When a current carrying loop is placed in a magnetic field, the coil experiences a torque given by τ=NBiAsinθ. Torque is maximum when θ=90∘, i. e., the plane of the coil is parallel to the field τmax=NBiA 
Forces F1 and F2 acting on the coil are equal in magnitude and opposite in direction. As the forces F1 and F2 have the same line of action, their resultant effect on the coil is zero. The two forces F3 and F4 are equal in magnitude and opposite in direction. As the two forces have different lines of action, they constitute a torque. Thus, if the force on one arc of the loop is F, the net force on the remaining three arms of the loop is −F.