What is hysteresis, what are the benefits and harms of this phenomenon. Definition of hysteresis: features, applications in boilers

Hysteresis (from the Greek hysteresis - lag) - physical phenomenon, in which there is a delay in the change in the state of the system from the change in the physical quantity that determines the external conditions.
For example, the delay of a change in the magnetization of a ferromagnet from a change in voltage magnetic field; delay of the change in the polarization of the ferroelectric from the change in the electric field.
It is observed in cases where the state of the system is determined by external conditions not only at a given moment in time, but also at previous moments. Hysteresis is observed in various branches of physics. The most important are: magnetic hysteresis, ferroelectric hysteresis and elastic hysteresis.

The essence of this phenomenon can be explained using the example of the operation of a thermostat.
Consider a thermostat set to maintain a temperature of 20°C using an electric heater. If the bimetallic plate that controls the heater, which deforms when the temperature changes, did not have hysteresis, the heater would turn on and off very often, which would lead to rapid wear of the contacts. In reality, the regulator turns on at 19 °C and turns off at approximately 21 °C. In this case, the mechanical inertia of the bimetallic plate and the thermal inertia of the heater give rise to the phenomenon of hysteresis, mode switching occurs with a low frequency, and the temperature in the thermostat fluctuates in a certain range near the set value ( rice. 1).

Picture 1

Hysteresis is characterized by the phenomenon of “saturation”, as well as the dissimilarity of trajectories between extreme states, hence the presence of an acute-angled loop in the graphs, called the hysteresis loop. The ambiguous dependence of the state of the system on a physical quantity (with cyclic change) is depicted by a hysteresis loop ( rice. 2)

Figure 2

In electronics and electrical engineering, devices are used that have magnetic hysteresis - various magnetic storage media, or electrical hysteresis, for example, a Schmitt trigger or a hysteresis motor. Hysteresis is used to suppress noise (fast oscillations, contact bounce) when switching logic signals. For example, a Schmitt trigger ( rice. 3).

Figure 3. The hysteresis loop for the Schmitt trigger is rectangular.
In electronic devices of all types, the phenomenon of thermal hysteresis is observed: after heating the device and its subsequent cooling to the initial temperature, its parameters do not return to initial values. Due to unequal thermal expansion of semiconductor crystals, crystal holders, chip packages and printed circuit boards Mechanical stresses arise in the crystals, which persist even after cooling. The phenomenon of thermal hysteresis is most noticeable in precision voltage references used in measurement analog-to-digital converters. In modern microcircuits, the relative shift of the reference voltage due to thermal hysteresis is on the order of 10-100

Any electromagnetic core after action electric current retains a magnetic field for some time (residual magnetism). This value depends on the properties of the material, but residual magnetism is always present. To remagnetize the core, a magnetic flux in the opposite direction is necessary. The change in magnetic induction does not keep pace with the change magnetic flux. This time delay in magnetization of the core due to a change in the direction of magnetic fluxes is called hysteresis.

To understand the essence of this phenomenon, it is necessary to consider the ability of substances to magnetize.

Magnetic properties of substances

All substances in the nature around us have magnetic properties to one degree or another. Even in ancient times, the amazing ability of some minerals to attract iron objects was known. Among the numerous navigational instruments necessary to plot the course of a ship or aircraft, a magnetic compass is always present.

In the most precise measuring instruments The main parts include permanent magnets. It is known that not only iron has strong magnetic properties. This includes cobalt, nickel, their alloys and some rare earth elements. All these substances and alloys are called ferromagnets. What they have in common is their ability to undergo spontaneous magnetization.

This property of ferromagnets is used to create permanent magnets. The presence of uncompensated magnetic moments in the atoms of a substance is a necessary condition occurrence of ferromagnetism.

In Einstein's experiment, based on the magnitude of twisting during magnetization of a sample, it was proven that ferromagnetism is associated with the spin magnetic moments of electrons. The exchange interaction of electrons at certain ratios of the diameter of the atom and the inner unfilled shell leads to parallel orientation of the spins.

