Force on 2 wires near each other with current

[D.B0004]

Homework Statement

Two wires are placed parallel near each other in free space at a distance of d=0.08m. They have a length of L=0.4m and each carry a current of i=3.7A (in the same direction.) Find the force exerted on each other. Consider attraction as positive force and repulsion as a negative force.

Homework Equations

F = B*I*L
B= = u*Hs = (u*I/(2*PI*R))*I*L

The Attempt at a Solution

4*PI*10^-7*3.7/(2*PI*.04)*3.7*.4 = 27.38uN
The correct answer is apparently 13.69N. Anyone see where I am going wrong? Is my B equation incorrect? Thanks!

 

[mfb]

B= = u*Hs = (u*I/(2*PI*R))*I*L

I don't understand what you calculated here.
And I wonder how you got units of tesla out of that calculation (a good check to see if everything went right).

A different issue: the approximation of very long cables (compared to R) you used might not be a good one.

 

[D.B0004]

I got the calculations from the website below. Not sure where else to find the solution to the problem.

http://dev.physicslab.org/Document.aspx?doctype=3&filename=Magnetism_CurrentCarryingWires.xml