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 N disc and the metal axle. What current, if any, flows through R?

Options

• Option A

  a current from P to Q P ω Q

• Option B

  a current from Q to P

• Option C

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

• Option D

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