Question

A magnet of moment 4Am${}^2$ is kept suspended in a magnetic field of induction $5\times {10}^{-5}T$. The workdone in rotating it through 180${}^0$ is

• A. $4\times {10}^{-4}J$
• B. $5\times {10}^{-4}J$
• C. $2\times {10}^{-4}J$
• D. ${10}^{-4}J$

Answer 1

The correct option is A $4\times {10}^{-4}J$

Torque acting on a magnetic dipole of moment $M$ kept in uniform magnetic field=$MBsin\theta$

Therefore work done in rotating a magnet of moment $M$ by an angle $\theta =MB(1-\cos \theta )=MB(1-(-1\left)\right)=2MB$

$=2\times 4\times 5\times {10}^{-5}=4\times {10}^{-4}J$