# Magnetic Flux Density (B)

Magnetic Flux density is the measure of the number of magnetic lines of force per unit of cross-sectional area.

The general symbol for magnetic flux density is B and the unit is the weber per square meter (Wb/m2). One weber per square meter is called a tesla (T).

If both the total flux and the area of the magnetic path are known, the flux density is found from:

Where:

• B = flux density in tesla (Wb/m2)
• Φ = total flux in webers
• A = area in m2

## Magnetic Flux Density Example

A magnetic circuit has a cross-sectional area of 100mm2 and a flux density of 0.01T. Calculate the total flux in the circuit.

Note: The answer is expressed in webers and not in lines of force.

## Example of Magnetic Flux Density 2

An air core coil has 0.65 μ Wb of flux in its core. Calculate the flux density if the core diameter is 4 cm.

Solution

First, we’ll calculate the core area:

$A=\pi ~{{r}^{2}}=3.14*{{\left( 0.02m \right)}^{2}}=1.256*~{{10}^{-3}}~{{m}^{2}}$

Now, we can calculate the magnetic flux density using the following formula:

$B=\frac{\varphi }{A}=~\frac{0.65*~{{10}^{-6}}~Wb}{1.256*~{{10}^{-3}}~{{m}^{2}}}=5.175*~{{10}^{-4}}~T$