# Magnetic Field given two wires

1. Oct 14, 2012

### jamesbiomed

1. The problem statement, all variables and given/known data
A long pair of insulated wires serves to conduct 29 A of dc current to and from an instrument.

(a) If the wires are of negligible diameter but are 2.8 mm apart, what is the magnetic field 10 cm from their midpoint, in their plane (see figure)?

http://www.webassign.net/gianpse3/28-31.gif

2. Relevant equations

B = µ0*I/(2*π) * (1/r1 - 1/r2).

µ0 = 4*π*10^-7 T*m/A

B = 2*10^-7*I*(1/r1 - 1/r2)

r1 = .0986 m
r2 = .1014 m

3. The attempt at a solution

The equations above are the correct method for solving this, its from yahoo answers. My question is about the values for r. They add up to .2, which I'm guessing is because the current is going in opposite directions. But I do not understand why they are not the same, given that all the dimensions are identical.

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution