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Basic Definitions in Magnetic circuit

by | Last updated Dec 22, 2020 | Basic Definitions

The magnetic circuit is analogous to the electric circuit. A magnetic circuit has a closed path, through which magnetic flux will flow. There are several terms involved in the magnetic circuit, which are discussed below.

Magnetic flux

The magnetic lines of force passing through a magnetic circuit is known as magnetic flux. It is denoted by a symbol Φ and given by a formula \phi = BA, where B is the magnetic flux density and A is the area of the cross-section in m2. The unit of magnetic flux is weber.

Magneto-motive force

Magneto-motive force or MMF is the cause for producing the magnetic flux. The MMF in a magnetic circuit depends on the number of turns(N) and the amount of current(I) flowing through it.

It is given by a formula, MMF = NI ant its unit is ampere turns.

Magnetic flux density

It is the amount of magnetic flux per unit area at right angles to the flux. The unit of magnetic flux density is weber/m2 and denoted by B. The formula is given by,


Magnetic field intensity

Magnetizing force or Magnetic field intensity or magnetic field strength is the MMF required to magnetize a unit length of the magnetic flux path. The unit of magnetic field intensity is AT/m and is denoted by H.



It is the opposition that the magnetic circuit offers for the flow of magnetic flux. We can also define the reluctance as the ratio of magneto-motive force to the magnetic flux. It is denoted by S and its unit is ampere-turns per weber.

    \[Reluctance(S) = \frac{MMF}{flux} \]


Permeance is the reciprocal of reluctance. The ease with which the flux can pass through the material is known as permeance. Weber/AT is the unit of permeance.

    \[Permeance = \frac{1}{reluctance}\]


It is the measure of the resistance of a material against the formation of a magnetic field. In simple words, the permeability of material means its conductivity for magnetic flux. The reciprocal of magnetic permeability is magnetic reluctivity. Greater permeability, greater is its conductivity.

Magnetic permeability is represented by a greek letter \mu. It is given by a formula,

    \[ \mu = \frac{B}{H}\]

Relative permeability

It is the ratio of flux density of a magnetic material to the flux density produced in air by the same magnetizing force.

The formula for relative permeability is,

    \[ \mu_r = \frac{\mu}{\mu_0}\]

where \mu_r – relative permeability of the magnetic material. \mu_0 – absolute permeability of air or vacuum. \mu – absolute permeability of the magnetic material.

Analogy between Magnetic circuit and Electric Circuit

Magnetic CircuitElectric Circuit
A closed path for a magnetic flux forms a magnetic circuit.A closed path for an electric current form an electric circuit.
Magnetic flux does not flow in a magnetic circuit.Electric current always flows in an electric circuit.
MMF is the cause for producing flux.EMF is the cause for producing current.
Weber is the unit of flux.Ampere is the unit of current.

    \[Flux = \frac{mmf}{reluctance}\]

    \[Current= \frac{emf}{resistance}\]

Reluctance opposes the flow of flux.Resistance opposes the flow of current.

    \[Reluctance= \frac{l}{\mu_0 \mu_r a}\]

    \[Resistance= \frac{\rho l}{a}\]

    \[Permeance = \frac{1}{reluctance}\]

    \[Conductance= \frac{1}{resistance}\]

Flux density,

    \[B = \frac{\phi}{a}\]

Current density,

    \[J = \frac{I}{a}\]

Magnetic field intensity,

    \[H = \frac{NI}{l}\]

Electric field intensity,

    \[E = \frac{V}{d}\]

Magnetic flux lines flow from the North pole to the South pole.Electric current flows from the positive to negative terminal.



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