Dielectric constant shows how many times. Electrical constant and dielectric loss angle

Permittivity permittivity

the value ε, showing how many times the force of interaction between two electric charges in a medium is less than in a vacuum. In an isotropic medium, ε is related to the dielectric susceptibility χ by the relation: ε = 1 + 4π χ. The dielectric constant of an anisotropic medium is a tensor. The dielectric constant depends on the field frequency; in strong electric fields, the dielectric constant begins to depend on the field strength.

PERMITTIVITY

DIELECTRIC CONTINUITY, a dimensionless quantity e, showing how many times the interaction force F between electric charges in a given medium is less than their interaction force F o in a vacuum:
e =F o /F.
Dielectric constant shows how many times the field is attenuated by the dielectric (cm. DIELECTRICS), quantitatively characterizing the property of a dielectric to polarize in an electric field.
The value of the relative dielectric constant of a substance, which characterizes the degree of its polarizability, is determined by the polarization mechanisms (cm. POLARIZATION). However, the value largely depends on state of aggregation substances, since during transitions from one state to another the density of the substance, its viscosity and isotropy change significantly (cm. ISOTROPY).
Dielectric constant of gases
Gaseous substances are characterized by very low densities due to large distances between molecules. Due to this, the polarization of all gases is insignificant and permittivity them is close to unity. The polarization of a gas can be purely electronic or dipole if the gas molecules are polar, however, in this case, the electronic polarization is of primary importance. The polarization of various gases is greater, the larger the radius of the gas molecule, and is numerically close to the square of the refractive index for this gas.
The dependence of a gas on temperature and pressure is determined by the number of molecules per unit volume of gas, which is proportional to pressure and inversely proportional to absolute temperature.
The air in normal conditions e =1.0006, and its temperature coefficient is about 2. 10 -6 K -1 .
Dielectric constant of liquid dielectrics
Liquid dielectrics can consist of non-polar or polar molecules. The e value of non-polar liquids is determined by electronic polarization, so it is small, close to the value of the square of the refraction of light and usually does not exceed 2.5. The dependence of e of a non-polar liquid on temperature is associated with a decrease in the number of molecules per unit volume, i.e., with a decrease in density, and its temperature coefficient is close to the temperature coefficient of volumetric expansion of the liquid, but differs in sign.
The polarization of liquids containing dipole molecules is determined simultaneously by the electronic and dipole-relaxation components. Such liquids have a higher dielectric constant, the greater the value of the electric moment of the dipoles (cm. DIPOLE) and with what larger number molecules per unit volume. The temperature dependence in the case of polar liquids is complex.
Dielectric constant of solid dielectrics
In solids it can take a variety of numeric values in accordance with the variety of structural features of the solid dielectric. In solid dielectrics all types of polarization are possible.
The smallest value of e is found in solid dielectrics consisting of non-polar molecules and having only electronic polarization.
Solid dielectrics, which are ionic crystals with densely packed particles, have electronic and ionic polarizations and have e values ​​that lie within a wide range (e rock salt- 6; e corundum - 10; e rutile - 110; e calcium titanate - 150).
e of various inorganic glasses, approaching the structure of amorphous dielectrics, lies in a relatively narrow range from 4 to 20.
Polar organic dielectrics have dipole-relaxation polarization in the solid state. e of these materials depends to a large extent on the temperature and frequency of the applied voltage, obeying the same laws as for dipole liquids.

Encyclopedic Dictionary. 2009 .

See what “dielectric constant” is in other dictionaries:

The value of e, showing how many times the force of interaction between two electric charges in a medium is less than in a vacuum. In an isotropic medium, e is related to the dielectric susceptibility with the relation: e = 1 + 4pc. Dielectric constant... ... Big Encyclopedic Dictionary

The value e characterizing the polarization of dielectrics under the influence of electricity. field E.D.p. is included in Coulomb’s law as a quantity showing how many times the force of interaction of two free charges in a dielectric is less than in a vacuum. Weakening of the... ... Physical encyclopedia

