The field exerts no force on the two arms AD and BC
of the loop It is perpendicular to the arm AB of the loop
and exerts a force F1 on it which is directed into the
plane of the loop Its magnitude is,
F1 = I b B
Similarly, it exerts a force F2 on the arm CD and F2
is directed out of the plane of the paper F2 = I b B = F1
Thus, the net force on the loop is zero