Decide the resulting force (F) per meter on L2

[DecoratorFawn82]

Homework Statement

Three long straight parallell wires that conduct electricity called L1, L2 and L3. The angle at L2 is 90 degrees and the distance between L2 and L1, L3 is 0,50 meters. The current through the wires L1 and L3 is 300 A while the current in L2 is 600 A. Decide the resulting power force (F) per meter at L2.

PICTURE:
O. the current goes towards you (out from the book for example)
Ox the current goes from you (into the book for example)
L3 O. ---<
|
| 0,50m
|
L1 L2 |
O. 0,50m ⊗ ----<
|<----------------->|

Homework Equations

F = I * l * B = [A] * [m] * [T]

B = k * (I/a)

The Attempt at a Solution

[/B]
First I thought that I was supposed to use pythagoras theorem to calculate the distance between L1 and L3:

l = √(0,50^2+0,50^2) = 0,71 m

Using this information I thought I could calculate the power for L1 and L3 and somehow calculate the power for L2 also.

I knew I (A) and l (m) but I didn't know B? How am I supposed to be able to calculate B when I don't know what F is?

The answer according to my physics book is F = 0,10 N at southeast.

 

[Simon Bridge]

I don't have a good picture of the geometry of the situation.
Are L1 and L3 parallel?
Which directions do the current point?

If you define x-y-z axes, so the the x-axis is to the right and the y-axis is up the page, then z axis points out of the page.
If L2 is along the z axis - and it's current points in the -z direction ... where are the others?
Would L1 and L3 be in the same plane? Then put them in the x-y plane perhaps?

Presumably you have an equation for the force on a current-carrying wire that is in a magnetic field.
You have another equation for the magnetic field due to a long straight wire carrying a current.