WHAT IS RELATIVE PERMEABILITY? - PHYSICAL - 2024

The relative permeability is the measure of the ability of a material way, being crossed by a stream without losing its features- respect to other material that serves as a reference. It is calculated as the ratio between the permeability of the material under study and that of the reference material. Therefore it is a quantity that lacks dimensions.

Generally speaking of permeability we think of a flow of fluids, commonly water. But there are also other elements capable of passing through substances, for example magnetic fields. In this case we speak of magnetic permeability and relative magnetic permeability.

Nickel has a high relative magnetic permeability, which is why coins adhere strongly to the magnet. Source: Pixabay.com.

The permeability of materials is a very interesting property, regardless of the type of flow that passes through them. Thanks to it, it is possible to anticipate how these materials will behave under very varied circumstances.

For example, the permeability of soils is very important when building structures such as drains, pavements and more. Even for crops, the permeability of the soil is relevant.

For life, the permeability of cell membranes allows the cell to be selective, by letting necessary substances such as nutrients pass through and rejecting others that may be harmful.

As for the relative magnetic permeability, it gives us information about the response of materials to magnetic fields caused by magnets or live wires. Such elements abound in the technology that surrounds us, so it is 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 much the wave is capable of penetrating the subsoil before being attenuated by it.

This provides a good idea of ​​the type of rocks that are in a certain place, since each rock has a different relative magnetic permeability, depending on its composition.

As it was said at the beginning, whenever we speak of relative permeability, the term "relative" requires comparing the magnitude in question of a certain material, with that of another that serves as a reference.

This is always applicable, regardless of whether it is permeability to a liquid or to a magnetic field.

Vacuum has permeability, since electromagnetic waves have no problem traveling there. It is a good idea to take this as a reference value to find the relative magnetic permeability of any material.

The permeability of the vacuum 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:

This magnitude describes how the magnetic response of a medium is compared to the response in a vacuum.

Now, the relative magnetic permeability can be equal to 1, less than 1 or greater than 1. That depends on the material in question and also on the temperature.

• Obviously, if μ _r = 1 the medium is the vacuum.
• If it is less than 1 it is a diamagnetic material
• If it is greater than 1, but not much, the material is paramagnetic
• And if it is much greater 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, so its magnetic response decreases.

Diamagnetic and paramagnetic materials

Diamagnetic materials respond negatively to magnetic fields and repel them. Michael Faraday (1791-1867) discovered this property in 1846, when he found that a piece of bismuth was repelled by any of the poles 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 exclusive to this element. All materials have it to some extent.

It is possible to show 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 all of them can have a diamagnetic response at some point.

Water, noble gases, gold, copper and many more are diamagnetic materials.

On the other hand, paramagnetic materials have some of their own magnetization. That is why they can respond positively to the magnetic field of a magnet, for example. They have a magnetic permeability similar to the value of μ _or.

Near a magnet, they can also become magnetized and become magnets on their own, but this effect disappears when the real magnet is removed from the vicinity. Aluminum and magnesium are examples of paramagnetic materials.

The truly magnetic materials: ferromagnetism

Paramagnetic substances are the most abundant in nature. But there are materials that are easily attracted to permanent magnets.

They are capable of acquiring magnetization on their own. These are iron, nickel, cobalt, and rare earths like gadolinium and dysprosium. In addition, some alloys and compounds between these and other minerals are known as ferromagnetic materials.

This type of material experiences a very strong magnetic response to an external magnetic field, such as a magnet, for example. This is why nickel coins stick to bar magnets. And in turn the bar magnets adhere to the refrigerators.

The relative magnetic permeability of ferromagnetic materials is much higher than 1. Inside they have small magnets called magnetic dipoles. As these magnetic dipoles align, they intensify the magnetic effect inside ferromagnetic materials.

When these magnetic dipoles are in the presence of an external field, they quickly align with it and the material sticks to the magnet. Although the external field is suppressed, moving the magnet away, a remanent magnetization remains inside 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 fades.

Curie temperature is the temperature at which the magnetic effect disappears completely 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 uses in research.

References

1. Tipler, P., Mosca G. (2003). Physics for Science and Technology, Volume 2. Editorial Reverte. Pages 810-821.
2. Zapata, F. (2003). Study of mineralogies associated with the Guafita 8x oil well belonging to the Guafita field (Apure State) using Mossbauer Magnetic Susceptibility and Spectroscopy measurements. Degree thesis. Central University of Venezuela.