Q. A bar magnet of magnetic moment ‘M’ is bent in the form of an arc which makes angle 60°. The percentage change in the magnetic moment is

(a) 9% Increase

(b) 9% Decrease

(c) 4.5% Decrease

(d) 4.5% Increase

Ans: (b)

Sol: When a thin bar magnet of magnetic moment M is bent into an arc of a circle subtending an angle ‘ θ ’ radians at the centre of the circle, then its new magnetic moment is given by

$ \displaystyle M’ = \frac{2 M sin(\theta /2)}{\theta}$ ; (θ must be in radians)