Question

A magnet of magnetic moment `M` is situated with its axis along the direction of a magnetic field of strength `B`. The work done in rotating it by an angle of `180^(@)` will be
A. ` - MB`
B. `+ MB`
C. zero
D. ` + 2 MB`

Answer 1

Correct Answer - D
Work done in rotating the magnet from angle `theta_(1) "to angle" theta_(2)` is given by,
`W=MB(costheta_(1)-costheta_(2))`
`Here, theta_(1)=0^(@) and theta_(2)=180^(@)`
`W=MB(cos 0^(@)-cos 180^(@))=2MB`