Problem 22.21

21.21.
By symmetry, we note that the magnetic forces on the top and bottom segments of the rectangle cancel and we have but to find the net force on the vertical segments of the rectangle.
$\vec{F} = \frac{\mu_{0}I_{1} I_{2} \ell}{2\pi} (\frac{1}{c+a} - \frac{1}{c}) \hat{i}$
Substituting in the numbers given in the problem we find
$\vec{F} = -2.70 \times 10^{-5} N \hat{i}$