It is possible only with a positive value of the exchange energy integral.

Ultimately, a spin orientation is established in a ferromagnet that ensures the minimum value of the sum of the magnetic and exchange interaction energies.

An area with uniform spontaneous magnetization is called a domain. The most energetically favorable arrangement of domains is one in which they create a closed magnetic circuit.

Between neighboring domains with different directions of magnetization there are transition layers called domain boundaries or walls. A gradual rotation of the magnetization vector occurs in them.

Ferromagnetic properties of substances exist only in a certain temperature range. The temperature at which ferromagnetic materials completely lose their ferromagnetic properties is called the Curie point. The shape and size of domains on the surface of a ferromagnet can be seen under a microscope

In the elementary crystalline cell of iron, the edges of the cube correspond to the direction of the easiest magnetization of the iron crystal. The diagonals of the faces determine the direction of the average magnetization.

The direction of the most difficult magnetization coincides with the diagonals of the cube. The area on the graph characterizes the energy of magnetic anisotropy.

In the absence of an external field, the magnetic moments of the domains are oriented along the directions of easy magnetization. In general, the sample is demagnetized.

In weak fields, the growth of domains occurs, the direction of magnetization of which makes a smaller angle with the direction of the external field.

This process is reversible. If the external field is removed, the sample will be demagnetized. As the external field increases, further growth of domains occurs, which is stopped due to crystal defects. When the field reaches a certain value, the walls of the growing domains jump over the obstacle. Due to this obstacle, the magnetization curve has a stepped character.

Abrupt changes in magnetization create voltage pulses in the solenoid coil. With a further increase in the field, the magnetization vector rotates from the easy magnetization axis towards the external field until they coincide.

Hysteresis

This area is called the region of technical saturation of the ferromagnet, and the corresponding field value is called the saturation field. If the field is reduced from this value to zero, the residual magnetization will remain in the sample.

Hysteresis is the phenomenon of magnetization lagging behind the external field strength. Closing domains, creating a closed magnetic circuit, reduce stray fields and reduce the free energy of the sample.

It is defined as the difference between the magnetic saturation of the ferromagnet and the magnetization of the closing domains. To demagnetize a sample, a negative field called coercive force must be applied to it. When the field reaches saturation value, complete magnetization reversal of the ferromagnet will occur.

On the graph you can determine another property that hysteresis has. With the next change in the field, the magnetization curve closes a loop, which is called a hysteresis loop.

The hysteresis loop for the saturation condition is called the limit loop. Its area is proportional to the energy loss due to magnetization reversal of the sample. Ferromagnets, when magnetized, change their linear dimensions. This phenomenon is called magnetostriction.

There are two main groups of ferromagnetic materials:

Magnetically hard.
Magnetic soft.

One of the main requirements for soft magnetic materials is their high coercivity. Soft magnetic materials are magnetized to saturation in low fields and have low magnetization reversal losses. The energy loss of the transformer depends on these parameters.

For example, in a 100 x 10 6 VA power line with transformers at the ends, the annual losses are about 5 million kilowatt-hours. One of best representatives Soft magnetic materials are considered permalloy, an alloy of iron and nickel. The magnetization of permalloy in weak fields is tens of times greater than the magnetization of iron. The magnetic ordered structures in some substances differ from the magnetic structure of ferromagnets.

If in iron, cobalt and nickel the spin magnetic moments are directed parallel, then in chromium and manganese they are antiparallel. Such substances are called antiferromagnets.

In this case, magnetic sublattices with spontaneous magnetization are compensated. If the crystals of a substance do not have complete compensation of the magnetic sublattices, then it is called a ferrimagnet. Ferrite is one example of ferrimagnets that is widely used in technology. The structure of ferrites is similar to the structure of spinel minerals, in which non-ferromagnetic metal ions are replaced by ferromagnetic ones.

Hysteresis in electrical engineering and electronics

From the variety of examples of the use of ferromagnetic materials, we will talk about their use in memory devices. For rapid storage of information, memory on ferrite rings is used. One ferrite core is enough to store one bit of information. Special magnetic disks (Schmidt triggers) serve as long-term high-capacity storage devices.

