Magnetic Circuit:

You know that electric circuit provides a path for electric current. Likewise, magnetic circuit provides a path for magnetic flux.

Example of a magnetic circuit is illustrated in Figure. Here we are considering an iron ring with a magnetic path of l meter, cross sectional area A m² and a coil of N turns carrying I Amperes wound on it as illustrated in Figure

Figure:  Magnetic Circuit

Now we will get acquainted with some significant terms associated to a magnetic circuit. These are magnetomotive force, flux density, magnetic field intensity, permeability, reluctance, etc.

Magnetic field is measured in terms of flux Φ which has unit weber (Wb). Flux density (B) is the flux per unit area, that menas. B = φ/ A. Magnetomotive Force (m.m.f.) is the force which drives flux through a magnetic circuit. Unit of m.m.f. is Ampere Turns (AT) and it is defined as the product of current and number of turns in a magnetic circuit. Thus, Magnetomotive force (m.m.f) = NI AT. Magnetomotive force per unit length of magnetic flux path is termed as magnetic field intensity (H) and it is described as

            H = NI/ l AT/m .

So,

B = μ₀ μ_r H = (μ₀  μ_r NI )/ l  )Wb/m²  

µ₀ is the absolute permeability = 4π × 10^{- 7} henry/m and µr is the relative permeability of the medium.

Total Flux produced

φ= B × A =( μ₀ μ_r A NI/ l) Wb

∴   φ= NI/( l/ μ₀ μ_r A)    Wb