Unit 3bb tủ tài liệu training

VERY SHORT AND SHORT-ANSWERS QUESTIONS
46. Name the law on which the tangent galvanometer works.
47. Where are the null points obtained when a bar magnet is placed horizontally with its north pole
towards earths’ magnetic south pole?
48. Where are the null points obtained when a bar magnet is placed horizontally with its north pole
towards earths’ magnetic north pole?
49. What is the basic difference between magnetic lines of force and electric lines of force ?
50. Write the unit of magnetic dipole moment in terms of joule.
51. Does a magnetic monopole exist ?
52. Write two basic properties of a magnet.
53. What do you mean by the poles of a magnet ?
54. Write the S.I. unit of pole strength.
55. What is magnetic length ?
56. Why does a paramagnetic sample display greater magnetization (for the same magnetizing
field) when cooled ?
57. Why are diamagnetic substances not affected by temperature ?
58. Two iron bars A and B are given, one of which is definitely known to be magnetized, but which
one we do not know. How would you ascertain whether or not both are magnetized ? If only
one is magnetized, how does one ascertain which one ? Use only the two bars A and B.
59. Magnetic field arises due to charges in motion. Can a system have magnetic moment even
though its net charge is zero ?
60. Calculate the dipole moment of a magnetic dipole of pole strength 30 A -m and of length 0.2 m.
61. What is the magnetic moment associated with a coil of one turn, having area of cross section
(AISSCE Delhi 1993)
10–4 m2, carrying a current of 2A?

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62. At a place the angle of dip is 30°. Find the horizontal and the vertical components of earth’s
magnetic field. Given the magnitude of the earth’s field = 0.4 gauss.

63. A bar magnet of magnetic moment 5.0 Am2 has poles 20 cm apart. Calculate its pole strength.
64. How can the magnetic moments of two bar magnets of the same size and mass be compared
using a vibration magnetometer ?
65. Describe how the horizontal components of earths’ magnetic field at two places may be compared by using a vibration magnetometer.
66. What are the differences between the magnetic properties of soft iron and steel ?
67. Draw lines of force of two magnets in the space between them, if
(a) the N pole of one magnet is directed towards the S pole of the other, their axes coinciding.
(b) the N pole of one magnet is directed towards the N pole of the other, their axes coinciding.
68. Draw pattern of lines of force of a bar magnet when the magnet is placed in the magnetic
meridian and the N pole of the magnet points towards the north pole of the earth.
69. Draw pattern of lines of force of a bar magnet when the magnet is placed in the magnetic
meridian and N pole of the magnet points towards the south pole of the earth.
70. Define retentivity.
71. Does an iron bar magnet retain its magnetisation when melted ? Explain. (AISSCE 1990 C)
72. Does the magnetisation of a paramagnetic salt depend on temperature ? Explain. (AISSCE 1990)
73. Interstellar space has an extermely weak magnetic field of the order of 10–12 T. Can such
a weak field be of any significant consequence ? Explain.
74. Write some properties of diamagnetic substances.
75. Write some properties of paramagnetic substances.
76. Define coercivity.

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What are the uses of ferromagnetic substances ?
Which direction would a compass point to if located near the north or the south pole of the earth ?
Why do we use steel or ALNICO for making permanent magnets ?
Name two alloys which can be used for making electromagnets or chokes.
Write an expression for the work done in rotating a bar magnet of moment M through an angle θ
from the equilibrium position in a magnetic field B.

82. What is the angle of dip at a place where the horizontal component of earth’s magnetic field is
equal to the vertical component?
83. A magnetic needle of magnetic moment M and pole strength m is broken into two pieces at the
middle. What are the magnetic moment and the pole strength of each piece ?
84. A freely suspended bar magnet oscillates with a period T. If it is cut into two equal parts parallel
to its length, what is the time period of each part ?
77.
78.
79.
80.
81.

