The relative permeability It is the measure of the capacity of a certain material to be traversed by a flow -without losing its characteristics-, with respect to that of another 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 of relative magnetic permeability.
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, soil permeability is relevant.
For life, the permeability of cell membranes allows the cell to be selective, by allowing necessary substances such as nutrients to 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.
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A very interesting application of electromagnetic waves is to facilitate oil prospecting. 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 said at the beginning, whenever you talk about 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..
The 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:
μor = 4π. 10 -7 T.m / A (Tesla. Meter / Ampere).
This constant is part of nature and is linked, together with electrical permittivity, to the value of the speed of light in vacuum.
To find the relative magnetic permeability, you have to compare the magnetic response of a material in two different media, one of which is vacuum..
In calculating magnetic induction B of a wire in vacuum, its magnitude was found to be:
And the relative permeability μr of said medium, is the quotient between B and Bor: μr= B / Bor. It is a dimensionless quantity, as you can see.
Relative magnetic permeability is a dimensionless and positive quantity, being the quotient of two positive quantities in turn. Remember that the modulus of a vector is always greater than 0.
μr= B / Bor = μ / μor
μ = μr . μor
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..
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.
The materials diamagnetic they 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, that is why all of them can have a diamagnetic response at some point.
Water, noble gases, gold, copper and many more are diamagnetic materials.
Instead the materials paramagnetic they have some magnetization of their own. That is why they can respond positively to the magnetic field of a magnet, for example. They have a magnetic permeability close to the value of μor.
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.
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 materials. ferromagnetic.
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 stick to the coolers.
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 gradually disappears..
The 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 uses in research.
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