Bodies made of dielectrics. Active dielectrics. Physical properties of dielectrics

electrification of bodies

2. Electrification of bodies.

These phenomena were discovered in ancient times. Ancient Greek scientists noticed that amber (petrified resin of coniferous trees that grew on Earth many hundreds of thousands of years ago), when rubbed with wool, begins to attract various bodies. In Greek, amber means electron, hence the name “electricity”.

A body which, after being rubbed, attracts other bodies to itself, is said to be electrified or to be given an electric charge.

Bodies made of different substances can become electrified. It is easy to electrify by rubbing sticks made of rubber, sulfur, ebonite, plastic, or nylon on wool.

Electrification of bodies occurs upon contact and subsequent separation of bodies. They rub their bodies against each other only to increase the area of ​​their contact.

Two bodies are always involved in electrification: in the experiments discussed above, a glass rod came into contact with a sheet of paper, a piece of amber came into contact with fur or wool, and a plexiglass rod came into contact with silk. In this case, both bodies are electrified. For example, when a glass rod and a piece of rubber come into contact, both glass and rubber become electrified. Rubber, like glass, begins to attract light bodies.

Electric charge can be transferred from one body to another. To do this, you need to touch another body with an electrified body, and then part of the electric charge will transfer to it. To make sure that the second body is also electrified, you need to bring small pieces of paper to it and see if they attract.

3. Two types of charges. Interaction of charged bodies.

All electrified bodies attract other bodies, such as pieces of paper. By the attraction of bodies, it is impossible to distinguish the electric charge of a glass rod rubbed against silk from the charge obtained on an ebonite rod rubbed against them. After all, both electrified sticks attract pieces of paper.

Does this mean that the charges obtained on bodies made of different substances are no different from each other?

Let's turn to experiments. Let's electrify an ebonite stick suspended on a thread. Let's bring another similar stick closer to it, electrified by friction against the same piece of fur. The sticks push off Since the sticks are the same and were electrified by friction against the same body, we can say that they had charges of the same kind. This means that bodies with charges of the same kind repel each other.

Now let’s bring a glass rod rubbed on silk to the electrified ebonite rod. We will see that the glass and ebonite rods are mutually attracted (Fig. No. 2). Consequently, the charge obtained on glass rubbed with silk is of a different kind than on ebonite rubbed with fur. This means that there is another kind of electric charge.

We will bring electrified bodies made of various substances: rubber, plexiglass, plastic, nylon closer to a suspended electrified ebonite stick. We will see that in some cases the ebonite rod is repelled by bodies brought to it, and in others it is attracted. If the ebonite stick is repelled, it means that the body brought to it has a charge of the same kind as that on it. And the charge of those bodies to which the ebonite stick is attracted is similar to the charge obtained on glass rubbed on silk. Therefore, we can assume that there are only two types of electric charges.

The charge obtained on glass rubbed on silk (and on all bodies where a charge of the same kind is obtained) was called positive, and the charge obtained on amber (as well as ebonite, sulfur, rubber) rubbed on wool was called negative, i.e. The charges were assigned the signs “+” and “-”.

And so, experiments have shown that there are two types of electric charges - positive and negative charges and that electrified bodies interact with each other differently.

Bodies with electric charges of the same sign repel each other, and bodies with charges of the opposite sign mutually attract.

4. Electroscope. Conductors and non-conductors of electricity.

If bodies are electrified, then they attract each other or repel each other. By attraction or repulsion one can judge whether the body has an electrical charge. Therefore, the device used to determine whether a body is electrified is based on the interaction of charged bodies. This device is called an electroscope (from the Greek words electron and skopeo - observe, detect).

In the electroscope, a metal rod is passed through a plastic plug (Fig. No. 3), inserted into a metal frame, at the end of which two sheets of thin paper are attached. The frame is covered with glass on both sides.

The greater the charge of the electroscope, the greater the repulsive force of the leaves and the greater the angle they will diverge. This means that by changing the angle of the divergence of the electroscope leaves, one can judge whether its charge has increased or decreased.

If you touch a charged body (for example, an electroscope) with your hand, it will discharge. Electrical charges will transfer to our body and through it they can go into the ground. A charged body can also be discharged if it is connected to the ground with a metal object, such as iron or copper wire. But if a charged body is connected to the ground with a glass or ebonite rod, then the electric charges along them will not go into the ground. In this case, the charged body will not discharge.

Based on their ability to conduct electrical charges, substances are conventionally divided into conductors and non-conductors of electricity.

All metals, soil, solutions of salts and acids in water are good conductors of electricity.

Non-conductors of electricity, or dielectrics, include porcelain, ebonite, glass, amber, rubber, silk, nylon, plastics, kerosene, air (gases).