It is also used in special hysteresis electric motors, noise reduction devices (contact bounce, oscillations, etc.) when switching logic circuits.

Many electronic devices have thermal hysteresis. During operation, the devices heat up, and after cooling, some properties no longer take their initial values. When a microcircuit, printed circuit board, or semiconductor crystals heat up, they expand and mechanical stress appears. During cooling, this tension remains to some extent.

Hysteresis

The phenomenon of magnetic hysteresis is observed not only when the field changes H in magnitude and sign, but also during its rotation (magnetic rotation hysteresis), which corresponds to a lag (delay) in changing direction M with change of direction H. Magnetic rotation hysteresis also occurs when the sample rotates relative to a fixed direction H.

The theory of the hysteresis phenomenon takes into account the specific magnetic domain structure of the sample and its changes during magnetization and magnetization reversal. These changes are due to the displacement of domain boundaries and the growth of some domains at the expense of others, as well as the rotation of the magnetization vector in domains under the influence of an external magnetic field. Anything that delays these processes and allows magnets to enter metastable states can cause magnetic hysteresis.

In single-domain ferromagnetic particles (in small-sized particles in which the formation of domains is energetically unfavorable) only rotation processes can occur M. These processes are hampered by magnetic anisotropy of various origins (anisotropy of the crystal itself, anisotropy of the particle shape and anisotropy elastic stresses). Thanks to anisotropy, M as if it is held by some internal field (the effective field of magnetic anisotropy) along one of the axes of easy magnetization, corresponding to the minimum energy. Magnetic hysteresis occurs because the two directions M(along and against) this axis in a magnetically uniaxial sample or several equivalent (in energy) directions M in a magnetically multiaxial sample correspond to states separated from each other by a potential barrier (proportional). When single-domain particles are remagnetized, the vector M a series of successive irreversible jumps turns in the direction H. Such rotations can occur both uniformly and non-uniformly in volume. With uniform rotation M coercive force. The mechanism of non-uniform rotation is more universal M. However, it has the greatest impact in the case where the main role is played by the anisotropy of the particle shape. In this case, the effective shape anisotropy field may be significantly smaller.

Ferroelectric hysteresis- ambiguous loop-shaped polarization dependence P ferroelectrics from an external electric field E when it changes cyclically. Ferroelectric crystals have spontaneous (spontaneous, that is, arising in the absence of an external electric field) electrical polarization in a certain temperature range P c. The direction of polarization can be changed by an electric field. At the same time, dependence P(E) in the polar phase is ambiguous, the value P given E depends on the background, that is, on what it was like electric field at previous points in time. Basic parameters of ferroelectric hysteresis:

residual crystal polarization P ost, at E = 0
field value E Kt (coercive field) at which repolarization

Elastic hysteresis

Hysteresis is used to suppress noise (fast oscillations, contact bounce) when switching logic signals.

In electronic devices of all types, the phenomenon of thermal hysteresis is observed: after heating the device and its subsequent cooling to the initial temperature, its parameters do not return to the initial values. Due to the unequal thermal expansion of semiconductor crystals, crystal holders, microcircuit packages and printed circuit boards, mechanical stresses arise in the crystals, which persist even after cooling. The phenomenon of thermal hysteresis is most noticeable in precision analog-to-digital converters used in measuring analog-to-digital converters. In modern microcircuits, the relative shift of the reference voltage due to thermal hysteresis is on the order of 10-100 ppm.

In biology

Hysteretic properties are characteristic of skeletal muscles mammals.

In soil science

One of them indicates the relationship between the efforts made by the subject of influence and the result achieved. The level of educational and propaganda work spent by the subject can be correlated with the level of “magnetization” (degree of involvement in a new idea) of the carrier object public opinion, social group, collective, social community or society as a whole; in this case, some lag between the object and the subject may be revealed. Persuasion, including those with supposed destructive consequences, is not always successful. It depends on one’s own moral values, customs, traditions, the nature of previous upbringing, the ethical standards dominant in society, etc.

