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Electromagnetism
4.
Release 07/08
1.
A 520-turn circular coil of radius 0.26 m is initially outside a 0.56 T
magnetic field. The coil is moved into the magnetic field, inducing an
average emf of 47 V. How much time does it take to move the coil to
its new position?
A particle of mass m carrying a negative charge q passes through a
perpendicular magnetic field B at speed v, as shown. The radius of
its circular path while it is within the magnetic field is r. Which of
the following is equal to the speed, v, of the particle?
(a) 2.5 x 10-3 s
(c) 1.6 s
(a) qr/mB
(c) qB/m
2.
Two magnetic poles of unequal strength are placed as shown. The
north magnetic pole is stronger than the south magnetic pole. What
is the direction of the magnetic field at point P on a line midway
between the poles?
(a) 1
(c) 3
3.
(b) qBr/m
(d) m/qBr
(b) 2
(d) 4
Which of the following is correct for the back emf of an electric
motor operating at a constant speed?
(a) the back emf is zero
(b) the back emf is increasing
(c) the back emf is decreasing
(d) the back emf remains constant
(b) 1.3 s
(d) 2.6 s
5.
A small transformer plugged into a 120 V outlet produces a voltage
of 9.2 V and a current of 0.025 A to power a toy car. What is the
ratio of primary windings to secondary windings in this transformer?
(a) 1 : 13
(b) 1 : 4800
(c) 13 : 1
(d) 4800 : 1
6.
A magnet is passed through an aluminium ring as shown.
What is the direction of the induced current at point P on the ring
as the north pole approaches the ring on the right side, and as the
south pole leaves the ring on the left side?
Which of the four dots shows the new position of the electron
beam?
(a) 1
(b) 2
(c) 3
(d) 4
Release 06/07
7.
Which of the following situations induces the greatest current flow
in the coil? (All magnets and coils are identical).
10. A conductor is initially at rest in a magnetic field.
In which direction should the conductor be moved so that its top end
becomes positively charged?
8.
A proton travelling at 2.0 x 106 m/s enters a 0.43 T magnetic field.
Which of the paths shown in the diagram will the proton follow?
(Diagram is not to scale).
(a) 1
(c) 3
11.
(a) P1, radius = 2.6 x 10-5 m
(c) P3, radius = 0.049 m
9.
(b) P2, radius = 2.6 x 10-5 m
(d) P4, radius = 0.049 m
(b) 2
(d) 4
A 55-turn circular coil of wire of radius 0.15 m is placed in a 0.76 T
magnetic field as shown. The magnetic field changes to 0.34 T in the
opposite direction in a time of 0.25 s.
What is the average induced emf in the coil during the change in
magnetic field?
An electron beam strikes the centre of a CRT screen as shown.
A magnet is then placed near the CRT.
(a) 6.5 V
(b) 14 V
(c) 17 V
(d) 36 V
12. Two solenoids are placed close together as shown.
(a) halve the charge
(c) double the speed
(b) double the charge
(d) double the B field strength
15. Which of the following single loops has the largest magnetic flux?
As the switch is closed, what is the direction of the current through
the ammeter, and what is the direction of the induced magnetic field
inside the second solenoid?
16. What is the magnitude of the force due to a current of 0.58 A, on
the L-shaped conductor in a 0.35 T field?
Release 05/06
13. A bar magnet is free to rotate in a magnetic field. It is initially
positioned as shown.
Which diagram show the final orientation of the magnet?
(a) 6.1 x 10-2 N
(c) 1.0 x 10-1 N
14. A charged particle travels in a circular path of radius R while in a
uniform magnetic field. Which change reduces the radius to ½ R?
(b) 8.1 x 10-2 N
(d) 1.4 x 10-1 N
17. A single loop of wire encloses an area of 3.5 x 10-2 m2. It is in a
magnetic field that changes from +0.72 T to -0.25 T in a time of
0.060 s.
What is the induced emf and the direction of current flow in the
loop?
What is the direction of the magnetic field at P?
21.
A current of 4.3 A flows through a solenoid. The 620-turn solenoid
is 14 cm long and has a 3.5 cm diameter.
