1. The problem statement, all variables and given/known data http://i.minus.com/1333003834/Q661ScjxBUxkrL2FfmVFPQ/iTEfM3UTAVtTa.png [Broken] 2. Relevant equations B = ∫ ([μ0 / 4pi] * I * ds-vector x r-hat) / r^2 3. The attempt at a solution I know the horizontal line will not add anything to the magnetic field (B), so focusing on the vertical line. I take a little bit of length (ds) which I will call dy. dy x r-hat = dy sin θ r = sqrt(x^2+y^2) sin θ = x / r do all your substitutions and get: B = ([μ0 / 4pi] * I * x ) ∫ dy / (x^2+y^2)^(3/2) At this point I am confused on my limits of integration, I know for an infinite long straight wire I use -∞ to ∞. In my notes I have an example where it goes from -y1 to y2 and comes out with B = ([μ0 / 4pi] * I ) / x * (cos θ1 - cos θ2) where θ1 is the angle between -y1 and the point i am finding, and θ2 is 180 - θ1. This whole θ thing is tripping me up, how did it get there ( I am assuming trig subsitutation). Further more how can I get a grasp on what θ1 would be? I guess -y1 in my situation is just y, and y2 is 0. so θ1 = inverse-tan (x/y) and thus θ2 = 180 - inverse-tan (x/y)?