The second circumstance is due to the fact that the new stage in the formation of public opinion can be correlated with the history of the object, its experience, its assessment by those who previously acted as the object of public opinion formation. In this case, one can find that the “reference point” of the time of formation of public opinion shifts relative to the previous one, which is a characteristic of the system itself and its current state.

Literature on the topic

Raddai Raikhlin Civil war, terror and banditry. Systematization of sociology and social dynamics. Section "Crowd Control"
Kapustin Valery Sergeevich Introduction to the theory of social self-organization. Topic 11. The phenomenon of hysteresis in the formation of national forms and methods of self-organization. Modern paradoxes and mysteries of the “beginning”

In philosophy

Mathematical models of hysteresis

The emergence of mathematical models of hysteresis phenomena was determined by a fairly rich set of applied problems (primarily in the theory of automatic control), in which the carriers of hysteresis cannot be considered in isolation, since they were part of a certain system. The creation of the mathematical theory of hysteresis dates back to the 60s of the 20th century, when Voronezh University A seminar began to work under the leadership of M. A. Krasnoselsky, on “hysteresis” topics. Later, in 1983, a monograph appeared in which various hysteretic phenomena received a formal description within the framework of systems theory: hysteretic converters were treated as operators depending on their initial state as a parameter, defined on a sufficiently rich functional space (for example, in the space of continuous functions), acting in a certain functional space. A simple parametric description of various hysteresis loops can be found in the work (replacing harmonic functions in this model with rectangular, triangular or trapezoidal pulses also allows us to obtain piecewise linear hysteresis loops, which are often found in discrete automation, see example in Fig. 2).

Literature

Notes

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Synonyms

See what “Hysteresis” is in other dictionaries: - (from the Greek hysteresis lag) the delay of a change in a physical quantity characterizing the state of a substance (magnetization M of a ferromagnet, polarization P of a ferroelectric, etc.) from a change in another physical quantity that determines... ...

Big Encyclopedic Dictionary Shift, lag Dictionary of Russian synonyms. hysteresis noun, number of synonyms: 2 lag (10) ...

Synonym dictionary HYSTERESIS, a phenomenon characteristic of elastic bodies; lies in the fact that the DEFORMATION of a body when STRESS increases is less than when it decreases due to a delay in the effect of deformation. When the mechanical stress is completely removed, what remains is... ... Scientific and technical

encyclopedic Dictionary - (from the Greek hysteresis lag, retardation) 1) G. in aerodynamics, the ambiguity of the structure of the flow field and, consequently, the aerodynamic characteristics of a streamlined body for the same values of kinematic parameters, but at ... ...

Let us consider the process of alternating magnetization of a ferromagnetic material. For this purpose, we will wind a winding around a steel core and pass direct current through it. Let us assume that the core of the electromagnet has not been previously magnetized.

Increasing the current passing through the turns of the winding I from zero, we will thereby increase the magnetizing force and field strength H. Magnetic induction value B in the core will also increase. Magnetization curve 0a in Figure 1 has a straight part, and then, due to saturation, the curve rises slowly, approaching the horizontal. If now, having reached the point A, reduce H, then it will decrease and B. However, the decrease B when decreasing H, that is, during demagnetization, will occur with a delay relative to the decrease H. The value of residual induction at H= 0 is characterized by the segment 0b.

In order for the magnetic induction in the core to become zero, it is necessary to magnetize the material in the opposite direction, that is, to remagnetize it. For this purpose, the direction of the current in the winding is reversed. The direction of magnetic lines and magnetic field strength also changes. At field strength H = 0v the induction in the core is zero and the core material is completely demagnetized. Field strength value H = 0v at B= 0 is a certain characteristic of the material and is called the retarding (coercive) force.

