Electrostatic permeability of a half ring. Dielectric constant

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

When the substance is not affected by any external electric field, then these particles are distributed in such a way that they balance each other and do not create an additional electric field in the entire total volume. In the case of external application of electrical energy inside molecules and atoms, a redistribution of charges occurs, which leads to the creation of its own internal electric field directed opposite to the external one.

If the vector of the applied external field is designated "E0", and the internal - "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.

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

conductors

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

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

This phenomenon is called electrostatic induction. The charges arising from it on the surface of metals are called induction charges.

The inductive charges formed in the conductor form their own field E ", compensating the action of the external E0 inside the conductor. Therefore, the value of the total, total electrostatic field is 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 with the 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 of fabrics with conductive threads, including headgear, is used in the power industry to protect personnel working in conditions of increased tension created by high-voltage equipment.

Dielectrics

So called substances with insulating properties. They contain only interconnected, not free charges. They have all positive and negative particles fastened inside a neutral atom, 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 the substance - inside the atoms and molecules the ratio of positive and negative particles changes, and on the surface of the substance there are excessive, unbalanced bound charges that form an internal electric field E ". It is directed counter applied from the outside tension.

This phenomenon has been named 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 inside dielectrics is of two types:

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, hydrogen sulfide.

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

This pattern 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 this polarization big influence processes are affected by temperature, which causes thermal motion and creates disorienting factors.

Electronic polarization, elastic mechanism

It manifests itself in non-polar dielectrics - materials of a different 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 works as an electric dipole oriented along the axis of the applied field. They, in this way, create their own field E "with the opposite direction on the outer surface.

In such substances, the deformation of molecules, and, consequently, the polarization from the action of the field from the outside 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 magnitude in direct proportion to the increase in the external field, and then, when saturation is reached, nonlinear effects appear. They come when all the molecular dipoles are lined up along lines of force in polar dielectrics, or there have been changes in the structure of a non-polar substance due to a strong deformation of atoms and molecules from a large energy applied from outside.

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

The dielectric constant

Among insulating materials, an important role is given to electrical characteristics and such an indicator as the dielectric constant . It can be assessed by two different characteristics:

1. absolute value;

2. relative value.

term absolute permittivity substances εa are used when referring to the mathematical notation of Coulomb's law. It, in the form of a coefficient εa, connects the vectors of induction D and intensity E.

Recall that the French physicist Charles de Coulomb used his own torsion balance to investigate the patterns of electric and magnetic forces between small charged bodies.

The determination of the relative permittivity 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 various conditions: in vacuum and working environment. In this case, the vacuum indicators are taken as 1 (εv=1), while for real substances they are always higher, εr>1.

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

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

The dielectric constant the dielectric constant

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

DIELECTRIC PERMITTIVITY, 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 vacuum:
e \u003d F about / F.
The dielectric constant shows how many times the field is weakened by the dielectric (cm. DIELECTRIC), quantitatively characterizing the property of a dielectric to be polarized in an electric field.
The value of the relative permittivity of a substance, which characterizes the degree of its polarizability, is determined by the mechanisms of polarization (cm. POLARIZATION). However, the value to a large extent also depends on the state of aggregation of the substance, 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 the large distances between molecules. Due to this, the polarization of all gases is negligible and their permittivity is close to unity. The polarization of a gas can be purely electronic or dipole if the gas molecules are polar, but in this case, too, the electron polarization is of primary importance. The polarization of various gases is the 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 the gas, which is proportional to the pressure and inversely proportional to the absolute temperature.
At 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 be composed of non-polar or polar molecules. The e value of non-polar liquids is determined by the electron polarization, so it is small, close to the value of the square of light refraction, 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 the volume expansion of the liquid, but differs in sign.
Polarization of liquids containing dipole molecules, is determined simultaneously by the electronic and dipole-relaxation components. Such liquids have the greater dielectric constant than more value electric moment of dipoles (cm. DIPOLE) and the greater the number of molecules per unit volume. The temperature dependence in the case of polar liquids is complex.
Dielectric constant of solid dielectrics
V solids ax can take a variety of numerical values ​​in accordance with the diversity structural features solid dielectric. In solid dielectrics, all types of polarization are possible.
The smallest value of e has solid dielectrics, consisting of non-polar molecules and having only electronic polarization.
Solid dielectrics, which are ionic crystals with close packing of particles, have electronic and ionic polarizations and have e values ​​that lie in 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 a 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 of two electric charges in a medium is less than in vacuum. In an isotropic medium, e is related to the dielectric susceptibility with the relation: e = 1 + 4pc. The dielectric constant… … Big Encyclopedic Dictionary

