Question

A magnet of magnetic moment $M$ is situated with its axis along the direction of a magnetic field of strength $B$. The work done in rotating it by an angle of $180^{\circ}$ will be

• A. -M B
• B. +M B
• C. 0
• D. +2 M B

Answer 1

Answer: (D)

Work done $=M B\left(\cos \theta_{1}-\cos \theta_{2}\right)$
$\theta_{1}=0^{\circ}$ and $\theta_{2}=180^{\circ}$
$\Rightarrow W =M B\left(\cos 0^{\circ}-\cos 180^{\circ}\right)=2\, M B$