A steel wire has a magnetic moment $M.$ What will be its magnetic moment if it is bent into a semi-circular arc ?
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Here, pole strength, $m=\frac{M}{l}$
If $\mathrm{r}$ is radius of semicircle, then
$\pi r=l, r=\frac{1}{\pi}$
New magnetic moment $M^{\prime}=m \times 2 r$
$=\frac{M}{l} \cdot \frac{2 l}{\pi}=\frac{2 M}{\pi}$
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