DIELECTRIC CONTINUITY, The value e, showing how many times the force of interaction of two electric charges in a medium is less than in a vacuum. The value of e varies widely: hydrogen 1.00026, transformer oil 2.24, ... ... Modern encyclopedia

- (designation e), in physics one of the properties various materials(see DIELECTRIC). It is expressed by the ratio of the density of the ELECTRIC FLOW in the medium to the intensity of the ELECTRIC FIELD that causes it. Dielectric constant of vacuum... ... Scientific and technical encyclopedic dictionary

permittivity- A quantity characterizing the dielectric properties of a substance, scalar for an isotropic substance and tensor for an anisotropic substance, the product of which by the electric field strength is equal to the electric displacement. [GOST R 52002 2003]… … Technical Translator's Guide

Permittivity- DIELECTRIC CONTINUITY, the value e, showing how many times the force of interaction of two electric charges in a medium is less than in a vacuum. The value of e varies widely: hydrogen 1.00026, transformer oil 2.24, ... ... Illustrated Encyclopedic Dictionary

Permittivity- a quantity characterizing the dielectric properties of a substance, scalar for an isotropic substance and tensor for an anisotropic substance, the product of which by the electric field strength is equal to the electric displacement... Source:... ... Official terminology

permittivity- absolute dielectric constant; industry dielectric constant A scalar quantity characterizing the electrical properties of a dielectric equal to the ratio of the magnitude of the electrical displacement to the magnitude of the electric field strength ... Polytechnic terminological explanatory dictionary

Absolute dielectric constant Relative dielectric constant Vacuum dielectric constant ... Wikipedia

permittivity- dielektrinė skvarba statusas T sritis chemija apibrėžtis Elektrinio srauto tankio tiriamojoje medžiagoje ir elektrinio lauko stiprio santykis. atitikmenys: engl. dielectric constant; dielectric permittivity; permittivity rus. dielectric... ... Chemijos terminų aiškinamasis žodynas

Books

• Properties of materials. Anisotropy, symmetry, structure. Per. from English , Newnham R.E. This book is devoted to anisotropy and the relationship between the structure of materials and their properties. It covers a wide range of topics and is a kind introductory course physical properties...

DIELECTRIC CONTINUITY, a value ε characterizing the polarization of dielectrics under the influence of an electric field of strength E. Dielectric constant is included in Coulomb’s law as a quantity showing how many times the force of interaction between two free charges in a dielectric is less than in a vacuum. The weakening of the interaction occurs due to the screening of free charges by bound ones formed as a result of polarization of the medium. Bound charges arise as a result of microscopic spatial redistribution of charges (electrons, ions) in an generally electrically neutral environment.

The relationship between the polarization vectors P, electric field strength E and electric induction D in an isotropic medium in the SI system has the form:

where ε 0 is the electrical constant. The value of dielectric constant ε depends on the structure and chemical composition substances, as well as pressure, temperature and other external conditions(table).

For gases its value is close to 1, for liquids and solids varies from several units to several tens; for ferroelectrics it can reach 10 4 . This scatter of ε values ​​is due to different polarization mechanisms that occur in different dielectrics.

Classical microscopic theory leads to an approximate expression for the dielectric constant of non-polar dielectrics:

where n i is the concentration of the i-th type of atoms, ions or molecules, α i is their polarizability, β i is the so-called internal field factor, due to the structural features of the crystal or substance. For most dielectrics with a dielectric constant in the range of 2-8, β = 1/3. Typically, the dielectric constant is practically independent of the magnitude of the applied electric field up to the electrical breakdown of the dielectric. The high values ​​of ε of some metal oxides and other compounds are due to the peculiarities of their structure, which allows, under the influence of the field E, a collective displacement of the sublattices of positive and negative ions in opposite directions and the formation of significant bound charges at the crystal boundary.

