Non-self-conductivity of air. Experimental setup

Air ionization

Atmospheric air is a mixture of many gaseous substances. In addition to oxygen and nitrogen, which form the bulk of air, it also contains small amounts of so-called inert gases, carbon dioxide and water vapor. In addition to the listed gases, the air contains even greater or lesser amounts of dust and some random impurities. Oxygen, nitrogen and inert gases are considered permanent components of air, since their content in the air is almost the same everywhere. On the contrary, the content of 2 CO, water vapor and dust may vary depending on different conditions. As is known, under normal conditions of pressure and temperature, various gases that make up the air are dielectrics.

If some of the molecules are ionized, the gas conducts current.

When we say that air is ionized, it means that some very large part of the gas molecules in the air carries an electrical charge of a negative or positive sign. Note that 1 cm 3 of air under normal conditions contains 2,710 19 molecules, the average number of light air ions under natural conditions in the same volume is approximately 500-700 pairs.

The concentration of air ions in the atmosphere is expressed by the number of positive and negative ions per 1 cm3. Hence, the conductivity of the atmosphere consists of polar conductivities - positive and negative, i.e.

- conductivity of the atmosphere,

n – number of positive and negative air ions,

k – mobility of positive and negative air ions,

Total conductivity of the atmosphere:
= + = nk e + n k e
Where:

Total conductivity of the atmosphere

Positive atmospheric conductivity

Negative atmospheric conductivity

n - number of positive air ions

n - number of negative air ions

k - mobility of positive air ions

k - mobility of negative air ions

e – air ion charge equal to 4.810 -10 absolute electrostatic units.
The density of the vertical atmospheric current can be expressed as follows:
I =
Where:

Total conductivity of the atmosphere,

I is the density of the vertical atmospheric current,

Vertical potential gradient.
The ratio of positive to negative air ions near the earth's surface is approximately 1.2, i.e.:
K= = 1,2
Where:

K – unipolarity coefficient,

n is the number of negative air ions.
The presence of a certain excess of positive air ions in the air is explained by the fact that the soil air, coming out through the soil capillaries, leaves predominantly negative air ions on them. As is known, the conductivity of soil air is 30 times greater than the conductivity of atmospheric air.

The electrical conductivity of the atmosphere is on average 110 4 electrical units.


Density of vertical conduction current of the atmosphere



The potential gradient of the earth's electric field undergoes sharp distortions due to various irregularities on the earth's surface. Equipotential surfaces bend around obstacles and condense over elevated objects. Inside buildings, the potential gradient of the electric field is zero; there is no electric field inside buildings even during strong atmospheric-electric phenomena. This circumstance is taken into account in the electroeffluvial method of aeroionification.

Due to the fact that atmospheric air contains, in addition to gas molecules, also suspended solid or liquid microparticles that adsorb light air ions, ionization equilibrium can be expressed as follows:
q = n + n - + n + N - + n+N0
Where:

n is the number of positive air ions,

N 0 - number of neutral particles.
But since the number of suspended microparticles is usually much greater than the number of light air ions, ionization equilibrium can be represented by the equation:
q = n + ( n - + N - + N 0) = / n t
Where:

q is the number of air ions formed per 1 cm 3 /s,

n is the number of positive air ions,

n - number of negative air ions,

Recombination coefficient of light air ions,

Coefficient of combination of light air ions with charged particles,

N - - number of charged particles,

N 0 - number of neutral particles,

t – time period,

n – total number of ions,

/ is the constant of disappearance of air ions.
The change in the number of air ions in the atmospheric air with a change in ion formation is expressed:

t – time period,

q is the number of air ions formed per 1 cm 3 /s,

/ - constant disappearance of air ions,

n is the total number of ions.
In the absence of ion formation, the number of ions decreases with time t according to the law:
n = n 0 e

The average lifetime of light air ions can be expressed as follows:

Numerous measurements of the number of light air ions made in many countries by hundreds of physicists, geophysicists, meteorologists and doctors cannot be considered absolutely reliable. The Ebert air ion counter, with which these measurements were made, does not satisfy the requirements for it.

The technique for measuring the number of air ions per unit volume has not yet received a final and accurate solution due to a complex set of factors accompanying ionic processes in atmospheric air.

Ionization consists of splitting molecules into an electron and an ion (charge +). Since gas molecules and atoms are quite stable, for ionization it is necessary to do work against the interaction forces between the electron and the ion. This work is called ionization work . The work of ionization depends on the nature of the gas and on the energy state of the electron.

The work of ionization can be determined by the ionization potential .
Ionization potential is the potential difference that an electron must undergo in an accelerating electric field so that the increase in its energy is equal to the ionization work.

