# Electromagnetic induction problem

## 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 dont know how to do it.

Thanks

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rude man
Homework Helper
Gold Member
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 ...