Magnetomotive force (SI Unit:
AmpereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
) is any physical driving (motive) force that produces
magnetic fluxMagnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
. In this context, the expression "driving force" is used in a general sense of "work potential", and is analogous, but distinct from
forceIn physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
measured in newtons. The name came about because in
magnetic circuitA magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other...
s it plays a role analogous to the role
electromotive forceIn physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
(
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
) plays in electric circuits.
SI versus CGS units
The SI unit of magnetomotive force is the
ampereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
(A), represented by a steady, direct
electric currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
of one
ampereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
flowing in a single-turn loop of electrically conducting material in a
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
.
The CGS unit of magnetomotive force is the gilbert (Gi), established by the
IECThe International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
in 1930
http://www.iec.ch/about/history/. The gilbert is defined differently, and is a slightly smaller unit than the ampere. The unit is named after William Gilbert (1544–1603) English physician, astronomer and natural philosopher.
The conversion factor between the SI and CGS units is
(≈ 0.795774715) ampere for every gilbert.
Between the CGS unit and SI unit, the
MKSThe MKS system of units is a physical system of units that expresses any given measurement using fundamental units of the metre, kilogram, and/or second ....
unit magnetomotive force was the ampere-turn
AtThe ampere-turn was the MKS unit of magnetomotive force , represented by a direct current of one ampere flowing in a single-turn loop in a vacuum...
.
Equations
The magnetomotive force
in an
inductorAn inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
or
electromagnetAn electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
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 lawOhm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
:
V = IR.