The polarization process of a dielectric when an electric field is applied does not develop instantly, but over a period of time τ (relaxation time). If the field E changes in time t according to a harmonic law with a frequency ω, then the polarization of the dielectric does not have time to follow it and a phase difference δ appears between the oscillations P and E. When describing oscillations of P and E using the method of complex amplitudes, the dielectric constant is represented as a complex quantity:

ε = ε’ + iε",

Moreover, ε' and ε" depend on ω and τ, and the ratio ε"/ε' = tan δ determines the dielectric losses in the medium. The phase shift δ depends on the ratio τ and the field period T = 2π/ω. At τ<< Т (ω<< 1/τ, низкие частоты) направление Р изменяется практически одновременно с Е, т. е. δ → 0 (механизм поляризации «включён»). Соответствующее значение ε’ обозначают ε (0) . При τ >> T (high frequencies), polarization does not keep pace with the change Ε, δ → π and ε’ in this case denote ε (∞) (the polarization mechanism is “turned off”). It is obvious that ε (0) > ε (∞), and in alternating fields the dielectric constant turns out to be a function of ω. Near ω = l/τ, ε’ changes from ε (0) to ε (∞) (dispersion region), and the tanδ(ω) dependence passes through a maximum.

The nature of the dependences ε’(ω) and tanδ(ω) in the dispersion region is determined by the polarization mechanism. In the case of ionic and electronic polarizations with elastic displacement of bound charges, the change in P(t) with stepwise inclusion of the field E has the character damped oscillations and the dependences ε’(ω) and tanδ(ω) are called resonant. In the case of orientational polarization, the establishment of P(t) is exponential, and the dependences ε’(ω) and tanδ(ω) are called relaxation.

Methods for measuring dielectric polarization are based on the phenomena of interaction of the electromagnetic field with the electric dipole moments of particles of matter and are different for different frequencies. Most methods at ω ≤ 10 8 Hz are based on the process of charging and discharging a measuring capacitor filled with the dielectric under study. With more high frequencies waveguide, resonant, multifrequency and other methods are used.

In some dielectrics, for example ferroelectrics, the proportional relationship between P and E [Ρ = ε 0 (ε ‒ 1)E] and, consequently, between D and E is violated already in ordinary electric fields achieved in practice. Formally, this is described as the dependence ε(Ε) ≠ const. In this case, an important electrical characteristic of the dielectric is the differential dielectric constant:

In nonlinear dielectrics, the value ε diff is usually measured in weak alternating fields with the simultaneous application of a strong constant field, and the variable component ε diff is called the reversible dielectric constant.

Lit. look at Art. Dielectrics.

Dielectrić chemical penetratioń capacity medium - a physical quantity that characterizes the properties of an insulating (dielectric) medium and shows the dependence of electrical induction on the electric field strength.

It is determined by the effect of polarization of dielectrics under the influence of an electric field (and with the value of the dielectric susceptibility of the medium characterizing this effect).

There are relative and absolute dielectric constants.

The relative dielectric constant ε is dimensionless and shows how many times the force of interaction between two electric charges in a medium is less than in a vacuum. This value for air and most other gases under normal conditions is close to unity (due to their low density). For most solid or liquid dielectrics, the relative permittivity ranges from 2 to 8 (for a static field). The dielectric constant of water in a static field is quite high - about 80. Its values ​​are large for substances with molecules that have a large electric dipole moment. The relative dielectric constant of ferroelectrics is tens and hundreds of thousands.

The absolute dielectric constant in foreign literature is denoted by the letter ε; in domestic literature, the combination is predominantly used, where is the electric constant. Absolute dielectric constant is used only in the International System of Units (SI), in which induction and electric field strength are measured in different units. In the SGS system there is no need to introduce absolute dielectric constant. The absolute dielectric constant (like the electrical constant) has the dimension L −3 M −1 T 4 I². In International System of Units (SI) units: =F/m.

It should be noted that the dielectric constant largely depends on the frequency of the electromagnetic field. This should always be taken into account since reference tables usually contain data for a static field or low frequencies down to a few units of kHz without specifying this fact. At the same time, there are also optical methods for obtaining the relative dielectric constant based on the refractive index using ellipsometers and refractometers. The value obtained by the optical method (frequency 10-14 Hz) will differ significantly from the data in the tables.