Bodies made of dielectrics are called insulators (from the Greek word isolaro - to seclude).

5. Divisibility of electric charge. Electron.

Let's charge a metal ball attached to the rod of the electroscope (Fig. No. 4a). Let's connect this ball with a metal conductor A, holding it by handle B, made of a dielectric, with another exactly the same, but uncharged ball, located on the second electroscope. Half of the charge will transfer from the first ball to the second (Fig. No. 4b). This means that the initial charge was discharged into two equal parts.

Now let's separate the balls and touch the second ball with our hand. This will cause it to lose its charge and discharge. Let's connect it again to the first ball, on which half of the original charge remains. The remaining charge will again be divided into two equal parts, and a fourth of the original charge will remain on the first ball.

In the same way you can get one eighth, one sixteenth of the charge, etc.

Thus, experience shows that electric charge can have different values. Electric charge is a physical quantity.

One coulomb is taken as a unit of electric charge (denoted 1 C). The unit is named after the French physicist C. Coulomb.

The experiment shown in Figure 4 shows that an electric charge can be divided into parts.

Is there a charge fission limit?

To answer this question, it was necessary to perform more complex and accurate experiments than those described above, since very soon the charge remaining on the electroscope ball becomes so small that it cannot be detected using an electroscope.

To divide the charge into very small portions, you need to transfer it not to balls, but to small grains of metal or droplets of liquid. By measuring the charge obtained on such small bodies, it was established that it is possible to obtain portions of the charge that are billions of billions of times smaller than in the described experiment. However, in all experiments it was not possible to separate the charge beyond a certain value.

This allowed us to assume that the electric charge has a limit of divisibility or, more precisely, that there are charged particles that have the smallest charge and are no longer divisible.

To prove that there is a limit for the fission of electric charge, and to establish what this limit is, scientists conducted special experiments. For example, the Soviet scientist A.F. Ioffe conducted an experiment in which small grains of zinc dust, visible only under a microscope, were electrified. The charge of the dust particles was changed several times, and each time they measured how much the charge had changed. Experiments showed that all changes in the charge of a dust particle were an integer number of times (i.e. 2, 3, 4, 5, etc.) greater than a certain certain smallest charge, i.e., the charge of a dust particle changed, although very small, but in whole portions. Since the charge from a dust grain leaves along with a particle of matter, Ioffe concluded that in nature there is a particle of matter that has the smallest charge, which is no longer divisible.

This particle was called an electron.

The value of the electron charge was first determined by the American scientist R. Millikan. In his experiments, similar to those of A.F. Ioffe, he used small droplets of oil.

The electron charge is negative, it is equal to 1.610 C (0.000 000 000 000 000 000 16 C). Electric charge is one of the main properties of an electron. This charge cannot be “removed” from the electron.

The mass of an electron is 9.110 kg, which is 3700 times less than the mass of a hydrogen molecule, the smallest of all molecules. A fly's wing has a mass approximately 510 times greater than the mass of an electron.

6. Nuclear model of atomic structure

The study of the structure of the atom practically began in 1897-1898, after the nature of cathode rays as a stream of electrons was finally established and the charge and mass of the electron were determined. The fact that electrons are released by a wide variety of substances led to the conclusion that electrons are part of all atoms. But the atom as a whole is electrically neutral, therefore, it must contain another component, positively charged, and its charge must balance the sum of the negative charges of the electrons.

This positively charged part of the atom was discovered in 1911 by Ernest Rutherford (1871-1937). Rutherford proposed the following diagram of the structure of the atom. At the center of the atom there is a positively charged nucleus, around which electrons rotate in different orbits. The centrifugal force arising during their rotation is balanced by the attraction between the nucleus and the electrons, as a result of which they remain at certain distances from the nucleus. The total negative charge of the electrons is numerically equal to the positive charge of the nucleus, so that the atom as a whole is electrically neutral. Since the mass of electrons is negligible, almost the entire mass of an atom is concentrated in its nucleus. On the contrary, the size of the nuclei is extremely small even compared to the size of the atoms themselves: the diameter of an atom is on the order of 10 cm, and the diameter of the nucleus is on the order of 10 - 10 cm. Hence it is clear that the share of the nucleus and electrons, the number of which, as we will see later, is relatively small, accounting for only an insignificant part of the total space occupied by the atomic system (Fig. No. 5)

Conductor resistance. Conductivity. Dielectrics. Application of conductors and insulators. Semiconductors.

Physical substances are diverse in their electrical properties. The most extensive classes of matter are conductors and dielectrics.

Conductors

Main feature of conductors– the presence of free charge carriers that participate in thermal motion and can move throughout the entire volume of the substance.
As a rule, such substances include salt solutions, melts, water (except distilled), moist soil, the human body and, of course, metals.

