An EMF is only generated when the magnetic flux changes. There are two ways to change the flux through a circuit in a magnetic field:

• Change the field strength while keeping the circuit stationary. The EMF is then

• Keep the field strength constant and move the circuit. The EMF is then

Self-inductance

• When an EMF is applied to a coil of wire, the current through the coil will increase, creating a magnetic field
• This changing magnetic field leads to a change in the magnetic flux through the coil
• The changing magnetic flux induces an EMF. Lenz' law tells us that this EMF must be in the opposite direction to the original applied EMF

The magnetic field is proportional to the current in the wire:

We can calculate the EMF from the magnetic flux:

Finally, we can relate this to the current in the wire:

We define the constant of proportionality between the EMF and the rate of change of the current as the self-inductance of the coil.

Self-inductance is given the letter L and is measured in henry (H).