# Homework Help: Hoop and Line

1. Mar 6, 2008

### GoColts

Hello guys I'm a first time poster here and this problem has had me stumped.

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?

So this is the question above and pretty much the picture shows a loop on the left of a straight infinently long wire with no distance in between specified.

The question offers several choices then:

a) The ring will spin clockwise and slide away from the wire.
b) The ring will spin clockwise and slide towards the wire.
c) The ring will spin clockwise and remain stationary.
d) The ring will not spin, but it will slide towards the wire.
e) The ring will spin counterclockwise and slide towards the wire.
f) The ring will spin counterclockwise and remain stationary.
g) The ring will not spin, but it will slide away from the wire.
h) The ring will spin clockwise and slide towards the top of the figure.
i) The ring will spin counterclockwise and slide towards the top of the figure.
j) The ring will not spin, but it will slide towards the bottom of the figure.
k) None of the above (please describe what does happen).

2. Relevant equations
For the equations I have been thinking that I should use the flux equation which is the

Flux = BA but since this is a conceptual problem I am wondering if using equations is even helpful in this problem.

3. The attempt at a solution

I have guessed that it could be choice E above since the positve charge in the ring would create an attractive force with the current but I don't really understand of how the ring will spin. Thanks for all the help.