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Magnetic reluctance or "magnetic resistance", is analogous to resistance
Electrical resistance

The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
 in an electrical circuit
Circuit

Circuit may mean:* Circuit * Circuit * Circuit court* Circuit , a 2001 gay-themed film* Circuit , from the Munna Bhai Series* Circuit , a group of theaters among which the same acts circulate, especially in vaudeville...
 (although it does not dissipate magnetic energy). In likeness to the way an electric field
Electric field

In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field ....
 causes an electric current
Electric current

Electric current is the flow of electric charge. The electric charge may be either electrons or ions.The International System of Units unit of electric current intensity is the ampere....
 to follow the path of least resistance, a magnetic field
Magnetic field

A magnetism field is a vector field which can exert a magnetic force on moving electric charges and on magnetic dipoles . When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field....
 causes magnetic flux
Magnetic flux

Magnetic flux, represented by the Greek letter F , is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field....
 to follow the path of least magnetic reluctance. It is a scalar
Scalar

A scalar is a variable that only has magnitude , e.g. a speed of 40 km/h. Compare it with vector, a quantity comprising both magnitude and Direction , e.g....
, extensive quantity
Intensive and extensive properties

In the physical sciences, an intensive property , is a physical property of a system that does not depend on the system size or the amount of material in the system....
, akin to electrical resistance.

term was coined in May 1888 by Oliver Heaviside
Oliver Heaviside

Oliver Heaviside was a autodidact English electrical engineering, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's equations in terms of electric and magnetic forces and flux, and independently co-f...
.






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Magnetic reluctance or "magnetic resistance", is analogous to resistance
Electrical resistance

The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
 in an electrical circuit
Circuit

Circuit may mean:* Circuit * Circuit * Circuit court* Circuit , a 2001 gay-themed film* Circuit , from the Munna Bhai Series* Circuit , a group of theaters among which the same acts circulate, especially in vaudeville...
 (although it does not dissipate magnetic energy). In likeness to the way an electric field
Electric field

In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field ....
 causes an electric current
Electric current

Electric current is the flow of electric charge. The electric charge may be either electrons or ions.The International System of Units unit of electric current intensity is the ampere....
 to follow the path of least resistance, a magnetic field
Magnetic field

A magnetism field is a vector field which can exert a magnetic force on moving electric charges and on magnetic dipoles . When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field....
 causes magnetic flux
Magnetic flux

Magnetic flux, represented by the Greek letter F , is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field....
 to follow the path of least magnetic reluctance. It is a scalar
Scalar

A scalar is a variable that only has magnitude , e.g. a speed of 40 km/h. Compare it with vector, a quantity comprising both magnitude and Direction , e.g....
, extensive quantity
Intensive and extensive properties

In the physical sciences, an intensive property , is a physical property of a system that does not depend on the system size or the amount of material in the system....
, akin to electrical resistance.

History

The term was coined in May 1888 by Oliver Heaviside
Oliver Heaviside

Oliver Heaviside was a autodidact English electrical engineering, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's equations in terms of electric and magnetic forces and flux, and independently co-f...
. The notion of “magnetic resistance” was first mentioned by James Joule and the term "magnetomotive force
Magnetomotive force

Magnetomotive force is any physical cause that produces magnetic flux, i.e. lines of forces emitted from a magnetic material. If a magnetic field passes through a cross sectional area A , it produces a flux given by the equation MMF?A = flux ....
” (MMF) was first named by Bosanquet. The idea for a magnetic flux
Magnetic flux

Magnetic flux, represented by the Greek letter F , is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field....
 law, similar to Ohm's law
Ohm's law

Ohm's law applies to electrical circuits; it states that the electric current through a conductor between two points is directly Proportionality to the potential difference or voltage across the two points, and inversely proportional to the Electrical resistance between them....
 for closed electric circuits, is attributed to H. Rowland.

Definition

The total reluctance is equal to the ratio of the "magnetomotive force
Magnetomotive force

Magnetomotive force is any physical cause that produces magnetic flux, i.e. lines of forces emitted from a magnetic material. If a magnetic field passes through a cross sectional area A , it produces a flux given by the equation MMF?A = flux ....
” (MMF) in a passive magnetic circuit
Magnetic circuit

A magnetic circuit is a closed path containing a magnetic flux. It generally contains magnetic elements such as permanent magnets, ferromagnetic materials, and electromagnets, but may also contain air gaps and other materials....
 and the magnetic flux
Magnetic flux

Magnetic flux, represented by the Greek letter F , is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field....
 in this circuit. In an AC field, the reluctance is the ratio of the amplitude values for a sinusoidal MMF and magnetic flux
Magnetic flux

Magnetic flux, represented by the Greek letter F , is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field....
. (see phasors)

The definition can be expressed as:

where

("R") is the reluctance in ampere-turn
Ampere-turn

The ampere-turn is the International System of Units unit of magnetomotive force, represented by a direct current of one ampere flowing in a single-turn loop in a vacuum....
s per weber
Weber (unit)

In physics, the weber is the SI physical unit of magnetic flux. It is named after the Germany physicist Wilhelm Eduard Weber ....
 (a unit that is equivalent to turns per henry). "Turns
Turn (geometry)

A turn is a unit of plane angle, equal to 360? or 2p radians. As an angular unit it is mainly useful for large angles, such as in connection with coils and rotation objects....
" refers to the winding number
Winding number

In mathematics, the winding number of a closed curve in the plane around a given point is an integer representing the total number of times that curve travels counterclockwise around the point....
 of an electrical conductor comprising an inductor. ("F") is the magnetomotive force (MMF) in ampere-turns
Φ ("Phi") is the magnetic flux in webers.


