Q. A magnet of magnetic moment M and length 2*l * is bent at its mid-point such that the angle of bending is 60°. The new magnetic moment is.

(a) M

(b) M/2

(c) 2M

(d) M/√2

Ans: (b)

Solution : When a magnet is bent at its mid-point , each half having magnetic moment M/2 & angle between them is 120°

The new magnetic moment is

$\displaystyle M’ = \sqrt{(\frac{M}{2})^2 + (\frac{M}{2})^2 + 2 \times \frac{M}{2} \times \frac{M}{2} \times cos120}$

$ \displaystyle M’ = \sqrt{(\frac{M^2}{4}) + (\frac{M^2}{4}) + 2 \times \frac{M}{2} \times \frac{M}{2}\times \frac{-1}{2}}$

$ \displaystyle M’ = \frac{M}{2} $