent to the ohm?
A.
V/C
B.
A/J
C.
J/s
D.
J•s/C2
E.
W/A


5.
The potential difference across the ends of a wire is doubled in magnitude. If Ohm's law is obeyed, which one of the following statements concerning the resistance of the wire is true?
A.
The resistance is one half of its original value.
B.
The resistance is twice its original value.
C.
The resistance is not changed.
D.
The resistance increases by a factor of four.
E.
The resistance decreases by a factor of four.


6.
Which one of the following circuits has the largest resistance?


7.
When a light bulb is connected to a 4.5 V battery, a current of 0.16 A passes through the bulb filament. What is the resistance of the filament?
A.
440 ?
B.
28 ?
C.
9.3 ?
D.
1.4 ?
E.
0.72 ?


8.
Which one of the following statements concerning resistance is true?
A.
The resistance of a semiconductor increases with temperature.
B.
Resistance is a property of resistors, but not conductors.
C.
The resistance of a metal wire changes with temperature.
D.
The resistance is the same for all samples of the same material.
E.
The resistance of a wire is inversely proportional to the length of the wire.


9.
Determine the length of a copper wire that has a resistance of 0.172 ? and cross-sectional area of 1 × 10–4 m2. The resistivity of copper is 1.72 × 10–8 ? • m.
A.
0.1 m
B.
10 m
C.
100 m
D.
1000 m
E.
10 000 m


10.
Complete the following statement: The unit kilowatt • hour measures
A.
current.
B.
energy.
C.
power.
D.
potential drop.
E.
voltage.


11.
Which one of the following quantities can be converted to kilowatt • hours (kWh)?
A.
2.0 A
B.
8.3 V
C.
5.8 J
D.
9.6 W
E.
6.2 C/V


12.
A 4-A current is maintained in a simple circuit with a total resistance of 2 ?. How much energy is dissipated in 3 seconds?
A.
3 J
B.
6 J
C.
12 J
D.
24 J
E.
96 J


13.
A 40-W and a 60-W light bulb are designed for use with the same voltage. What is the ratio of the resistance of the 60-W bulb to the resistance of the 40-W bulb?
A.
1.5
B.
0.67
C.
2.3
D.
0.44
E.
3.0


14.
A 5-A current is maintained in a simple circuit that consists of a resistor between the terminals of an ideal battery. If the battery supplies energy at a rate of 20 W, how large is the resistance?
A.
0.4 ?
B.
0.8 ?
C.
2 ?
D.
4 ?
E.
8 ?


15.
A resistor dissipates 1.5 W when it is connected to a battery with a potential difference of 12 V. What is the resistance of the resistor?
A.
0.13 ?
B.
220 ?
C.
18 ?
D.
8.0 ?
E.
96 ?


16.
Which one of the following statements concerning resistors in series is true?
A.
The voltage across each resistor is the same.
B.
The current through each resistor is the same.
C.
The power dissipated by each resistor is the same.
D.
The rate at which charge flows through each resistor depends on its resistance.
E.
The total current through the resistors is the sum of the current through each resistor.


17.
Two wires, A and B, and a variable resistor, R, are connected in series to a battery. Which one of the following results will occur if the resistance of R is increased?
A.
The current through A and B will increase.
B.
The voltage across A and B will increase.
C.
The voltage across the entire circuit will increase.
D.
The power used by the entire circuit will increase.
E.
The current through the entire circuit will decrease.


18.
Three resistors, 50-?, 100-?, 200-?, are connected in series in a circuit. What is the equivalent resistance of this combination of resistors?
A.
350 ?
B.
250 ?
C.
200 ?
D.
120 ?
E.
29 ?


19.
A 4.5-V battery is connected to two resistors connected in series as shown in the drawing. Determine the total power dissipated in the resistors.
A.
0.033 W
B.
0.090 W
C.
0.15 W
D.
0.60 W
E.
4.7 W


20.
Two 15-? and three 25-? light bulbs and a 24 V battery are connected in a series circuit. What is the current that passes through each bulb?
A.
0.23 A
B.
0.51 A
C.
0.96 A
D.
1.6 A
E.
The current will be 1.6 A in the 15-? bulbs and 0.96 A in the 25-? bulbs.