Consider, for example, the case of water. In the case of a static field (frequency zero), the relative dielectric constant under normal conditions is approximately 80. This is the case down to infrared frequencies. Starting at approximately 2 GHz ε r starts to fall. In the optical range ε r is approximately 1.8. This is quite consistent with the fact that in the optical range the refractive index of water is 1.33. In a narrow frequency range, called optical, dielectric absorption drops to zero, which actually provides a person with the mechanism of vision [ source not specified 1252 days] in the earth's atmosphere saturated with water vapor. With further increase in frequency, the properties of the medium change again. You can read about the behavior of the relative dielectric constant of water in the frequency range from 0 to 10 12 (infrared region) at (English)

The dielectric constant of dielectrics is one of the main parameters in the development of electrical capacitors. The use of materials with high dielectric constant can significantly reduce the physical dimensions of capacitors.

The capacitance of the capacitors is determined:

Where ε r- dielectric constant of the substance between the plates, ε O- electrical constant, S- area of ​​the capacitor plates, d- distance between the plates.

The dielectric constant parameter is taken into account when developing printed circuit boards. The value of the dielectric constant of the substance between the layers, in combination with its thickness, affects the value of the natural static capacitance of the power layers, and also significantly affects the characteristic impedance of the conductors on the board.

RESISTANCE electrical, physical quantity equal to electrical resistance ( cm. ELECTRICAL RESISTANCE) R of a cylindrical conductor of unit length (l = 1 m) and unit cross-sectional area (S = 1 m 2).. r = R S/l. In Si, the unit of resistivity is Ohm. m. Resistivity can also be expressed in Ohms. cm. Resistivity is a characteristic of the material through which current flows and depends on the material from which it is made. Resistivity equal to r = 1 Ohm. m means that a cylindrical conductor made of of this material, length l = 1 m and with a cross-sectional area S = 1 m 2 has a resistance R = 1 Ohm. m. The value of resistivity of metals ( cm. METALS), which are good conductors ( cm. CONDUCTORS), can have values ​​of the order of 10 - 8 – 10 - 6 Ohms. m (for example, copper, silver, iron, etc.). The resistivity of some solid dielectrics ( cm. DIELECTRICS) can reach a value of 10 16 -10 18 Ohm.m (for example, quartz glass, polyethylene, electroporcelain, etc.). The resistivity value of many materials (especially semiconductor materials ( cm. SEMICONDUCTOR MATERIALS)) significantly depends on the degree of their purification, the presence of alloying additives, thermal and mechanical treatments, etc. The value s, the reciprocal of the resistivity, is called conductivity: s = 1/r Specific conductivity is measured in siemens ( cm. SIEMENS (conductivity unit)) per meter S/m. Electrical resistivity (conductivity) is a scalar quantity for an isotropic substance; and tensor - for an anisotropic substance. In anisotropic single crystals, the anisotropy of electrical conductivity is a consequence of the anisotropy of the inverse effective mass ( cm. EFFECTIVE MASS) electrons and holes.

1-6. ELECTRICAL CONDUCTIVITY OF INSULATION

When turning on the insulation of a cable or wire on constant voltage U a current i passes through it, varying with time (Fig. 1-3). This current has constant components - conduction current (i ∞) and absorption current, where γ is the conductivity corresponding to the absorption current; T is the time during which the current i abs drops to 1/e of its original value. For infinitely long time i abs →0 and i = i ∞. The electrical conductivity of dielectrics is explained by the presence in them of a certain amount of free charged particles: ions and electrons.

The most characteristic feature of most electrical insulating materials is ionic electrical conductivity, which is possible due to contaminants inevitably present in the insulation (impurities of moisture, salts, alkalis, etc.). In a dielectric with an ionic conductivity, Faraday's law is strictly observed - the proportionality between the amount of electricity passing through the insulation and the amount of substance released during electrolysis.

As the temperature increases, the resistivity of electrical insulating materials decreases and is characterized by the formula

where_ρ o, A and B are constants for a given material; T - temperature, °K.

A greater dependence of insulation resistance on moisture occurs with hygroscopic insulating materials, mainly fibrous (paper, cotton yarn, etc.). Therefore, fibrous materials are dried and impregnated, as well as protected by moisture-resistant shells.

