# Electromagnetic induction

production of voltage by a varying magnetic field

Electromagnetic induction is where a voltage or current is produced in a conductor by a changing magnetic flux. It may happen when a magnet is moved in a solenoid, thus changing the magnetic flux.

## Magnetic flux

When a coiled wire is introduced near a magnet, the magnetic lines of force pass through the coil. This causes the magnetic flux to change. Magnetic flux is represented by the symbol ${\Phi }$ , therefore we can say that ${\Phi }$  = BAcos(a) and the resulting unit will be $Tm^{2}$ , where T is the unit for magnetic field and $m^{2}$  is the unit for area.

The changing magnetic flux generates an electromotive force (EMF). This force moves free electrons in a certain way, which constitute a current.

Michael Faraday found that an electromotive force is generated when there is a change in magnetic flux in a conductor.

His laws state that:

${\mathcal {E}}={-{d\Phi } \over dt}$

where,

${\mathcal {E}}$  is the electromotive force, measured in volts;

${d\Phi }$  is the change in magnetic flux, measured in webers;

$dt$  is the change in time, measured in seconds.

In the case of a solenoid:

${\mathcal {E}}={-N{d\Phi } \over dt}$

where,

N is the number of loops in the solenoid.

## Lenz's law

The negative sign in both equation above is a result of Lenz's law, named after Heinrich Lenz. His law states that the electromotive force (EMF) produces a current that opposes the motion of the changing magnetic flux.