The unit of $ \frac{\mu_{0}}{4 \pi} $ is

$ \mathrm{Nm}^{2} \mathrm{C}^{2} $

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NEET • PHYSICS • MAGNETISM AND MATTER

Magnetism and matter practice · Medium

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91 Medium

91 of 128

Remembering +4 | -1

\mathrm{M}

is magnetic moment and

\mathrm{B}

is the magnetic field, then the torque is given by

View answer and explanation

Correct option: C

\overrightarrow{\mathrm{M}} \times \overrightarrow{\mathrm{B}}

Option C (

\overrightarrow{\mathrm{M}} \times \overrightarrow{\mathrm{B}}

) is correct because the torque on a magnetic dipole in a magnetic field is given by the cross product of the magnetic moment and the magnetic field.

92 Medium

92 of 128

Analyzing +4 | -1

Which of the following statements are correct?
A. Electric monopoles do not exist, whereas magnetic monopoles exist.
B. Magnetic field lines due to a solenoid at its ends and outside cannot be completely straight and confined.
C. Magnetic field lines are completely confined within a toroid.
D. Magnetic field lines inside a bar magnet are not parallel.
E.

\chi = - 1

is the condition for a perfect diamagnetic material, where

\chi

is its magnetic susceptibility.
Choose the correct answer from the options given below.

View answer and explanation

Correct option: A

C and E

Option A is correct because electric monopoles exist, magnetic monopoles are theorized but not yet observed, and χ = -1 is the condition for perfect diamagnetism.

93 Medium

93 of 128

Remembering +4 | -1

Which among the following have negative susceptibility?

View answer and explanation

Correct option: C

Bismuth

Option C (Bismuth) is correct because Bismuth is a diamagnetic material, which has negative susceptibility.

94 Medium

94 of 128

Understanding +4 | -1

The magnetic lines of force inside a bar magnet

View answer and explanation

Correct option: D

Are from south-pole to north-pole of the magnet

Option D (Are from south-pole to north-pole of the magnet) is correct because inside a bar magnet, the magnetic field lines travel from the south pole to the north pole.

95 Medium

95 of 128

Application +4 | -1

A very long magnet is placed vertically with one pole on the table. A neutral point was found at 20 cm from the pole. What is the pole strength if the vertical component of earth’s field is

0.4 \times {10^{ - 4}}\mathrm{Wb}{\mathrm{m}^{ - 2}}

View answer and explanation

Correct option: A

16 A-m

Option A (16 A-m) is correct because the pole strength can be calculated using the formula for the magnetic field at a neutral point.

96 Medium

96 of 128

Application +4 | -1

The magnetic moment of a magnet is 0.1 amp

\times {\mathrm{m}^2}

. It is suspended in a magnetic field of intensity

3 \times {10^{ - 4}}\;\mathrm{Wb}{\mathrm{m}^{ - 2}}

. The couple acting upon it when deflected by 30

^ \circ

from the magnetic field is

View answer and explanation

Correct option: B

1.5

\times {10^{ - 5}}

N m

Option B is correct because the couple (torque) is calculated using the formula τ = MBsinθ, and substituting the given values yields 1.5 × 10⁻⁵ N m.

97 Medium

97 of 128

Application +4 | -1

A current of 1A is flowing on the sides of an equilateral triangle of side

4.5 \times {10^{ - 2}}m

. The magnetic field at the centre of the triangle will be:

View answer and explanation

Correct option: A

4 \times {10^{ - 5}}\,Wb/{m^2}

Option A is correct because the magnetic field at the center of the triangle is calculated using the formula B = (3μ₀I)/(2πd), where d is the perpendicular distance from the side to the center, resulting in 4 × 10⁻⁵ Wb/m².

98 Medium

98 of 128

Understanding +4 | -1

Assertion(A): It is not necessary that every magnet has one north pole and one south pole.

Reason(R): It is a basic fact that magnetic mono poles exists.

View answer and explanation

Correct option: D

If both (A) and (R) are false

Option D is correct because every magnet must have both a north and south pole, and magnetic monopoles have not been observed.

99 Medium

99 of 128

Analyzing +4 | -1

Assertion(A): When a magnet is brought near iron nails, only translatory force act on it.

Reason(R): The field due to a magnet is generally uniform.