, (1)

Ionization potential (eV),

Electron-volt (eV) is the energy acquired by a particle having a charge equal to the charge of an electron after passing through a potential difference of 1 V. This extra-system unit of energy is currently approved for use in physics. 1eV= 1.6021892·10 -19 J

Ionization work,

e– electron charge.

(2)

m - electron mass (kg)

V - electron speed (m/sec.)

e– electron charge.
If the kinetic energy of the electron is:

, (2.1)

The energy W that an electron acquires when passing through a potential difference U is equal to:

W=eU (2.2)
And the ionization potential (the energy possessed by an electron when colliding with another electron can ionize it) is equal to:

T+W, (2.3)
Then, substituting (2.1) and (2.2) into (2.3) we get:

U is the potential difference that 1 electron needs to pass through,

to have enough energy to ionize the electron it collides with.

e– electron charge,

m - electron mass (kg),

V - electron speed (m/sec.),

Ionization potential (eV).

In some gases, such as oxygen, carbon dioxide, water vapor,

a separated electron during one of the closest encounters with another neutral

molecule combines with it, turning it into an electronegative ion.

Addition, “the attachment of an electron to a neutral molecule, leads to

In such cases, to such a rearrangement of its electronic shell that, as a result, the energy of the molecule that has captured an extra electron turns out to be less than the energy of the neutral molecule by a certain amount, which is called the electron affinity energy.

It ranges from 0.75 to 4.5 eV for most different gases. In inert gases - argon, neon, helium, krypton, xenon, and also in nitrogen - negative ions do not appear.
The values ​​for some molecules of various components of atmospheric air are given in Table 1.
Table 1.

Speed ​​of an electron (kilometers per second) passing without collisions

potential difference U (volts) is determined by the expression:

Substituting ionization potentials into this formula, we see that an electron ionizes gas molecules when its speed is above 1000 km/s.

Depending on how ionization is carried out, the following types of ionization are distinguished:

1) Photoionization (exposure to X-rays and gamma rays);

It is known that air ionization and the formation of partial surface discharges (PSDs) can occur, for example, during photoionization. For exposure to radiation to lead to ionization of air, the following condition must be met:

With- speed of light;

Radiation wavelength;

h- Planck's constant;

Wi- ionization energy

Determining the radiation wavelength using the above formula, we obtain

 10–7 m, or 103 Å.

Waves with such lengths lie on the border of ultraviolet and x-ray radiation (the so-called vacuum ultraviolet), while visible light cannot lead to ionization of air.

2) Impact ionization (impact
And particles (electron, positron);

1. 
   Thermal ionization (heating to high temperature).

4) Ionization by an electric field. In order for negative and positive ions to be formed as a result of electrostatic emission, an external electric field of more than 1000 kV/cm is required. This type of ionization is the most common and is used for artificial ionization of air in domestic premises, using devices called air ionizers. Next we will consider this type of ionization.

As a result of all these types of ionization, current carriers appear. In this case they talk about non-self-conductivity gas. If current carriers arise in a gas, which are caused only by an electric field applied to the gas, conductivity is called independent.
Let's consider dependent gas discharge. Gas discharge called the passage of current through a gas.

Under the influence of an external ionizer, a gas molecule is split into an electron and an ion . The electron can be captured by a neutral molecule, which will become an ion.

Number of pairs of ionized molecules per unit volume V and per unit of time t denote by
. Part of ionized molecules recombine, i.e. Neutralization of opposite pairs occurs when they meet.

The presence of recombination prevents an unlimited increase in the number of ions in the gas and explains the establishment of a certain concentration of ions a short time after the start of the action of the external ionizer.

The probability of two ions of opposite signs meeting is proportional to both the number of positive and negative ions. Therefore, the number of ion pairs recombining per second per unit volume
proportional to the square of the number of ion pairs present per unit volume n:

Number of recombining ion pairs (per second per unit volume).

r

n

Ion concentration in gas:

Where:

n is the number of simultaneously generated ions in the gas

v – recombination coefficient.

In the absence of an external field, equilibrium occurs: the number of pairs of ionized molecules is equal to the number of pairs of recombined molecules, i.e.

, (3)
whence the number of ion pairs per unit volume is equal to:

.

V and per unit of time t.

r– proportionality coefficient.

n - the number of ion pairs present per unit volume.

