Chapter 19 Magnetic Forces and Fields
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3. The magnetism of the Earth acts approximately as if it originates from a huge bar magnet within the Earth.
Which of the following statements are true?
A. The north magnetic pole of the Earth is located at the Earth's North Pole.
B. The south magnetic pole of the Earth is located at the Earth's South Pole.
C. The Earth's magnetic field is vertical at the equator.
D. The Earths north magnetic pole is magnetically a south pole.
E. None of these statements is true.
4. The magnitude of the magnetic force on a charge depends on:
A. the velocity component perpendicular to the magnetic field.
B. the magnitude of the charge.
C. the magnitude of the magnetic field.
D. the sign of the charge.
E. three of these items.
6. In diagrams of magnetic fields, the symbol "×" is used to represent:
A. the region of no magnetic field.
B. the north magnetic field.
C. the magnetic field coming out of the paper.
D. the magnetic field going into the paper.
E. the magnetic field.
7. A proton is moving at 2.0 × 107 m/s in a magnetic field of 30 mT. What is the magnetic force on the proton?
A. 9.6 × 10-14 N
B. 4.8 × 10-14 N
C. 0.0
D. 6.0 × 10-5 N
E. More information is needed.
9. An electron is moving at 3.0 × 106 m/s at an angle of 40° to a 0.80 T magnetic field. What is the magnitude of
the force on the electron?
A. 4.8 × 10-13 N
B. 3.8 × 10-13 N
C. 2.5 × 10-13 N
D. 2.9 × 10-13 N
E. 1.2 × 10-13 N
11. A proton is moving perpendicular to a magnetic field at 3.0 × 106 m/s and experiences a magnetic force of
magnitude 2.5 × 10-16 N. What is the magnitude of the magnetic field?
A. 0.52 T
B. 3.8 × 10-2 T
C. 1.9 × 10-3 T
D. 2.6 × 10-4 T
E. 5.2 × 10-4 T
13. An electron is moving at 2.5 × 106 m/s perpendicular to a 3.5 mT magnetic field. What is the radius of its
path?
A. 5.1 cm
B. 3.9 cm
C. 1.6 cm
D. 4.1 mm
E. 2.0 mm
14. An electron is moving at 3.6 × 106 m/s perpendicular to a uniform magnetic field. If the radius of the motion
is 18 mm, what is the magnitude of the magnetic field?
A. 70 mT
B. 1.14 mT
C. 9.3 mT
D. 1.6 μT
E. 8.9 nT
15. If a charged particle is moving through a magnetic field with its velocity at an angle of 45 degrees with
respect to the field, what happens to the magnetic force on it if both the speed and magnetic field are doubled?
A. It stays the same.
B. It doubles.
C. It quadruples.
D. It halves.
E. It increases by a factor of 5.66.
18. Doubling the magnetic field in a proton synchrotron changes the maximum kinetic energy of the protons by
what amount?
A. It is quadrupled.
B. It is doubled.
C. It remains the same.
D. It is halved.
E. It is quartered.
19. What is the speed of a proton in a cyclotron having a 0.50 T magnetic field when the proton is at a distance
of 25 cm from the center?
A. 3.7 × 107 m/s
B. 1.2 × 107 m/s
C. 6.0 × 106 m/s
D. 3.2 × 106 m/s
E. 1.1 × 104 m/s
20. A cyclotron with a magnetic field of 1.2 T is used to accelerate protons. How long does it take for the
protons to make one complete trip around the cyclotron at a radius of 25 cm?
A. 11 ns
B. 28 ns
C. 32 ns
D. 48 ns
E. 55 ns
22. An electric field perpendicular to a magnetic field is used as a velocity selector for Li+ ions, selecting the
velocity v1. Next, Li++ ions are sent through the same velocity selector, selecting velocity v2. If neither the
electric nor magnetic fields have changed between the two processes, how do v1 and v2 compare?
A. v2 = v1
B. v2 = 2 v1
C. v2 = v1/2
D. v2 = 4 v1
E. v2 = v1/4
24. A velocity selector for ions uses perpendicular electric and magnetic fields. If the electric field used were
800 V/m, what magnetic field would allow the selection of a velocity of 4000 m/s?
A. 0.0400 T
B. 0.0800 T
C. 0.160 T
D. 0.200 T
E. 0.500 T
26. In a velocity selector for ions, when the velocity of the ions is double the value for which the selector is set,
how do the magnetic force FM and the electric force FE compare?
A. FM = 4 FE
B. FM = 2 FE
C. FM = FE/2
D. FM = FE/4
E. FM = 1.414 FE
31. A long straight wire is carrying a current of 2.4 A in a region where the magnetic field is perpendicular to
the wire. If the magnetic field is 0.52 mT, what is the magnitude of the force on 3.0 m of the wire?
