Two contours whose planes are parallel to each other and are separated by a certain distance. Both are carrying current in the same direction. A is fixed & B can be positioned in different manner with respect to the first.

Different positions are such that the plane of B tuned by 90º, or by 180º, and in third case it is just moved parallel to itself over a certain distance. One has to do work to bring B in any of above positions. These works are W 1 for 90º, W 2 for 180º and for third case W 3 . Among W 1 W 2 , W 3 –