# Homework Help: Easy physics problem, need help finding forumula

1. Sep 13, 2004

### rdn98

I have included a picture.

Question:
What is the force on each wire if B = 1.656T, i = 10A, L1 = 7cm and L2 = 14cm?

Now, I have checked my physics book, and the equation I have found is
F_12 = mu_0*L*I1*I2/(2*pi*D)

where D is the distance between the two wires, and L is the length of one wire.

However, I tried that formula in the computer, and it doesn't work. I let L =L2 (m), D=L1 (m), I have tried I1=10 and I2=+/- 10...but it didn't work.

I'm thinking there is another round about way to do this, but I don't know what. CAn someone lend me a hand?

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2. Sep 13, 2004

### Juntao

Hey

Ok, the formula is right, you got all the information right under your nose. You're just interpreting wrong.

We know that the wires, lets denote them A and B, are going to have equal and opposite forces.

The equation is F_BA = I_B*L X(cross) B

F_BA = I_B*L * mu_0*I_A/(2*pi*D)
Ok, well, your B is already given to you, which you can substitute for the mu_0*I_A/(2*pi*D).

Let I_B = current, and your L = L2 and that should do the trick.

Well

3. Sep 13, 2004

### Tide

Did you include the external magnetic field?