Answer
$3.2 \times10^{-4}m\cdot N$
Work Step by Step
See Figure 20-34b. Use equation 20–10 to find the torque.
$$\tau=NIABsin\theta$$
Use Ohm’s law to equate the current to the voltage divided by the resistance.
$$\tau=N\frac{V}{R}ABsin\theta $$
The torque is maximum when the angle between the perpendicular to the loop, and the magnetic field, is 90 degrees. The sine is 1.
$$\tau_{max}= N\frac{V}{R}AB $$
$$\tau_{max}= 20\frac{9.0V}{28\Omega}(0.050m)^2(0.020T) $$
$$ =3.2 \times10^{-4}m\cdot N$$