I'm confused. Here is the problem I'm working on. Imagine a long, straight wire crossing the screen or page horizontally. It is carrying a current i. Now imagine a rectangular loop underneath the wire a distance of d away. The loop has a height of a and a width of b. I am supposed to find the the flux that passes through the loop. i is a constant, as are b, a, and d. My confusion arrises from that fact that the flux = ((mu(naught)*i*b)/2*pi)(ln((d+a/a)). Since ampere's law is (flux)= integral(B dot dA) (where B is the B of the wire), shouldn't there be an a in the formula for flux? I know that there shouldn't be, but not why. Could some one please help me?