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 .
B = μ0 μr H = (μ0 μr 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 μr A NI/ l) Wb
∴ φ= NI/( l/ μ0 μr A) Wb