Q 12-05-013NEETJEE MainMedium
The work done in turning a magnet of moment $M$ from the direction of a uniform field $B$ to the opposite direction is
Answer: (D) $2MB$
Concept. Work done in rotating a dipole from $\theta_1$ to $\theta_2$ equals the rise in potential energy: $W = MB(\cos\theta_1 - \cos\theta_2)$.
Step 1. Substitute. From $\theta_1 = 0°$ to $\theta_2 = 180°$:
$$W = MB(\cos 0° - \cos 180°) = MB(1 + 1) = 2MB$$
Answer. $2MB$ (Option D).
Solution by Sreeraj P, M.Sc Physics