# Homework Help: Force on Paralell Wires

1. Jul 16, 2008

### myersb05

I felt like this problem was simple. We submit our answers in WebAssign however, and I can not find the correct solution.

Two parallel wires are 20.0 cm apart, and each carries a current of 40.0 A.

(a) If the currents are in the same direction, find the force per unit length exerted on one of the wires by the other.

(b) Repeat the problem with the currents in opposite directions.

I know that Bwire=4*10^-7(constant)*I/2piR
So I calculated Bwire by using 4*10^-7*40/2pi*.2 and got a value of .000013 for each of the wires because their distances apart and currents are equal.

I also know that F/L=IBsin(theta). I plugged my current of 40 amps and my newly calculated B wire in. I thought that there is no angle between two paralell wires so using 0, my values for force came out as 0. After finding that was not correct, I disregarded the sin part of the equation thinking that it would probably be 1 or -1. I came up with a F/L value of .00052 N/m which is not correct. Where am I going wrong?

2. Jul 17, 2008

### alphysicist

Hi myersb05,

I don't believe this formula is quite right. Do you see what's missing?