Magnetism and Matter | Physics JEE Main questions with Solutions

Q 1 :

A straight magnetic strip has a magnetic moment of 44 ${Am}^{2}$ . If the strip is bent in a semicircular shape, its magnetic moment will be ____ ${Am}^{2}$ (Given $π = \frac{22}{7}$ ) [2024]

0%

(28)

Magnetic moment of straight wire $M = 44 {Am}^{2}$

We know that, pole strength $m = \frac{M}{ℓ} \Rightarrow M = m ℓ$

If $r$ is the radius of semicircle formed by bending the strip of length $l,$ then

$π r = ℓ \Rightarrow r = \frac{ℓ}{π}$

Distance between the poles; $2 r = \frac{2 l}{π}$

Hence, new magnetic moment $(M') = m \times 2 r = \frac{M}{l} \cdot \frac{2 l}{π}$

$\Rightarrow M' = \frac{2 M}{π} = \frac{2 \times 44}{(\frac{22}{7})} = 28 {Am}^{2}$

Q 2 :

A bar magnet has total length $2 l = 20$ units and the field point P is at a distance d = 10 units from the centre of the magnet. If the relative uncertainty of length measurement is 1%, then uncertainty of the magnetic field at point P is: [2025]

10%
4%
3%
5%

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(2)

Magnetic field at P, $B = \frac{μ_{0}}{4 π} \frac{m (2 l)}{d^{3}}$ , where m is the pole strength

$\Rightarrow (\frac{△ B}{B} \times 100) = (\frac{△ l}{l}) \times 100 + 3$ $| \frac{△ d}{d} |$ $\times 100$

= 1% + 3% = 4%

Q 3 :

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : Magnetic monopoles do not exist.

Reason (R) : Magnetic field lines are continuous and form closed loops.

In the light of the above statements, choose the most appropriate answer from the options given below: [2025]

Both (A) and (R) are correct but (R) is not the correct explanation of (A)
(A) is correct but (R) is not correct
Both (A) and (R) are correct and (R) is the correct explanation of (A)
(A) is not correct but (R) is correct

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(3)

The magnet does not exist in form of monopole and magnetic lines never intersect, are continuous and form a closed loop. Line initiates from north pole and end to south pole outside while inside vice versa.

It means both statements are correct and reason is also the correct explanation of assertion.

Q 4 :

A bar magnet is released from rest along the axis of a very long vertical copper tube. After some time the magnet will [2023]

Move down with almost constant speed
Oscillate inside the tube
Move down with an acceleration greater than g
Move down with an acceleration equal to g

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(1)

After some time both force becomes equal.

Q 5 :

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.

Assertion A: A bar magnet dropped through a metallic cylindrical pipe takes more time to come down compared to a non-magnetic bar with the same geometry and mass.

Reason R: For the magnetic bar, Eddy currents are produced in the metallic pipe which oppose the motion of the magnetic bar.

In the light of the above statements, choose the correct answer from the options given below [2023]

Both A and R are true but R is NOT the correct explanation of A
A is true but R is false
Both A and R are true and R is the correct explanation of A
A is false but R is true

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(3)

Conceptual

Q 6 :

A bar magnet with a magnetic moment 5.0 ${Am}^{2}$ is placed in parallel position relative to a magnetic field of 0.4 T. The amount of required work done in turning the magnet from parallel to antiparallel position relative to the field direction is ________. [2023]

4 J
1 J
2 J
Zero

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(1)

$u = - M B \cos θ$

$W = Δ u$

$W = - M B \cos 180 ° (- m B \cos 0 °)$

$W = 2 M B = 2 \times 5 \times 0.4 = 4 J$

Q 7 :

The magnetic intensity at the center of a long current carrying solenoid is found to be $1.6 \times 10^{3} {Am}^{- 1}$ . If the number of turns is 8 per cm, then the current flowing through the solenoid is ________ A. [2023]

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(2)

$H = \frac{B}{μ_{0}} = \frac{μ_{0} n i}{μ_{0}} = n i$

$i = \frac{H}{n} = \frac{1.6 \times 10^{3}}{(\frac{8}{10^{- 2}})} = 2 A$

Q 8 :

The current required to be passed through a solenoid of 15 cm length and 60 turns in order to demagnetise a bar magnet of magnetic intensity $2.4 \times 10^{3} {Am}^{- 1}$ is _________ A. [2023]

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(6)

$I = H$

$Given, I = 2.4 \times 10^{3} A/m$

$2.4 \times 10^{3} = H = n i$

$Number of turns per unit length, n = \frac{N}{ℓ}$

$2.4 \times 10^{3} = \frac{60}{15 \times 10^{- 2}} i$

$i = \frac{2.4 \times 15 \times 10}{60} = \frac{36}{6} = 6 A$

Q 9 :

A short bar magnet placed with its axis at $30 °$ with an external field of 800 Gauss experiences a torque of 0.016 N.m. The work done in moving it from the most stable to the most unstable position is $α \times 10^{- 3} J$ . The value of $α$ is _______. [2026]

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(64)

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