Metals are considered the best conductors of electrical charge.
There are also very good conductors that are not metals.
Among such conductors, the best example is carbon.
All conductors have properties such as resistance And conductivity . Due to the fact that electric charges, colliding with atoms or ions of a substance, overcome some resistance to their movement in an electric field, it is customary to say that conductors have electrical resistance ( R).
The reciprocal of resistance is called conductivity ( G).

G = 1/ R

That is, conductivity – It is the property or ability of a conductor to conduct electric current.
You need to understand that good guides represent very low resistance to the flow of electrical charges and, accordingly, have high conductivity. The better the conductor, the greater its conductivity. For example, a copper conductor has b O higher conductivity than an aluminum conductor, and the conductivity of a silver conductor is higher than the same conductor made of copper.

Dielectrics

Unlike conductors, in dielectrics at low temperatures there are no free electric charges. They consist of neutral atoms or molecules. Charged particles in a neutral atom are bound to each other and cannot move under the influence of an electric field throughout the entire volume of the dielectric.

Dielectrics include, first of all, gases that conduct electrical charges very poorly. As well as glass, porcelain, ceramics, rubber, cardboard, dry wood, various plastics and resins.

Items made from dielectrics are called insulators. It should be noted that the dielectric properties of insulators largely depend on the state of the environment. Thus, in conditions of high humidity (water is a good conductor), some dielectrics may partially lose their dielectric properties.

About the use of conductors and insulators

Both conductors and insulators are widely used in technology to solve various technical problems.

Eg, all electrical wires in the house are made of metal (usually copper or aluminum). And the sheath of these wires or the plug that is plugged into the socket must be made of various polymers, which are good insulators and do not allow electrical charges to pass through.

It should be noted that the terms “conductor” or “insulator” do not reflect quality characteristics: the characteristics of these materials actually range from very good to very bad.
Silver, gold, platinum are very good conductors, but these are expensive metals, so they are used only where price is less important compared to the function of the product (space, defense).
Copper and aluminum are also good conductors and at the same time inexpensive, which predetermined their widespread use.
Tungsten and molybdenum, on the contrary, are poor conductors and for this reason cannot be used in electrical circuits (they will disrupt the operation of the circuit), but the high resistance of these metals, combined with refractoriness, predetermined their use in incandescent lamps and high-temperature heating elements.

Insulators there are also very good ones, just good ones and bad ones. This is due to the fact that real dielectrics also contain free electrons, although there are very few of them. The appearance of free charges even in insulators is due to thermal vibrations of electrons: under the influence of high temperature, some electrons still manage to break away from the core and the insulating properties of the dielectric deteriorate. Some dielectrics have more free electrons and their insulation quality is correspondingly worse. It is enough to compare, for example, ceramics and cardboard.

The best insulator is an ideal vacuum, but it is practically unattainable on Earth. Absolutely pure water will also be an excellent insulator, but has anyone seen it in reality? And water with the presence of any impurities is already a fairly good conductor.
The criterion for the quality of an insulator is its compliance with the functions that it must perform in a given circuit. If the dielectric properties of a material are such that any leakage through it is negligible (does not affect the operation of the circuit), then such a material is considered a good insulator.

Semiconductors

There are substances, which in their conductivity occupy an intermediate place between conductors and dielectrics.
Such substances are called semiconductors. They differ from conductors in the strong dependence of the conductivity of electrical charges on temperature, as well as on the concentration of impurities, and can have the properties of both conductors and dielectrics.

Unlike metal conductors, in which conductivity decreases with increasing temperature; in semiconductors, conductivity increases with increasing temperature, and resistance, as the inverse value of conductivity, decreases.

At low temperatures resistance of semiconductors, as can be seen from rice. 1, tends to infinity.
This means that at absolute zero temperature, a semiconductor has no free carriers in the conduction band and, unlike conductors, behaves like a dielectric.
With increasing temperature, as well as with the addition of impurities (doping), the conductivity of the semiconductor increases and it acquires the properties of a conductor.

Rice. 1. Dependence of resistance of conductors and semiconductors on temperature

A dielectric is a material or substance that practically does not allow electric current to pass through. This conductivity is due to the small number of electrons and ions. These particles are formed in a non-conducting material only when high temperature properties are achieved. What a dielectric is will be discussed in this article.

Description

Each electronic or radio conductor, semiconductor or charged dielectric passes electric current through itself, but the peculiarity of the dielectric is that even at high voltages above 550 V, a small current will flow in it. Electric current in a dielectric is the movement of charged particles in a certain direction (can be positive or negative).

