Question

A magnet of magnetic moment M is rotated by ${360}^0$ in a magnetic field B. What will be the work done?

• A. $180$
• B. $0$
• C. $360$
• D. $90$

Answer 1

The correct option is B $0$

Work done, $W={\int }_0^{2\pi }\tau \cdot d\theta$

$={\int }_0^{2\pi }MB\sin \theta d\theta =MB{\int }_0^{2\pi }\sin \theta d\theta$

$=MB{[-\cos \theta ]}_0^{2\pi }$

$=MB[-1+1]=0$

W is the work done in rotating it from $0$ to $360$ degrees.

$\tau$ is the torque acting on it.