A bar magnet of magnetic moment `12 A-m^2` is free to rotate about a vertical axis passing through its centre. The magnetic released from rest from east-west direction. Find kinetic energy of the magnet as it takes North-South position if horizontal composed of earth's magnetic field is `30 muT`.

Explanation

`DeltaK = Delta U = U_(f) - U_(i) =-mB (cos theta_(2) -cos theta_(1))`
`=-mB(1-0) =-mB rArr 12 xx 30 = 360 mu`