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!

F = BIL sin θ

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 3D (the complete picture) 
In 2D (cross-section) 
In 2D (side view) 

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.

Low magnetic flux density
High magnetic flux density

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

Using a current balance
The wire is held perpendicular to the permanent magnetic field.

When current is switched on, the reading on the balance (F) changes as a force is created between the wire & the magnet due to the motor effect.

The current is varied.
Values of F for each value of I are recorded.

Graph of F against I is plotted.

Gradient of the graph gives BL.

L of the wire passing through the magnetic field is be measured.

Thus, B of this specific permanent magnet can be calculated.
Using a Hall probe

(a measurement device which uses the principal of Hall voltage)

Hall probe must first be calibrated to Earth’s magnetic field by rotating it until it reaches a maximum reading.

This reading is noted, & the probe is rotated 180° until another maximum is reached.

The difference between the 2 readings is calculated, & the Earth’s magnetic field is given by HALF of this difference.

To measure B of a magnet, the Hall probe is held so that the field lines pass directly through it.

To ensure accurate readings, the probe is rotated until a maximum is reached.

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

One thought on “PHY C22: Magnetic Flux Density

  1. Pingback: PHY C23: Magnetic Flux – ProDuckThieves

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