Derived Magnetism SI Units: UNIZOR.COM - Physics4Teens - Units in Physic...

Notes to a video lecture on http://www.unizor.com

For a short introduction to the International System of units (SI) see the previous lecture SI Intro & Time within topic Base SI Units.

Magnetism Units

All SI units used in the science of Magnetism are derived from base units introduced in the previous chapters of this part Units in Physics of the course.


Intensity (B)

Recall the Lorentz force that magnetic field exhorts on a conductor with an electric current flowing in it.
In short, when a conductor with an electric current I and length L is positioned perpendicular to uniform magnetic field lines, the force F of the magnetic field onto a conductor is perpendicular to both, magnetic field lines and a conductor and its magnitude is proportional to both the amperage of an electric current and the length of a conductor
F = B·I·L
where coefficient B is called the intensity or magnetic flux density of the magnetic field.

The above equation allows to establish a unit of measurement of the intensity of the magnetic field as follows.
A uniform magnetic field that exerts a strength of 1 newton (N) on a thin perpendicular to field lines wire of 1 meter (m) long with an electric current of 1 ampere (A) running through this wire has a strength of 1 tesla (T).
Tesla is a unit of measurement of the magnitude of the intensity of a magnetic field.
1 T = 1 N/(A·m)
Its direction is defined as being tangential and co-directional to magnetic field lines.


Magnetic Flux

The strength of a magnetic field is defined by its intensity (or magnetic flux density) at each point of space where the field is present regardless of whether there is a probing device (a conductor with an electric current running through it) or not.
In a sense, it's similar to a concept of pressure of air that exists at any point where air is present regardless of the presence of a manometer that measures this pressure. It represents a force that molecules in their chaotic motion exert onto a unit of area.

While magnetic flux density can be compare to a force that molecules exert onto a unit of area (air pressure), magnetic flux is comparable to a total pressure on any area, which is a product of a pressure onto a unit of area by a size of an area.

So, magnetic flux Φ is a product of intensity B (magnetic flux density) of a magnetic field by an area A perpendicular to magnetic field lines that coincide with a direction of vectors of intensity of the magnetic field
Φ = B·A

From the formula above, the unit of magnetic flux is the magnetic flux of a field of intensity of 1 tesla (T) with its lines going through an area of 1 m² positioned perpendicularly to magnetic field lines.
This unit is called weber (Wb)
1 Wb = 1 T·m²