Question

Magnetic moment of bar magnet is $M$. The work done to turn the magnet by $90^{\circ }$ of magnet in direction of magnetic field $B$ will be

• A. zero
• B. $\frac{1}{2}MB$
• C. 2 MB
• D. MB

Answer 1

Answer: (D)

Potential energy of a magnetic moment at an angle $\theta$ with the magnetic field $=-MB\cos \theta$
Thus work done in rotating from angle ${\theta }_1$ to ${\theta }_2$
$=MB\left(\cos {\theta }_1-\cos {\theta }_2\right)$
$=MB\left(\cos 0^{\circ }-\cos 90^{\circ }\right)$
$=MB(1-0)$
$=MB$