Why Does Gauss' Law for Magnetism Apply to Current-Carrying Wires and Squares?

[greenbean]

I am trying to answer all the odd problems at the end of the chapter and I can't seem to get one of them.
A long, current-carrying wire is oriented vertically; next to it is drawn a square whoe area lies in the same plane as the wire. Using the distances indicated, find the magnetic flux through the square.

The figure shows a long wire with I pointing upward, and a square at a distance d from the wire. Each side of the square is of length a.

The answer is (u0=4pi*10^-7*I*a)/2pi * ln[a+d/a]
Please Help!

 

[whozum]

Well you know that there is a clockwise magnetic field around the wire that loses strength with a certain proportionality. It wants you to find how much of that magnetic field flows through the square. Start with finding the expression for the magnetic field around the wire, and try setting up an integral.