View answer and explanation

Correct option: D

If both (A) and (R) are false

Option D (If both (A) and (R) are false) is correct because both translational and rotational forces can act on iron nails near a magnet, and the field due to a magnet is generally non-uniform.

100 Medium

100 of 128

Application +4 | -1

The magnetic potential at a point on the axial line of a bar magnet of dipole moment

M

V

. What is the magnetic potential due to a bar magnet of dipole moment

\frac{M}{4}

at the same point

View answer and explanation

Correct option: D

\frac{V}{4}

Option D (

\frac{V}{4}

) is correct because the magnetic potential is directly proportional to the magnetic dipole moment.

101 Medium

101 of 128

Understanding +4 | -1

Assertion(A): The poles of magnet cannot be separated by breaking into two pieces.

Reason(R): The magnetic moment will be reduced to half when a magnet is broken into two equal pieces.

View answer and explanation

Correct option: B

If both (A) and (R) are true but (R) is not a correct explanation of the (A)

Option B is correct because the poles of a magnet cannot be separated, and while the magnetic moment is reduced when broken, it's not the reason why poles can't be separated.

102 Medium

102 of 128

Understanding +4 | -1

Assertion (A): Magnetic moment of helium atom is zero.

Reason (R): All the electron are paired in helium atom orbitals.

View answer and explanation

Correct option: A

If both (A) and (R) are true and the (R) is a correct explanation of the (A)

Option A is correct. Helium atom's magnetic moment is zero because all its electrons are paired, and the reason correctly explains this.

103 Medium

103 of 128

Understanding +4 | -1

The unit of

\frac{\mu_{0}}{4 \pi}

View answer and explanation

Correct option: B

N A^{-2}

Option B is correct because the unit of

\frac{\mu_{0}}{4 \pi}

N A^{-2}

, derived from the Biot-Savart law or Ampere's law.

104 Medium

104 of 128

Analyzing +4 | -1

Assertion (A): The properties of paramagnetic and ferromagnetic substance are not effected by heating.

Reason (R): As temperature rises, the alignment of molecular magnets gradually increases.

View answer and explanation

Correct option: D

If both (A) and (R) are false

Option D (If both (A) and (R) are false) is correct because heating *does* affect the properties of paramagnetic and ferromagnetic substances, and the alignment of molecular magnets *decreases* with rising temperature.

105 Medium

105 of 128

Application +4 | -1

A bar magnet is held at right angles to a uniform magnetic field. The couple acting on the magnet is to be halved by rotation it from this position. The angle of rotation is

View answer and explanation

Correct option: A

60^\circ

Option A is correct because the angle of rotation is found by setting the new torque (τ/2) equal to MBsin(90-θ) and solving for θ, which yields 60°.

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106 Medium

106 of 128

Application +4 | -1

A bar magnet is demagnetized by inserting it inside a solenoid of length 0.2m, 100 turns, and carrying a current of 5.2 A. The coercivity of the bar magnet is:

View answer and explanation

Correct option: B

2600 A/m

Option B is correct. The coercivity is calculated using the formula

H = \frac{{NI}}{{L}}

, where N is the number of turns, I is the current, and L is the length of the solenoid.

107 Medium

107 of 128

Analyzing +4 | -1

Assertion(A): The earth's magnetic field does not affect the working of a moving coil galvanometer.

Reason(R): Earth's magnetic field is very weak.

View answer and explanation

Correct option: A

If both (A) and (R) are true and the (R) is a correct explanation of the (A)

Option B (If both (A) and (R) are true but (R) is not a correct explanation of the (A)) is correct because the Earth's magnetic field *does* affect the galvanometer, but its effect is minimized by the design of the galvanometer.

108 Medium

108 of 128

Application +4 | -1

A long magnet is placed vertically with its

S -

pole resting on the table. A neutral point is obtained 10 cm from the pole the geographic north of it. If

H = 3.2 \times {10^{ - 5}}

T ,then the pole strength of magnet is

View answer and explanation

Correct option: C

32 ab-A-

\mathrm{c}{\mathrm{m}^{ - 1}}

Option C is correct because the pole strength is calculated using the formula for the magnetic field at a neutral point, resulting in 32 ab-A/cm.

