# Basic Concepts of Magnetic Circuits

When a current flows in a wire, it creates a magnetic field around it. And, if the current-carrying wire is a coil, it creates a magnetic field that induces a voltage in it.

In other words, when the current-carrying conductor faces another magnetic field, it induces a force. And when a current-carrying conductor, moves in a magnetic field, it induces a voltage.

## Magnetic field intensity around a closed path

Magnetic field intensity H is the ability of current to create a magnetic field in a circuit. To calculate the magnetic field intensity, caused by a current, we can use the below equation, Ampere’s law.

If there is a ferromagnetic metal inside a coil, the magnetic field created by the coil is absorbed into the core.

So, the Ampere’s law

H is the magnitude of the magnetic field intensity vector,

## Magnetic field intensity and the core

If there is a ferromagnetic metal inside the coil, the magnetic field created by the coil is absorbed into the core. However, the strength of the magnetic field depends on the core.

## Total flux in a given area ϕ

dA is the differential unit of area.

If the flux density is a constant and has a θ angle to a plane of area A, we can write the equation as,

ϕ =BA cos⁡(θ)

If the flux density is perpendicular to the area and a constant, we can rewrite the above equation as,

ϕ = BA

So, to get to know the total magnetic flux generated by the current, we can substitute,

Into the equation, therefore the magnetic flux density,

## Force that drives the current flow

In a normal electrical circuit, the voltage or the EMF (electromotive force) drives the current flow. So it can be described using Ohm’s law,

V=IR

But in magnetic circuits, the force that drives the current flow is MMF (magnetomotive force).  And it’s represented by the script letter F and measured by Ampere turns. The equation for magnetomotive force is,

F=Ni

So, we can write the magnetic flux as,

### MMF, flux and reluctance

Magnetomotive force F (MMF) is the cause of the generation of magnetic flux ∅. Magnetic reluctance is a similar concept to electrical resistance. And the units of reluctance s ampere-turns per weber. Just like in Ohm’s law, we can build a relationship as follows,

So, we can write the magnetic flux as,

#### Permeance

As there is reluctance for the resistance, what is there for the conductance. There is permeance for the resistance.