The value of e, which characterizes the polarization of dielectrics under the action of electric. field E. D. p. enters the Coulomb law as a quantity showing how many times the force of impact of two free charges in a dielectric is less than in vacuum. The weakening of the ... ... Physical Encyclopedia

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

- (notation 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. Vacuum permittivity ... ... Scientific and technical encyclopedic dictionary

the dielectric constant- The value characterizing di electrical properties substance, scalar for an isotropic substance and tensor for an anisotropic substance, the product of which and the electric field strength is equal to the electric displacement. [GOST R 52002 2003]… … Technical Translator's Handbook

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

The dielectric constant- 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

the dielectric constant- absolute permittivity; industry dielectric permittivity 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 permittivity Relative permittivity Vacuum permittivity ... Wikipedia

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

Books

• Material properties. Anisotropy, symmetry, structure. Per. from English. , Newnham RE. This book is about anisotropy and the relationship between the structure of materials and their properties. It covers a wide range of topics and is a kind of introductory course on physical properties...

As you know, the air around us is a combination of several gases, therefore it is a good dielectric. In particular, this avoids in many cases the need to organize additional insulating layers of any material around the conductor. Today we'll talk about air permeability. But first, perhaps, let's start with a definition of what exactly is meant by the term "dielectric".

All substances, depending on the ability to conduct electricity conditionally divided into three large groups: conductors, semiconductors and dielectrics. The former provide minimal resistance to the directed passage of charged particles through them. Their largest group is metals (aluminum, copper, iron). The latter conduct current under certain conditions (silicon, germanium). Well, the third is so large that the current does not pass through them. A prime example is air.

What happens when a substance enters the zone of action of an electric field? For conductors, the answer is obvious - an electric current arises (of course, in the presence of closed circuit, providing a "path" for the particles). This is due to the fact that the way the charges interact changes. Completely different processes occur when a field is applied to a dielectric material. When studying the interaction of particles possessing, it was noticed that the strength of interaction depends not only on numerical value charge, but also from the medium separating them. This important characteristic is called the “dielectric constant of a substance”. In fact, it is a correction factor, since it has no dimension. It is defined as the ratio of the value of the interaction force in vacuum to the value in any medium. The physical meaning of the term "dielectric constant" is as follows: this value shows the degree of weakening of the electric field by the dielectric material compared to vacuum. The reason for this phenomenon lies in the fact that the molecules of the material expend the energy of the field not on the conduction of particles, but on polarization.

It is known that air is equal to one. Is it a lot or a little? Let's figure it out. Now there is no need to independently calculate the numerical value of the permeability for most common substances, since all these data are given in the corresponding tables. By the way, it is from such a table that equal to one is taken. The dielectric constant of air is almost 8 times less than that of, for example, getinaks. Knowing this number, as well as the value of the charges and the distance between them, it is possible to calculate the strength of their interaction, subject to separation by an air medium or a getinax plate.

The formula for strength is as follows:

F = (Q1*Q2) / (4* 3.1416* E0*Es*(r*r)),

where Q1 and Q2 are the values ​​of the charges; E0 - vacuum permeability (constant equal to 8.86 to the power of -12); Es - dielectric constant of air ("1" or value for any other substance, according to the table); r is the distance between the charges. All dimensions are taken in accordance with the SI system.