Types of currents

The electrical conductivity of dielectrics is based on:

• Absorption currents are a current that flows in a dielectric at a constant current until it reaches a state of equilibrium, changing direction when turned on and voltage is applied to it and when turned off. With alternating current, the voltage in the dielectric will be present in it the entire time it is in the action of the electric field.
• Electronic conductivity is the movement of electrons under the influence of a field.
• Ionic conductivity is the movement of ions. Found in solutions of electrolytes - salts, acids, alkalis, as well as in many dielectrics.
• Molion electrical conductivity is the movement of charged particles called molions. Found in colloidal systems, emulsions and suspensions. The phenomenon of the movement of molions in an electric field is called electrophoresis.

They are classified according to their state of aggregation and chemical nature. The former are divided into solid, liquid, gaseous and solidifying. Based on their chemical nature, they are divided into organic, inorganic and organoelement materials.

According to the state of aggregation:

• Electrical conductivity of gases. Gaseous substances have a fairly low current conductivity. It can occur in the presence of free charged particles, which appears due to the influence of external and internal, electronic and ionic factors: X-ray and radioactive radiation, collisions of molecules and charged particles, thermal factors.
• Electrical conductivity of a liquid dielectric. Dependency factors: molecular structure, temperature, impurities, presence of large charges of electrons and ions. The electrical conductivity of liquid dielectrics largely depends on the presence of moisture and impurities. The conductivity of electricity in polar substances is also created using a liquid with dissociated ions. When comparing polar and non-polar liquids, the former have a clear advantage in conductivity. If you clean a liquid of impurities, this will help reduce its conductive properties. With an increase in conductivity and its temperature, a decrease in its viscosity occurs, leading to an increase in ion mobility.
• Solid dielectrics. Their electrical conductivity is determined by the movement of charged dielectric particles and impurities. In strong fields of electric current, electrical conductivity is revealed.

Physical properties of dielectrics

When the specific resistance of the material is less than 10-5 Ohm*m, they can be classified as conductors. If more than 108 Ohm*m - to dielectrics. There may be cases when the resistivity will be several times greater than the resistance of the conductor. In the range of 10-5-108 Ohm*m there is a semiconductor. Metal material is an excellent conductor of electric current.

Of the entire periodic table, only 25 elements are classified as non-metals, and 12 of them may have semiconductor properties. But, of course, in addition to the substances in the table, there are many more alloys, compositions or chemical compounds with the properties of a conductor, semiconductor or dielectric. Based on this, it is difficult to draw a definite line between the values ​​of various substances and their resistances. For example, at a reduced temperature factor, a semiconductor will behave like a dielectric.

Application

The use of non-conductive materials is very extensive, because it is one of the most popular classes of electrical components. It has become quite clear that they can be used due to their properties in active and passive form.

In their passive form, the properties of dielectrics are used for use in electrical insulating materials.

In their active form, they are used in ferroelectrics, as well as in materials for laser emitters.

Basic dielectrics

Commonly encountered types include:

• Glass.
• Rubber.
• Oil.
• Asphalt.
• Porcelain.
• Quartz.
• Air.
• Diamond.
• Pure water.
• Plastic.

What is a liquid dielectric?

Polarization of this type occurs in the field of electric current. Liquid non-conducting substances are used in technology for pouring or impregnating materials. There are 3 classes of liquid dielectrics:

Petroleum oils are slightly viscous and mostly non-polar. They are often used in high-voltage equipment: high-voltage water. is a non-polar dielectric. Cable oil has found application in the impregnation of insulating paper wires with a voltage of up to 40 kV, as well as metal-based coatings with a current of more than 120 kV. Transformer oil has a purer structure than capacitor oil. This type of dielectric is widely used in production, despite the high cost compared to analogue substances and materials.

What is a synthetic dielectric? Currently, it is banned almost everywhere due to its high toxicity, as it is produced on the basis of chlorinated carbon. And the liquid dielectric, which is based on organic silicon, is safe and environmentally friendly. This type does not cause metal rust and has low hygroscopic properties. There is a liquefied dielectric containing an organofluorine compound, which is especially popular due to its non-flammability, thermal properties and oxidative stability.

And the last type is vegetable oils. They are weakly polar dielectrics, these include flax, castor, tung, and hemp. Castor oil is highly hot and is used in paper capacitors. The remaining oils are evaporable. Evaporation in them is not caused by natural evaporation, but by a chemical reaction called polymerization. Actively used in enamels and paints.

Conclusion

The article discussed in detail what a dielectric is. Various types and their properties were mentioned. Of course, in order to understand the subtlety of their characteristics, you will have to study the physics section about them in more depth.

When studying thermal phenomena, it was said that according to their ability to conduct heat, substances are divided into good and bad heat conductors.

Based on their ability to transfer electrical charges, substances are also divided into several classes: conductors, semiconductors And non-conductors electricity.

