19(b) is a view
of the arrangement from the AD end and it illustrates these two forces
constituting a couple The magnitude of the torque on the loop is,
1
2
sin
sin
2
2
a
a
F
F
τ
θ
θ
=
+
= I ab B sin q
   = I A B sin q
(4 27)
FIGURE 4 18 (a) A rectangular
current-carrying coil in uniform
magnetic field