Calculating Force of Parallel Wires: 1 Wire & 1 Loop

[Marioqwe]

Homework Statement

So let's say that we have 2 wires that are parallel. To find the force that one exerts on the other I can use the equation B = μ0 I / (2 * π r), where μ0 is equal to 4(pi)e-7. But what can I do when I have 1 wire parallel to a wire that forms a loop. Will the equation still work?

Homework Equations

B = μ0 I / (2 * π r)

The Attempt at a Solution

 

[vela]

Depends. The derivation of the equation relies on a symmetry that arises because you assume the wire is infinitely long. The equation is only applicable to situations where that assumption is a good approximation, so if you want to use it to calculate the magnetic field of the wire, that's fine. You can't use it, however, to find the magnetic field due to the loop.