Electromagnetism Resource

A magnet is any device or object that can produce a magnetic field.

Magnets are two main types:

• permanent magnets: permanent magnets can retain a magnetic field even without an external magnetising force;
• temporary magnets: temporary magnets require an applied magnetising force to create a magnetic field. When the magnetising force disappears, the magnetic field also disappears.

The images below show two types of magnets. The red magnet is a permanent magnet.

The black-and-white image is an electromagnet. An electromagnet uses electric current to generate a temprary magnetic field. The electromagnet image shows a "horseshoe" core with a coil of wire on each "leg".

The magnetic permeability \( \mu \), or permeability describes the magnetic behaviour of a substance. The unit of permeability is the henry per metre, or H.m^-1. The henry is the unit of magnetic inductance.

The permeability is the ratio between the magnetic field strength, and the magnetising force.

The magnetic permeability is usually broken into two parts such that \( \mu = \mu_0 \mu_{\mathrm{r}} \).

The two parts are:

• \( \mu_0 \): the permeability of free space. The permeability of free space (or a vacuum) is a fundamental constant of the universe. The permeability of free space has the approximate (since 2019, but still accurate to better than 1 part in a billion) value of \( 4\pi \cdot 10^{-7} \) H.m^-1. The numerical value is \( 1.2566 \cdot 10^{-7} \) H.m^-1.
  
  
• \( \mu_{\mathrm{r}} \): the relative permeability, the ratio of the permeability of the substance of interest to the permeability of free space.

Magnetic materials have a relative permeability (\( \mu_{\mathrm{r}} \)) greater than 1. Materials that have high permeability are called ferromagnetic or ferrimagnetic materials, depending exactly how they achieve their high permeability.

Not many materials than iron, nickel, cobalt or their compounds and alloys have a relative magnetic permeability significantly more than 1. Most other substances can be treated like free space (i.e. \( \mu_{\mathrm{r}} = \) 1 ).