1
IB - MAGNETISM MCQ and SMALL PROBLEMS
1. A suitable unit of magnetic field strength is
A.
B.
C.
D.
A N-1 m-1
kg s-2 A-1
A m N-1
kg A s2
2. The diagram below shows a point P on the Earth’s surface at which a compass needle
is
suspended freely.
Which one of the following gives the correct direction in which the needle of the
compass will point?
3. An electron and a proton travelling with the same velocity are injected into a region of uniform magnetic field at 900 to the
magnetic field direction. The initial magnetic forces on them are
A. equal in magnitude and direction.
B. equal in magnitude and opposite in direction.
C. equal in magnitude and perpendicular to each other.
D. in opposite directions and differing in magnitude by the ratio of their masses.
4. A charged particle is injected into a region of uniform magnetic field and travels in a circular arc.
If the particle were to be injected with a greater speed, what would be true of the magnetic force
on it and the radius of its path?
A.
B.
C.
D.
Force
Arc radius
greater
greater
smaller
smaller
greater
smaller
greater
smaller
5. A magnetic force acts on an electric charge in a magnetic field when
A. the charge is not moving.
B. the charge moves in the direction of the magnetic field.
C. the charge moves in the opposite direction to the magnetic field.
D. the charge moves at right angles to the lines of the magnetic field.
2
6.
A positively charged particle enters a region of uniform magnetic field. The direction of the particle’s velocity is parallel to the
direction of the magnetic field as shown in the diagram below.
Which of the following diagrams correctly shows the path of the charged particle while in the region of magnetic field?
7. A strip of aluminium foil is held between the poles of a strong magnet, as shown below.
When a current is passed through the aluminium foil in the direction shown, the foil is deflected. In which direction is this
deflection?
A. Vertically downwards
B. Vertically upwards
C. Towards the North pole of the magnet
D. Towards the South pole of the magnet
8.
An electron is travelling in the direction shown and enters a region of uniform magnetic field.
On entering the field the direction of the force acting on the electron is
A. into the plane of the paper.
B. out of the plane of the paper.
C. towards the top of the page.
D. towards the bottom of the page.
3
9. The diagram below shows a charged particle about to enter a region of uniform magnetic field directed into the page.
Which of the following correctly describes the change, if any, in the kinetic energy and the momentum of the particle in the
magnetic field?
A.
B.
C.
D.
Kinetic energy
Momentum
Changed
Changed
Unchanged
Unchanged
Changed
Unchanged
Changed
Unchanged
10. The diagram shows a coil of wire that can rotate between the poles of a magnet about the axis XY.
A current is passed through the coil by means of a commutator connected to the ends of the coil. What is the position of the
coil in the magnetic field so that its turning effect is a maximum and what is the position of the coil when the current is
reversed so that the coil rotates continuously?
plane of coil for maximum
turning effect
plane of coil for
reversal of current
A. parallel to direction of field
parallel to direction of field
B. normal to direction of field
parallel to direction of field
C. parallel to direction of field
normal to direction of field
D. normal to direction of field
normal to direction of field
11. A charged particle of mass m and charge q is travelling in a uniform magnetic field with speed v such that the magnetic force on
the particle is F. The magnetic force on a particle of mass 2m, charge q and speed 2v travelling in the same direction in the
magnetic field is
A. 4F.
B. 2F.
C. F.
D. ½ F.
12. A proton is in a region where a uniform electric field of 5 × 104 V/m is perpendicular to a uniform magnetic field of 0.8 T. If its
acceleration is zero then its speed must be:
A)
B)
C)
D)
E)
0
6.3 × 104 m/s
1.6 × 104 m/s
4.0 × 105 m/s
any value but 0
4
13. The diagram show a straight wire carrying current i in a uniform magnetic field. The magnetic force on the wire is indicated by
an arrow but the magnetic field is not shown. Of the following possibilities, the direction of the magnetic field is:
A)
B)
C)
D)
E)
to the right
opposite the direction of ¢
in the direction of ¢
into the page
out of the page
14. The diagram below shows a current I in a wire placed at an angle  inside a uniform magnetic field of field strength B.
