Physics Lesson 16.2.4 - Magnetic Permeability. Relative Permeability. Diamagnetic, Paramagnetic and Ferromagnetic Materials

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Magnetic Permeability. Relative Permeability. Diamagnetic, Paramagnetic and Ferromagnetic Materials

In all formulae discussed in the previous paragraphs, the effect of the surrounding medium is assumed as negligible. This occurs when this medium is the vacuum. We included in all formulae the effect of vacuum by inserting the quantity known as vacuum magnetic permeability (or magnetic permeability of free space), which value is μ0 = 4π × 10^-7 N/A2.

What if the magnetic field is not produced in vacuum but in another medium instead? In this case, we have to consider the effect of the other medium as well. We don't expect the magnetic field have the same value as if there was vacuum, due to the presence of extra matter, which affects the magnitude of magnetic field. Such a presence is represented through a quantity known as relative permeability, μr, which is calculated by

where B is the magnetic field in presence of a certain matter, B₀ is the corresponding magnetic field in vacuum; μ and μ0 are the values of permeability in the presence of the given matter and in vacuum respectively.

The values of relative magnetic permeability of some materials are given in the table below.

If we look carefully the table above, we observe in the list three kind of materials:

1. The first category includes materials that have a relative permeability slightly smaller than 1. They are known as diamagnetic materials. The magnetic field produced in them is slightly weaker than the magnetic field produced in the same circumstances but in vacuum.
   Examples of diamagnetic materials include carbon, bismuth, silver, copper, etc. When a diamagnetic is placed inside a magnetic field, it is weakly magnetised in the opposite direction of magnetic field. As a result, the overall magnetic field decreases. Therefore, diamagnetic materials are used for decreasing the magnetic field of a given magnet when necessary. This means the magnetic field lines diverge from each other when approaching the material as shown in the figure below.
2. The second category includes materials that have a relative permeability slightly higher than 1. They are known as paramagnetic materials. Examples of paramagnetic materials include aluminium, magnesium, air, etc.
   When a diamagnetic is placed inside a magnetic field, it is slightly magnetised in the direction of this field. Therefore, the magnetic field lines slightly converge when approaching the material. However, this convergence cannot be seen at naked eye as it is too small to be noticeable and as a result, the magnetic field lines are parallel to each other as shown in the figure below.
3. Ferromagnetic materials are those materials that have their relative permeability much greater than 1 (a few hundred to a few thousand times greater than 1). They are strongly magnetised when placed inside a magnetic field. As a result, the magnetic field enforces and the field lines converge when approaching the material, as shown in the figure below. Iron cobalt and nickel are examples of ferromagnetic materials. Due to their strong magnetic properties, ferromagnetic materials are used to produce artificial magnets.

You have reached the end of Physics lesson 16.2.4 Magnetic Permeability. Relative Permeability. Diamagnetic, Paramagnetic and Ferromagnetic Materials. There are 4 lessons in this physics tutorial covering Magnetic Field Produced by Electric Currents, you can access all the lessons from this tutorial below.

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