21.
Two resistors are arranged in a circuit that carries a total current of 15 A as shown in the figure. Which one of the entries in the following table is correct?


A.
A
B.
B
C.
C
D.
D
E.
E


22.
What is the total power dissipated in the two resistors in the circuit shown?
A.
10 W
B.
15 W
C.
33 W
D.
67 W
E.
670 W


23.
Three resistors, 6.0-?, 9.0-?, 15-?, are connected in parallel in a circuit. What is the equivalent resistance of this combination of resistors?
A.
30 ?
B.
10 ?
C.
3.8 ?
D.
2.9 ?
E.
0.34 ?


24.
What is the equivalent capacitance of the combination of capacitors shown in the circuit?
A.
0.37 µF
B.
3.3 µF
C.
4.6 µF
D.
0.67 µF
E.
2.1 µF


25.
Five resistors are connected as shown. What is the equivalent resistance between points A and B?
A.
6.8 ?
B.
9.2 ?
C.
3.4 ?
D.
2.1 ?
E.
16 ?


26.
Three parallel plate capacitors, each having a capacitance of 1.0 µF are connected in parallel. The potential difference across the combination is 100 V. What is the equivalent capacitance of this combination?
A.
0.3 ?F
B.
1 ?F
C.
3 ?F
D.
6 ?F
E.
30 ?F


27.
How much energy is stored in the combination of capacitors shown?
A.
0.01 J
B.
0.02 J
C.
0.03 J
D.
0.04 J
E.
0.05 J


28.
Which one of the following statements concerning permanent magnets is false?
A.
The north pole of a permanent magnet is attracted to a south pole.
B.
All permanent magnets are surrounded by a magnetic field.
C.
The direction of a magnetic field is indicated by the north pole of a compass.
D.
Magnetic field lines outside a permanent magnet originate from the north pole and end on the south pole.
E.
When a permanent magnet is cut in half, one piece will be a north pole and one piece will be a south pole.


29.
Which combination of units can be used to express the magnetic field?
A.
kg • m2/C
B.
kg • s/C2
C.
N • m2/C
D.
kg/(C • s)
E.
kg • m/(C • s2)


30.
Which one of the following statements concerning the magnetic force on a charged particle in a magnetic field is true?
A.
It is a maximum if the particle is stationary.
B.
It is zero if the particle moves perpendicular to the field.
C.
It is a maximum if the particle moves parallel to the field.
D.
It acts in the direction of motion for a positively charged particle.
E.
It depends on the component of the particle's velocity that is perpendicular to the field.


31.
Complete the following statement: The magnitude of the magnetic force that acts on a charged particle in a magnetic field is independent of
A.
the sign of the charge.
B.
the magnitude of the charge.
C.
the magnitude of the magnetic field.
D.
the direction of motion of the particle.
E.
the velocity components of the particle.


32.
A proton traveling due east in a region that contains only a magnetic field experiences a vertically upward force away from the surface of the earth. What is the direction of the magnetic field?
A.
north
B.
east
C.
south
D.
west
E.
down


33.
A charged particle is fired from a gun with a velocity of 5.2 × 104 m/s at an angle of 35° with respect to a 0.0045-T magnetic field. If the magnetic field exerts a force of 0.0026 N on the particle, determine the magnitude of the charge.
A.
11 µC
B.
15 µC
C.
19 µC
D.
23 µC
E.
27 µC


34.
An electron traveling due north enters a region that contains a uniform magnetic field that points due east. In which direction will the electron be deflected?
A.
east
B.
west
C.
down
D.
up
E.
south


35.
Two electrons are located in a region of space where the magnetic field is zero. Electron A is at rest; and electron B is moving westward with a constant velocity. A non-zero magnetic field directed eastward is then applied to the region. In what direction, if any, will each electron be moving after the field is applied?
 
electron A
electron B
A.
 
at rest
 
westward
 
B.
 
northward
 
eastward
 
C.
 
at rest
 
eastward
 
D.
 
southward
 
downward, toward the earth
 
E.
 
upward, away from earth
 
westward
 


36.
An electron is moving with a speed of 3.5 × 105 m/s when it encounters a magnetic field of 0.60 T. The direction of the magnetic field makes an angle of 60.0° with respect to the velocity of the electron. What is the magnitude of the magnetic force on the electron?
A.
4.9 × 10–13 N
B.
3.2 × 10–13 N
C.
1.7 × 10–13 N
D.
3.4 × 10–14 N
E.
2.9 × 10–14 N


