Magnetic cores guide magnetic flux through paths whose geometry and material strongly influence the field. A magnetic circuit is a lumped model that describes this flux path using quantities analogous to those used in electric circuits.

Flux in a core

If magnetic flux $\Phi$ is approximately uniform through a cross-sectional area $A$, the average flux density is

$$B=\frac{\Phi}{A}.$$

Magnetic field intensity

The magnetic field intensity $H$ is related to flux density by the material law. For a linear material,

$$B=\mu H,$$

where $\mu$ is the permeability.

A path model

A coil wrapped around a core drives magnetic flux around the core. If leakage and fringing are small, the same total flux passes through successive sections, much as current passes through series elements in an electric circuit.

Limits of the analogy

Magnetic flux is not a flow of magnetic charge, and real fields are distributed continuously in space. The magnetic-circuit model is an approximation that becomes useful when the flux is strongly confined to identifiable core paths.

This model provides the framework for magnetomotive force, reluctance, saturation and transformer and machine cores.