magnetomotive force

Magnetic Circuits
Conventional Magnetic Circuits
Magnetomotive force
Magnetic flux Φ
Magnetic reluctance
Phasor Magnetic Circuits
Complex reluctance Zμ
Related Concepts
Magnetic permeability μ
Gyrator-Capacitor Model variables
Magnetic impedance zM
Effective resistance rM
Magnetic inductivity LM
Magnetic capacitivity CM
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Magnetomotive force (MMF) (SI Unit: At) is any physical force that produces magnetic flux. In this context, the word "force" is used in a general sense of "work potential", and is analogous to, but distinct from mechanical force measured in newtons. The name came about because in magnetic circuits it plays a role analogous to the role electromotive force (voltage) plays in electric circuits.

SI versus CGS units

The SI unit of magnetomotive force is the ampere-turn (At), represented by a steady, direct electric current of one ampere flowing in a single-turn loop of electrically conducting material in a vacuum.

The CGS unit of magnetomotive force is the gilbert (Gi), established by the IEC in 1930 [1]. The gilbert is defined differently, and is a slightly smaller unit than the ampere-turn. The unit is named after William Gilbert (1544–1603) English physician and natural philosopher.

The conversion factor between the SI and CGS units is (≈ 0.795774715) Ampere-turns for every gilbert.

Equations

The magnetomotive force in an inductor or electromagnet consisting of a coil of wire is given by:

where N is the number of turns of wire in the coil and I is the current in the wire.

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

where Φ is the magnetic flux and 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.

References

• The Penguin Dictionary of Physics, 1977, ISBN 0-14-051071-0

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