A steel wire of length $l$ and magnetic moment $M$ is bent into a semicircular arc of radius $R$. The new magnetic moment is

When wire is bent in the form of semicircular arc then, $l=\pi r$

$\therefore$ The radius of semicircular arc, $r=\frac{l}{\pi }$

Distance between two end points of semicircular wire

$=2r=\frac{2l}{\pi }$

$\therefore$Magnetic moment of semicircular wire

$=m\times 2r=m\times \frac{2l}{\pi }=\frac{2}{\pi }ml$

But $ml$ is the magnetic moment of straight wire i.e., $ml=M$

$\therefore$ New magnetic moment  $=\frac{2}{\pi }M$