# How to Determine Magnetic Flux Density?

The magnetic flux density is the measure of amount of magnetic flux in each square meter of the magnetic field. Therefore, the magnetic flux density is defined as the magnetic flux passing normally (at 90°) per unit area and is denoted by B, i.e.,

Magnetic flux density,

$$B=\frac{\phi}{A}$$

Where,

• Φ is the magnetic flux,

• A is area normal (or perpendicular) to flux.

The magnetic flux density is a vector quantity i.e. it possesses both magnitude and direction.

## Unit of Magnetic Flux Density

As we know, the magnetic flux density is

$$B=\frac{\phi}{A}=\frac{Weber}{meter^{2}}=Wb/m^{2}=Tesla(T)$$

Therefore, the magnetic flux density is measured 𝑊𝑏/𝑚2 or Tesla (T), named in honour of Nikola Tesla.

Important – The magnetic flux density can also be given in terms of magnetic intensity (H) as follows,

$$\mathrm{Magnetic\:fulx\:density\:\left ( B \right ) \propto\: Magnetic\:intensity (H))}$$

$$B=\mu\:H=\mu_{0}\mu_{r}H$$

## Numerical Example

Determine the flux density in an air gap of cross-section area of 25 cm2 when a total flux of 100 μWb passes through it perpendicularly.

Solution

Magnetic flux density,

$$B=\frac{\phi}{A}=\frac{100\times\:10^{-6}}{25\times\:10^{-4}}$$

$$\Rightarrow\:B=0.04\:Wb/m^{2}=4\times\:10^{-2}\:Wb/m^{2}$$