Faraday's Law

Electromagnetic Induction

Physics 9

Electromagnetic Induction

The effect of producing electricity from magnetism was discovered in 1831 by Faraday and is called electromagnetic induction. It led to the construction of generators for producing electrical energy in power stations.

Two ways of investigating the effect follow

• a) Straight wire and U-shaped magnet

• b) Bar magnet and coil

Bar magnet and coil

The magnet is pushed into the coil, one pole fi rst, then held still inside it. It is then withdrawn. The meter shows that current is induced in the coil in one direction as the magnet is moved in and in the opposite direction as it is moved out. There is no defl ection when the magnet is at rest. The results are the same if the coil is moved instead of the magnet, i.e. only relative motion is needed. 

Faraday’s law

To ‘explain’ electromagnetic induction Faraday suggested that a voltage is induced in a conductor whenever it ‘cuts’ magnetic fi eld lines, i.e. moves across them, but not when it moves along them or is at rest. If the conductor forms part of a complete circuit, an induced current is also produced.

Faraday found, and it can be shown with apparatus like that in Figure, that the induced p.d. or voltage increases with increases of

• the speed of motion of the magnet or coil,

• the number of turns on the coil,

• the strength of the magnet.

These facts led him to state a law:

The size of the induced p.d. is directly proportional to the rate at which the conductor cuts magnetic field lines.

Open Ended

What do you get from todays lesson?