# Equilibrium, charge, and reaction question

1. Mar 12, 2009

### carrotcake10

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

http://img25.imageshack.us/img25/4573/hoopline.gif [Broken]
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.

3. The attempt at a solution
So obviously when the ring is stationary, the two objects are in static equilibrium, but I still don't know precisely how to answer this question. I think that if the current is halved, the electric field will be weaker.

What it comes down to is that I don't know the nature of two such objects given any conditions. Any links/explanations would be very helpful. My textbook, in my opinion, is fairly poor at conceptual explanations.

Thanks!

Last edited by a moderator: May 4, 2017
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