# Homework Help: Magnetic Force Per Unit Length

1. Oct 23, 2008

### Ithryndil

1. The problem statement, all variables and given/known data
A packed bundle of 100 long, straight, insulated wires forms a cylinder of radius R = 0.600 cm.

If each wire carries 4.50 A, what are the magnitude and direction of the magnetic force per unit length acting on a wire located 0.200 cm from the center of the bundle?

2. Relevant equations
$$F/L = IB$$
$$B = (\frac{(\mu)I}{2(\pi)R^{2}})r$$

3. The attempt at a solution

I plugged in the various values getting:

$$F/L = (450)((4\pi X 10^{-7}*450)/(2\pi(.006^{2}))(.002)$$

That equals 2.25 N/m which is orders of magnitude off. Can anyone tell me what I am doing wrong here? This problem is in the book with slightly different numbers and the answer is is X.XX mN/m. Thanks for the help.

Last edited: Oct 23, 2008
2. Oct 23, 2008

### Ithryndil

Anyone know where I am going wrong?

3. Oct 25, 2008

### alphysicist

Hi Ithryndil,

I don't think this is correct. The B-field part looks right to me, but if you then compare this to your first equation F/L = IB, this would be the force per unit length on a single wire that has a current of 450A.