Conductors are bodies through which electrical charges can pass from a charged body to an uncharged one.

Good conductors of electricity are metals, soil, water with salts, acids or alkalis dissolved in it, and graphite. The human body also conducts electricity. This can be discovered through experience. Let's touch the charged electroscope with our hand. The leaves will drop immediately. The charge from the electroscope goes through our body through the floor of the room into the ground.

a - iron; b - graphite

The best conductors of electricity among metals are silver, copper, and aluminum.

Nonconductors are those bodies through which electric charges cannot pass from a charged body to an uncharged one.

Non-conductors of electricity, or dielectrics, are ebonite, amber, porcelain, rubber, various plastics, silk, nylon, oils, air (gases). Bodies made of dielectrics are called insulators (from the Italian insulator - to isolate).

a - amber; b - porcelain

Semiconductors are bodies that, in terms of their ability to transfer electrical charges, occupy an intermediate position between conductors and dielectrics.

Semiconductors are quite widespread in nature. These are metal oxides and sulfides, some organic substances, etc. Germanium and silicon are most widely used in technology.

Semiconductors at low temperatures do not conduct electricity and are dielectrics. However, as the temperature rises, the number of electric charge carriers in the semiconductor begins to increase sharply, and it becomes a conductor.

Why is this happening? In semiconductors such as silicon and germanium, atoms in the crystal lattice oscillate around their equilibrium positions, and already at a temperature of 20 ° C this movement becomes so intense that chemical bonds between neighboring atoms can be broken. With a further increase in temperature, the valence electrons (electrons located on the outer shell of an atom) of semiconductor atoms become free, and under the influence of an electric field, an electric current arises in the semiconductor.

A characteristic feature of semiconductors is that their conductivity increases with increasing temperature. In metals, as the temperature increases, the conductivity decreases.

The ability of semiconductors to conduct electric current also arises when they are exposed to light, a flow of fast particles, the introduction of impurities, etc.

a - germanium; b- silicon

The change in the electrical conductivity of semiconductors under the influence of temperature has made it possible to use them as thermometers for measuring ambient temperature; they are widely used in technology. With its help, the temperature is controlled and maintained at a certain level.

An increase in the electrical conductivity of a substance under the influence of light is called photoconductivity. Devices based on this phenomenon are called photoresistors. Photoresistors are used for signaling and in controlling production processes at a distance and sorting products. With their help, in emergency situations, machines and conveyors are automatically stopped, preventing accidents.

Due to the amazing properties of semiconductors, they are widely used in the creation of transistors, thyristors, semiconductor diodes, photoresistors and other complex equipment. The use of integrated circuits in television, radio and computer devices makes it possible to create devices of small and sometimes negligibly small sizes.

Questions

1. What groups are substances divided into based on their ability to transfer electrical charges?
2. What characteristic feature do semiconductors have?
3. List the applications of semiconductor devices.

Exercise 22

1. Why does a charged electroscope discharge when its ball is touched by hand?
2. Why is the rod of an electroscope made of metal?
3. A positively charged body is brought to the ball of an uncharged electroscope without touching it. What charge will appear on the leaves of the electroscope?

This is interesting...

The body's ability to electrify is determined by the presence of free charges. In semiconductors, the concentration of free charge carriers increases with increasing temperature.

Conduction, which is carried out by free electrons (Fig. 43), is called electronic conductivity of a semiconductor or n-type conductivity (from Latin negativus - negative). When electrons are separated from germanium atoms, free spaces are formed at the breakpoints that are not occupied by electrons. These vacancies are called “holes.” An excess positive charge appears in the area where the hole is formed. The vacant place can be occupied by another electron.

An electron, moving in a semiconductor, creates the opportunity to fill some holes and form others. The appearance of a new hole is accompanied by the appearance of a free electron, i.e., there is a continuous formation of electron-hole pairs. In turn, filling holes leads to a decrease in the number of free electrons. If a crystal is placed in an electric field, then not only electrons will move, but also holes. The direction of movement of holes is opposite to the direction of movement of electrons.

Conduction, which occurs as a result of the movement of holes in a semiconductor, is called hole conductivity or p-type conductivity (from the Latin positivus - positive). Semiconductors are divided into pure semiconductors, n-type impurity semiconductors, and p-type impurity semiconductors.

Pure semiconductors have their own conductivity. Free charges of two types participate in the creation of current: negative (electrons) and positive (holes). In a pure semiconductor, the concentration of free electrons and holes is the same.

When impurities are introduced into a semiconductor, impurity conductivity occurs. By changing the impurity concentration, it is possible to change the number of charge carriers of one or another sign, i.e., create semiconductors with a predominant concentration of negative or positive charge. n-type impurity semiconductors have electronic conductivity. The majority charge carriers are electrons, and the minority charge carriers are holes.

