back to directory
NEET PHYSICSMAGNETISM AND MATTERMedium

Question

A bar magnet is hung by a thin cotton thread in a uniform horizontal magnetic field and is in the equilibrium state. The energy required to rotate it by 60∘60^\circ is WW. Now the torque required to keep the magnet in this new position is:

A

W3\frac{W}{\sqrt{3}}

B

3W\sqrt{3}W

C

3W2\frac{\sqrt{3}W}{2}

D

2W3\frac{2W}{\sqrt{3}}

Step-by-Step Solution

  1. Work Done (Energy): The work done (WW) in rotating a magnetic dipole with magnetic moment mm in a uniform magnetic field BB from an initial angle θ1\theta_1 to a final angle θ2\theta_2 is equal to the change in potential energy: W=Uf−Ui=−mBcos⁡θ2−(−mBcos⁡θ1)=mB(cos⁡θ1−cos⁡θ2)W = U_f - U_i = -mB \cos \theta_2 - (-mB \cos \theta_1) = mB(\cos \theta_1 - \cos \theta_2) Given that the magnet starts from the equilibrium state, θ1=0∘\theta_1 = 0^\circ. It is rotated by 60∘60^\circ, so θ2=60∘\theta_2 = 60^\circ. W=mB(cos⁡0∘−cos⁡60∘)=mB(1−12)=mB2W = mB(\cos 0^\circ - \cos 60^\circ) = mB\left(1 - \frac{1}{2}\right) = \frac{mB}{2} From this, we deduce: mB=2WmB = 2W.
  2. Torque Calculation: The torque (τ\tau) magnitude required to hold the magnet at an angle θ\theta is given by: τ=mBsin⁡θ\tau = mB \sin \theta At the new position θ=60∘\theta = 60^\circ: τ=mBsin⁡60∘\tau = mB \sin 60^\circ
  3. Substitution: Substitute the value of mBmB derived from the work equation: τ=(2W)(32)=3W\tau = (2W) \left( \frac{\sqrt{3}}{2} \right) = \sqrt{3}W

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from MAGNETISM AND MATTER. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSMAGNETISM AND MATTERmagnetcottonthreaduniformhorizontal

More MAGNETISM AND MATTER Questions

View all

If a diamagnetic substance is brought near the north or the south pole of a bar magnet, it is

A.repelled by both the poles
B.repelled by the north pole and attracted by the south pole
C.attracted by the north pole and repelled by the south pole
D.attracted by both the poles
EasySolve

The variation of susceptibility ($\chi$) with absolute temperature ($T$) for a paramagnetic material is represented as:

A.Option 1
B.Option 2
C.Option 3
D.Option 4
EasySolve

If $\theta_1$ and $\theta_2$ be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip $\theta$ is given by:

A.$\cot^2 \theta = \cot^2 \theta_1 + \cot^2 \theta_2$
B.$\tan^2 \theta = \tan^2 \theta_1 + \tan^2 \theta_2$
C.$\cot^2 \theta = \cot^2 \theta_1 - \tan^2 \theta_2$
D.$\tan^2 \theta = \tan^2 \theta_1 - \tan^2 \theta_2$
MediumSolve

A magnetic needle suspended parallel to a magnetic field requires $\sqrt{3}$ J of work to turn it through $60^\circ$. The torque needed to maintain the needle in this position will be:

A.$2\sqrt{3}$ J
B.3 J
C.$\sqrt{3}$ J
D.$\frac{3}{2}$ J
MediumSolve

An iron bar of length $L$ has a magnetic moment $M$. It is bent at the middle of its length such that the two arms make an angle $60^{\circ}$ with each other. The magnetic moment of this new magnet is:

A.M/2
B.2M
C.M\sqrt{3}
D.M
MediumSolve

Above Curie temperature:

A.a ferromagnetic substance becomes paramagnetic.
B.a paramagnetic substance becomes diamagnetic.
C.a diamagnetic substance becomes paramagnetic.
D.a paramagnetic substance becomes ferromagnetic.
EasySolve

The magnetic susceptibility is negative for:

A.paramagnetic material only.
B.ferromagnetic material only.
C.paramagnetic and ferromagnetic materials.
D.diamagnetic material only.
EasySolve

The following figures show the arrangement of bar magnets in different configurations. Each magnet has a magnetic dipole moment $\mathbf{m}$. Which configuration has the highest net magnetic dipole moment?

A.Option 1
B.Option 2
C.Option 3
D.Option 4
EasySolve

Why 32,000+ students choose TopperSquare

Everything you need to crack NEET

15,000+ Chapter MCQs

Physics, Chemistry, Biology — chapter-wise, topic-wise practice

AI Performance Analytics

Know your weak topics. Get a personalised study plan.

Full NEET Mock Tests

180 questions, timed, with detailed score analysis

PYQ Sets 2010–2024

15 years of past papers with solved explanations

Start Practising Free

No credit card · 30-second signup · Free forever plan included

This neet physics practice question is part of the TopperSquare free question bank. TopperSquare offers 15,000+ chapter-wise NEET MCQs across Physics, Chemistry, and Biology with detailed step-by-step explanations, full mock tests, NEET PYQs (2010–2024), and an AI-powered performance analytics dashboard. browse all neet practice questions → · practice physics sets →

Explore more subjects:Physics MCQs•Chemistry MCQs•Biology MCQs•Botany MCQs•Zoology MCQs•NEET Mock Tests•NEET PYQs
A bar magnet is hung by a thin cotton thread in a ... | NEET MCQ | TopperSquare