A bar magnet is held perpendicular to a uniform magnetic field. If the couple acting on the magnet is to be halved by rotating it, then the angle by which it is to be rotated is

Options

A. `30^(@)`
B. `45^(@)`
C. `60^(@)`
D. `90^(@)`

Answer

C

Explanation

`tau= mB sin theta`
`rArr (tau_(1))/(tau_(2)) = (sin theta_(1))/(sin theta_(2)) rArr (tau)/(tau//2) = (sin 90^(@))/(sin theta)= theta= 30^(@)`
Angle of rotation `=90^(@) - 30^(@)= 60^(@)`