A thin non-conducting disc of radius R is rot | vedclass

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(B) The rotating charged disc behaves like a collection of current loops. The magnetic field $B$ is present only on the right half of the disc. Consid

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The net torque vector on the disc is directed leftwards.

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(B) The rotating charged disc behaves like a collection of current loops. The magnetic field $B$ is present only on the right half of the disc. Consider a small element of the disc at a distance $r$ from the center. The rotation creates a current $dI$ in the circular path. The magnetic force on a charge element $dq$ moving with velocity $v$ is $dF = dq(v 	imes B)$. Since the disc is rotating clockwise,on the right half,the velocity vectors $v$ of the charges are directed downwards in the upper quadrant and upwards in the lower quadrant. Using the right-hand rule for $F = q(v 	imes B)$,where $B$ is directed downwards (as per the figure),the force on the upper right quadrant is directed out of the plane,and the force on the lower right quadrant is directed into the plane. These forces create a torque about the horizontal axis passing through the center. Specifically,the torque vector is directed towards the left. Thus,the net torque vector on the disc is directed leftwards.

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