A bar magnet of magnetic moment 2.0 A`m^(2) ` is free to rotate about a vertical axis passing through its centre, the magnet is released from rest from east-west position. Then, the kinetic energy of the magnet as it takes north-south position is (horizontal component of earth field is `25 mu`T)

Options

A. `25muj`
B. `50muj`
C. `100muj`
D. `12.5muj`

Answer

B

Explanation

Kinetic energy of the magnet =`MB(1-"cos"theta)`
Given `M=2 A-m^(2), B=25mu" "T`
` KE=2xx25xx10^(-6)(1-"cos"90^(@))=50xx10^(-6)=50muJ`