By repeating the magnetization reversal process, we obtain a closed curve a b c d e f a, which is called a hysteresis loop or magnetic hysteresis loop. Hysteresis from the Greek means lagging, retarded. From this experiment, it is easy to verify that the magnetization and demagnetization of the core (the appearance and disappearance of poles, magnetic induction or magnetic flux) lags behind the moment of the appearance and disappearance of the magnetizing and demagnetizing force (current in the electromagnet winding). The phenomenon of hysteresis can be characterized in other words as a lag between changes in magnetic induction and changes in field strength. Magnetization reversal of a material involves the expenditure of a certain amount of energy, which is released in the form of heat that heats the material.

Magnetic hysteresis is especially severe if the core material has high residual magnetism (for example, hard steel). The phenomenon of hysteresis is harmful in most cases. It causes hysteresis losses expressed in core heating and extra costs power of the voltage source, and is also accompanied by the hum of the core due to the change in polarity and rotation of the elementary particles of the core material.

The first serious study of the processes of magnetization of steel was carried out by Alexander Grigorievich Stoletov (1839 – 1896) in 1872 and published in the work “On the function of magnetization of soft iron.”

A.G. Stoletov, in addition, investigated and explained the nature of the external photoelectric effect and manufactured the first photocell.

Video 1. Hysteresis

Hysteresis (from the Greek hysteresis - lag, delay), a phenomenon that consists in the fact that a physical quantity characterizing the state of a body (for example, magnetization) ambiguously depends on physical quantities, characterizing external conditions (for example, magnetic field). G. is observed in cases where the state of the body at a given moment in time is determined by external conditions not only at the same time, but also at previous points in time. An ambiguous dependence of quantities is observed in any process, because Changing the state of the body always requires a certain time (relaxation time) and the body’s reaction lags behind the causes that cause it. This lag is smaller the slower the external conditions change. However, for some processes the lag when the change slows down external conditions does not decrease. In these cases, the ambiguous dependence of quantities is called hysteresis, and the phenomenon itself is called hysteresis.

G. is observed in various substances and during various physical processes. Of greatest interest are: magnetic g., dielectric g., and elastic g.

Magnetic magnetism is observed in magnetic materials, for example, in ferromagnets. The main feature of ferromagnets is the presence of spontaneous (spontaneous) magnetization. Typically, a ferromagnet is not uniformly magnetized, but is divided into domains - areas of uniform spontaneous magnetization, in which the magnitude of magnetization (magnetic moment per unit volume) is the same, but the directions are different. Under the influence of an external magnetic field, the number and size of domains magnetized by the field increase at the expense of other domains. In addition, the magnetic moments of individual domains can rotate along the field. As a result, the magnetic moment of the sample increases.

In Fig. Figure 1 shows the dependence of the magnetic moment M of a ferromagnetic sample on the strength H of the external magnetic field (magnetization curve). In a sufficiently strong magnetic field, the sample is magnetized to saturation (with a further increase in the field, the value of M practically does not change, point A). In this case, the sample consists of one domain with a saturation magnetic moment Ms directed along the field. As the external magnetic field strength H decreases, the magnetic moment of the sample M will decrease along curve I primarily due to the emergence and growth of domains with a magnetic moment directed against the field. The growth of domains is due to the movement of domain walls. This movement is difficult due to the presence of various defects in the sample (impurities, inhomogeneities, etc.), which fix the domain walls in certain positions; strong enough magnetic fields are required to move them. Therefore, when the field H is reduced to zero, the sample retains the so-called residual magnetic moment M r (point B).

The sample is completely demagnetized only when sufficiently strong field in the opposite direction, called the coercive field (coercive force) H c (point C). With a further increase in the magnetic field in the opposite direction, the sample is again magnetized along the field until saturation (point D). The magnetization reversal of the sample (from point D to point A) occurs along curve II. Thus, with a cyclic change in the field, the curve characterizing the change in the magnetic moment of the sample forms a magnetic loop. If the field H is changed cyclically within such limits that saturation magnetization is not achieved, then an unlimited magnetic loop is obtained (curve III). By reducing the amplitude of the field change H to zero, the sample can be completely demagnetized (arrive at point O). The magnetization of the sample from point O occurs along curve IV.