The magnetic force per unit of length of wire is M. The magnetic field strength B is given
by
A.
B.
C.
D.
15. A straight conductor is in the plane of a uniform magnetic field as shown.
The current in the conductor is I and the conductor is at an angle è to the magnetic field.
The force per unit length on the conductor due to the current in the magnetic field is P.
Which is the correct expression for the magnitude of the magnetic field strength?
A. P sin 
I
B. Pcos 
I
C.
P
Isin 
D.
P
Icos
16. A long straight wire is in the plane of a rectangular conducting loop of wire. The straight wire carries a constant current I as
shown in the figure below and is moved towards the rectangular loop.
While the wire is being moved towards the rectangular loop, the current in the loop
A. is always zero.
B. flows clockwise around the loop.
C. flows counterclockwise around the loop.
D. alternates, first one way then the opposite way around the loop.
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17. Four long straight parallel wires carry equal currents directed vertically out of the page. They are arranged on the corners of a
square as shown in the figure below.
The direction of the resultant magnetic force exerted on the wire labelled X is
A. south.
B. north.
C. west.
D. east.
18. A current-carrying solenoid is placed with its axis pointing east-west as shown below. A small compass is situated near one end
of the solenoid.
The axis of the needle of the compass is approximately 45o to the axis of the solenoid. The current in the solenoid is then
doubled. Which of the following diagrams best shows the new position of the compass needle?
19. The diagram below represents a solenoid in which there is no electric current.
Which one of the following best represents the magnetic field pattern due
to an electric current in the solenoid?
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20. A long wire that carries a current I is bent into five loops as shown in the figure.
If the observer could "see" the magnetic field inside this arrangement of loops, how would it appear?
21. The currents in two parallel wires are I and 3I in the directions shown in the diagram below.
The magnetic force on wire 2 due to the current in wire 1 is F. The magnitude of the force on wire 1 due to the current in wire 2
is
A.
B.
C.
D.
F/3.
F/2.
F.
3F.
22. The diagram below shows three parallel wires P, Q and R that are equally spaced.
The currents in the wires are each of the same magnitude I and are in the directions shown. The resultant force on wire Q due
to the current in wire P and in wire R is
A. perpendicular and into the plane of the paper.
B. perpendicular and out of the plane of the paper.
C. in the plane of the paper to the right.
D. in the plane of the paper to the left.
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23. Two long, parallel, straight wires X and Y carry equal currents into the plane of the page as shown.
The diagram shows arrows representing the magnetic field strength B at the position of each wire and the magnetic force F on
each wire.
The current in wire Y is doubled. Which diagram best represents the magnetic field strengths and forces?
24.
The diagram shows part of a long, straight vertical wire in which there is a current I. P is a
charged particle at a distance r from the wire. The magnitude of the charge of P is q.
The particle P is moving normally into the plane of the paper with speed v. The magnitude of the
force on P due to the magnetic field of the wire is
A. zero.
25.
B.
0 I
qv
2 r
C.
0 Iq
2 rv
D.
 0 Iv
2 rq
A long, straight current-carrying wire is placed normal to the plane of the page. The current in the wire is into the plane of the
page.
Which of the following diagrams best represents the magnetic field around
the wire?
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26. The diagram below shows two long parallel wires, 1.0 m apart, on the plane of the page. Each wire carries a current I in the
same direction.
Point P is on the plane of the page midway between the two wires. The magnitude of the magnetic field strength at point P due
to wire 1 alone is B0.
The magnitude of the magnetic field strength at point P due to both wires is
A. 0.
B. ½ B0 .
C. B0.
D. 2 B0 .
27. The diagram below represents four long straight wires perpendicular to the plane of the paper.
The magnitude of the direct current in each wire is the same. Wires with + have current into the
plane of the paper and wires with • have current out of the plane of the paper. Point P is the same
distance from each wire.
