Dielectric constant of vacuum. Dielectric constant of air as a physical quantity

Relative dielectric constant environment ε - dimensionless physical quantity characterizing the properties of the insulating (dielectric) medium. It is associated with the effect of polarization of dielectrics under the action of an electric field (and with the value of the dielectric susceptibility of the medium characterizing this effect). The value of ε shows how many times the force of interaction of two electric charges in a medium is less than in a vacuum. The relative dielectric constant of 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 great for substances with molecules that have a large electric dipole. The relative dielectric constant of ferroelectrics is tens and hundreds of thousands.

Practical use

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

Parameter dielectric constant taken into account in the design of 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.

Frequency dependence

It should be noted that the dielectric constant is highly dependent on the frequency electromagnetic field... This should always be taken into account, since the tables of the handbooks usually contain data for a static field or low frequencies up to several kHz units without specifying this fact... At the same time, there are optical methods for obtaining the relative permittivity from 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 equal to zero), the relative permittivity under normal conditions is approximately 80. This is the case up to infrared frequencies. From about 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, the dielectric absorption drops to zero, which in fact provides a person with a vision mechanism in the earth's atmosphere saturated with water vapor. WITH further growth the frequencies of the properties of the medium change again.

Dielectric constant values ​​for some substances

DIELECTRIC PERMISSIBILITY, the value of ε, which characterizes the polarization of dielectrics under the action of an electric field of intensity E. The dielectric constant enters into the Coulomb's law as a value showing how many times the force of interaction of 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 electrically neutral medium as a whole.

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

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

For gases, its value is close to 1, for liquids and solids it 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 take place in different dielectrics.

The 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 sort of atoms, ions or molecules, α i is their polarizability, β i is the so-called internal field factor, due to the peculiarities of the structure of the crystal or substance. For most dielectrics with a dielectric constant lying in the range of 2-8, β = 1/3. Usually, the dielectric constant is practically independent of the applied electric field up to the electric 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 action 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 process of polarization of a dielectric when an electric field is applied does not develop instantaneously, but within a certain 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 of P and E. When describing oscillations P and E by the method of complex amplitudes, the dielectric constant is represented by a complex value:

ε = ε ’+ iε",

where 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 of τ and the field period T = 2π / ω. For 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 in Ε, δ → π and ε 'in this case denote ε (∞) (the polarization mechanism is "off"). Obviously, ε (0)> ε (∞), and in variable 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 character of the ε '(ω) and tanδ (ω) dependences in the dispersion region is determined by the polarization mechanism. In the case of ionic and electronic polarizations with an elastic displacement of bound charges, the change in P (t) with a stepwise switching on of the field E has the character of damped oscillations and the dependences ε '(ω) and tanδ (ω) are called resonant. In the case of orientational polarization, the establishment of Р (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 the interaction of an electromagnetic field with the electric dipole moments of particles of a substance and are different for different frequencies. Most of the methods at ω ≤ 10 8 Hz are based on the process of charging and discharging a measuring capacitor filled with a dielectric under study. At higher frequencies, waveguide, resonant, multi-frequency and other methods are used.

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

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

Lit. see at Art. Dielectrics.

• determining the strength of the electric field in vacuum;
• included in the expressions of some laws of electromagnetism, including Coulomb's law, when written in a form corresponding to the International System of Units.

Through the dielectric constant, the connection between the relative and absolute dielectric constant is carried out. It is also included in the entry for Coulomb's law:

see also

Notes (edit)

Literature

Links

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See what "Dielectric constant" is in other dictionaries:

dielectric constant- dielectric constant - [YN Luginsky, MS Fezi Zhilinskaya, YS Kabirov. English Russian Dictionary of Electrical Engineering and Electric Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts Synonyms dielectric constant ... ...

- (notation e0), a physical quantity indicating the ratio of the force acting between electric charges in a vacuum with the size of these charges and the distance between them. Initially, this indicator was called DIELECTRIC ... ... Scientific and technical encyclopedic dictionary

dielectric constant- absolute dielectric constant (for an isotropic substance); branch. dielectric constant Scalar quantity characterizing electrical properties dielectric and equal ratio electrical displacement in it to tension ... ...

dielectric constant- dielektrinė skvarba statusas T sritis fizika atitikmenys: angl. dielectric constant; permittivity vok. dielektrische Leitfähigkeit, f; Dielektrizitätskonstante, f; Permittivität, f rus. dielectric constant, f; dielectric constant ... Fizikos terminų žodynas

Obsolete name for dielectric constant (See Dielectric constant) ... Great Soviet Encyclopedia

Dielectric constant ε for some liquids (at 20 ° С)- Solvent ε Acetone 21.5 Benzene 2.23 Water 81.0 ... Chemical handbook

initial dielectric constant- - [Ya.N. Luginsky, M.S.Fezi Zhilinskaya, Y.S.Kabirov. The English Russian Dictionary of Electrical Engineering and Electric Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts EN initial dielectric constant ... Technical translator's guide

relative dielectric constant- - [Ya.N. Luginsky, M.S.Fezi Zhilinskaya, Y.S.Kabirov. The English Russian Dictionary of Electrical Engineering and Electric Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts EN relative permittivityrelative dielectric constant ... Technical translator's guide

specific dielectric constant- - [Ya.N. Luginsky, M.S.Fezi Zhilinskaya, Y.S.Kabirov. English Russian Dictionary of Electrical Engineering and Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts EN simultaneous interchange capabilitySIC ... Technical translator's guide

the dielectric constant- absolute dielectric constant; branch. dielectric constant A scalar quantity characterizing the electrical properties of a dielectric equal to the ratio of the electric displacement to the electric field strength ... Polytechnic Terminological Explanatory Dictionary

THE DIELECTRIC CONSTANT (dielectric constant) is a physical quantity characterizing the ability of a substance to reduce the forces of electrical interaction in this substance in comparison with a vacuum. Thus, the differential pressure shows how many times the forces of electrical interaction in a substance are less than in a vacuum.

