In
physics, the magnetomotive force (abbreviated mmf or MMF, symbol ) is a quantity appearing in the equation for the
magnetic flux in a
magnetic circuit, Hopkinson's law.[1] It is the property of certain substances or phenomena that give rise to
magnetic fields:
where Φ is the magnetic flux and 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, since it is the cause of magnetic flux in a magnetic circuit:[2]
where N is the number of turns in a
coil and I is the
electric current through the coil.
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.
Units
The
SI unit of mmf is the
ampere, the same as the unit of current[3] (analogously the units of
emf and
voltage are both the
volt). Informally, and frequently, this unit is stated as the
ampere-turn to avoid confusion with current. This was the unit name in the
MKS system. Occasionally, the
cgs system unit of the
gilbert may also be encountered.
History
The term magnetomotive force was coined by
Henry Augustus Rowland in 1880. Rowland intended this to indicate a direct analogy with
electromotive force.[4] The idea of a magnetic analogy to electromotive force can be found much earlier in the work of
Michael Faraday (1791–1867) and it is hinted at by
James Clerk Maxwell (1831–1879). However, Rowland coined the term and was the first to make explicit an
Ohm's law for magnetic circuits in 1873.[5]
Ohm's law for magnetic circuits is sometimes referred to as Hopkinson's law rather than Rowland's law as some authors attribute the law to
John Hopkinson instead of Rowland.[6] According to a review of magnetic circuit analysis methods this is an incorrect attribution originating from an 1885 paper by Hopkinson.[7] Furthermore, Hopkinson actually cites Rowland's 1873 paper in this work.[8]
Schmidt, Robert Munnig; Schitter, Georg,
"Electromechanical actuators", ch. 5 in Schmidt, Robert Munnig; Schitter, Georg; Rankers, Adrian; van Eijk, Jan, The Design of High Performance Mechatronics, IOS Press, 2014
ISBN1614993688.