Under the influence of cosmic radiation and traces of radioactive substances present in the earth's crust, 1 cm 3 at an equilibrium ion concentration has a value of the order of
. This concentration is not sufficient to cause significant conductivity (clean, dry air is a very good insulator).
If, every second on the ionizer electrodes neutralized
pairs of ions, then the current strength in the circuit will be equal to:

, (4)

I

Ionizer,

S– electrode area,

l

Current between ionizer electrodes:

j – current density

S is the area of ​​each electrode in the space between which the ion generation effect takes place

From expression (4) we obtain that the concentration of ion pairs neutralized on the electrodes per unit time is equal to

, (5)

The number of ion pairs that are neutralized by the electrodachionizer,

I - current strength between the emitting electrodes of the ionizer,

– charge of the current carrier (ion),

S– electrode area,

l– distance between electrodes;

j– current density.

In the presence of current, the condition for ion equilibrium will be written as follows: E = Ohm's law obtained from expression (8).

j– current density,

- specific electrical conductivity of gas,

E– field strength.

In the second region on the dependence curve
the linear relationship between current density and voltage is violated due to the fact that the concentration of ions in the gas decreases.

In the third region, starting from a certain voltage value, the current density remains constant as E increases. This is due to the fact that, with a constant ionization intensity in strong electric fields, all ions formed per unit time in the gas reach the electrodes. The current density value is called current density saturation:

. (10)

J us– saturation current density,

– charge of the current carrier (ion),

Number of pairs of ionized molecules per unit volume V and per unit of time t,

l– distance between electrodes.

The real value of the saturation current in air is very small and is approximately J us =10 -15 Vehicle 2 .

Beyond the saturation region lies a region of sharp increase in current density (in Fig. 2 this region is depicted by a dashed line). This increase is explained by the fact that, starting from a certain value E, the electrons generated by the external ionizer manage, during their free path, to acquire energy sufficient to collide with a molecule and cause its ionization, i.e.

, (11)
Where
– kinetic energy of the electron;
– work of ionization of a molecule. The electrons generated during ionization, having accelerated, in turn cause ionization. Thus, an avalanche-like multiplication of primary ions that arise under the influence of an external ionizer occurs. However, the process does not lose the character of a non-self-sustaining discharge.

Electric potential gradient in the atmosphere

On a normal day over a desert plain or over the sea, the electrical potential increases by about 100 V with each meter as you rise. There is a vertical electric field E of 100 V/m in the air. The sign of the field corresponds to the negative charge of the earth's surface. This means that on the street the potential at your nose is 200 volts higher than the potential at your heels! One can, of course, ask: “Why not place a pair of electrodes in the air a meter apart from each other and use these 100 V for electric lighting? “And you might be surprised: “If there really is a voltage of 200 V between my nose and my heel, then why don’t I get an electric shock as soon as I go outside? ”

Your body is a pretty good conductor. When you stand on the ground, you and the ground form an equipotential surface. Normally, equipotential surfaces are parallel to the ground, but when you are on the ground, they shift, so that the potential difference between the top of your head and your heels is almost zero. Charges pass from the ground to your head and change the field around you. Some of them are discharged by air ions, but the ion current is very small, because air is a poor conductor.

How to measure such a field, since it is distorted by everything that falls into it? There are several ways. One way is to place the insulated conductor at some height above the ground and not touch it until it reaches air potential. If you wait long enough, then even with very low air conductivity, charges will drain from the conductor (or flow onto it), equalizing its potential with the potential of the air at this level. Then we can lower it to the ground and measure the change in its potential. Another faster way is to use a bucket of water that has a small leak as a guide. As the water flows out, it carries away the excess charge, and the bucket quickly acquires the potential of air. (Charges, as you know, spread over the surface, and water drops are moving “pieces of the surface”) The potential of a bucket can be measured with an electrometer.

There is also a way to directly measure the potential gradient. Since there is an electric field, there must also be a surface charge on the ground (y = e0E). If we place a flat metal plate A at the surface of the earth and ground it, then negative charges will appear on it. If we then cover the plate with another grounded conductive cover B, then charges will appear on cover B and disappear on plate A. If we measure the charge flowing from plate A to the ground (say, using a galvanometer in the ground wire circuit) at the moment when A is covered with a lid, then we will find the density of the surface charge that was on A, and hence the electric field.

Electric currents in the atmosphere

In addition to the potential gradient, another quantity can be measured - the current in the atmosphere. Its density is low: about 10-6 microns passes through every square meter parallel to the earth's surface. Air is apparently not a perfect insulator; Because of this conductivity, a weak current flows all the time from heaven to earth, caused by the electric field we have described.

Why does the atmosphere have conductivity? Because in it, among the air molecules, there are ions, for example, oxygen molecules, sometimes equipped with an extra electron, and sometimes deprived of one of their own. These ions do not remain alone; Thanks to their electric field, they tend to gather other molecules near them. Each ion then becomes a small lump, which, together with other similar lumps, drifts into the field, slowly moving up or down, creating the current that we were talking about.