It is sometimes known as Hopkinson's law
Hopkinson's law

Hopkinson's law is the magnetic counterpart to the electrical Ohm's law. The law is named after the British electrical engineer, John Hopkinson....
 and is analogous to Ohm's Law
Ohm's law

Ohm's law applies to electrical circuits; it states that the electric current through a conductor between two points is directly Proportionality to the potential difference or voltage across the two points, and inversely proportional to the Electrical resistance between them....
 with resistance replaced by reluctance, voltage by MMF and current by magnetic flux.

Magnetic flux always forms a closed loop, as described by Maxwell's equations
Maxwell's equations

In electromagnetism, James Clerk Maxwell equations are a set of four partial differential equations that describe the properties of the electric field and magnetic field fields and relate them to their sources, charge density and current density....
, but the path of the loop depends on the reluctance of the surrounding materials. It is concentrated around the path of least reluctance. Air and vacuum have high reluctance, while easily magnetized materials such as soft iron have low reluctance. The concentration of flux in low-reluctance materials forms strong temporary poles and causes mechanical forces that tend to move the materials towards regions of higher flux so it is always an attractive force(pull).

The reluctance of a uniform magnetic circuit can be calculated as:

where

l is the length of the circuit in metre
Metre

The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes....
s
' is the permeability of free space, equal to henry per metre
' is the relative magnetic permeability of the material (dimensionless)
A is the cross-sectional area of the circuit in square metre
Square metre

The square metre is the SI derived unit of area, with symbol m?. It is defined as the area of a square whose sides measure exactly one metre....
s


The inverse of reluctance is called permeance
Permeance

Permeance, in general, is the degree to which a material admits a flow of matter or energy....
.

Its SI
Si

Si, si, or SI may refer to :...
 derived unit is the henry (the same as the unit of inductance
Inductance

Inductance is the property in an electrical circuit where a change in the current flowing through that circuit induces an Electromotive force that opposes the change in current ....
, although the two concepts are distinct).

Applications

  • Air gaps can be created in the cores of certain transformers to reduce the effects of saturation
    Saturation (magnetic)

    Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off....
    . This increases the reluctance of the magnetic circuit, and enables it to store more energy
    Energy

    In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
     before core saturation. This effect is also used in the flyback transformer
    Flyback transformer

    A flyback or line output transformer is a type of transformer used in the power supply of a cathode ray tube that generates the high voltage needed as part of the supply of a Cathode ray tube display or for other devices such as plasma globes....
    .


  • Variation of reluctance is the principle behind the reluctance motor
    Reluctance motor

    A reluctance motor is a type of synchronous electric motor which induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance....
     (or the variable reluctance generator) and the Alexanderson alternator
    Alexanderson alternator

    An Alexanderson alternator is a alternator invented by Ernst Alexanderson for the generation of high frequency alternating current up to 100 kHz, for the purpose of radio communication....
    .


  • Multimedia
    Multimedia

    Multimedia is media and content that utilizes a combination of different content format. The term can be used as a noun or as an adjective describing a medium as having multiple content forms....
     loudspeaker
    Loudspeaker

    A loudspeaker, speaker, or speaker system is an electroacoustical transducer that converts an electricity signal processing to sound....
    s are typically shielded magnetically, in order to reduce magnetic interference caused to television
    Television

    Television is a widely used telecommunication mass-media for transmitting and receiving moving , either monochrome or color, usually accompanied by sound....
    s and other CRT
    CRT

    CRT may refer to:In computing:* Transport_Layer_Security, in computing* The C runtime library , in programming* The C++ Curiously recurring template pattern, in programming....
    s. The speaker magnet is covered with a material such as soft iron to minimize the stray magnetic field.


Reluctance can also be applied to:
  • Reluctance motors
    Electric motor

    An electric motor uses electrical energy to produce mechanical energy, nearly always by the interaction of magnetic fields and current-carrying conductors....
  • Variable reluctance (magnetic) pickup
    Pickup (music)

    A pickup device acts as a transducer that captures mechanical vibrations and converts them to an electrical signal, which can be instrument amplifier and sound recording....
    s


See also

  • Magnetic capacitivity
    Magnetic capacitivity

    Magnetic capacitivity is a scalar measure of permeance for an element in a magnetic circuit. It is either numerically equal to the magnetic conductivity for a magnetic flux or to the relation between the magnetic flux and the difference of the magnetic potentials across the element....
  • Magnetic capacitance
    Magnetic capacitance

    Magnetic capacitance is the magnetic reactance to magnetic current, which is attributed by the magnetic capacitivity of the element of a magnetic circuit....
  • Magnetic circuit
    Magnetic circuit

    A magnetic circuit is a closed path containing a magnetic flux. It generally contains magnetic elements such as permanent magnets, ferromagnetic materials, and electromagnets, but may also contain air gaps and other materials....
  • Magnetic complex reluctance
    Magnetic complex reluctance

    Magnetic complex reluctance is the complex number, which is equal to the relation of the complex effective or amplitude value of a sinusoidal magnetic tension on the passive magnetic circuit or its element and accordingly the complex effective or amplitude value of a sinusoidal magnetic flux in this circuit or in this element....
  • Reluctance motor
    Reluctance motor

    A reluctance motor is a type of synchronous electric motor which induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance....


References



External links