37.
A proton traveling due north enters a region that contains both a magnetic field and an electric field. The electric field lines point due west. It is observed that the proton continues to travel in a straight line due north. In which direction must the magnetic field lines point?
A.
up
B.
down
C.
east
D.
west
E.
south


38.
An electron traveling horizontally enters a region where a uniform magnetic field is directed into the plane of the paper as shown. Which one of the following phrases most accurately describes the motion of the electron once it has entered the field?
A.
upward and parabolic
B.
upward and circular
C.
downward and circular
D.
upward, along a straight line
E.
downward and parabolic


39.
An electron enters a region that contains a magnetic field directed into the page as shown. The velocity vector of the electron makes an angle of 30° with the +y axis. What is the direction of the magnetic force on the electron when it enters the field?
A.
up, out of the page
B.
at an angle of 30° below the positive x axis
C.
at an angle of 30° above the positive x axis
D.
at an angle of 60° below the positive x axis
E.
at an angle of 60° above the positive x axis


40.
Two charged particles of equal mass are traveling in circular orbits in a region of uniform, constant magnetic field as shown. The particles are observed to move in circular paths of radii R1 and R2 with speeds v1 and v2, respectively.

As the figure shows, the path of particle 2 has a smaller radius than that of particle 1. Which one of the following statements about this system is false?
A.
|v1/Q1| < |v2/Q2|
B.
Particle 2 carries a positive charge.
C.
Particle 1 carries a negative charge.
D.
Neither particle gains energy from the magnetic field.
E.
The particle velocities have no components parallel to the magnetic field.


A beam consisting of five types of ions labeled A, B, C, D, and E enters a region that contains a uniform magnetic field as shown in the figure below. The field is perpendicular to the plane of the paper, but its precise direction is not given. All ions in the beam travel with the same speed. The table below gives the masses and charges of the ions. Note: 1 mass unit = 1.67 × 10–27 kg and e = 1.6 × 10–19 C
Reference: Ref 21-1

41.
Which ion falls at position 2?
A.
A
B.
B
C.
C
D.
D
E.
E


A beam consisting of five types of ions labeled A, B, C, D, and E enters a region that contains a uniform magnetic field as shown in the figure below. The field is perpendicular to the plane of the paper, but its precise direction is not given. All ions in the beam travel with the same speed. The table below gives the masses and charges of the ions. Note: 1 mass unit = 1.67 × 10–27 kg and e = 1.6 × 10–19 C
Reference: Ref 21-1

42.
What is the direction of the magnetic field?
A.
toward the right
B.
toward the left
C.
into the page
D.
out of the page
E.
toward the bottom of the page


A beam consisting of five types of ions labeled A, B, C, D, and E enters a region that contains a uniform magnetic field as shown in the figure below. The field is perpendicular to the plane of the paper, but its precise direction is not given. All ions in the beam travel with the same speed. The table below gives the masses and charges of the ions. Note: 1 mass unit = 1.67 × 10–27 kg and e = 1.6 × 10–19 C
Reference: Ref 21-1

43.
Determine the magnitude of the magnetic field if ion A travels in a semicircular path of radius 0.50 m at a speed of 5.0 × 106 m/s.
A.
1.0 T
B.
0.84 T
C.
0.42 T
D.
0.21 T
E.
0.11 T


44.
A 0.150-m wire oriented horizontally between the poles of an electromagnet carries a direct current of 12.5 A. The angle between the direction of the current and that of the magnetic field is 25.0°. If the magnetic field strength is 0.625 T, what is the magnitude and direction of the magnetic force on the wire between the poles?
A.
1.17 N, upward
B.
3.30 N, downward
C.
0.792 N, upward
D.
1.44 N, downward
E.
0.495 N, upward


45.
A long, straight wire carries a 6.0-A current that is directed in the positive x direction. When a uniform magnetic field is applied perpendicular to a 3.0-m segment of the wire, the magnetic force on the segment is 0.36 N, directed in the negative y direction, as shown. What are the magnitude and direction of the magnetic field?
A.
0.020 T, out of the paper
B.
0.020 T, into the paper
C.
0.060 T, out of the paper
D.
0.060 T, into the paper
E.
0.65 T, out of the paper


