Question

Three wires lie in a plane, placed parallel to one another and equally spaced by 0.20 m. If the wires are oriented up and down on
the page, the right-hand wire carries 100 A to the top, the middle wire 300 A to the bottom, and the left-hand wire carries 200 A to the top. What are the forces per unit length on each of the wires?

Answer 1

Ugh. Such tedium...
First find the magnetic field exerted by the other two wires:
B = (mu)i/2pi r
for example for the 2nd on the left wire-
B = pi4e-7(300)/2pi(.2)
(you can cancel the pi's)
Sum the two B fields (they're opposite direction so careful)
Next find the force/length
F/L = IB
Lather rinse repeat