# 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:[1]

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

## Notes

1. ^ Smith, R.J. (1966), Circuits, Devices and Systems, pp 495-506, Wiley International Edition, New York.

## References

• The Penguin Dictionary of Physics, 1977, ISBN 0-14-051071-0
• A textbook of Electrical Technology, 2008, ISBN 81-219-2440-5
• Smith, R.J. (1966), Circuits, Devices and Systems, Chapter 15, Wiley International Edition, New York. Library of Congress Catalog Card No. 66-17612