46.
A loop of wire with a weight of 1.47 N is oriented vertically and carries a current I = 1.75 A. A segment of the wire passes through a magnetic field directed into the plane of the page as shown. The net force on the wire is measured using a balance and found to be zero. What is the magnitude of the magnetic field?
A.
zero tesla
B.
0.51 T
C.
0.84 T
D.
1.5 T
E.
4.2 T


A long straight vertical segment of wire traverses a magnetic field of magnitude 2.0 T in the direction shown in the diagram. The length of the wire that lies in the magnetic field is 0.060 m. When the switch is closed, a current of 4.0 A flows through the wire from point P to point Q.
Reference: Ref 21-2

47.
Which one of the following statements concerning the effect of the magnetic force on the wire is true?
A.
The wire will be pushed to the left.
B.
The wire will be pushed to the right.
C.
The wire will have no net force acting on it.
D.
The wire will be pushed downward, into the plane of the paper.
E.
The wire will be pushed upward, out of the plane of the paper.


A long straight vertical segment of wire traverses a magnetic field of magnitude 2.0 T in the direction shown in the diagram. The length of the wire that lies in the magnetic field is 0.060 m. When the switch is closed, a current of 4.0 A flows through the wire from point P to point Q.
Reference: Ref 21-2

48.
What is the magnitude of the magnetic force acting on the wire?
A.
0.12 N
B.
0.24 N
C.
0.48 N
D.
67 N
E.
zero newtons


49.
A current-carrying, rectangular coil of wire is placed in a magnetic field. The magnitude of the torque on the coil is not dependent upon which one of the following quantities?
A.
the magnitude of the current in the loop
B.
the direction of the current in the loop
C.
the length of the sides of the loop
D.
the area of the loop
E.
the orientation of the loop


50.
Which one of the following statements concerning the magnetic field well inside a long, current-carrying solenoid is true?
A.
The magnetic field is zero.
B.
The magnetic field is non-zero and nearly uniform.
C.
The magnetic field is independent of the number of windings.
D.
The magnetic field is independent of the current in the solenoid.
E.
The magnetic field varies as 1/r as measured from the solenoid axis.


51.
Complete the following statement: The magnetic field around a current-carrying, circular loop is most like that of
A.
the earth.
B.
a short bar magnet.
C.
a current-carrying, rectangular loop.
D.
a long straight wire that carries a current.
E.
two long straight wires that carry currents in opposite directions.


52.
Two loops carry equal currents I in the same direction. They are held in the positions shown in the figure and project above and below the plane of the paper. The point P lies exactly halfway between them on the line that joins their centers. The centers of the loops and the point P lie in the plane of the paper. Which one of the figures below shows the position of a compass needle if the compass were placed in the plane of the paper at P?



A long, straight wire is carrying a current of 5.0 A in the direction shown in the figure. The point P is 0.040 m from the wire.

Reference: Ref 21-3

53.
What is the direction of the magnetic field at point P due to the current in the wire?
A.
to the right of page
B.
to the left of the page
C.
toward the bottom of the page
D.
into the plane of the page
E.
out of the plane of the page


A long, straight wire is carrying a current of 5.0 A in the direction shown in the figure. The point P is 0.040 m from the wire.

Reference: Ref 21-3

54.
What is the magnitude of the magnetic field at the point P?
A.
1.3 × 10–5 T
B.
1.9 × 10–5 T
C.
2.5 × 10–5 T
D.
7.9 × 10–5 T
E.
9.4 × 10–5 T


Two long, straight, parallel wires separated by a distance d carry currents in opposite directions as shown in the figure. The bottom wire carries a current of 6.0 A. Point C is at the midpoint between the wires and point O is a distance 0.50d below the 6-A wire as suggested in the figure. The total magnetic field at point O is zero tesla.
Reference: Ref 21-5

55.
Determine the value of the current, I, in the top wire.
A.
2 A
B.
3 A
C.
6 A
D.
18 A
E.
This cannot be determined since the value of d is not specified


