Faraday's Law

Faraday's Law of Induction

Changing Magnetic Fields

A stationary magnetic field has no effect on a wire or current-loop,

However, . . . .

A moving or changing magnetic field does produce a current in a current loop or a voltage across the ends of a current loop. This is called electromagnetic induction and the current or voltage is called an induced current or an induced voltage.

Magnetic Flux

You have seen electric flux and magnetic flux before. We can better understand or describe a "moving magnet" or a "changing magnetic field" in terms of magnetic flux. As you already know, we define the magnetic flux as

You might think of the "air flux" as air blows through a window. The size of the window (A), the speed of the air (B) and the direction (theta) all determine how much air comes through the window.

Faraday's Law

The potential difference induced across the ends of a coil of wire is equal to the time rate of change of the magnetic flux through that coil of wire. This potential difference is known as an induced "electromotive force" or as an induced emf.

The flux through a loop of wire can change due to many different situations.

Anything that causes a change the the flux through a loop of wire produces a potential difference across the ends of the loop. If there are N loops, the potential differences across each of the loops will be added and the total potential difference will be N times that across the ends of a single loop.

Homework Assignment

Motional EMF

Return to ToC

(c) Doug Davis, 2002; all rights reserved