Question

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

• A. 0.064 J
• B. -0.064 J
• C. zero
• D. 0.082 J

Answer 1

Answer: (B)

-0.064 J

Potential energy of a magnet with magnetic moment $\overrightarrow{M}$ placed in a magnetic field $\overrightarrow{B}$ is given by

$U=-\overrightarrow{M}.vecB$

$=-MBcos\theta$

which is smallest for $\theta =0$

Hence $U_{min}=-MB=-0.4\times 0.16J$

$=-0.064J$