Two long, straight, parallel wires separated by a distance d carry currents in opposite directions as shown in the figure. The bottom wire carries a current of 6.0 A. Point C is at the midpoint between the wires and point O is a distance 0.50d below the 6-A wire as suggested in the figure. The total magnetic field at point O is zero tesla.
Reference: Ref 21-5

56.
Determine the magnitude of the magnetic field at point C if d = 0.10 m.
A.
2.4 × 10–5 T
B.
4.8 × 10–5 T
C.
9.6 × 10–5 T
D.
1.1 × 10–4 T
E.
1.4 × 10–4 T


A long, coaxial cable, shown in cross-section in the drawing, is made using two conductors that share a common central axis, labeled C. The conductors are separated by an electrically insulating material that is also used as the outer cover of the cable. The current in the inner conductor is 2.0 A directed into the page and that in the outer conductor is 2.5 A directed out of the page. The distance from point C to point A is 0.0015 m; and the distance from C to B is 0.0030 m. The radii a and b of the conductors are 6.0 × 10–4 m and 1.9 × 10–3 m, respectively.
Reference: Ref 21-7

57.
What is the magnitude and direction of the magnetic field at point A?
A.
3.3 × 10–5 T, clockwise
B.
3.3 × 10–5 T, counterclockwise
C.
6.8 × 10–5 T, clockwise
D.
6.8 × 10–5 T, counterclockwise
E.
2.7 × 10–4 T, clockwise


A long, coaxial cable, shown in cross-section in the drawing, is made using two conductors that share a common central axis, labeled C. The conductors are separated by an electrically insulating material that is also used as the outer cover of the cable. The current in the inner conductor is 2.0 A directed into the page and that in the outer conductor is 2.5 A directed out of the page. The distance from point C to point A is 0.0015 m; and the distance from C to B is 0.0030 m. The radii a and b of the conductors are 6.0 × 10–4 m and 1.9 × 10–3 m, respectively.
Reference: Ref 21-7

58.
What is the magnitude and direction of the magnetic field at point B?
A.
3.3 × 10–5 T, clockwise
B.
3.3 × 10–5 T, counterclockwise
C.
6.8 × 10–5 T, clockwise
D.
6.8 × 10–5 T, counterclockwise
E.
2.7 × 10–4 T, clockwise


A wire is bent into the shape of a circle of radius r = 0.10 m and carries a 20.0-A current in the direction shown.
Reference: Ref 21-8

59.
What is the direction of the magnetic field at the center of the loop?
A.
to the right of the page
B.
to the left of the page
C.
toward the top of the page
D.
into the plane of the paper
E.
out of the plane of the paper


A wire is bent into the shape of a circle of radius r = 0.10 m and carries a 20.0-A current in the direction shown.
Reference: Ref 21-8

60.
What is the magnitude of the magnetic field at the center of the loop?
A.
2.0 × 10–5 T
B.
1.3 × 10–5 T
C.
2.0 × 10–4 T
D.
1.3 × 10–4 T
E.
zero tesla


Here are several solved problems. Your exam problem will be one of these with different values.


61.
Three resistors and two batteries are connected as shown in the circuit diagram. What is the magnitude of the current through the 12-V battery?


62.
Three resistors are connected in a circuit as shown. Using Kirchhoff's rules, determine the current in one of the 16-? resistors.


A long, straight wire carries a 10.0-A current in the +y direction as shown in the figure.

Next to the wire is a square copper loop that carries a 2.00-A current as shown. The length of each side of the square is 1.00 m.
Reference: Ref 21-9

63.
What is the magnitude of the net magnetic force that acts on the loop?


Answer Key

1. D
2. E
3. D
4. D
5. C
6.
E
7. B
8. C
9. D
10. B
11. C
12. E
13. B
14. B
15. E
16. B
17. E
18. A
19. C
20. A
21. C
22. B
23. D
24. D
25. A
26. C
27. C
28. E
29. D
30. E
31. A
32. A
33. C
34. C
35. A
36. E
37. A
38. C
39. B
40. A
41. B
42. C
43. D
44. E
45. A
46. E
47. D
48. C
49. B
50. B
51. B
52.
D
53. E
54. C
55. D
56. C
57. E
58. B
59. D
60. D
61.
0.52 A
62.
0.75 A
63.
1.7 × 10–5 N