A disc of radius $r$ and carrying positive charge $q$ is rotating with an angular speed $l$ in a uniform magnetic field $B$ about a fixed axis as shown in figure, such that angle made by axis of disc with magnetic field is $\theta $. Torque applied by axis on the disc is
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$\frac{M}{L}=\frac{q}{2 m}$
$\Rightarrow M=\frac{q}{2 m} \times \frac{m r^{2}}{2} \omega \Rightarrow \tau=\bar{M} \times \bar{B}=\frac{q \omega r^{2} B \sin \theta}{4}(\text { clockwise })$
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