Impurity p-type semiconductors have hole conductivity. The majority charge carriers are holes, and the minority charge carriers are electrons.

It is a combination of p- and l-type semiconductors. The resistance of the contact area depends on the direction of the current. If a diode is connected to the circuit so that the region of the crystal with n-type electronic conductivity is connected to the positive pole, and the region with p-type hole conductivity to the negative pole, then there will be no current in the circuit, since the transition of electrons from the n-region to p -the area becomes difficult.

If the p-region of a semiconductor is connected to the positive pole, and the n-region to the negative, then in this case the current passes through the diode. Due to the diffusion of the main current carriers into the foreign semiconductor, a double electrical layer is formed in the contact area, preventing the movement of charges. The external field directed from p to n partially compensates for the action of this layer, and as the voltage increases, the current increases rapidly.

DEFINITION, PURPOSE AND CLASSIFICATION

ELECTRICAL INSULATING MATERIALS

Dielectrics- substances in which electrostatic fields can exist for a long time. These materials, in contrast to conductive ones, practically do not conduct electric current under the influence of a constant voltage applied to them.

The purpose of electrical insulation is primarily to prevent the passage of current along paths undesirable for the operation of an electrical device. In addition, dielectrics in electrical devices, in particular capacitors, play an active role, providing the required capacitance.

Dipole dielectrics are those whose molecules are arranged asymmetrically in space; they generally have a higher dielectric constant than neutral dielectrics. Dipole dielectrics are more hygroscopic and are more easily wetted by water than neutral ones.

Dielectrics are also divided into heteropolar (ionic), whose molecules are relatively easily split into oppositely charged parts (ions), and homeopolar, not split into ions.

Based on their chemical composition, electrical insulating materials are divided into organic, V whose composition includes carbon, and on inorganic, containing no carbon. Usually, inorganic materials have higher heat resistance, than organic.

ELECTRICAL CONDUCTIVITY OF DIELECTRICS

By their very purpose, dielectrics under the influence of constant voltage should not allow current to pass at all, i.e. they should be non-conductors. However, all practically used electrical insulating materials, when applying a constant voltage, pass some insignificant current, the so-called leakage current. Thus, the resistivity of electrical insulating materials is not infinite, although it is very large.

Resistance section of insulation is equal to the ratio of the DC voltage applied to this section of insulation U (in volts) to leakage current I(in amperes) through this section:

Conductivity of insulation

.

Distinguish volumetric resistance isolation R V , numerically determining the obstacle created by the insulation to the passage of current through its thickness, and surface resistanceR S defining an obstacle to the passage of current along the insulation surface and characterizing the presence of increased conductivity of the surface layer of the dielectric due to moisture, contamination, etc.

Impedance insulation is defined as the result of two resistances connected in parallel between the electrodes, volume and surface:

For a flat section of insulation with a cross section S[cm 2 ] and thickness h[cm] volumetric resistance (excluding the influence of edges) is equal to:

.

Numerically ρ V equal to the resistance (in Ohms) of a cube with an edge of 1 cm of a given material, if the current passes through two opposite faces of the cube:

.

1 Ohm∙cm= 10 4 Ohm∙mm 2 /m= 10 6 μΩ∙cm= 10 -2 Ohm∙m.

The reciprocal of volumetric resistivity

,

called specific volume conductivity material.

Values ρ V practically used solid and liquid electrical insulating materials range from approximately 10 8 -10 10 Ohm∙cm for relatively low-quality materials used in unimportant cases (wood, marble, asbestos cement, etc.) up to 10 16 -10 18 Ohm∙cm for materials such as amber, polystyrene, polyethylene, etc. For non-ionized gases ρ V about 10 19 -10 20 Ohm∙cm The ratio of the resistivity of a high-quality solid dielectric and a good conductor (at normal temperature) is expressed by a colossal number - on the order of 10 22 -10 24.

Specific surface resistanceρ S characterizes the property of an electrical insulating material to create surface resistance in the insulation made from it. Surface resistance (neglecting the influence of edges) between electrodes with parallel straight edges of length b, located at a distance from each other A, when excluding the volumetric leakage current through the thickness of the material, it is equal to , Where .

Magnitude ρ S numerically equal to the resistance of a square (of any size) on the surface of a given material , if the current is supplied to the electrodes limiting the two opposite sides of this square .

Physical nature of electrical conductivity of dielectrics

The electrical conductivity of dielectrics is explained by the presence in them of free (i.e., not associated with certain molecules and able to move under the influence of an applied electric field) charged particles: ions, molions (colloidal particles), and sometimes electrons.

