Force from a rectangular loop of wire?

[cdotter]

Homework Statement

[PLAIN]http://img710.imageshack.us/img710/3483/71071851.png

Homework Equations

n/a

The Attempt at a Solution

I understand everything except "the forces on the top and bottom segments cancel."

How do I figure out that they cancel? I understand that a force is orthogonal to both the current flow and the magnetic field...but there's no magnetic field given?

 

[cdotter]

Nevermind, I think I figured it out.

The direction of the magnetic field can be found by using the right hand rule. Place your thumb along the direction of current and curl your fingers around the wire.

I1's magnetic field goes into the page on the right side. It comes out of the page on the left side, but there's nothing on the left side of the page so it's inconsequential.

To find the force on the rectangular section by the straight section use the right hand rule again in accordance to F=IdL X B. That is to say, cross the length vector (which is in the same direction of I) into the magnetic field vector you just found.

Top section: Force vector points up
Right section: Force vector points right
Bottom section: Force vector points down
Left section: Force vector points left

Top and bottom cancel out. Left and right would also cancel but one side (the left) is closer to the straight wire, which happens to be exerting a force on the loop.
It's sad that it took me an hour of searching Google to explain something so simple that I couldn't find in my textbook...I hate textbooks so much.