In magnetic resonance, the same value of the external magnetic field strength H corresponds to different meanings magnetic moment M. This ambiguity is due to the influence of states of the sample preceding this one (i.e., the magnetic prehistory of the sample).

The type and dimensions of the magnetic loop and the value of Hc in different ferromagnets can vary within wide limits. For example, in pure iron Hc = 1 Oe, in a magnetic alloy Hc = 580 Oe. The magnetic loop is strongly influenced by material processing, which changes the number of defects (Fig. 2).

The area of the magnetic loop is equal to the energy lost in the sample during one cycle of field change. This energy is ultimately used to heat the sample. Such energy losses are called hysteresis. In cases where G. losses are undesirable (for example, in the cores of transformers, in the stators and rotors of electrical machines), magnetically soft materials with a small Hc and a small G loop area are used. For the manufacture of permanent magnets, on the contrary, magnetically hard materials with a large Ns.

With increasing frequency of the alternating magnetic field (the number of magnetization reversal cycles per unit time), other losses associated with eddy currents and magnetic viscosity are added to the hysteresis losses. Accordingly, the area of the G. loop increases at high frequencies. This type of loop is sometimes called a dynamic loop, in contrast to the static loop described above.

Many other properties of a ferromagnet, such as electrical resistance and mechanical deformation, depend on the magnetic moment. A change in the magnetic moment causes a change in these properties. Accordingly, for example, galvanomagnetic gyration and magnetostrictive hysteria are observed.

Dielectric hydrogen is usually observed in ferroelectrics, such as barium titanate. The dependence of polarization P on the electric field strength E in ferroelectrics (Fig. 3) is similar to the dependence of M on H in ferromagnets and is explained by the presence of spontaneous electric polarization, electric domains and the difficulty of rearranging the domain structure. Hysteresis losses are most dielectric losses in ferroelectrics.

Since other characteristics of ferroelectrics, such as deformation, are associated with polarization, other types of hysteresis are associated with dielectric hysterics, for example, piezoelectric hysterics (Fig. 4) and electro-optical effect hysterics. In some cases, double dielectric loops are observed (Fig. 5). This is explained by the fact that, under the influence of an electric field, a phase transition occurs in the sample with a restructuring of the crystal structure. This kind of dielectric hysteria is closely related to hysteria during phase transitions.

Elastic hysteresis, that is, the hysteretic dependence of deformation on mechanical stress s, is observed in any real materials at sufficiently high stresses (Fig. 6). Elastic distortion occurs whenever plastic (inelastic) deformation occurs (see Plasticity). Plastic deformation is caused by the movement of defects, such as dislocations, which are always present in real materials. Impurities, inclusions and other defects, as well as the crystal lattice itself, tend to hold dislocations in certain positions in the crystal. Therefore, stresses of sufficient magnitude are required to move the dislocation. Mechanical processing and the introduction of impurities lead to the pinning of dislocations, as a result of which the material is strengthened; plastic deformation and elastic distortion are observed at high stresses. The energy lost in a sample during one cycle ultimately goes to heating the sample. Elastic hydraulic losses contribute to internal friction. In the case of elastic deformations, in addition to hysteresis, there are other losses, for example due to viscosity. The magnitude of these losses, in contrast to hysteresis, depends on the frequency of change s (or u). Sometimes the concept of "elastic G." is used more widely - they talk about the dynamic loop of an elastic band, which includes all losses at a given frequency.

Lit.: Kirensky L.V., Magnetism, 2nd ed., M., 1967; Vonsovsky S.V., Modern doctrine of magnetism, M. - L., 1952; Bozort R., Ferromagnetism, trans. from English, M., 1956; Jonah F., Shirane D., Ferroelectric crystals, trans. from English, M., 1965; Postnikov V.S., Internal friction in metals, M., 1969; Physical encyclopedic dictionary, vol. 1, M., 1960.

A. P. Levanyuk, D. G. Sannikov.