# Magnetic field of 2 infinite wires.

1. Nov 6, 2011

### dikmikkel

1. The problem statement, all variables and given/known data
Two thin, long wires situated in y = -a and y = a carrying two equal in magnitude and opposite running currents I.
Find the field strength in therms of y.
Find the average Field strength in a region -b to b, where 0 is between the 2 points.

2. Relevant equations
Amperes law.

3. The attempt at a solution
I find via. Amperes' law, that the field of the wire in -a (current running down) has the magnitude:
$B(y) = \dfrac{\mu_0 I}{2\pi r_1}$
Where r1 is the distance from the wire, now i find that r1 = a-y.
The field from the other is the same with the distance r2 instead of r1. r2 = a-y too so the field is zero and the average is of course also 0, and that is annoying. What have i done wrong.

Last edited by a moderator: Nov 6, 2011
2. Nov 6, 2011

### rude man

Better check if, for the downgoing wire, r1 = a - y.