Which arrow shows the direction of the magnetic field at P?
28.
A direct current (dc) motor is connected to a battery by means of two leads. What is the function of the commutator of the
motor?
A. To allow the motor to produce a uniform turning effect.
B. To prevent too large a current in the coil of the motor.
C. To reverse the direction of current in the leads to the motor.
D. To reverse the direction of current in the coil of the motor.
29. The function of the split-ring commutator in a simple direct-current (d.c.) motor is to reverse the direction of the current
A. in the coil, so as to control the speed of the coil.
B. in the coil, so that the coil will rotate continuously.
C. in the external circuit, so as to control the speed of the coil.
D. in the external circuit, so as to change the direction of the magnetic field.
30. The function of the commutator of a d.c. electric motor is
A. to reverse the current through the rotating coils each half turn.
B. to step up the voltage from the electrical source.
C. to convert the motor into an a.c. motor.
D. to enable the rotational speed of the motor to be varied.
31. A bar magnet is placed in a uniform magnetic field as shown.
(a) Is there a net force on the bar magnet?
(b) Will it move? If so, how?
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32. Find the direction of the missing quantity from B, v and F in each of the cases shown. The circle represents a positive charge.
33. A proton moves past a bar magnet as shown. Find the direction of the force it experiences in each case.
34. What is the direction of a magnetic field in each of the four cases that results in a force on the current as shown?
35. A high-tension electricity wire running along a north—south line carries a current of 3000.0 A. If the magnetic field of the earth
at the position of the wire has a magnitude of 5.00 X 10-5 T and makes an angle of 30° below the horizontal, what is the force
experienced by a length of 30.0 m of the wire?
36. A rectangular coil of size 20cm X 10cm is placed in a horizontal uniform magnetic field of magnitude 0.050 T, as shown in Figure.
A current of 2.0 A flows in the coil in a counter-clockwise direction as shown.
(a) Find the force on sections AB, BC, CD and DA.
(b) What is the net force on the coil?
37. An electron of speed v enters a region of magnetic held B directed normally to its velocity and is deflected into a circular path.
Find an expression for the number of revolutions per second the electron will make. If the electron is replaced by a proton, how
does your answer change?
10
38. Figure shows two parallel plates with a potential difference of 120 V a distance 5.0 cm apart. The top plate is at the higher
potential and the shaded region is a region of magnetic field normal to the page.
(a) What should the magnetic field magnitude and direction be such that an electron experiences zero net force when shot
through the plates with a speed of 2 x 105 ms-1
(b) Would a proton shot with the same speed through the plates experience zero net force?
(c) If the electron’s speed were doubled, would it still be undeflected if the magnetic field took the value you found in (a)?
39. A wire that is carrying a current of 3.50 A east has 2.00 m of its length in a uniform magnetic field of magnetic flux density of
5.00 x 10-7 T directed vertically into the paper.
Determine the magnitude and direction of the force it experiences.
40. An electron is moving with a speed of 3.0 x105 ms-1 in a direction that is at right angles to a uniform magnetic field of 3.0 x 10-3
T. Calculate
a. the force exerted on the electron.
b. the radius of the path of the electron.
41. Draw the magnetic field lines for two parallel wires carrying equal currents into the page. Repeat for antiparallel currents.
42. Two long, straight wires lie on the page and carry currents of 3.0 A and 4.0 A as shown. Find the magnetic field at point P.
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43. Find the magnetic field at points P, Q and R The currents are parallel and each carry 5.00 A. Point Q is equidistant from the
wires. (The three points lie on the same plane as the wires.)
44. A rectangular loop of wire of size 5 cm x 15 cm is placed near a long straight wire with side CD at a distance of 5 cm from it as
shown. What is the net force exerted on the loop (magnitude and direction)? How does your answer change if the current in
the loop is reversed?