Insulation resistance can decrease with increasing voltage due to the formation of space charges in the insulating materials. The additional electronic conductivity created in this case leads to an increase in electrical conductivity. There is a dependence of conductivity on voltage in very strong fields (Ya. I. Frenkel’s law):

where γ o - conductivity in weak fields; a is constant. All electrical insulating materials are characterized by certain values ​​of insulation conductivity G. Ideally, the conductivity of insulating materials is zero. For real insulating materials, the conductivity per unit cable length is determined by the formula

In cables with an insulation resistance of more than 3-10 11 ohm-m and communication cables, where losses due to dielectric polarization are significantly greater than thermal losses, conductivity is determined by the formula

Insulation conductivity in communications technology is an electrical parameter of a line that characterizes energy loss in the insulation of cable cores. The dependence of the conductivity value on frequency is shown in Fig. 1-1. The reciprocal of conductivity, the insulation resistance, is the ratio of the DC voltage applied to the insulation (in volts) to the leakage voltage (in amperes), i.e.

where R V is the volumetric insulation resistance, which numerically determines the obstacle created by the passage of current through the thickness of the insulation; R S - surface resistance, which determines the obstacle to the passage of current along the insulation surface.

A practical assessment of the quality of the insulating materials used is the specific volumetric resistance ρ V expressed in ohm-centimeters (ohm*cm). Numerically, ρ V is equal to the resistance (in ohms) of a cube with a 1 cm edge made of a given material, if the current passes through two opposite faces of the cube. Specific surface resistance ρ S is numerically equal to the surface resistance of a square (in ohms) if current is supplied to the electrodes delimiting two opposite sides of this square.

The insulation resistance of a single-core cable or wire is determined by the formula

Humidity properties of dielectrics

Moisture resistance – this is the reliability of the insulation when it is in an atmosphere of water vapor close to saturation. Moisture resistance is assessed by changes in electrical, mechanical and other physical properties after the material is in an atmosphere with high and high humidity; on moisture and water permeability; on moisture and water absorption.

Moisture permeability – the ability of a material to transmit moisture vapor in the presence of a difference in relative air humidity on both sides of the material.

Moisture absorption – the ability of a material to sorb water when exposed for a long time in a humid atmosphere close to a state of saturation.

Water absorption – the ability of a material to absorb water when immersed in water for a long time.

Tropical resistance and tropicalization equipment – protection of electrical equipment from moisture, mold, rodents.

Thermal properties of dielectrics

To characterize the thermal properties of dielectrics, the following quantities are used.

Heat resistance– the ability of electrical insulating materials and products to withstand high temperatures and sudden temperature changes without harm to them. Determined by the temperature at which a significant change in mechanical and electrical properties is observed, for example, tensile or bending deformation under load begins in organic dielectrics.

Thermal conductivity– the process of heat transfer in a material. It is characterized by an experimentally determined thermal conductivity coefficient λ t. λ t is the amount of heat transferred in one second through a layer of material 1 m thick and a surface area of ​​1 m 2 with a temperature difference between the surfaces of the layer of 1 °K. The thermal conductivity coefficient of dielectrics varies over a wide range. The lowest values ​​of λ t have gases, porous dielectrics and liquids (for air λ t = 0.025 W/(m K), for water λ t = 0.58 W/(m K)), high values have crystalline dielectrics (for crystalline quartz λ t = 12.5 W/(m K)). The thermal conductivity coefficient of dielectrics depends on their structure (for fused quartz λ t = 1.25 W/(m K)) and temperature.

Thermal expansion dielectrics are assessed by the temperature coefficient of linear expansion: . Materials with low thermal expansion, as a rule, have higher heat resistance and vice versa. The thermal expansion of organic dielectrics significantly (tens and hundreds of times) exceeds the expansion of inorganic dielectrics. Therefore, the dimensional stability of parts made of inorganic dielectrics during temperature fluctuations is significantly higher compared to organic ones.

1. Absorption currents

Absorption currents are displacement currents of various types of slow polarization. Absorption currents at a constant voltage flow in the dielectric until an equilibrium state is established, changing their direction when the voltage is turned on and off. With an alternating voltage, absorption currents flow during the entire time the dielectric is in the electric field.

