Electric generator in a hydroelectric power plant: an alternating electric current is induced in the stator by the rotation of electromagnets or permanent magnets of the rotor.

When a loop of conducting wire moves in a magnetic field, or the magnetic field in the loop changes, an electric current is generated in the loop. The magnitude of the voltage generated is proportional to the magnitude of the magnetic field change. Thus, an alternating electric current can be generated by a periodic change of the magnetic field. The principle is also called electromagnetic induction and is used by transformers, electric generators and electric motors.