Question

A long thin magnet of magnetic moment $M$ is bent into a semi-circle. The decrease in the magnetic moment is

• A. $\frac{2 M}{\pi }$
• B. $\frac{\pi M}{2}$
• C. $\frac{M \left(\pi - 2\right)}{\pi }$
• D. $\frac{M \left(2 - \pi \right)}{2}$

Answer 1

Answer: (C)

Magnetic length is the minimum distance between the north pole and the south pole, which is decreased in the second case.
$M^{′}=\frac{2 M}{\pi }$ , decrease in magnetic moment
$=M-\frac{2 M}{\pi }$