2013
1. When a magnetic needle is kept in a uniform magnetic field, it experiences
a. neither a force nor a torque.
b. a force and not a torque.
c. a torque and a force.
d. only a torque.
2. Magnetic lines of force prefer to pass through ferromagnetic substances than through air. Why?
Ans.: It is because Ferromagnetic substances have very high relative permeability.
3. Distinguish between paramagnetic and diamagnetic materials based on their behaviour in a
uniform magnetic field and in a non-uniform magnetic field.
Ans.:
The paramagnetic material placed in a uniform magnetic field is feebly magnetized in the direction
of the field and magnetic flux density within it is slightly greater than outside.
The diamagnetic material placed in a uniform magnetic field is feebly magnetized opposite to the
field and magnetic flux density within it is slightly smaller than outside.
In a non-uniform magnetic field, paramagnetic material tends to move from weaker to stronger part
and diamagnetic materials tends to move from stronger to weaker part of the field.
2012
1. In order to experience a magnetic force, the test charge must be
a. at rest in the magnetic field.
b. in motion in the magnetic field.
c. at rest outside the magnetic field.
d. in motion outside the magnetic field.
2. When the current flowing through a wire loop is halved, its magnetic moment will become
a. half.
b. one-fourth.
c. double.
d. quadruple.
3. When a uniform magnetic field is in a vertically downward direction and an electron is projected
in a horizontal direction, the electron will move in a circular path with a constant speed,
a. clockwise in a vertical plane.
b. clockwise in a horizontal plane.
c. anticlockwise in a vertical plane.
d. anticlockwise in a horizontal plane.
4. A dipole in a non-uniform electric field experiences only a torque. (True or False)
Ans.: False. It is because in a non-uniform electric field, a dipole experiences both torque and force.
OR The dipole experiences only a torque when placed in a uniform field.
5. Define magnetic field lines and mention two of its properties.
Ans.: The magnetic field line is the path along which an isolated north pole will tend to move if it is free
to do so.
Properties: (Can write any two properties)
i. The magnetic field lines are closed continuous loops extending through the body of the magnet.
ii. No two magnetic field lines intersect each other.
6. A short magnet placed with its axis 30° to a uniform magnetic field of 0.2T experiences a torque
of 5 Nm2.Compute the magnetic moment of the magnet.
Ans.:
= MBSin
M=
BSin
5Nm 2
M=
0.2T×Sin300
5Nm 2
=
1
0.2T×
2
-1
= 50JT
7. Why are permanent magnets made of steel while electromagnets are made of soft iron?
Ans.: Permanent magnets have high retentivity as compared to soft irons.
For steel, coercivity is very large and remanence is fairly large. Therefore, it cannot be easily
demagnetised and hence is suitable for making permanent magnets. For soft iron coercivity and
hysterisis loss are small and permeability is high, thus it is suitable for making electromagnets.
Note:
Coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Ferromagnetic
materials with high coercivity are called magnetically hard materials, and are used to make permanent
magnets.
Remanence is the magnetic flux that stays in a substance when the magnetizing force has been taken
away.
2011
1. If the relative permeability of a substance A is slightly less than unity and that of substance B is
slightly more than unity, then
a. A is paramagnetic and B is ferromagnetic.
b. A is diamagnetic and B is ferromagnetic.
c. A is paramagnetic and B is diamagnetic.
d. A is diamagnetic and B is paramagnetic.
2. If the current flowing through a wire loop is doubled, then its magnetic moment will become
a. half.
b. double.
c. one-fourth.
d. one-third.
3. A charged particle enters perpendicularly in a uniform magnetic field. Show that its radius is
directly proportional to its momentum.
Ans.: Force on the particle due to magnetic field = centripetal force
m 2
q B
r
m
m
r
qB
qB
r
= ..
Therefore
4. A bar magnet placed in a uniform magnetic field of 0.3T with its axis at 90° to the field experiences
a torque Find the magnetic moment of the bar magnet.
Ans.:
MB sin
M
0.06
0.3sin 900
B sin
0.2 Am 2 OR Nm / T
5. Distinguish between intensity of magnetism ( M ) and magnetic susceptibility ( m ) . What is the
value of magnetic susceptibility of aluminium if it’s relative permeability is 1.000031?
Ans.: Intensity of magnetism (M) is the magnetic moment per unit volume of a magnetised substance.
Pm
M
V
=
Magnetic susceptibility (
the magnetising field.
