A metal disc rotates freely, between the poles of a magnet in the direction indicated. Brushes P and Q make contact with the edge of the disc and the metal axle. What current, if any, flows through R

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no current, because the emf induced in one side of the disc is opposed by the emf induced in the other side

no current, because the emf in the disc is opposed by the back emf

a current from P to Q

a current from Q to P

answer is A.

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Detailed Solution

OA is the elementary part (rod) of disc which is rotator in anticlock wise direction as seen from N – pole of given magnet. From fleming’s right hand rule, the motional emf and hence current in that element is form ‘P’ to ‘Q’. Disc is equivalent to parallel connection of infinite no. of such rods. In R, current is from P to Q. (outside the cell)

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