Mutual inductance between square and twin-lead.

[Lavabug]

Homework Statement

Find the coefficient of mutual inductance between the twin lead and square circuit if they're being circulated by currents I1 and I2 respectively. Also find the interaction energy and force between the two. See image

[PLAIN]http://img94.imageshack.us/img94/1277/induct.png

The Attempt at a Solution

I know I have to calculate the flux of B produced by the two wires on the area demarcated by the square circuit and divide by I1 to get the inductance, but my question is, what is the total field (vector sum of the fields) produced by each of the wires in the region where the square circuit is?. Been drawing the field crosses and dots and it looks like it should be a subtraction between the two, am I correct in this assumption, or should they be added? Does it matter which one I subtract from the other?

I guess the magnitude of either one will determine what will be the resulting sign (into or out of the square), which will determine the direction of the current?

 

[jbsteele]

Also, how do I calculate the interaction energy and force?Any help on this will be greatly appreciated.