**Magnetic flux $\phi$ through a plane surface is the product of the magnetic flux density normal to the surface B _{N} and the area A of the surface.**

- S.I. unit for $\phi$ is weber(Wb) or T m
^{2} - Base Units for Wb: kg m
^{2}s^{-1}C^{-1}

$\phi = AB_{N} \, = AB \, cos \, \theta$, where

$\theta$ is the angle between the B-field and the normal to the area A.

**The weber is defined as the magnetic flux through a surface if a magnetic field of flux density 1 T exists perpendicularly to an area of 1 m ^{2}.**

**When a coil of N turns is placed instead,**

**Magnetic Flux Linkage $\Phi$ is defined as the product of the number of turns N of the coil and the magnetic flux $\phi$** **linking each turn.**

$\Phi = N \phi \, = NAB_{N} \, = NAB \, cos \, \theta$, where

- A is area of coil
- $\theta$ is the orientation of the coil with respect to the direction of B

The magnetic flux linkage through a coil depends on:

- number of turns in the coil
- area of the coil
- external magnetic flux density
- the orientation of the coil with respect to the direction of B