18. An AC transformer converts 120 V into 3.0 V. A small electronic
device draws 4.5 x 10-3 A from this transformer. If the secondary
has 50 turns, what is the number of turns and current in the primary
coil?
What are the direction and magnitude of the magnetic field inside
the solenoid?
Release 04/05
19.
In a step-down transformer, which of the following is greater in the
secondary than in the primary?
(a) power
(b) current
(c) voltage
(d) number of turns
20. Two identical bar magnets are placed as shown.
22. A conductor is moved to the right through four magnetic fields as
shown. In which case would the largest emf be induced?
(a) 3.5 x 10-2 m
(c) 5.9 x 10-2 m
(b) 5.1 x 10-2 m
(d) 6.2 x 10-2 m
August 04
25.
23.
A bar magnet is moved away from a coil as shown. What is the
direction of the current through the resistor and the polarity of the
left end of the coil?
What particles make up the beam in a cathode ray tube?
(a) atoms
(b) protons
(c) neutrons
(d) electrons
26. A coil of copper wire is wrapped around a compass as shown. Which
way will the compass needle point when current flows through the
coil?
(a) W
(c) Y
24.
A 200-turn coil has a 15.2 V potential difference induced in it when
the magnetic field changes from 0.42 T to 0.22 T in the opposite
direction in 3.2 x 10-2 s. What is the radius of this coil?
(b) X
(d) Z
27. Protons move in a circular path of radius 1.2 x 10-2 m when traveling
perpendicular to a 8.5 x 10-3 T magnetic field. What is the speed of
these protons?
(a) 9.9 x 101 m/s
(b) 4.2 x 103 m/s
3
(c) 9.8 x 10 m/s
(d) 1.8 x 107 m/s
28. A magnet is moving towards an aluminum ring.
Which of the following best shows the emf vs. time graph for the
emf induced in the coil as it moves from 1 to 2?
In which way will the current flow in the labelled portion of the ring
as the magnet moves towards the ring?
(a) towards 1
(b) towards 2
(c) towards 3
(d) towards 4
29. A 240 V motor draws 9.0 A of current at start up. At normal
operating speeds, the same motor only draws 2.5 A. What is the
back emf of this motor at normal operating speeds?
(a) 0.0 V
(b) 67 V
(c) 170 V
(d) 240 V
30. A transformer is made up of 200 turns in the primary windings and
of 50 turns in the secondary windings. The primary voltage is 120 V
and the secondary current is 0.12 A. What is the primary current
and the secondary voltage for this transformer?
June 04
32. Protons and electrons travelling at high speeds enter a magnetic
field parallel to their direction of travel. Which of the following is
correct?
(a) only protons are deflected
(b) only electrons are deflected
(c) both protons and electrons are deflected
(d) neither protons nor electrons are deflected
33. Which of the following diagrams best shows the orientation for a
set of four compasses placed around a current-carrying wire?
31. A coil of wire moves at constant velocity across a region of magnetic
field as shown.
34. An electron travels at a speed of 1.4 x 104 m/s while following a
circular path of radius 0.020 m perpendicular to a magnetic field.
What is the strength of the magnetic field?
(a) 0.0 T
(b) 4.0 x 10-6 T
(c) 6.3 x 10-3 T
(d) 4.1 x 10-2 T
38. A 0.034 kg copper rod is hung by two wires and placed in a constant
magnetic field. A current of 14 A runs through the 0.050 m long
copper rod, making it hang at an angle of 34º from the vertical as
shown below.
35. A potential difference of 22 V is placed across a 570-turn solenoid
that has a resistance of 4.9 Ω. The solenoid has a diameter of 0.052
m and is 0.37 m long. What is the magnetic field strength in the
centre of this solenoid?
(a) 8.7 x 10-3 T
(b) 6.2 x 10-2 T
(c) 1.2 x 10-1 T
(d) 3.0 x 10-1 T
36. A 200-turn copper coil of radius 0.050 m is perpendicular to a 0.087
T magnetic field. The magnetic field collapses to zero in 0.63 s.
What was the average induced emf in the coil during the collapse?
(a) 0.0 V
(b) 1.1 x 10-3 V
(c) 0.22 V
(d) 1.4 V
37. The coil of a motor has a resistance of 4.1 Ω. The motor is plugged
into a 120 V outlet, and the coil develops a back emf of 118 V when
rotating at normal speeds. Find the current when the motor first
starts up and the current when it is operating at normal speeds.
