As loop of magnetic moment `M` is placed in the orientation of unstable equilbirum position in a uniform magnetic field `B`. The external work done in rotating it through an angle `theta` is

Options

A. `-MB(1-costheta)`
B. `-Mbcostheta`
C. `Mbcostheta`
D. `MB(1-costheta)`

Answer

A

Explanation

`theta_i=180^@, theta_f=180^@-theta`
`W=U_f-U_i`
`=-Mbcos(180^@-theta)-(-MBcos180^@)`
`=Mbcostheta-MB`