# Magnetic Flux

Show/Hide Sub-topics (Electromagnetic Induction | A Level)

Magnetic flux $\phi$ through a plane surface is the product of the magnetic flux density normal to the surface BN and the area A of the surface.

• S.I. unit for $\phi$ is weber(Wb) or T m2
• Base Units for Wb: kg m2 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 m2.

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

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