Where do ions come from? At first it was thought that ions create radioactivity on the Earth. (It was known that radiation from radioactive substances makes air conductive by ionizing air molecules.) Particles emerging from the atomic nucleus, say b-rays, move so fast that they snatch electrons from the atoms, leaving behind a trail of ions. This view, of course, assumes that at higher altitudes the ionization should become less, because all the radioactivity is all traces of radium, uranium, sodium, etc. - is found in the dust of the earth.

To test this theory, physicists flew up in balloons and measured ionization (Hess, in 1912). It turned out that everything was happening just the opposite - ionization per unit volume increased with height; two plates were periodically charged to potential V. Due to the conductivity of the air, they were slowly discharged; the rate of discharge was measured with an electrometer) This incomprehensible result was the most stunning discovery in the entire history of atmospheric electricity. The discovery was so important that it required the creation of a new branch of science - cosmic ray physics. And atmospheric electricity itself remained among the less surprising phenomena. The ionization was apparently generated by something outside the Earth; the search for this unearthly source led to the discovery of cosmic rays. We will not talk about them now and will only say that they are the ones that support the supply of ions to the air. Although ions are constantly carried away, cosmic particles, bursting from outer space, continually create new ions.

To be precise, we must note that, in addition to ions made up of molecules, there are other types of ions. Tiny clumps of soil, like extremely fine particles of dust, float in the air and become charged. They are sometimes called “nuclei”. For example, when waves splash in the sea, small splashes fly into the air. When such a droplet evaporates, a small crystal of NaCl remains floating in the air. These crystals can then attract charges and become ions; they are called “large ions.”

Small ions, i.e. those created by cosmic rays, are the most mobile. Because they are so small, they travel quickly through the air, at a speed of about 1 cm/sec in a field of 100 V/m, or 1 V/cm. Large and heavy ions move much more slowly. It turns out that if there are a lot of “nuclei”, then they intercept charges from small ions. Then, since the “large ions” move very slowly in the field, the overall conductivity decreases. Therefore, the conductivity of the air is very changeable - it is very sensitive to its “clogging”. There is much more of this “litter” over land than over the sea; the wind raises dust from the ground, and people also pollute the air in every possible way. It is not surprising that from day to day, from moment to moment, from one place to another, conductivity near the earth's surface changes significantly. The electric field at each point above the earth's surface also changes, because the current flowing from top to bottom is approximately the same in different places, and changes in conductivity near the earth's surface lead to variations in the field.

Air conductivity, resulting from ion drift, also increases rapidly with altitude. This happens for two reasons. Firstly, the ionization of air by cosmic rays increases with altitude. Secondly, as the density of the air decreases, the free path of the ions increases, so that they can travel further in the electric field before colliding. As a result, at altitude the conductivity jumps sharply.

The density of electric current in the air itself is equal to only a few micro-microamperes per square meter, but there are a lot of such square meters on Earth. The entire electric current reaching the earth's surface is approximately 1800 A. This current is, of course, “positive” - it transfers a positive charge to the Earth. So the result is a current of 1800 A at a voltage of 400,000 V. Power 700 MW!

With such a strong current, the negative charge of the Earth should soon disappear. In fact, it would only take about half an hour to discharge the entire Earth. But much more than half an hour has passed since the discovery of the electric field in the atmosphere. How does it hold up? How is the tension maintained? And between what and what is it? The Earth is on one electrode, and what is on the other? There are many such questions.

The earth is negatively charged, but the potential in the air is positive. At a high enough altitude, the conductivity is so great that the probability of horizontal voltage changes becomes zero. Air, at the time scale we are talking about now, actually turns into a conductor. This occurs at an altitude of about 50 km. This is not yet as high as what is called the “ionosphere,” where there are a very large number of ions formed due to the photoelectric effect from the sun’s rays. For our purposes, when discussing the properties of atmospheric electricity, we can assume that at an altitude of about 50 km the air becomes sufficiently conductive and there exists a practically conducting sphere from which currents flow downward. The question is how the positive charge is maintained there. How does it pump back up? Since it flows to Earth, then it must somehow be pumped back? For a long time this was one of the main mysteries of atmospheric electricity.

