We know that moving charges in a magnetic field experience a force. It is possible for this to create an electric potential difference or electromotive force (see Electricity - Electromotive force  ).

Imagine a straight wire, at right angles to a magnetic field, moving through the field. The direction of motion is perpendicular both to the wire and to the field. The force on the electrons in the wire will cause them to move towards one end. The accumulation of charge creates a potential difference between the ends of the wire.

It is also possible to generate currents this wire, by connecting the ends of the wire to form a loop.

This effect is called electromagnetic induction. It only occurs when changes in flux are taking place.

Magnetic flux

Using our earlier definition of magnetic flux, the flux through this single loop is

When several loops are joined together to make a coil of N turns, the total flux through the coil becomes

This is because each turn contributes AB to the flux.

The laws of electromagnetic induction

• This law is due to Faraday, and concerns the magnitude of the electromotive force (EMF). It states: "The induced EMF across a conductor is equal to the rate at which magnetic flux is cut by the conductor."
• Due to Lenz, this law describes the direction of the EMF. It states: "The direction of any induced EMF is such as to oppose the change which caused it."

These laws may be combined in the mathematical form

E is the induced EMF, while

is the rate of change of magnetic flux.