The two should not be confused different concepts- "magnetic permeability of air" and its dielectric constant. Magnetic is another characteristic of any substance, which is also a coefficient, but its meaning is different - the relationship and values ​​​​in a particular substance. The formulas use a reference indicator - magnetic permeability for pure vacuum. Both the first and second concepts are used to perform calculations of various electrical devices.

DIELECTRIC PERMITTIVITY, the value of ε, which characterizes the polarization of dielectrics under the action of an electric field of strength E. The dielectric constant is included in the Coulomb law as a quantity showing how many times the force of interaction of two free charges in a dielectric is less than in vacuum. The weakening of the interaction occurs due to the screening of free charges by the bound charges formed as a result of the polarization of the medium. Bound charges arise as a result of a microscopic spatial redistribution of charges (electrons, ions) in an electrically neutral medium as a whole.

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

where ε 0 is an electrical constant. The value of the permittivity 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 it varies from several units to several tens, for ferroelectrics it can reach 10 4 . Such a spread of ε values ​​is due to various mechanisms polarizations occurring in different dielectrics.

The classical microscopic theory leads to an approximate expression for the permittivity of nonpolar dielectrics:

where n i is the concentration of the i-th kind 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 a crystal or substance. For most dielectrics with permittivity ranging from 2-8, β = 1/3. Usually, the permittivity is practically independent of the magnitude of the applied electric field up to the electrical breakdown of the dielectric. High valuesε of some metal oxides and other compounds are due to the peculiarities of their structure, which allows, under the action of the field E, the 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 dielectric polarization when an electric field is applied does not develop instantly, but over 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 P and E. When describing the oscillations P and E by the method of complex amplitudes, the permittivity is represented by a complex value:

ε = ε’ + iε",

moreover, ε' and ε" depend on ω and τ, and the ratio ε"/ε' = tg δ determines the dielectric losses in the medium. The phase shift δ depends on the ratio τ and the field period Т = 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) the polarization does not keep up with the change in Ε, δ → π and ε' in this case denote ε (∞) (the polarization mechanism is “off”). It is obvious that ε (0) > ε (∞), and in alternating fields the permittivity turns out to be a function of ω. Near ω = l/τ, ε' changes from ε (0) to ε (∞) (dispersion region), and the dependence tgδ(ω) passes through a maximum.

The nature of the dependences ε'(ω) and tgδ(ω) 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 inclusion of the field E has the character of damped oscillations, and the dependences ε'(ω) and tgδ(ω) are called resonant. In the case of orientational polarization, the establishment of P(t) is exponential, and the dependences ε'(ω) and tgδ(ω) are called relaxation.

Methods for measuring dielectric polarization are based on interaction phenomena electromagnetic field with electrical dipole moments particles of matter 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 the investigated dielectric. At higher frequencies, waveguide, resonant, multifrequency and other methods are used.

In some dielectrics, for example, ferroelectrics, the proportional relationship between P and Ε [P = ε 0 (ε – 1)E] and, consequently, between D and E is violated even 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 permittivity:

In nonlinear dielectrics, the value of ε diff 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 permittivity.

Lit. see at st. Dielectrics.

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 permittivity shows the dependence of the electric induction in the dielectric on the strength of the electric field acting on it. At the same time, its value is influenced not only 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 permittivity: absolute and relative.

Relative permittivity shows the ratio of the insulating (dielectric) properties of the material under study to the similar properties of vacuum. It characterizes the insulating properties of a substance in gaseous, liquid or solid states. That is, it is applicable to almost all dielectrics. The value of the relative permittivity for substances in the gaseous state, as a rule, is in the range of 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 equal to 80, and ferroelectrics - tens or even hundreds of units, depending on the properties of the material.

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

Usage

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