What is the magnetic field strength holding the copper rod in this
position?
(a) 2.7 x 10-1 T
(b) 3.2 x 10-1 T
-1
(c) 3.9 x 10 T
(d) 4.8 x 10-1 T
January 04
39. Which of the following best represents the magnetic field between
two magnets?
40. Which of the four arrows indicates the direction of the magnetic
field when current flows in the solenoid shown below?
(a) W
(c) Y
(b) X
(d) Z
41. A 0.40 m length of copper rod is held perpendicularly to a 0.082 T
magnetic field as shown.
42. A 0.35 m length of a conducting rod is moving perpendicular to a
0.026 T magnetic field as shown.
What is the potential difference as measured across the ends of the
conducting rod?
(a) 0.0 V
(b) 0.064 V
(c) 0.091 V
(d) 0.13 V
43. Which of the four situations below shows the greatest amount of
magnetic flux for a rectangular coil?
The copper rod is connected to a 12 V power supply and a 47 ohm
resistor. What are the magnitude and direction of the magnetic
force on the copper rod when the switch is closed?
44. An ideal 2.25 W transformer changes 120 V to 4.5 V for use in
portable electronic devices. What is the current in the secondary
windings and the ratio of primary coils to secondary coils in this
transformer?
August 03
45. Identify the magnetic poles 1 and 2 of the current-carrying solenoid
in the diagram below.
47. A beam made up of ions of various charges and masses enters a
uniform magnetic field as shown.
One type of ion is observed to follow path 2. Which path describes
the one taken by an oppositely charged ion with twice the mass and
twice the charge? (Assume all ions have the same speed.)
(a) Path 1
(b) Path 3
(c) Path 4
(d) Path 5
48. A step-down transformer is required to operate a 12 V, 25 W
halogen lamp. Which of the following sets of conditions could apply
to this transformer?
(a) Np = 20, Ns = 200
(b) Vp = 120 V, Ip = 0.21 A
(c) Ip = 2.1 A, Is = 2.1 A
(d) Vp = 120 V, Ip = 0.21 A
49. A 0.25 m wire is perpendicular to a uniform 0.20 T magnetic field.
What force is exerted on this wire when it carries a 15 A current?
(a) 0.12 N
(b) 0.75 N
(c) 3.0 N
(d) 6.0 N
46. Determine the direction of the magnetic force on the currentcarrying conductor in the diagram below.
(a) towards the left
(b) towards the right
(c) towards the top of the page
(d) towards the bottom of the page
50. As switch S is closed, in what direction does the compass needle
point and what is the direction of the current through resistor R?
53. What is the direction of the magnetic field at point P due to the bar
magnet?
(a) 1
(c) 3
(b) 2
(d) 4
54. The diagrams below each illustrate a magnet and a conductor. In
each case, the current in the conductor is out of the page. In which
of these situations will there be a force on the conductor that
points towards the top of the page?
51. One method for determining masses of heavy ions involves timing
their orbital period in a known magnetic field. What is the mass of a
singly charged ion that makes 7.0 revolutions in 1.3 x 10-3 s in a 4.5 x
10-2 T field?
(a) 2.1 x 10-25 kg
(b) 1.3 x 10-24 kg
-23
(c) 6.5 x 10 kg
(d) 5.0 x 10-20 kg
June 03
52. Which one of the following diagrams best illustrates the magnetic
field produced by a current-carrying wire?
55. A positively charged object (q = 1.6 x 10-19 C) is travelling at 1.9 x
104 m/s perpendicular to a 1.0 x 10-3 T magnetic field. If the radius
of the resulting path is 0.40 m, what is the object’s mass?
(a) 3.4 x 10-27 kg
(b) 3.1 x 10-19 kg
(c) 2.1 x 10-9 kg
(d) 0.77 kg
56. A conducting rod of length 0.25 m is moved to the right at 6.0 m/s
as shown in the diagram. The induced emf is 3.0 V.
January 03
59. A conductor is placed in a magnetic field as shown.
What is the magnitude of the magnetic field and which end of the
conducting rod, 1 or 2, becomes positively charged?
