The torque on a current loop in a magnetic field is the basis for how an electric motor works. As FIGURE 29.49 shows, the armature of a motor is a coil of wire wound on an axle. When a current passes through the coil, the magnetic field exerts a torque on the armature and causes it to rotate. If the current were steady, the armature would oscillate back and forth around the equilibrium position until (assuming there’s some friction or damping) it stopped with the plane of the coil perpendicular to the field. To keep the motor turning, a device called a commutator reverses the current direction in the coils every $180\text{°}$. (Notice that the commutator is split, so the positive terminal of the battery sends current into whichever wire touches the right half of the commutator.) The current reversal prevents the armature from ever reaching an equilibrium position, so the magnetic torque keeps the motor spinning as long as there is a current.

FIGURE 29.49 A simple electric motor.