Question

A magnetized needle of the magnetic moment 20 JT^-1 is placed at 60^o with the direction of the uniform magnetic field of magnitude 9 T. What is the torque acting on the needle?

(a) 225 J

(b) 625 J

(c) 155.8 J

(d) 318 J

I had been asked this question in examination.

This question is from Bar Magnet in portion Magnetism and Matter of Physics – Class 12

Answer 1

Correct choice is (c) 155.8 J

Easiest explanation: Torque, τ = mBsin (θ).

Given: m = 20 J/T; B =9 T; θ = 60^o

τ = 20 JT^-1 × 9 T × sin(60^o)

τ = 155.8J.