Question

A short bar magnet of magnetic moment $0.4{\text{JT}}^{-1}$ is placed in a uniform magnetic field of $0.16\text{T}$. The magnet is in unstable equilibrium when the potential energy is

• A. $-0.064\text{J}$
• B. $0\text{J}$
• C. $-0.032\text{J}$
• D. $0.064\text{J}$

Answer 1

The correct option is D $0.064\text{J}$

For unstable equilibrium, angle between magnetic moment and magnetic field is $\theta ={180}^{\circ }$

Potential energy
$U=-MB\cos \theta =-MB\cos {180}^{\circ }$

$U=-0.4\times 0.16\times (-1)$

$U=0.064\text{J}$