The figure shows an insulating ring and an infinite straight wire resting on the
ID: 2141371 • Letter: T
Question
The figure shows an insulating ring and an infinite straight wire resting on the surface of an infinite plane. The wire is fixed in position, but the ring slides without friction on the surface of the plane. The ring is uniformly charged, with net positive charge Q. Initially, the wire carries a constant current I towards the top of the figure, and the ring is stationary. At time t=0, the current in the wire is reduced to I/2. Which of the following best describes the motion of the ring after the current is reduced?
Hint: Think of the induced electric fields. If they can make current move, they can also apply a force or torque to a charge on an insulator.
Explanation / Answer
The ring will spin clockwise and move away from the wire.This is because the charge on the ring is +Q and the current in the wire is reduced to I/2.The change in current causes an induced emf in the ring and the ring spins clockwise and moves away from the wire.
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