A bar magnet with a magnetic moment 5 . 0A m ^ 2 is placed in...

For a bar magnet, the work done in turning from parallel to antiparallel position relative to the field direction is

$W = MB\left(cos{\theta}_{1} - cos{\theta}_{2}\right)$, here, ${\theta}_{1} = 0 ° and{\theta}_{1} = 180 °$.

$W = - MBcos180 ° \left(- mBcos0 °\right)$

$W = 2MB = 2 \times 5 \times 0 . 4 = 4J$