# Hoop, Line and Stinker

1. Oct 28, 2009

### cpcolts

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?

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).

my solution:

OK so i feel like the answer is j. because when the current decreases the magnetic field will also decrease which will let the ring go and it slide down.
But I am not sure that the answer is right though.