A. 1.2 × 10-3 N
B. 3.7 × 10-3 N
C. 7.2 × 10-3 N
D. 3.7 N
E. 7.2 N
32. A long straight wire is carrying a current of 2.4 A in a region where the magnetic field is perpendicular to
the wire. If the magnetic field is 0.52 mT and makes a 60° angle with the wire, what is the magnitude of the
force on 3.0 m of the wire?
A. 3.2 N
B. 6.5 N
C. 1.6 × 10-3 N
D. 3.2 × 10-3 N
E. 6.5 × 10-3 N
33. A 3.0 cm section of a horizontal wire carrying a 6.7 A current from east to west is place in a north to south
0.42 T magnetic field in the lab. In what direction is the magnetic force on the 3.0 cm section?
A. north
B. south
C. up
D. down
E. southwest
34. A 3.0 cm section of a horizontal wire carrying a 6.7 A current from east to west is placed in a north to south
0.42 T magnetic field in the lab. What is the magnitude of the force on the 3.0 cm section of the wire?
A. 0.084 N
B. 0.17 N
C. 0.25 N
D. 0.0017 N
E. 0.0025 N
36. A 300 turn coil of area 25 cm2 is placed in an 80 mT magnetic field. If the coil carries a current of 3.3 A,
what is the magnitude of the maximum magnetic torque it could experience?
A. 20 N·m
B. 6.0 N·m
C. 0.13 N·m
D. 0.20 N·m
E. 0.060 N·m
37. A 300 turn coil of area 25 cm2 is placed in an 80 mT magnetic field. If the coil carries a current of 3.3 A and
if the normal to the area of the coil makes an angle of 20° with the direction of the magnetic field, what is the
magnitude of the torque on the coil?
A. 0.20 N·m
B. 0.19 N·m
C. 0.072 N·m
D. 0.068 N·m
E. 0.039 N·m
38. Doubling the current, number of turns, the area, and the ambient magnetic field for a coil changes the
maximum torque it experiences by what factor?
A. 2
B. 4
C. 8
D. 16
E. 32
39. A straight wire is carrying a current upward. Observed from above (i.e., looking downward toward the
wire), the magnetic field lines are:
A. radially outward.
B. radially inward.
C. forming clockwise circles.
D. forming counter-clockwise circles.
E. directed toward the observer.
40. If the magnetic field from a current-carrying long straight wire has a magnitude B0 at a distance d, what is
the magnitude of the field at a distance 2d?
A. also B0
B. 0.707 B0
C. 0.693 B0
D. B0/2
E. B0/4
41. A power line carries 1000 A at a height of 20 m above the ground. What is the resulting magnetic field at
ground level?
A. 50 mT
B. 0.13 mT
C. 13 μT
D. 10 μT
E. 5.0 μT
42. Two parallel wires are each carrying 10 A and are separated by 4.0 m. If the currents are in opposite
directions, what is the magnitude of the magnetic field halfway between them?
A. 0.0
B. 1.0 μT
C. 2.0 μT
D. 4.0 μT
E. 1.4 μT
43. Two parallel wires run in a north-south direction. The eastern wire carries 15.0 A southward while the
western wire carries 6.0 A northward. If the wires are separated by 30 cm, what is the magnetic field magnitude
at a point between the wires at a distance of 10 cm from the western wire?
A. 3.0 μT
B. 6.0 μT
C. 15 μT
D. 27 μT
E. 90 μT
44. Find the circulation for the path shown.
A. -(4 A)μ0
B. (4 A)μ0
C. (28 A)μ0
D. -(28 A)μ0
E. (7 A)μ0
45. Find the circulation for the path shown.
A. -(4 A)μ0
B. (4 A)μ0
C. -(28 A)μ0
D. (28 A)μ0
E. -(7 A)μ0
47. The magnetic field inside a solenoid having 100 turns per cm at a current of 5.0 A is:
A. 2.0 × 10-2 T.
B. 1.0 × 10-2 T.
C. 1.0 × 10-4 T.
D. 6.3 × 10-4 T.
E. 6.3 × 10-2 T.
52. One wire, lying on the x-axis, carries a current of 8.0 A in the positive x-direction. Another wire, lying on
the y-axis, carries a current of 12 A in the positive y-direction. What is the magnitude of the magnetic field at
(x, y) = (8.0 cm, 12.0 cm)?
A. 1.3 × 10-5 T
B. 3.0 × 10-5 T
C. 1.7 × 10-5 T
D. 4.3 × 10-5 T
E. 4.0 × 10-10 T