Force per unit length acting on four parallel wires

[joeyc]

Homework Statement

An arrangement of four wires, in cross section, with θ=30° and u=5cm. Wire 2 and wire 4 carry the same magnitude of current (I₂=I₄=10mA) directed out of the page. Wire 3 carries current I₃=25mA directed into the page.

If the resultant force per unit length on wire 1 is zero due to the other three wires, determine s, the distance between the origin and wire 3. Write your final answer in centimeters.

Homework Equations

F/L = (μ_oI_aI_b)/2πd

The Attempt at a Solution

I try to equate F₂/L + F₃/L + F₄/L =0
but now what's makes me confusing is I don't have the value of I₁, so how should I continue it?

 

[gneill]

Look at the magnetic fields rather than the forces. The force on any current in wire 1 is due to the external magnetic field at its location.