# PHY C22: Magnetic Flux Density

Now that we got the basics down, let’s introduce a new term:

• Magnetic Flux Density
• Tesla
• Measuring magnetic flux density

What is magnetic flux density?

## The force experienced per unit length by a straight conductor carrying unit current & placed at right angles to a magnetic field.

### It is represented by B.

This official definition relies on the motor effect explained here!

Remember:
F = BIL sin θ

So,
B = F/(IL sin θ)

If θ = 90°,

## B = F/IL

Magnetic flux density has units of the Tesla (T).

## 1 Tesla is the uniform magnetic flux which, when acting perpendicularly to a long straight wire carrying a current of 1 Ampere, causes a force per unit length of 1 Nm-1 on the conductor.

This is why we use B as a measure of how strong a magnetic field is.

• Large magnetic flux density = more force per length per current = stronger magnetic field
• Although it is defined by the force on a current-carrying conductor, B is also used to describe the force on a permanent magnet
• We can represent B using Field Lines: the closer together field lines are, the larger the magnetic flux density

In these representation, B is represented by the DENSITY of LINES passing through a cross-sectional area. You can visualise this by counting the number of lines per unit area – this is why we name it flux density.

How do you measure magnetic flux density?
There are 2 ways to practically do so:

This concept of Magnetic Flux Density is often paired with a similar term: Magnetic Flux.
See here for an explanation on that.