M
m
) is the ratio of intensity of the magnetisation to the magnetic intensity of
H
I
H
m
1
r
m
m
I
H
1.000031 1
m
m
1.000031 1
= 0.000031
m
6. A charged particle moving in a straight line parallel to a uniform magnetic field enters a field.
What will be its path in the field? Will there be any change in its speed or direction of motion?
Ans.: The particle moving parallel to the field will not experience any force in the field. Hence, it will
continue to move in a straight line without any change in speed or direction of motion.
F qvB sin
00
F
0
7. A rectangular loop of area is placed in a magnetic field of 0.3T with its plane:
i.
normal to the field.
ii.
parallel to the field.
iii.
inclined at 30° to the field.
Find the flux linked with the coil in each case.
Ans.:
BA cos
0.3 500 10
1.5 10 2Wb
4
cos 00
BA cos
4
cos 900
0.3 500 10
4
cos (90 30) 0
0.3 500 10
4
cos 600
0.3 500 10
0
BA cos
7.5 10 3Wb
2010
1. The magnetic field lines
a. cannot intersect at all.
b. intersect at the neutral point.
c. intersect near the north and south poles.
d. depend upon the position of the magnet.
2. Write down the relationship between relative permeability and magnetic susceptibility.
Ans.:
1
r
m
OR
1 K
r
3. Distinguish between paramagnetic and diamagnetic substances.
Ans.:
Paramagnetic
Diamagnetic
0
0
m
m
Independent of temperature
1
T
Weakly magnetised in the direction of Weakly magnetised in the direction opposite to
the magnetising field.
magnetising
field.
It moves from stronger to weaker field.
It moves from weaker to stronger field.
4. What is magnetic Lorentz force?
Ans.: It is the force experienced by a moving charge in a magnetic field.
OR
F q(vxB )
5. The magnetic induction (B) and magnetizing field (H) in a sample of magnetic material are 1.2T
and 2000 Am-1 respectively. Find the:
i.
magnetic permeability ( )
ii.
relative magnetic permeability ( r) and
iii.
magnetic susceptibility ( m).
Ans.:
Magnetic Permeability
B
H
1.2
2000
6 x10 4 TmA 1
Relative Permability
r
0
6 x10 4
4 x10 7
= 4.77 x102
= 477.8
Magnetic Susceptibility
1
m
r
= 477.8 – 1
=476.8
2009
1. A moving electric charge produces
a. electric field only.
b. magnetic field only.
c. both electric field and magnetic field.
d. neither of these two fields.
2. A conducting loop carrying a current ‘i’ is placed in uniform magnetic field pointing into the plane
of the paper as shown in the figure. The loop will have a tendency to
a. expand.
Y
b. contract.
XX X XXX
c. move towards +ve x-axis.
X X X XX
d. move towards –ve ax-axis.
X
X
X
X XX
X X X
X
X
i
XX
X X X
X XX X
X
X
3. A diamagnetic substance is …….. of temperature and its relative permeability is slightly ……….
than unity. (dependent, independent, less, more)
Ans.: independent, less
4. An air solenoid has 500 turns of wire in its 40 cm length. If the current in the wire is 1.0A, find
the magnetic field at the axis inside the solenoid.
Ans.:
0 IN
B
4 x10 7 x1x500
0.4
1.57 x10 3 N Am
5. Derive the expression for comparison of the magnetic moments of two bar magnets of the same
size and mass using a vibration magnetometer.
Ans.: We know that
I
T 2
MBH
M
4 2I
T 2 BH
OR
And M1, M2 are the magnetic moments, I1, I2 are the respective moments of inertia and T1 and T2 are the
respective time periods of oscillation of the two bar magnets, then from equation (1),
4 2 I1
M1
T12 BH
and
4 2 I2
M2
T 22 BH
Dividing eqn. (2) by eqn. (3)
M 1 I1 T12
M 2 I 2 T12
If the mass and size of the magnets are the same, then I1 =I2
M1 T22
M 2 T12
6. Distinguish between diamagnetic, paramagnetic and ferromagnetic substances in terms of their
relative permeability and susceptibility.
Ans.:
Material
(magnetic susceptibility)
Diamagnetic
Slightly less than unity
Small, -ve
Paramagnetic
Slightly more than unity
Small, +ve
3
Ferromagnetic
Much greater than unity ( 10 )
Large, +ve