# Magnetomotive force

The equation for the magnetic flux in a magnetic circuit, sometimes known as Hopkinson's law, is:

$\mathcal{F} = \Phi \mathcal{R}$

where Φ is the magnetic flux and $\mathcal{R}$ is the reluctance of the magnetic circuit. It can be seen that the magnetomotive force plays a role in this equation analogous to the voltage V in Ohm's law: V = IR.

Magnetomotive force is analogous to electromotive force, emf( = difference in electric potential, or voltage, between the terminals of a source of electricity, e.g., a battery from which no current is being drawn) since it is the cause of magnetic flux in a magnetic circuit.

i.e.,

     1: m m f = NI
where  N = no of turns in the coil,
I =  electric current through the circuit

     2: m m f = ΦR  where Φ = magnetic flux
R= reluctance

     3: m m f = Hl  where H = magnetizing force
l = mean length of solenoid,circumference of toroid