A magnetic dipole is placed at right angles to the direction of magnetic induction B . If it is rotated through an angle of 180 ° , then the work done is

Step-by-step solution

Work done in rotating a dipole W = M B cos ⁡ θ 1 - cos ⁡ θ 2 To rotate it from 90 ° to 0 ° θ 1 = 90 ° θ 2 = 0 ° W = M B cos ⁡ 9 0 ° - cos ⁡ 0 ° W 1 = M B 0 - 1 = - M B W 1 = - M B .........(i) To rotate it from 0 ° to - 90 ° in same manner. θ 1 = 0 ° θ 2 = - 90 ° W 2 = M B cos ⁡ 0 ° - cos ⁡ - 90 ° W 2 = M B ........(ii) Total workdone W = W 1 + W 2 W = - M B + M B W = 0