Practice Midterm on Magnetism
1) An electron is moving at 3.0 x 106 m/s at an angle of 40o to a 0.80-T magnetic field. What is
the magnitude of the acceleration of the electron?
a) 2.7 x 1017 m/s2
b) 1.4 x 1017 m/s2
c) 5.2 x 1017 m/s2
d) 7.8 x 1017 m/s2
e) 9.8 x 1017 m/s2
Ans: a
2) A proton is moving at 2.0 x 107 m/s in a magnetic field of 30 mT. What is the magnetic force
on the proton.
a) 9.6 x 10-14 N
b) 4.8 x 10-14 N
c) 0
d) 6.0 x 10-5 N
e) More information is needed.
Ans: e
3) 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.
Ans: a
4) 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
Ans: a
5) 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
Ans: d
6) 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 60o 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 x 10-3 N
d) 3.2 x 10-3 N
e) 6.5 x 10-3 N
Ans: d
7) 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
Ans: d
8) 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
b) 1.0 T
c) 2.0 T
d) 4.0 T
e) 1.4 T
Ans: c
9) What current is necessary to generate a field of 0.20 T inside a solenoid of 300 turns per cm?
a) 530 A
b) 5.3 A
c) 33 A
d) 0.33 A
e) 3.1 A
Ans: b
10) Two long, parallel wires carry currents of different magnitudes. If the amount of current in
each wire is halved, what happens to the magnitude of the force between the wires?
A) It is doubled.
B) It is quadrupled.
C) It is reduced by a factor of 2.
D) It is reduced by a factor of 4.
Answer: D
11) A bar magnet is placed with its north pole pointing toward a coil of wire that has a crosssectional area of 0.02 m2 and 6 turns, as shown in Figure above (a). What is the magnitude of the
induced emf in the coil of wire?
A) 0.04 V
B) 0.01 V
C) 0.02 V
D) 0 V
Answer: D
12) A conducting rod with a length of 28.0 cm is placed on a U-shaped metal wire that includes
a light bulb with a resistance of 60.0 Ω as shown in Figure above. A constant magnetic field with
a strength of 0.700 T is applied perpendicular into the paper. An applied force moves the rod to
the right with a constant speed of 6.00 m/s. What is the power consumed by the light bulb?
A) 4.00 mW
B) 23.1 mW
C) 7.57 mW
D) 25.4 mW
Answer: B
13) The primary coil of a transformer has 100 turns and its secondary coil has 400 turns. If the
ac current in the secondary coil is 2 A, what is the current in its primary coil?
A) 2 A
B) 8 A
C) 1/2 A
D) 4 A
Answer: B
14) A rectangular coil with N turns, length L, and width w, as shown in Figure above, is rotating
in a magnetic field B with an angular frequency ω. If the area of the coil is A, what is the induced
emf in the coil?
A) ε = NB (Lw) ω sin ωt
B) ε = NBA ω sin ωt
C) ε = NBA (2πf) sin 2πft
D) All of these answers are correct.
Answer: D
15) Find the circulation for the path shown.
a) –(4 A)0
b) (4 A)0
c) –(28 A)0
d) (28 A)0
e) –(7 A)0
Ans: e
16) 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 T
d) 10 T
e) 5.0 T
Ans: d
17) A cyclotron with a magnetic field of 1.2 T is used to accelerate protons. How does the time
for the protons to complete one orbit change when the radius of their orbit changes from 10 cm
to 22 cm?
a) It increases by a factor of 2.2.
b) It decreases by a factor of 2.2.
c) It increases by a factor of 4.8.
d) It decreases by a factor of 4.8.
e) It stays the same.
Ans: e
18) A mass spectrometer splits a beam of 10B and 11B singly charged ions having the same
velocity by passing the beam through a uniform magnetic field. If the radius of the path for the
10
B ions is 12.0 cm, what will be the radius for the 11B ions?
a) 9.9 cm
b) 10.9 cm
c) 12.0 cm
d) 13.2 cm
e) 14.5 cm
Ans: d
19) A rectangular coil lies flat on a horizontal surface. A bar magnet is held above the center of
the coil with its south pole pointing down. If the magnet is dropped from this position, what is
the direction of the induced current in the coil?
A) clockwise
B) counterclockwise
C) There is no current in the coil.
D) None of the other answers is correct.
Answer: A
20) The magnetic field and the number of turns in a coil of wire are doubled. Assuming that all
the other parameters remain the same, what happens to the induced emf in that coil of wire?
A) It is doubled.
B) It is reduced by a factor of 2.
C) It is quadrupled.
D) It stays the same.
Answer: C
21) The area of a rectangular loop of wire is 3.4 × 10-3 m2. The loop is placed in a magnetic field
that changes from 0.20 T to 1.2 T in 1.6 s. The plane of the loop is perpendicular to the direction
of the magnetic field. What is the magnitude of the induced emf in that loop?
A) 2.8 × 10-3 V
B) 2.1 × 10-3 V
C) 0 V
D) 1.8 × 10-3 V
Answer: B
22) A constant magnetic field of 0.70 T is applied to a rectangular loop of area 3.4 × 10-3 m2. If
the area of this loop changes from its original value to a new value of 1.6 × 10-3 m2 in 1.6 s, what
is the emf induced in the loop?
A) 1.6 × 10-2 V
B) 0 V
C) 7.5 × 10-2 V
D) 7.9 × 10-4 V
Answer: D
23) Which one of the following is the correct expression for the time constant of an RL circuit?
A) L/R
B) R/L
C) LR
D) None of the other answers is correct.
Answer: A
24) A 1.50-H inductor is connected in series with a 200-Ω resistor through a 15.0-V dc power
supply and a switch. If the switch is closed at t = 0 s, what is the energy stored by the inductor
20.0 ms after the switch is closed?
A) 0.910 mJ
B) 1.83 mJ
C) 3.65 mJ
D) 7.31 mJ
Answer: C
25) The primary coil of a transformer has 100 turns and its secondary coil has 400 turns, if the
ac voltage applied to the primary coil is 120 V, what voltage is present in its secondary coil?
A) 100 V
B) 30 V
C) 480 V
D) 400 V
Answer: C
26) Which one of the following is the correct unit for the time constant of an RL circuit?
A) H/Ω
B) (Vs)/(AΩ)
C) s
D) All of the other answers are correct.
Answer: D