What are the magnitude and direction of the magnetic force acting
on this conductor when it carries a 15 A current?
57. A coil consisting of 50 loops of radius 4.0 x 10-2 m is placed with its
plane perpendicular to a magnetic field that is increasing at a rate
of 0.20 T/s. What is the magnitude of the emf induced in the coil?
(a) 0.0010 V
(b) 0.050 V
(c) 0.40 V
(d) 1.3 V
58. One hundred turns of wire are wrapped around an iron core with a
cross-sectional area of 0.0015 m2. The ends of the wire are
connected to a resistor producing a circuit with a total resistance of
10.0 Ω.
If the magnetic field in the iron core changes from 3.0 T towards
the left to 1.0 T towards the right, how much charge flows in the
circuit?
(a) 0.030 C
(b) 0.060 C
(c) 0.30 C
(d) 0.60
60. Which of the following are correct units for magnetic flux?
(a) T
(b) T/m
(c) Wb
(d) Wb•m2
61. Consider the 800-turn solenoid shown in the diagram below.
What is the current in the windings that would produce a magnetic
field of 0.060 T at the centre of this solenoid?
(a) 3.0 A
(b) 8.0 A
(c) 18 A
(d) 290 A
Written:
62. A proton enters a 0.65 T magnetic field. The velocity of the proton
is perpendicular to the field causing the proton to travel in a
circular arc of radius 1.1 x 10-2 m. What is the momentum of the
proton? (Release 07/08)
63. An 18 cm long metal rod, of mass 35 g, is suspended from the ceiling
with light wire. A uniform 0.22 T magnetic field is directed
vertically upward. When there is a current in the rod, it swings
outward 15º to the vertical as shown. (Release 06/07).
65. A 0.75 m metal rod is suspended as shown. A current of 13 A then
flows as indicated.
(a) Is the tension in the springs increased or decreased?
(b) How much does the tension change?
What are the magnitude and the direction of the current in the rod?
(5 marks)
64. A proton travelling at a high velocity enters a 0.45 T magnetic field
and travels in a circular path of radius 0.28 m as shown. What is the
kinetic energy of the proton? (Release 05/06)
66. A 480-turn circular coil of radius 0.075 m is placed in a
perpendicular magnetic field of 0.72 T. The coil is connected to a
resistor of 35 Ω as shown. (August 04)
a.
b.
Calculate the average current through the resistor as the
coil is removed from the magnetic field in a time of 0.22 s.
(6 marks)
In which direction will current flow in the coil, clockwise or
counterclockwise? (1 mark)
67. A 0.13 kg aluminum rod maintains contact with two vertical metal
rails. A voltage is applied across the metal rails and a horizontal
magnetic field of 0.65 T exists across the whole apparatus as shown.
(June 04)
a.
The electrons that make up the beam travel at 4.7 x 107
m/s. What is the acceleration of the electrons in this field?
(5 marks)
b.
The electron beam is then made to strike the screen at
position 2. What two changes were made to the current in
the solenoid? State the effect on the electron beam
produced by each change. (4 marks)
69. A 0.120 m diameter coil consisting of 200 loops is placed in a 0.35 T
magnetic field. The magnetic field is changed to 0.25 T in the
opposite direction in 0.80 s. (August 03)
What current must flow through the aluminum rod to have it remain
stationary? (7 marks)
68. A solenoid placed beneath a cathode ray tube as shown below
produces a magnetic field of 0.011 T on the electron beam causing it
to hit the screen at position 1. (January 04)
What is the magnitude of the current through the 33 Ω resistor
connected to the coil? (Ignore the resistance of the coil.) (7
marks)
70. A 420-turn rectangular coil is positioned as shown in a 0.14 T
magnetic field.
The magnetic field strength is increased over a 0.20 s interval,
inducing an average emf of 1.8 V in the coil. What is the final
magnetic field strength? (June 2003) (7 marks)
71. A rectangular wire loop with a resistance of 0.60 Ω is pulled out of a
magnetic field at 7.0 m/s as shown in the diagram. (January 03)
a.
b.
What is the current in the loop? (5 marks)
What is the direction of the current in the loop, CW or
CCW (2 marks)