# A magnetised wire is bent into an arc of a circle subtending an angle 60° at its centre. Then its magnetic moment is x…..

Q. A magnetised wire is bent into an arc of a circle subtending an angle 60° at its centre. Then its magnetic moment is x . If the same wire is bent into an arc of a circle subtending an angle 90° at its centre then its magnetic moment will be

(a) $\displaystyle \frac {x\sqrt2}{3}$

(b) $\displaystyle \frac {x}{3}$

(c) $\displaystyle \frac {2\sqrt2 x }{3}$

(d) $\displaystyle \frac {3 x}{2\sqrt2}$

Ans: (c)
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

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

In first case ,

$\displaystyle x = \frac{2M sin(\pi/6)}{\pi/3}$

$\displaystyle x = \frac{3 M}{\pi}$ …(i)

In second case ,

$\displaystyle x’ = \frac{2M sin(\pi/4)}{\pi/2}$

$\displaystyle x’ = \frac{2\sqrt{2} M}{\pi}$ …(ii)

From(i)

$\displaystyle x’ = \frac{2\sqrt{2} x}{3}$