Magnetic permeability of substance

The relationship between the magnetic field (H) and the magnetic induction (B) in matter is characterized by a physical quantity called magnetic permeability . Absolute magnetic The permeability of the medium is the ratio of B to H. According to the International System of Units, it is measured in units called 1 Henry per meter.

Its numerical value is expressed by the ratio of its magnitude to the value of the magnetic permeability of the vacuum and is denoted by μ. This quantity is called the relative magnetic Permeability (or simply magnetic permeability) of the medium. As a relative quantity, it does not have a unit of measurement.

Consequently, the relative magnetic permeability μ is a quantity that indicates the number of times the field induction of a given medium is inducted (or larger) than the induction of a vacuum magnetic field.

When the substance is exposed to an external magnetic field, it becomes magnetized. How does this happen? According to Ampere's hypothesis, microscopic electric currents are constantly circulating in each substance, caused by the motion of electrons along their orbits and the presence of their own magnetic moment. Under normal conditions, this movement is disordered, and the fields "extinguish" (compensate) each other. When the body is placed in an external field, the currents are ordered, and the body becomes magnetized (ie, having its own field).

The magnetic permeability of all substances is different. Based on its magnitude, the substances are divided into three large groups.

In diamagnets, the magnetic permeability μ is slightly less than unity. For example, in bismuth, μ = 0.9998. Diamagnets include zinc, lead, quartz, rock salt, copper, glass, hydrogen, benzene, water.

The magnetic permeability of paramagnets is a little more than one (for aluminum, μ = 1.000023). Examples of paramagnetic substances are nickel, oxygen, tungsten, ebonite, platinum, nitrogen, and air.

Finally, a number of substances (mainly metals and alloys) belong to the third group, whose magnetic permeability significantly (by several orders of magnitude) exceeds unity. These substances are ferromagnets. This mainly includes nickel, iron, cobalt and their alloys. For steel, μ = 8 ∙ 10 ^ 3, for a nickel alloy with iron, μ = 2.5 ∙ 10 ^ 5. Ferromagnets have properties that distinguish them from other substances. First, they have residual magnetism. Secondly, their magnetic permeability depends on the magnitude of the induction of the external field. Third, for each of them there exists a certain temperature threshold, called the Curie point , at which it loses its ferromagnetic properties and becomes a paramagnet. For nickel, the Curie point is 360 ° C, for iron - 770 ° C.

The properties of ferromagnets determine not only the magnetic permeability, but also the quantity I, called the magnetization of a given substance. This is a complex nonlinear function of magnetic induction, the growth of magnetization is described by a line called the magnetization curve . At the same time, reaching a certain point, the magnetization practically ceases to grow ( magnetic saturation occurs). The lag of the magnetization of the ferromagnet from the increasing value of the induction of the external field is called magnetic hysteresis . In this case, there is a dependence of the magnetic characteristics of the ferromagnet not only on its state at the present time, but also on its previous magnetization. The graphic representation of the curve of this dependence is called the hysteresis loop .

Due to their properties, ferromagnets are widely used in engineering. They are used in the rotors of generators and electric motors, in the manufacture of transformer cores and electromagnetic relays, in the manufacture of parts of electronic computers. The magnetic properties of ferromagnets are used in tape recorders, telephones, magnetic tapes and other media.