Here’s How to Crack It
Ignoring the tiny gap in the vertical left-hand wire, we have two wires of length ℓ 1
and two of length ℓ 2. There is no magnetic force on either of the sides of the loop of
length ℓ 2 , because the current in the top side is parallel to B and the current in the
bottom side is antiparallel to B. The magnetic force on the right-hand side points
out of the plane of the page, while the magnetic force on the left-hand side points
into the plane of the page.