In general electric current j in a dielectric is the sum of the through current j sk and absorption current j ab

j = j sk + j ab.

The absorption current can be determined through the bias current j cm - rate of change of the electrical induction vector D

The through current is determined by the transfer (movement) of various charge carriers in the electric field.

2. Electronic electrical conductivity is characterized by the movement of electrons under the influence of a field. In addition to metals, it is present in carbon, metal oxides, sulfides and other substances, as well as in many semiconductors.

3. Ionic – caused by the movement of ions. It is observed in solutions and melts of electrolytes - salts, acids, alkalis, as well as in many dielectrics. It is divided into intrinsic and impurity conductivity. Intrinsic conductivity is due to the movement of ions obtained during dissociation molecules. The movement of ions in an electric field is accompanied by electrolysis – transfer of a substance between electrodes and its release on the electrodes. Polar liquids are more dissociated and have higher electrical conductivity than non-polar liquids.

In nonpolar and weakly polar liquid dielectrics (mineral oils, silicone liquids), electrical conductivity is determined by impurities.

4. Molion electrical conductivity – caused by the movement of charged particles called molions. It is observed in colloidal systems, emulsions , suspensions . The movement of molions under the influence of an electric field is called electrophoresis. During electrophoresis, unlike electrolysis, no new substances are formed; the relative concentration of the dispersed phase in different layers of the liquid changes. Electrophoretic conductivity is observed, for example, in oils containing emulsified water.

Any substance or body that surrounds us has certain electrical properties. This is explained by the molecular and atomic structure: the presence of charged particles that are in a mutually bound or free state.

When no external electric field acts on the substance, these particles are distributed in such a way that they balance each other and do not create an additional electric field throughout the total volume. In the case of an external application electrical energy inside molecules and atoms, a redistribution of charges occurs, which leads to the creation of its own internal electric field, directed counter to the external one.

If the vector of the applied external field is denoted by “E0”, and the internal field by “E””, then the total field “E” will be the sum of the energy of these two quantities.

In electricity, it is customary to divide substances into:

conductors;

dielectrics.

This classification has existed for a long time, although it is rather arbitrary because many bodies have other or combined properties.

Conductors

Media that have free charges act as conductors. Most often, metals act as conductors, because their structure always contains free electrons, which are able to move within the entire volume of the substance and, at the same time, are participants in thermal processes.

When a conductor is isolated from the action of external electric fields, a balance of positive and negative charges is created in it from ionic lattices and free electrons. This equilibrium is immediately destroyed upon application - thanks to the energy of which, the redistribution of charged particles begins and unbalanced charges of positive and negative quantities appear on the outer surface.

This phenomenon is usually called electrostatic induction. The charges that arise on the surface of metals are called induction charges.

The inductive charges formed in the conductor form their own field E, compensating the effect of external E0 inside the conductor. Therefore, the value of the total, total electrostatic field compensated and equal to 0. In this case, the potentials of all points both inside and outside are the same.

The resulting output indicates that inside the conductor, even when connected external field, there is no potential difference and no electrostatic fields. This fact is used in shielding - the application of a method of electrostatic protection of people and electrical equipment sensitive to induced fields, especially high-precision measuring instruments and microprocessor technology.

Shielded clothing and footwear made from fabrics with conductive threads, including headwear, are used in the energy sector to protect personnel working in conditions of increased tension created by high-voltage equipment.

Dielectrics

This is the name given to substances that have insulating properties. They contain only interconnected charges and not free charges. For them, all positive and negative particles are held together inside a neutral atom and are deprived of freedom of movement. They are distributed inside the dielectric and do not move under the action of the applied external field E0.

However, its energy still causes certain changes in the structure of matter - inside atoms and molecules the ratio of positive and negative particles, and on the surface of the substance, excess, unbalanced bound charges appear, forming an internal electric field E." It is directed counter to the externally applied tension.

This phenomenon is called dielectric polarization. It is characterized by the fact that an electric field E appears inside the substance, formed by the action of external energy E0, but weakened by the counteraction of internal E."

