Generally speaking, when talking about permeability, one thinks of a flow of fluids, usually water. But there are also other elements capable of passing through substances, for example, magnetic fields. In this case, we talk about magnetic permeability and relative magnetic permeability .
The permeability of materials is a very interesting property, regardless of the type of flux that passes through them. Thanks to it, it is possible to predict how these materials will behave in very different circumstances.
For example, soil permeability is very important when it comes to building structures like drains, pavements and more. Even for crops, soil permeability is relevant.
Throughout life, the permeability of cell membranes allows the cell to be selective, passing necessary substances such as nutrients and rejecting others that can be harmful.
As for the relative magnetic permeability, it gives us information about the response of materials to magnetic fields caused by magnets or wires with current. Such elements abound in the technology around us, so it’s worth investigating what effects they have on materials.
relative magnetic permeability
A very interesting application of electromagnetic waves is to facilitate oil exploration. It is based on knowing how far the wave is able to penetrate underground before being attenuated by it.
This gives you a good idea of the type of rocks found at a particular location, as each rock has a different relative magnetic permeability depending on its composition.
As mentioned at the beginning, whenever we talk about relative permeability , the term “relative” requires comparing the magnitude in question of a given material with that of another that serves as a reference.
This is always applicable, regardless of the permeability to a liquid or a magnetic field.
Vacuum has permeability, as electromagnetic waves have no problem moving there. It is a good idea to take this as a reference value to find the relative magnetic permeability of any material.
Vacuum permeability is none other than the well-known constant of the Biot-Savart law, which is used to calculate the magnetic induction vector. Its value is:
μ o = 4π. 10 -7 Tm / A (Tesla. Metro / Ampere).
This constant is part of nature and is linked, along with electrical permittivity, to the value of the speed of light in a vacuum.
To find the relative magnetic permeability, you must compare the magnetic response of a material in two different media, one of which is a vacuum.
In calculating the magnetic induction B of a wire in vacuum, it was found that its magnitude is:
And the permeability in relation to μ r of the medium is the relationship between B and B or : μ r = B / B or . It’s a dimensionless quantity, as you can see.
Classification of materials according to their relative magnetic permeability
The relative magnetic permeability is a dimensionless and positive quantity, being the quotient of two positive quantities in turn. Remember that the magnitude of a vector is always greater than 0.
μ r = B / B or = μ / μ or
μ = μ r . μ the
This magnitude describes how the magnetic response of a medium is compared to the response in a vacuum.
However, the relative magnetic permeability can be equal to 1, less than 1 or greater than 1. This depends on the material in question and also the temperature.
- Obviously, if µ r = 1, the medium is the vacuum.
- If it’s less than 1, it’s a diamagnetic material.
- If it is greater than 1, but not much, the material is paramagnetic.
- And if it’s much larger than 1, the material is ferromagnetic .
Temperature plays an important role in the magnetic permeability of a material. In fact, this value is not always constant. As the temperature of a material increases, it becomes internally disordered, decreasing its magnetic response.
Diamagnetic and paramagnetic materials
The materials diamagnetic respond adversely to magnetic fields and repel. Michael Faraday (1791-1867) discovered this property in 1846 when he discovered that a piece of bismuth was repelled by either pole of a magnet.
Somehow, the magnetic field of the magnet induces a field in the opposite direction within the bismuth. However, this property is not unique to this element. All materials have to some extent.
It is possible to demonstrate that the net magnetization in a diamagnetic material depends on the characteristics of the electron. And the electron is part of the atoms of any material, so everyone can have a diamagnetic response at some point.
Water, noble gases, gold, copper and many others are diamagnetic materials.
On the other hand, the material paramagnetic have their own magnetization. That’s why they can respond positively to the magnetic field of a magnet, for example. They have a magnetic permeability similar to the µ o value .
Next to a magnet, they can also be magnetized and become magnets on their own, but this effect disappears when the real magnet is removed from nearby. Aluminum and magnesium are examples of paramagnetic materials.
Truly Magnetic Materials: Ferromagnetism
Paramagnetic substances are the most abundant in nature. But there are materials that are easily attracted by permanent magnets.
They are able to acquire magnetization by themselves. It’s iron, nickel, cobalt and rare earths like gadolinium and dysprosium. Also, some alloys and compounds between these and other minerals are known as ferromagnetic materials .
This type of material undergoes a very strong magnetic response to an external magnetic field, such as a magnet, for example. That’s why nickel coins stick to bar magnets. In turn, the bar magnets adhere to refrigerators.
The relative magnetic permeability of ferromagnetic materials is much greater than 1. Inside, they have small magnets called magnetic dipoles . By aligning these magnetic dipoles, they intensify the magnetic effect within the ferromagnetic materials.
When these magnetic dipoles are in the presence of an external field, they quickly line up beside it and the material adheres to the magnet. Although the external field is suppressed, pushing the magnet away, there remains a remaining magnetization within the material.
High temperatures cause internal disorder in all substances, producing what is called “thermal agitation”. With heat, the magnetic dipoles lose their alignment and the magnetic effect disappears.
Curie temperature is the temperature at which the magnetic effect completely disappears from a material. At this critical value, ferromagnetic substances become paramagnetic.
Devices for storing data, such as magnetic tapes and magnetic memories, make use of ferromagnetism. Also with these materials, high intensity magnets are manufactured with many research uses.