Magnetomotive force
In physics, the magnetomotive force (mmf) is a quantity appearing in the equation for the magnetic flux in
a magnetic circuit, often called Ohm's law for magnetic circuits.[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 the coil and I is the electric current through the circuit.
where Φ is the magnetic flux and is the magnetic reluctance
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[edit]
The SI unit of mmf is the ampere, the same as the unit of current (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[edit]
The term magnetomotive force was coined by Henry Augustus Rowland in 1880. Rowland intended this to
indicate a direct analogy with electromotive force.[3] 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.[4]
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.[5] According to a review of magnetic circuit
analysis methods this is an incorrect attribution originating from an 1885 paper by Hopkinson.[6] Furthermore,
Hopkinson actually cites Rowland's 1873 paper in this work.[7]
Permeance
From Wikipedia, the free encyclopedia
Jump to navigationJump to search
Permeance, in general, is the degree to which a material admits a flow of matter or energy.
Permeance is usually represented by a curly capital P: .
Contents
Electromagnetism[edit]
In electromagnetism, permeance is the inverse of reluctance. In a magnetic circuit,
permeance is a measure of the quantity of magnetic flux for a number of current-turns.
A magnetic circuit almost acts as though the flux is conducted, therefore permeance is larger
for large cross-sections of a material and smaller for smaller cross section lengths. This
concept is analogous to electrical conductance in the electric circuit.
Magnetic permeance is defined as the reciprocal of magnetic reluctance
(in analogy with the reciprocity between electric conductance and resistance):
, which can also be re-written:
using Hopkinson's law (magnetic circuit analogue of Ohm's law for electric circuits) and the
definition of magnetomotive force (magnetic analogue of electromotive force):
where:
Φ , magnetic flux,
B
I, current, in amperes,
N, winding number of, or count of turns in the electric coil.
Alternatively in terms of magnetic permeability (analogous to electric conductivity):
where:
μ, permeability of material,
A, cross-sectional area,
, magnetic path length.
The SI unit of magnetic permeance is the henry (H), that is webers per ampere-turn.
Materials science[edit]
In materials science, permeance is the degree to which a material transmits another
substance.
Magnetic reluctance
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis
of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux.
It represents the opposition to magnetic flux, and depends on the geometry and composition
of an object.
Magnetic reluctance in a magnetic circuit is analogous to electrical resistance in an electrical
circuit in that resistance is a measure of the opposition to the electric current. The definition
of magnetic reluctance is analogous to Ohm's law in this respect. However, magnetic flux
passing through a reluctance does not give rise to dissipation of heat as it does for current
through a resistance. Thus, the analogy cannot be used for modelling energy flow in
systems where energy crosses between the magnetic and electrical domains. An alternative
analogy to the reluctance model which does correctly represent energy flows is the gyrator–
capacitor model.
Magnetic reluctance is a scalar extensive quantity, akin to electrical resistance. The unit for
magnetic reluctance is inverse henry, H .−1
History[edit]
The term reluctance was coined in May 1888 by Oliver Heaviside. The notion of "magnetic
[1]
resistance" was first mentioned by James Joule in 1840. The idea for a magnetic flux law,
[2]
similar to Ohm's law for closed electric circuits, is attributed to Henry Augustus Rowland in
an 1873 paper. Rowland is also responsible for coining the term magnetomotive force in
[3]
1880, also coined, apparently independently, a bit later in 1883 by Bosanquet.
[4] [5]
Reluctance is usually represented by a cursive capital .
Definitions[edit]
In physics, the magnetomotive force (mmf) is a quantity appearing in the equation for the magnetic flux in
a magnetic circuit, often called Ohm's law for magnetic circuits.[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 the coil and I is the electric current through the circuit.
where Φ is the magnetic flux and is the magnetic reluctance
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[edit]
The SI unit of mmf is the ampere, the same as the unit of current (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[edit]
The term magnetomotive force was coined by Henry Augustus Rowland in 1880. Rowland intended this to
indicate a direct analogy with electromotive force.[3] 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.[4]
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.[5] According to a review of magnetic circuit
analysis methods this is an incorrect attribution originating from an 1885 paper by Hopkinson.[6] Furthermore,
Hopkinson actually cites Rowland's 1873 paper in this work.[7]
Permeance
From Wikipedia, the free encyclopedia
Jump to navigationJump to search
Permeance, in general, is the degree to which a material admits a flow of matter or energy.
Permeance is usually represented by a curly capital P: .
Contents
Electromagnetism[edit]
In electromagnetism, permeance is the inverse of reluctance. In a magnetic circuit,
permeance is a measure of the quantity of magnetic flux for a number of current-turns.
A magnetic circuit almost acts as though the flux is conducted, therefore permeance is larger
for large cross-sections of a material and smaller for smaller cross section lengths. This
concept is analogous to electrical conductance in the electric circuit.
Magnetic permeance is defined as the reciprocal of magnetic reluctance
(in analogy with the reciprocity between electric conductance and resistance):
, which can also be re-written:
using Hopkinson's law (magnetic circuit analogue of Ohm's law for electric circuits) and the
definition of magnetomotive force (magnetic analogue of electromotive force):
where:
Φ , magnetic flux,
B
I, current, in amperes,
N, winding number of, or count of turns in the electric coil.
Alternatively in terms of magnetic permeability (analogous to electric conductivity):
where:
μ, permeability of material,
A, cross-sectional area,
, magnetic path length.
The SI unit of magnetic permeance is the henry (H), that is webers per ampere-turn.
Materials science[edit]
In materials science, permeance is the degree to which a material transmits another
substance.
Magnetic reluctance
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis
of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux.
It represents the opposition to magnetic flux, and depends on the geometry and composition
of an object.
Magnetic reluctance in a magnetic circuit is analogous to electrical resistance in an electrical
circuit in that resistance is a measure of the opposition to the electric current. The definition
of magnetic reluctance is analogous to Ohm's law in this respect. However, magnetic flux
passing through a reluctance does not give rise to dissipation of heat as it does for current
through a resistance. Thus, the analogy cannot be used for modelling energy flow in
systems where energy crosses between the magnetic and electrical domains. An alternative
analogy to the reluctance model which does correctly represent energy flows is the gyrator–
capacitor model.
Magnetic reluctance is a scalar extensive quantity, akin to electrical resistance. The unit for
magnetic reluctance is inverse henry, H .−1
History[edit]
The term reluctance was coined in May 1888 by Oliver Heaviside. The notion of "magnetic
[1]
resistance" was first mentioned by James Joule in 1840. The idea for a magnetic flux law,
[2]
similar to Ohm's law for closed electric circuits, is attributed to Henry Augustus Rowland in
an 1873 paper. Rowland is also responsible for coining the term magnetomotive force in
[3]
1880, also coined, apparently independently, a bit later in 1883 by Bosanquet.
[4] [5]
Reluctance is usually represented by a cursive capital .
Definitions[edit]