D. p.- a characteristic that depends on the structure of the substance-dielectric. Electrons, ions, atoms, molecules or their individual parts and larger sections of any substance in an electric field are polarized (see Polarization), which leads to partial neutralization of the external electric field. If the frequency of the electric field is comparable with the polarization time of the substance, then in certain range frequency dispersion takes place, i.e., the dependence of its magnitude on frequency (see Dispersion). The diapason of a substance depends both on the electrical properties of atoms and molecules and on their mutual arrangement, that is, on the structure of the substance. Therefore, the definition of the diapason or its change depending on the surrounding conditions is used in the study of the structure of a substance, and in particular of various tissues of the body (see. Electrical conductivity of biological systems).

Various substances (dielectrics), depending on their structure and state of aggregation, have different values ​​for the D. p. (Table).

Table. The value of the dielectric constant of some substances

Of particular importance for medical biol, researches D.'s study and. in polar liquids. Their typical representative is water, consisting of dipoles, which are oriented in an electric field due to the interaction between the charges of the dipole and the field, which leads to the appearance of dipole or orientational polarization. The high value of D. p. Water (80 at t ° 20 °) determines high degree dissociation in it of various chem. substances and good solubility of salts, to - t, bases and other compounds (see. Dissociation, Electrolytes). With an increase in the concentration of electrolyte in water, the value of its D. p. Decreases (for example, for monovalent electrolytes, the D. p. Of water decreases by one when the salt concentration increases by 0.1 M).

Most biol, objects belong to heterogeneous dielectrics. When the ions of biol, an object interact with an electric field, the polarization of the interfaces is of significant importance (see. Biological membranes). In this case, the lower the frequency of the electric field, the greater the magnitude of the polarization. Since the polarization of the interfaces of a biol, an object depends on their permeability (see) for ions, it is obvious that the effective D. p. In to a greater extent is determined by the state of the membranes.

Since the polarization of such a complex heterogeneous object as a biological one has a different nature (concentration, macrostructural, orientational, ionic, electronic, etc.), then it becomes clear that with an increase in frequency, the change in the DP (dispersion) sharply expressed. Conventionally, three dispersion regions are distinguished: alpha dispersion (at frequencies up to 1 kHz), beta dispersion (frequencies from several kHz to tens of MHz), and gamma dispersion (frequencies above 109 Hz); in biol, objects there is usually no clear border between the areas of dispersion.

In case of deterioration of funkts, the state of biol, the object, the dispersion of D. p. On low frequencies decreases up to complete disappearance(with the dying off of tissues). At high frequencies, the magnitude of the D. p. Does not change significantly.

D. p. Is measured in a wide frequency range and, depending on the frequency range, measurement methods also change significantly. At frequencies electric current Less than 1 Hz, the measurement is carried out using the method of charging or discharging a capacitor filled with the substance under test. Knowing the dependence of the charging or discharge current on time, it is possible to determine not only the value of the electrical capacitance of the capacitor, but also the losses in it. At frequencies from 1 to 3 10 8 Hz for measuring D. and. special resonance and bridge methods are used, which make it possible to comprehensively study the changes in the diaphragm. various substances the most complete and versatile.

In medical biology, research, symmetrical AC bridges with a direct reading of the measured values ​​are most often used.

Bibliography: High-frequency heating of dielectrics and semiconductors, ed. A. V. Netushila, M. -L., 1959, bibliogr .; With edunov BI and Fran to-K and m e-N e c and y DA Dielectric constant of biological objects, Usp. physical Sciences, v. 79, v. 4, p. 617, 1963, bibliogr .; Electronics and Cybernetics in Biology and Medicine, trans. from English, ed. P.K. Anokhin, p. 71, M., 1963, bibliogr .; E m e F. Dielectric measurements, trans. from it., M., 1967, bibliogr.

The dielectric constant- This is one of the main parameters characterizing the electrical properties of dielectrics. In other words, it determines how good an insulator a particular material is.

The value of the dielectric constant shows the dependence of the electrical induction in the dielectric on the strength of the electric field acting on it. Moreover, its value is influenced not only by physical properties the material or medium itself, but also the frequency of the field. As a rule, reference books indicate the value measured for a static or low-frequency field.

There are two types of dielectric constant: absolute and relative.

Relative dielectric constant shows the ratio of the insulating (dielectric) properties of the test material to the analogous properties of vacuum. It characterizes the insulating properties of a substance in a gaseous, liquid or solid state. That is, it is applicable to almost all dielectrics. The value of the relative permittivity for substances in gaseous state, as a rule, is in the range 1. For liquids and solids it can be in a very wide range - from 2 and almost to infinity.

For example, the relative permittivity fresh water is 80, and ferroelectrics - tens, or even hundreds of units, depending on the properties of the material.

Absolute dielectric constant Is a constant. It characterizes the insulating properties of a particular substance or material, regardless of its location and external factors affecting it.

Usage

Dielectric constant, or rather its values, are used in the development and design of new electronic components, in particular capacitors. Its value determines the future dimensions and electrical characteristics of the component. This value is also taken into account when developing whole electrical circuits(especially in high-frequency electronics) and even