Types of polarization

It is of two types inside dielectrics:

1. orientation;

2. electronic.

The first type has the additional name of dipole polarization. It is inherent in dielectrics with displaced centers of negative and positive charges, which form molecules from microscopic dipoles - a neutral combination of two charges. This is typical for water, nitrogen dioxide, and hydrogen sulfide.

Without the action of an external electric field, the molecular dipoles of such substances are oriented in a chaotic manner under the influence of existing temperature processes. In this case, at any point in the internal volume and on the outer surface of the dielectric there is no electric charge.

This picture changes under the influence of externally applied energy, when the dipoles slightly change their orientation and regions of uncompensated macroscopic bound charges appear on the surface, forming a field E" with the opposite direction to the applied E0.

With such polarization, temperature has a great influence on the processes, causing thermal movement and creating disorienting factors.

Electronic polarization, elastic mechanism

It manifests itself in non-polar dielectrics - materials of another type with molecules devoid of a dipole moment, which, under the influence of an external field, are deformed so that positive charges are oriented in the direction of the E0 vector, and negative charges in the opposite direction.

As a result, each of the molecules acts as an electric dipole, oriented along the axis of the applied field. In this way, they create their own field E" on the outer surface in the opposite direction.

In such substances, the deformation of molecules, and, consequently, polarization from the influence of an external field does not depend on their movement under the influence of temperature. An example of a non-polar dielectric is methane CH4.

The numerical value of the internal field of both types of dielectrics initially changes in direct proportion to the increase in the external field, and then, when saturation is reached, nonlinear effects appear. They occur when all the molecular dipoles are lined up along the field lines of polar dielectrics or changes in the structure of a non-polar substance have occurred due to strong deformation of atoms and molecules from large externally applied energy.

In practice, such cases rarely occur - usually a breakdown or insulation failure occurs first.

Permittivity

Among insulating materials, an important role is played by electrical characteristics and such an indicator as permittivity. It can be assessed by two different characteristics:

1. absolute value;

2. relative size.

The term absolute dielectric constant substances εa are used when referring to the mathematical notation of Coulomb's law. It, in the form of coefficient εа, connects the induction vector D and the voltage E.

Let us remember that the French physicist Charles de Coulomb, using his own torsion balances, studied the patterns of electric and magnetic forces between small charged bodies.

Determination of the relative dielectric constant of a medium is used to characterize the insulating properties of a substance. It estimates the ratio of the interaction force between two point charges at two different conditions: in vacuum and working environment. In this case, the vacuum indicators are taken as 1 (εv=1), and for real substances they are always higher, εr>1.

The numerical expression εr is displayed as a dimensionless quantity, is explained by the polarization effect of dielectrics, and is used to evaluate their characteristics.

Dielectric constant values ​​for individual media(at room temperature)

Substance	ε	Substance	ε
Rochelle salt	6000	Diamond	5,7
Rutile (along the optical axis)	170	Water	81
Polyethylene	2,3	Ethyl alcohol	26,8
Silicon	12,0	Mica	6
Glass	5-16	Carbon dioxide	1,00099
NaCl	5,26	water vapor	1,0126
Benzene	2,322	Air (760 mmHg)	1,00057

Lecture No. 19

1. The nature of electrical conductivity of gaseous, liquid and solid dielectrics

Permittivity

Relative dielectric constant, or dielectric constant ε- one of the most important macroscopic electrical parameters of a dielectric. Permittivityε quantitatively characterizes the ability of a dielectric to be polarized in an electric field, and also evaluates the degree of its polarity; ε is a constant of a dielectric material at a given temperature and frequency of electrical voltage and shows how many times the charge of a capacitor with a dielectric is greater than the charge of a capacitor of the same size with a vacuum.

