Question

The work done in rotating a bar magnet of magnetic moment $M$ from its unstable equilibrium position to its stable equilibrium position in a uniform magnetic field $B$ is

• A. $2MB$
• B. $MB$
• C. $-MB$
• D. $-2MB$

Answer 1

The correct option is D $-2MB$

$u_i=-MB\left(cos\pi \right)$

$=MB$

$u_f=-MB$

$w=u_f-u_i$

Hence,

$w=-2MB$

so, option $D$ is correct.