A bar magnet of magnetic moment $1.5A\,m^{2}$ is initially placed in the direction of a uniform magnetic field of $18\times10^{-2}$ T. The work to be done to rotate the magnet so that its magnetic moment becomes perpendicular to the direction of the magnetic field is}
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For magnetic dipoles,
\[
U=-MB\cos\theta
\]
and
\[
W=\Delta U
\]
Always calculate the change in potential energy between initial and final orientations.