85. Horizontal component of earth’s magnetic field at a place is 3 times the vertical component.
What is the value of the angle of dip at this place ?
(AISSCE Delhi 1997)
86. What is the angle of dip at a place where the horizontal and the vertical components of the
earth’s magnetic field are equal ?
87. What is the use of dip circle ?
88. What is the magnetic axis of the earth ?
89. A bar magnet is placed in a uniform magnetic field. In which position the potential energy of
the magnet is (i) maximum (ii) zero.
90. Angles of dip at two places on the surface of the earth are 0° and 90°, respectively. Where are
these places located ?
91. What material is used in making the core of a moving coil galvanometer ? (AISSCE 1996)
92. Which type of magnetic material is used in making permanent magnets ?
(AISSCE 1996)

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ANSWERS
46.
47.
48.
49.

A tangent galvanometer works on the tangent law.
The null points are obtained on the equatorial line of the magnet.
The null points are obtained on the axial line of the magnet.
An electric line of force starts at one point (positive charge) and ends at some other point
(negative charge). A magnetic line of force has no beginning and end. It forms closed loop.

50. joule/testa (JT–1)
51. No, the most elementary magnetic entity is a dipole.
52. (i) A magnet attracts certain substances like iron, nickel, cobalt etc.
(ii) A freely suspended magnet always aligns itself nearly in the geographic North-South direction.
53. Poles are two points near the two ends of a magnet where the magnetic force is maximum.
54. The S. I. unit of pole strength is Ampere–meter (Am).
55. The distance between the two poles of a magnet is called its magnetic length. The magnetic
length is always less then the actual length of a magnet.
56. At lower temperatures, the tendency to disrupt the alignment of atomic dipoles arising from
random thermal motion is reduced.
57. Diamagnetism is due to induced dipole moment in a sample which is always opposite to the
magnetizing field, no matter what the internal motion of the atom is.
58. Bring different ends of the bars closer. A repulsive force in some situations shows that both are
magnetized. If the force is always attractive then only one bar is magnetized. To see which one,
pick up one, say A, and lower one of its ends first on one of the ends of B, and then on the
middle of B. If you find that in the middle of B, A experiences no force, then B is magnetized.
If you do not find any change from the end to the middle of B, then A is magnetized.

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59. Yes, It is possible. The net charge in a system may be zero but it is not necessary that the net
magnetic moment due to all the current loops will also be zero. For example, a neutron has zero
charge, but non-zero magnetic moment.
60. Dipole moment M = Pole strength × Length of magnet
= 30 Am × 0.2 m
= 6.0 Am2
61. Magnetic dipole moment M = niA
Here
n = 1, i = 2, A = 10–4 m2
M = 1 × 2 × 10–4
= 2 × 10–4 Am2
62. B = 0.4 Gauss = 4 × 10–4 T
Horizontal component

BH = B cos θ
= 0.4 × cos 30°
= 0.4 ×

3

2
= 0.35 gauss

Vertical component

BV = B sin θ
= 0.4 × sin 30°

= 0.4 ×

1
2

= 0.2 gauss

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63. We have
M = Pole strength × Length of magnet
Here

M = 5.0 Am2

Length of magnet = 0.2 m
5.0

= 25 Am
0.2
64. The time period of oscillation of a magnet in a vibration magnetometer is

∴ Pole strength =

I

T = 2π

MBH

2

or

M=

4π I
2

...(1)

T BH

In order to compare the magnetic moments of two magnets, they are made to oscillate in turn in
the magnetometer. If M1, M2 are their magnetic moments, I1, I2, are their respective moments of
inertia, and T1, T2 are their respective time periods of oscillation, then from Eq. (1)

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2

4π I1

M1 =

...(2)

2

T1 BH
2

M2 =

4π I 2

...(3)

2

T2 BH

Dividing Eq. (2) by Eq. (3)
M1
M2

=

I1 T2

2

2

I2 T
1
If the mass and size are same, then I1 = I2
In that case
M1

M2

=

T2
T1

2
2

65. A given magnet is made to oscillate in a vibration magnetometer at the two places, in turn. If T1
and T2 are the time periods and B1 and B2 are the horizontal components of the earth’s field at
the two places, then

T1 = 2π
T2 = 2π

I

, and

...(1)