Dielectric constant determines the value of the electrical capacitance of a product (capacitor, cable insulation, etc.). For a parallel plate capacitor, the electric capacitance is WITH,Ф, expressed by formula (1)

where S is the area of ​​the measuring electrode, m2; h is the thickness of the dielectric, m. From formula (1) it is clear that the larger the value ε dielectric used, the greater the electrical capacitance of the capacitor with the same dimensions. In turn, electrical capacitance C is the coefficient of proportionality between the surface charge QK, accumulated capacitor, and an electrical voltage applied to it

yarning U(2):

From formula (2) it follows that electric charge QK, accumulated by the capacitor is proportional to the value ε dielectric. Knowing QK and the geometric dimensions of the capacitor can be determined ε dielectric material for a given voltage.

Let us consider the mechanism of charge formation QK on the electrodes of a capacitor with a dielectric and what components make up this charge. To do this, we take two flat capacitors of the same geometric dimensions: one with a vacuum, the other with an interelectrode space filled with a dielectric, and apply the same electrical voltage to them U(Fig. 1). A charge is formed on the electrodes of the first capacitor Q0, on the electrodes of the second - QK. In turn, the charge QK is the sum of charges Q0 And Q(3):

Charge Q 0 is formed by the external field E0 by accumulating third-party charges with surface density σ 0 on the electrodes of the capacitor. Q- this is an additional charge on the electrodes of the capacitor, created by an electrical voltage source to compensate for the bound charges formed on the surface of the dielectric.

In a uniformly polarized dielectric, the charge Q corresponds to the value surface density bound charges σ. The charge σ forms a field E сз, directed opposite to the field E O.

The dielectric constant of the dielectric in question can be represented as the charge ratio QK capacitor filled with dielectric to charge Q0 the same capacitor with vacuum (3):

From formula (3) it follows that the dielectric constant ε - the quantity is dimensionless, and for any dielectric it is greater than unity; in case of vacuum ε = 1. From the considered example also

it can be seen that the charge density on the electrodes of a capacitor with a dielectric in ε times the charge density on the electrodes of a capacitor with a vacuum, and the voltages at the same voltages for both

their capacitors are the same and depend only on the voltage U and distances between electrodes (E = U/h).

In addition to the relative dielectric constant ε differentiate absolute dielectric constant ε a, F/m, (4)

which has no physical meaning and is used in electrical engineering.

The relative change in dielectric constant εr with an increase in temperature by 1 K is called the temperature coefficient of dielectric constant.

ТКε = 1/ εr d εr/dT К-1 For air at 20°С ТК εr = -2.10-6К-

Electrical aging in ferroelectrics is expressed as a decrease in εr with time. The reason is the regrouping of domains.

A particularly sharp change in dielectric constant over time is observed at temperatures close to the Curie point. Heating ferroelectrics to a temperature above the Curie point and subsequent cooling returns εr to its previous value. The same restoration of the dielectric constant can be achieved by exposing the ferroelectric to an electric field of increased intensity.

For complex dielectrics - a mechanical mixture of two components with different εr in a first approximation: εrх = θ1 · εr1х · θ · εr2х, where θ is the volumetric concentration of the mixture components, εr is the relative dielectric constant of the mixture component.

Dielectric polarization can be caused by: mechanical loads (piezopolarization in piezoelectrics); heating (pyropolarization in pyroelectrics); light (photopolarization).

The polarized state of a dielectric in an electric field E is characterized by the electric moment per unit volume, polarization P, C/m2, which is related to its relative dielectric constant eg: P = e0 (eg - 1)E, where e0 = 8.85∙10-12 F /m. The product e0∙eг =e, F/m, is called the absolute dielectric constant. In gaseous dielectrics eg differs little from 1.0, in non-polar liquids and solids it reaches 1.5 - 3.0, in polar ones it has large values; in ionic crystals eg - 5-MO, and in those having perovskite crystal lattice reaches 200; in ferroelectrics eg - 103 and more.

In non-polar dielectrics, eg decreases slightly with increasing temperature; in polar dielectrics, changes are associated with the predominance of one or another type of polarization; in ionic crystals, it increases; in some ferroelectrics, at the Curie temperature it reaches 104 or more. Temperature changes eg are characterized by a temperature coefficient. Polar dielectrics are characterized by a decrease in eg in the frequency range where the time t for polarization is comparable to T/2.

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