Now that we got the basics down, let’s introduce a new term:
- Magnetic Flux Density
- 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.
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.
F = BIL
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.