The length of a steel wire is $L$ and its magnetic moment is $M$. On bending it into the shape of a semicircle, its’ magnetic moment will be
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Initial magnetic moment $M=m l$
when it is bent in a semi circular wire the effective length between the poles become $2 R$
where $\pi R=l$
$R=l / \pi$
Therefore, new $M_{\text {new}}=m l_{e f f}=m \times 2 \times R=\frac{m \times 2 \times l}{\pi}=\frac{2 M}{\pi}$
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