Magnetic Circuit

The closed path followed by magnetic flux is known as magnetic circuit. A magnetic circuit usually consists of materials having high permeability.

The amount of magnetic coil depends upon current (I) and number of turns (N). If we increase the current or number of turns, the amount of magnetic flux also increases and vice versa. The product of NI is called magnetomotive force (mmf).

Analysis of Magnetic Circuit

l =  mean length of the magnetic circuit in meters

a =  effective area of the core in m2

μr  = relative permeability of the core material

N = number of turns

I = current passed through the circuit in ampere

Φ = flux set up in the core in weber

Important Terms of Magnetic Circuit

Flux:- The number of lines of force in a magnetic circuit is called flux. The unit of magnetic flux is the Weber (Wb). It is denoted by Φ.

Magnetomotive force:- It is a force that set up magnetic flux in a magnetic circuit. The unit magnetomotive force is AT.

mmf = NI ampere-turns (AT)

Reluctance:- The opposition that the magnetic circuit offers to magnetic flux is called reluctance. The unit of reluctance is AT/wb. It is denoted by S.

Reluctance depends upon the following factors:-

  • Reluctance of the magnetic circuit is directly proportional to the length of the magnetic path.
  • Reluctance of the magnetic circuit is inversely proportional to the effective area of the magnetic circuit.
  • The reluctance of the magnetic circuit is also inversely proportional to the relative permeability of the material.
  • Nature of material.

Permeance: – it is reciprocal of reluctance. The unit of permeance is Wb/AT.

Reluctivity: – It is the specific reluctance.

Analogy of Electric Circuits and Magnetic Circuits

Sr No.

Electric Circuit

Magnetic Circuit





Current (I)

Flux (Φ)


Resistance (R)

Reluctance (S)


Resistivity (Rho)

Reluctivity (1/μ)


Current Density (A/m)

Flux Density (Φ/m)


Dissimilarities of Electric Circuits and Magnetic Circuits

  • The resistivity of conductors is more or less constant but the permeability of the ferromagnetic materials varies greatly with magnetic field strength.
  • The flux does not actually flow in the sense in which an electric current flows.
  • In an electric circuit energy must be supplied, to maintain the flow of electric current in the circuit, whereas the magnetic flux once, it is set up, does not require any further supply of energy.

B-H Curve

The curve drawn between magnetic flux density (B) and magnetizing force (H) is called B-H curve.