45. A long straight wire carries current as shown. Two electrons move with velocities that are parallel and perpendicular to the
current. Find the direction of the magnetic force experienced by each electron.
46. An electron is shot along the axis of a solenoid that carries current. Will it experience a magnetic force?
47. Draw the magnetic field lines that result when the magnetic field of a long straight wire carrying current into the page is
superimposed on a uniform magnetic field pointing to the right that lies on the page.
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48. A tightly wound solenoid of length 30 cm is to produce a magnetic field of 2.26 x 10-3 T along its axis when a current of 15.0
A flows in it. If the radius of the solenoid is 12.0 cm, what length of wire is required to make the solenoid?
49. What is the direction of the magnetic field at points P and Q in the plane of a circular loop carrying a counterclockwise current,
as shown?
50. Two parallel wires a distance of 20.0 cm apart carry currents of 2.0 A and 3.0 A as shown.
(a) At which points is the magnetic field zero?
(b) How would your answer change if the direction of the 3.0 A current were reversed?
51. Figure shows two parallel conductors carrying current out of the page. Conductor 1 carries double the current of conductor 2.
Draw to scale the magnetic fields created by each conductor at the position of the other and the forces on each conductor
52. Find the magnetic field at point P due to three currents as shown.
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53. Three parallel wires carry currents as shown. Find the force per unit length that wires 1 and 3 exert on wire 2.
54. A uniform magnetic field is established in the plane of the paper and has magnitude 0.3 mT. Two parallel wires separated by 5.0
cm carry currents of 200 A and 100 A into the plane of the paper as shown. Find the magnetic force per unit length on the 100 A
wire.
55. A uniform magnetic field is established in the plane of the paper as shown in Figure 6.49. Two wires carry parallel currents of
equal magnitudes normally to the plane of the paper at P and Q. Point R is on the line joining P to Q and closer to Q. The
magnetic field at position R is zero.
(a) Are the currents going into the paper or out of the paper?
(b) If the current is increased slightly, will the point where the magnetic field is zero move to the right or to the left of R?
14
IB - MAGNETISM MCQ and SMALL PROBLEMS
1. B. B = F / IL = kg m s-2 / A m
9. C
10.C
11.B
12.B
20. C
21.C
22.D
23.D
2.C
13.E
24.A
3.B
14.B
25.D
4.A
15.C
26.A
5.D
16.C
27.D
6.A
17.D
28.D
7.B
18.B
29.B
8.B
19.A
30.A
31.
magnetic field lines point from north toward south pole outside the magnet and opposite inside the magnet, so magnet placed
into mag. field will rotate ccw , because there will be two forces acting on it in opposte directions.
32.
33.
34.
35.
40.
A. B into the page B. F into the page C. B out of the page
A. into the page
B. F = 0
A. out of the page
nadji
36.nadji
D F = 0 E. F = 0
C. south
B. out of the page
C. west
37.nadji
38.nadji
D. east
39.F = IlB = (3.5 A) x (2.00 m) x (5.00 x 10-7 T ) = 3.50 x 10-6 N.
a. F = qv B = (1.6 x 10-19 C) x ( 3.0 x105 ms-1 ) x ( 3.0 x 10-3 T) = 1.44 x 10-16 N
at right angles to the magnetic field and the path of its motion.
b. Fmag = qv B
Fcp = mv2 / r.
2
qv B = mv / r
q B = mv / r
r = mv / q B
r = [(9.11 x 10-31) x (3.0 x 105 ms-1)]/[(1.6 x 10-19 C) x (3.0 x 10-3 T)]
r = 5.69 x 10-4 m
41.
42.
46.
48.
49.
50.
51.
vector sum of two fields
no
nadji
nadji
nadji
nadji
52.
53.
54.
55.
nadji
nadji
nadji
nadji
47.vector sum
43.vector sum of two fields 44.budemo
45.A. south
B. west