,

...(2)

MB1
I
MB2

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where I and M are the moments of inertia and the magnetic moment of the magnet. Dividing Eq.
(2) by Eq. (1),
B1
B2

=

T2
T1

2
2

66. Soft iron has low coercivity and small hysteresis loss. It can be easily magnetized and demagnetized. Steel has high coercivity and large hysteresis loss. It cannot be easily magnetized and
demagnetized.
67. (a)

S

N

Fig. 3 B.2

(b)

S

N

N

N

S

Fig.3 B.3

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68.
N

N
X

X
S

S
Fig. 3 B.4

69.

N

S

N

S
Fig. 3 B.5

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70. When a piece of ferromagnetic material is magnetised to saturation and then the magnetising
field is removed, some magnetic induction remains in the material. This remaining field is
called remanence or retentivity. This is due to the tendency of domains to stay together once
they have been aligned.
71. Iron melts at a temperature higher than its curie temperature. Therefore a bar magnet loses its
magnetisation on melting as the alignment of its atomic magnets is destroyed.
72. Yes, the magnetisation of a paramagnetic salt depends on temperature. The atoms of a paramagnetic salt have non-zero magnetic moments. As the temperature of such a substances is
increased, the alignment of these atomic magnets decreases due to thermal agitation and hence
the magnetisation also decreases.
73. From the relation r = mv/Bq, an extermely weak field bends charged particles in circles of very
large radii. Over a small distance this bending is not noticeable, but over large interstellar distances, there can be significant deflection in the path of, e.g., cosmic rays.
74. Properties of diamagnetic substances:
(a) When placed in a non-uniform magnetic field, they move from stronger to weaker parts of
the field.
(b) They are feebly repelled by a magnet.
(c) Diamagnetic behaviour is independent of temperature.
(d) Their relative permeability is slightly less than 1.
(e) Their susceptibility is small and negative.
75. Properties of paramagnetic substances :
(a) They are feebly attracted by a magnet.

(b) When placed in a non-uniform magnetic field, they move from weaker to stronger parts of
the field.
(c) Paramagnetic property decreases with rise in temperature.
(d) Their relative permeability is slightly greater than 1.
(e) Their susceptibility is small and positive.

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76. The reverse magnetising field required to destroy the residual magnetic induction in a material is
called coercivity. It is a measure of the difficulty of breaking up the alignment of the domains.
77. Ferro-magnetic substances are mainly used in making permanent magnets, electromagnets and
transformer cores.
78. The compass can point in any direction, because at the poles, earth’s field is vertical and compass is free to move in a horizontal plane.
79. For steel and ALNICO, the values of coercivity and retentivity are high.
80. (a) Mu-metal (alloy of nickel, iron, copper and chromium).
(b) Radio-metal (alloy of nickel, iron, copper and manganese).
81. W = MB (1 – cos θ)
82. We have tan δ =

BV
BH

=1

⇒ δ = 45°
83. Magnetic moment of each piece =

M
2

Pole strength of each piece = m
84. We know T = 2π

I
MH

When the magnet is cut parallel to its length the moment of inertia and the magnetic moment of
each part will become half. The new time period

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T ′ = 2π

I /2
( M / 2) H

= 2π

I
MH

=T

Thus the time period remains the same.
85.
86.

tan δ =

BV
BH

=

1
3

⇒ δ = 30°

BH = BV

or B cos δ = B sin δ
or tan δ = 1
or
δ = 45°.
87. It is used to measure the angle of dip at a place.
88. The straight line passing through the magnetic north and south poles of the earth is called the
magnetic axis of the earth.
89. Potential energy U = – MB cos θ
(i) U is maximum when θ = 180°
Umax = – MB cos 180°
= MB
→ →
(ii) U is zero when θ = 90°, i.e., M & B are perpendicular to each other.
90. Angle of dip is zero at the magnetic equator and 90° is at the magnetic poles.
91. Soft iron (It has high magnetic permeability).
92. Materials having high remanence and high coercivity are used for making permanent magnet,
e.g., steel.

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