A metallic ring of radius r with a uniform metallic spoke of negligible mass and length r is rotated about its axis with angular velocity in a perpendicular uniform magnetic field B as shown in figure. The central end of the spoke is connected to the rim of the wheel through a resistor R as shown. The resistor does not rotate, its one end is always at the center of the ring and the other end is always in contact with the ring. A force F as shown is needed to maintain constant angular velocity of the wheel. F is equal to (the ring and the spoke has zero resistance)
Induced emf in the spoke is shown in figure below.
There will be no induced emf separately in parts ADC or AEC.
Balancing torque about E:
Note: Force due to current s I/2 will act on circular parts also, but their torque about E will be zero.