Electromagnetic induction problem

[oportunity]

Homework Statement

http://imageshack.us/photo/my-images/194/55132039.png/

http://imageshack.us/photo/my-images/194/55132039.png/

How much curret flows trough circuit ABCD when the center of the circuit (O) is distance r from the straight wire? I think these are the relevant equations:
∅=B.dS
E=-d∅ /dt
B= μoI/2∏r

But i have trouble with dS. i know that is has to be rewritten like e.g. ds= ydx=yvdt and that way eliminate dt
but in this case i don't know how to do it.

Thanks

 

[rude man]

Well, obviously v is associated with dψ/dt where ψ = ∫BdS (apperently you are using S for area.) (Sorry, the symbol table does not have a Greek letter for "phi", can't imagine how that fell thru the cracks).

You have the right formula for B(r), you know v = dr/dt, right? So how does ∫BdS change with time? S doesn't change with time, it's just the area of the square ... so it must be B that is changing ...