109 Medium

109 of 128

Understanding +4 | -1

The torque on a bar magnet due to the earth’ magnetic field is maximum when the axis of the magnet is

View answer and explanation

Correct option: A

Perpendicular to the field of the earth

Option A is correct because the torque on a bar magnet is maximum when the angle between the magnetic moment and the magnetic field is 90 degrees, meaning they are perpendicular.

110 Medium

110 of 128

Remembering +4 | -1

The correct measure of magnetic hardness of a material is

View answer and explanation

Correct option: C

Coercivity

Option C (Coercivity) is correct because coercivity is the measure of a material's resistance to becoming demagnetized, which is a key indicator of its magnetic hardness.

111 Medium

111 of 128

Understanding +4 | -1

The relative permeability of a substance

X

is slightly less than unity and that of substance

Y

is slightly more than unity, then

View answer and explanation

Correct option: D

X

is diamagnetic and

Y

is paramagnetic

Option D is correct. A relative permeability slightly less than 1 indicates diamagnetism, while a value slightly more than 1 indicates paramagnetism.

112 Medium

112 of 128

Remembering +4 | -1

Water is

View answer and explanation

Correct option: A

Diamagnetic

Option A (Diamagnetic) is correct because water is repelled by magnetic fields, which is a characteristic of diamagnetic substances.

113 Medium

113 of 128

Application +4 | -1

The susceptibility of a paramagnetic material is

K

at 27

^ \circ C

. At what temperature will its susceptibility be

K/2

View answer and explanation

Correct option: D

327

^ \circ C

Option D is correct. Using Curie's law, susceptibility is inversely proportional to absolute temperature, so

T_2 = T_1 * (\chi_1 / \chi_2)

114 Medium

114 of 128

Application +4 | -1

The earth’s magnetic field at a certain place has a horizontal component

0.3\;gauss

and the total strength

0.5\;gauss

. The angle of dip is

View answer and explanation

Correct option: C

{\tan ^{ - 1}}\frac{4}{3}

Option C (

{\tan ^{ - 1}}\frac{4}{3}

) is correct because the angle of dip is given by

\tan{\delta} = \frac{B_V}{B_H}

, where

B_V

is the vertical component and

B_H

is the horizontal component.

115 Medium

115 of 128

Application +4 | -1

The time period of a vibration magnetometer is

{T_0}

. Its magnet is replaced by another magnet whose moment of inertia is 3 times and magnetic moment is 1/3 of the initial magnet. The time period now will be

View answer and explanation

Correct option: A

3{T_0}

Option A (

3{T_0}

) is correct because the time period is proportional to the square root of the moment of inertia and inversely proportional to the square root of the magnetic moment.

116 Medium

116 of 128

Understanding +4 | -1

In sum and difference method vibration magnetometer, the time period is more if

View answer and explanation

Correct option: B

Opposite poles of both magnets are on same sides

Option B is correct because when opposite poles are on the same side, the effective magnetic moment is smaller, leading to a longer time period.

117 Medium

117 of 128

Application +4 | -1

The distance of two points on the axis of a magnet from its centre is

10\;cm

and

20\;cm

respectively. The ratio of magnetic intensity at these points is

12.5\;:1

. The length of the magnet will be

View answer and explanation

Correct option: C

cm

Option C (10

cm

) is correct because the length of the magnet is derived from the ratio of magnetic intensities at two points on its axis.

118 Medium

118 of 128

Understanding +4 | -1

A bar magnet has magnetic moment

M

. If a hole is drilled normally through the centre of length of magnet, then the new magnetic moment of the magnet is

View answer and explanation

Correct option: A

Less than

M

Option A (Less than

M

) is correct because removing material from the magnet reduces its overall magnetic moment.

119 Medium

119 of 128

Application +4 | -1

Magnetic moment of bar magnet is

M

. The work done to turn the magnet by

90^\circ

of magnet in direction of magnetic field

B

will be

View answer and explanation

Correct option: D

MB

Option D is correct because the work done to turn the magnet by 90° is given by MB(cosθ₁ - cosθ₂), where θ₁ = 0° and θ₂ = 90°, resulting in MB.

120 Medium

120 of 128

Understanding +4 | -1

Due to the earth’s magnetic field, charged cosmic ray particles

View answer and explanation

Correct option: C

Can never reach the equator

Option A (Require greater kinetic energy to reach the equator than the poles) is correct because charged particles are deflected by the Earth's magnetic field, and this deflection is greatest at the equator.

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