# Insulating ring and straight wire

1. Mar 9, 2009

### svayl

1. The problem statement, all variables and given/known data

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?

1. The ring will spin clockwise and slide away from the wire.
2. The ring will spin clockwise and slide towards the wire.
3. The ring will spin clockwise and remain stationary.
4. The ring will not spin, but it will slide towards the wire.
5. The ring will spin counterclockwise and slide towards the wire.
6. The ring will spin counterclockwise and remain stationary.
7. The ring will not spin, but it will slide away from the wire.
8. The ring will spin clockwise and slide towards the top of the figure.
9. The ring will spin counterclockwise and slide towards the top of the figure.
10. The ring will not spin, but it will slide towards the bottom of the figure.
11. None of the above (please describe what does happen).

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.

2. Relevant equations

3. The attempt at a solution

All I know is that it would spin...since is loses current. I am having trouble figuring out its direction. Any help/advice would be greatly appreciated!