Most typical for most electrical insulating materials ionic conductivity. It should be noted that in some cases the main substance of the dielectric is subjected to electrolysis; An example is glass, in which, due to its transparency, the release of electrolysis products can be directly observed. When direct current is passed through glass, heated to reduce conductivity, characteristic tree-like deposits (“dendrites”) of the metals that make up the glass, primarily sodium, form at the cathode. Even more often, cases are observed when the molecules of the main substance of the dielectric do not have the ability to be easily ionized, but ionic electrical conductivity occurs due to impurities almost inevitably present in the dielectric - impurities of moisture, salts, acids, alkalis, etc. Even very small ones, sometimes with impurities that are difficult to detect by chemical analysis can significantly affect the conductivity of a substance; Therefore, in the manufacture of dielectrics and in general in electrical insulation technology, the purity of the starting products and the cleanliness of the workplace are so important. In a dielectric with an ionic conductivity, Faraday's law is strictly observed, i.e., the proportionality between the amount of electricity passed through the insulation (at constant current) and the amount of substance released during electrolysis.

When increasing temperature The resistivity of electrical insulating materials, as a rule, is greatly reduced. Obviously, the operating conditions of electrical insulation become more severe. At low temperatures, on the contrary, even very poor dielectrics acquire high values ρ V .

The presence of even small amounts of water can significantly reduce ρ V dielectric. This is explained by the fact that impurities present in water dissociate into ions, or the presence of water can contribute to the dissociation of the molecules of the substance itself. Thus, the operating conditions of electrical insulation become more difficult when hydration. Humidification has a very strong effect on the change ρ V fibrous and some other materials in which moisture can form continuous films along the fibers - “bridges” that penetrate the entire dielectric from one electrode to another.

To protect against moisture after drying, hygroscopic materials are impregnated or coated with non-hygroscopic varnishes, compounds, etc. When drying electrical insulation, moisture is removed from it, and its resistance increases. Therefore, as the temperature increases ρ V moistened material can even grow at first (if the effect of removing moisture outweighs the effect of increasing temperature), and only after removing a significant part of the moisture does a decrease begin ρ V .

Insulation resistance may decrease with increase in voltage, which has significant practical significance: by measuring the insulation resistance (of a machine, cable, capacitor, etc.) at a voltage that is lower than the operating voltage, we can obtain an overestimated resistance value.

Addiction R from on the voltage value is explained by a number of reasons:

formation of space charges in the dielectric;

poor contact between the electrodes and the measured insulation, etc.

At sufficiently high voltages, electrons can be released by electric field forces; the additional electronic conductivity created in this case leads to a significant increase in the overall electrical conductivity. This phenomenon precedes the development of dielectric breakdown.

When a constant voltage is applied to a solid dielectric, in most cases the current gradually decreases over time, asymptotically approaching a certain steady-state value. Thus, gradually the conductivity of the dielectric increases and the resistance decreases. The change in conductivity over time is associated with the influence of the formation of space charges, with electrolysis processes in the dielectric and other reasons.

Character of changes in specific surface resistance ρ S dielectrics from various factors (temperature, humidity, voltage, time of exposure to voltage) is similar to the nature of the change ρ V discussed above. Magnitude ρ S hygroscopic dielectrics are very sensitive to moisture.

Polarization of dielectrics

The most important property of dielectrics is their ability to polarize under the influence of externally applied electrical voltage. Polarization comes down to a change in the spatial position of charged material particles of a dielectric, and the dielectric acquires induced electric torque, and an electric charge is formed in it. If we consider some section of insulation with electrodes to which voltage is applied U [V], then the charge of this section Q [Cl] is determined by the expression

Q= C.U. .

Here WITH is the capacitance of a given section of insulation, measured in farads (f).

The insulation capacity depends both on the material (dielectric) and on the geometric dimensions and configuration of the insulation.

The ability of a given dielectric to form electrical capacitance is called its dielectric constant and is designated ε . Magnitude ε vacuum is taken as one.

Let WITH O- capacity of a vacuum capacitor of arbitrary shape and size. If, without changing the size, shape and relative position of the capacitor plates, the space between its plates is filled with a material with a dielectric constant ε , then the capacitance of the capacitor will increase and reach the value

C=ε C O .

Thus, the dielectric constant of a substance is a number showing how many times the capacity of a vacuum capacitor will increase if, without changing the size and shape of the capacitor electrodes, the space between the electrodes is filled with a given substance. The capacitance of a capacitor of given geometric dimensions and shape is directly proportional ε dielectric.

