# Magnetomotive force

In physics, the magnetomotive force is a quantity appearing in the equation for the magnetic flux in a magnetic circuit, sometimes known as Hopkinson's law:

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

where Φ is the magnetic flux and R is the reluctance of the 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. = NI
where N is the number of turns in the coil and
I is the electric current through the circuit
2. = ΦR
where Φ is the magnetic flux and
R is the reluctance
3. = HL
where H is the magnetizing force (the strength of the magnetizing field) and
L is the mean length of a solenoid or the circumference of a toroid