How Does Lenz's Law Explain the Movement of a Loop Near a Current-Carrying Wire?

[joriarty]

Homework Statement

The diagram below shows two circuits: a very long straight wire, and a single loop rectangle of dimensions a and b. The rectangle lies in a plane through the wire and is placed a distance c from the long wire as shown. The long straight wire carries a current of I.

[PLAIN]http://img121.imageshack.us/img121/6890/screenshot20101107at259.png

a. Derive an expression for the magnetic flux Φ through the area of the rectangle. Hint: Consider small strips of area inside the rectangle of length b and width dr.

b. Using the diagram for the previous problem explain what would happen to the position of the rectangular loop if the current through the long straight wire was suddenly switched off and the rectangular loop was free to move.

The Attempt at a Solution

For part (a) I derived $$B=1/2\,\mu_{{0}}ib\ln \left( {\frac {c+a}{c}} \right) {\pi }^{-1}$$.

But I'm not sure how to do part (b). I'm thinking I could use Lenz's Law, if I could find the current direction for the loop. Using the right hand rule I think this current would be going clockwise, same direction as the wire. A current in two wires going in the same direction will mean the loop moves towards the wire.

 

[Redbelly98]

I agree with your part (a) answer.

For part (b), I think you are correct and I agree that the loop current is clockwise. But I have some reservations, because it says the wire current is turned off suddenly. The attraction towards the wire only holds true while the wire's current is reduced, but not yet zero. I guess "suddenly" still allows a small amount of time to take place while the current drops to zero, otherwise there would be no force on the loop.

Also, I wouldn't say that the straight wire has a clockwise current -- that does not make sense.