The value of dielectric constant is included in many basic equations of electrostatics. Yes, according to the law pendant force of mutual repulsion of two point electric charges of magnitude Q 1 and Q 2 (absolute charge units) located in a medium with dielectric constant ε at a distance from each other h[cm] , is:

Dielectric constant is a dimensionless quantity. For gases it is very close to 1. So, for air under normal conditions ε= 1.00058. For most liquid and solid electrical insulating materials ε – on the order of several units, less often tens and very rarely exceeds 100. Some substances of a special class - ferroelectrics - under certain conditions have exceptionally high values ​​of dielectric constant.

The physical essence of polarization

Polarization, like conductivity, is caused by the movement of electrical charges in space. The differences between these two phenomena:

polarization causes a shift related with certain molecules of charges that cannot go beyond the boundaries of a given molecule, while conductivity is due to the movement (drift) of free charges that can move in a dielectric over a relatively large distance;

polarization displacement – ​​elastic shift of charges; upon termination of the voltage applied to the dielectric, the displaced charges tend to return to their original positions, which is not typical for conductivity;

polarization of a homogeneous material occurs in almost all dielectric molecules, while the electrical conductivity of dielectrics is often determined by the presence of a small amount of impurities (contaminants).

While the conduction current exists as long as a constant voltage is applied to the dielectric from the outside, bias current (capacitive current) occurs only when the direct voltage is turned on or off, or even when the magnitude of the applied voltage changes; for a long time there is a capacitive current only in the dielectric under the influence alternating voltage.

The most typical types of polarization are electronic, ionic and dipole.

Electronic polarization- displacement of electron orbits relative to the atomic nucleus. Electronic polarization when an external electric field is applied occurs in an extremely short time (about 10 -15 sec).

Ionic polarization(for ionic dielectrics) - the displacement relative to each other of the ions that make up the molecule. This polarization occurs in periods longer than electronic polarization, but also in very short periods - about 10 -13 seconds.

Electronic and ion polarization - varieties deformation polarization, representing a shift of charges relative to each other in the direction of the external electric field.

Dipole (orientation) polarization comes down to the rotation (orientation) of dipole molecules of a substance. This polarization is numerically large compared to deformation polarization and occurs completely over time intervals that are different for molecules of different substances, but significantly longer than the duration of deformation polarization.

It is obvious that in neutral dielectrics only deformation polarization can occur. These dielectrics have a relatively low dielectric constant (for example, for liquid and solid hydrocarbons ε about 1.9-2.8).

Table 1.1

The dielectric constant of some substances

Dipole dielectrics, in which, in addition to deformation polarization, orientation polarization is also observed, have higher values ​​of dielectric constant compared to neutral dielectrics, and in dipole dielectrics, for example, for water, ε = 82.

The dielectric constant of a dipole substance, generally speaking, is greater, the smaller the size of the molecule (or molecular weight). Yes, quite big ε water is due to the very small size of its molecule.

Dependence of dielectric constant on frequency. Since the time of establishment of deformation polarization is very short compared to the time of change in the sign of the voltage even at the highest frequencies used in modern radio electronics, the polarization of neutral dielectrics manages to be fully established in a time that can be neglected in comparison with the half-cycle of an alternating voltage. Therefore, there is practically no significant dependence ε from frequency neutral dielectrics do not.

For dipole dielectrics, as the frequency of the alternating voltage increases, the value ε at first also remains unchanged, but starting from a certain critical frequency, when the polarization does not have time to fully establish itself in one half-cycle, ε begins to decrease, approaching at very high frequencies the values ​​characteristic of neutral dielectrics; As the temperature increases, the critical frequency increases.

In sharply inhomogeneous dielectrics, in particular, in dielectrics with inclusions of water, the phenomenon of the so-called interlayerNoah polarization. Interlayer polarization is reduced to the accumulation of electric charges at the interfaces between dielectrics (in the case of a moistened dielectric, on the surface of disseminated water). The processes of establishing interlayer polarization are very slow and can take place over minutes and even hours. Therefore, the increase in insulation capacity due to moistening of the latter is greater, the lower the frequency of the alternating voltage applied to the insulation.

HeadThe dependence of dielectric constant on temperature. For neutral dielectrics ε weakly depends on temperature, decreasing as the latter increases due to thermal expansion of the substance, i.e., a decrease in the number of polarizable molecules per unit volume of the substance.

In dipole dielectrics at low temperatures, when the substance has high viscosity, the orientation of dipole molecules along the field is in most cases impossible or, in any case, difficult. As the temperature increases and the viscosity decreases, the possibility of dipole orientation becomes easier, resulting in ε increases significantly. At high temperatures, due to increased thermal chaotic thermal vibrations of molecules, the degree of orderliness of molecular orientation decreases, which again leads to a decrease ε .

In crystals with ionic polarization, glasses, porcelain and other types of ceramics with a high content of the glassy phase, the dielectric constant increases with increasing temperature.