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  4. A bar magnet has a magnetic moment 2.5 J T -1 and is placed in a magnetic field of 0.2 T. The work done in turning the magnet from parallel to antiparallel position relative to field direction is

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Q. A bar magnet has a magnetic moment $2.5 J T ^{-1}$ and is placed in a magnetic field of $0.2 T$. The work done in turning the magnet from parallel to antiparallel position relative to field direction is

Magnetism and Matter

Solution:

Work done in changing the orientation of a dipole of moment $M$ in a magnetic field $B$ from position $\theta_{1}$ to $\theta_{2}$ is given by
$W=M B\left(\cos \theta_{1}-\cos \theta_{2}\right)$
Here, $\theta_{1}=0^{\circ}$ and $\theta_{2}=180^{\circ}$
So, $ W=2 M B=2 \times 2.5 \times 0.2=1\, J$