Question

A thin non-conducting disc of radius $R$ is rotating clockwise (see figure) with an angular velocity $\omega$ about its central axis, which is perpendicular to its plane. Both its surfaces carry +ve charges of uniform surface density. Half the disc is in a region of a uniform, unidirectional magnetic field $B$ parallel to the plane of the disc, as shown. Then,

• A. The net torque on the disc is zero
• B. The net torque vector on the disc is directed leftwards
• C. The net torque vector on the disc is directed rightwards
• D. The net torque vector on the disc is parallel to $B$

Answer 1

Answer: (B)

$\vec{\tau}=\vec{M} \times \vec{B}=\frac{I A}{2}(-\hat{k}) \times B(-\hat{j})=\frac{I A B}{2}(-\hat{i})$ (leftward)