A short bar magnet of magnetic moment $0.4 \mathrm{~J} \mathrm{~T}^{-1}$ is place in a uniform magnetic…

A short bar magnet of magnetic moment $0.4 \mathrm{~J} \mathrm{~T}^{-1}$ is place in a uniform magnetic field of $0.16 \mathrm{~T}$. The magnet is stable equilibrium when the potential energy is
  1. $-0.64 \mathrm{~J}$
  2. zero
  3. $-0.082 \mathrm{~J}$
  4. $-0.064 \mathrm{~J}$

Solution

We know that
$U=-M B \cos \theta$
For stable equilibrium $\theta=0^{\circ}$
$\begin{aligned}
U & =-M B \\
& =-(0.4)(0.16)=-0.064 \mathrm{~J}
\end{aligned}$ ^

Asked in: NEET 2011 (Mains)

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