A circular coil having $N\,turns$ and radius $r$ carries a current. It is held in the $XZ$ plane in a magnetic field $B\hat i$ . The torque on the coil due to the magnetic field is
JEE MAIN 2019, Diffcult
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$ \text { Magnetic moment of coil } =\mathrm{NIA} \hat{\mathrm{j}}$
$=\mathrm{Nl}\left(\pi \mathrm{r}^{2}\right) \hat{\mathrm{j}} $
Torque on loop (coil) $=\overline{\mathrm{M}} \times \overline{\mathrm{B}}$
$=\mathrm{Nl}\left(\pi \mathrm{r}^{2}\right) \mathrm{B} \sin 90^{\circ}(-\hat{\mathrm{k}})$
$=\mathrm{NI} \pi \mathrm{r}^{2} \mathrm{B}(-\hat{\mathrm{k}})$
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