Electromagnetic induction

The phenomenon in which an electric field is generated in an electric circuit when the number of magnetic field lines passing through the circuit changes.

The voltage induced is proportional to the rate of change of the field, and large voltages can be produced by switching off quite small magnetic fields suddenly. Frequently, the magnetic field is itself generated by an electric current in a coil, in which case the voltage induced is proportional to the rate of change of the current.

A changing magnetic field passing through a conductor causes a current to flow in the conductor. This current in turn induces a magnetic field. The induced magnetic field points in the opposite direction of the changing magnetic field, opposing the changing field. (This is called Lenz's Law.) The external field must be changing! The induced field opposes the change in the external field, not the field itself. If the field is not changing, there is nothing to oppose and there is no current.

Electric motor

Generator

The Grand Coulee Dam

Power house Generator of the Dam