Any information on this matter can provide a clue to the mystery, or at least tell us something about it. Here is one interesting phenomenon: if we measure the current (and it, as we know, is more stable than the potential gradient), say over the sea, and with careful observance of precautions, very carefully average everything and get rid of all errors, then we find that there remain There are still some daily variations. The average of many measurements over the oceans has a temporal variation. The current varies by approximately ±15% and reaches its greatest value at 7 pm London time. The strangest thing here is that, no matter where you measure the current - in the Atlantic, Pacific or Arctic Ocean - its peak hours are when the clock in London shows 7 pm! Throughout the world the current reaches its maximum at 19:00 London time and its minimum at 4:00 London time. In other words, the current depends on absolute terrestrial time, and not on local time at the observation point. In one respect this is not so strange after all; this is quite consistent with our idea that at the very top there is a very large horizontal conductivity, which excludes local changes in the potential difference between the Earth and the top. Any change in capacity must be worldwide, and so it is. So, now we know that the voltage “above” with a change in absolute earthly time either rises or falls by 15%.

Origin of currents in the atmosphere

Now we need to answer the question about the source of the large negative currents that must flow from the “top” to the earth’s surface in order to maintain its negative charge. Where are the batteries that do this? It's a thunderstorm, or rather lightning. It turns out that lightning flashes do not “discharge” the potential difference that we were talking about (and as it might seem at first glance). Lightning supplies the Earth with a negative charge. If we saw lightning, we can bet ten to one that it brought a large number of negative charges to the Earth. It is thunderstorms that charge the Earth with an average of 1800 A of electricity, which is then discharged in areas with good weather.

On Earth, about 300 thunderstorms thunder every day. They can be considered the batteries that pump electricity into the upper layers of the atmosphere and maintain the potential difference. Now consider the geography - midday thunderstorms in Brazil, tropical thunderstorms in Africa, etc. Scientists have made estimates of how many lightning strikes the Earth every second; Needless to say, their estimates are more or less consistent with the potential difference measurements: the overall degree of thunderstorm activity reaches its maximum throughout the Earth at 19.00 London time. However, estimates of thunderstorm activity are very difficult to make; they were made only after it became known that such variations must exist. The difficulty is that there are not enough observations in the oceans, and elsewhere in the world, to accurately determine the number of thunderstorms. But those scientists who think they “got it all right” claim that maximum activity occurs at 19:00 GMT.

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Gases in their normal state are good dielectrics (for example, clean, non-ionized air). However, if gases contain moisture mixed with organic and inorganic particles and are ionized, then they conduct electricity.

In all gases, even before electrical voltage is applied to them, there is always a certain amount of electrically charged particles - electrons and ions, which are in random thermal motion. These can be charged gas particles, as well as charged particles of solid and liquid substances - impurities found, for example, in the air.

The formation of electrically charged particles in gaseous dielectrics is caused by ionization of gas by external energy sources (external ionizers): cosmic and solar rays, radioactive radiation from the Earth, etc.

The electrical conductivity of gases depends mainly on the degree of their ionization, which can be achieved in various ways. Basically, the ionization of gases occurs as a result of the removal of electrons from a neutral gas molecule.

An electron released from a gas molecule will mix in the intermolecular space of the gas, and here, depending on the type of gas, it can retain the “independence” of its movement for a relatively long time (for example, in such gases as hydrogen H 2, nitrogen N 2) or, conversely, quickly penetrate into a neutral molecule, turning it into a negative ion (for example, in oxygen).

The greatest effect of ionization of gases is achieved by irradiating them with X-rays, cathode rays or rays emitted by radioactive substances.

Atmospheric air in summer is very intensely ionized under the influence of sunlight. Moisture in the air condenses on its ions, forming tiny droplets of water charged with electricity. Ultimately, thunderclouds are formed from individual electrically charged droplets of water, accompanied by lightning, i.e. electrical discharges of atmospheric electricity.

The process of gas ionization by external ionizers is that they impart part of the energy to the gas atoms. In this case, valence electrons acquire additional energy and are separated from their atoms, which turn into positively charged particles - positive ions.

The resulting free electrons can maintain independent movement for a long time in a gas (for example, in hydrogen, nitrogen) or after some time they attach to electrically neutral atoms and molecules of the gas, turning them into negatively charged ions.

The appearance of electrically charged particles in a gas can also be caused by the release of electrons from the surface of metal electrodes when they are heated or exposed to radiant energy. Being in random thermal motion, some of the oppositely charged (electrons) and positively charged (ions) particles reunite with each other and form electrically neutral atoms and gas molecules. This process is called restoration or recombination.

If a certain volume of gas is enclosed between metal electrodes (disks, balls), then when an electrical voltage is applied to the electrodes, electric forces will act on the charged particles in the gas - electric field strength.

Under the influence of these forces, electrons and ions will move from one electrode to another, creating electric current in gas.

The current in the gas will be greater, the more different dielectrics there are, the more charged particles are formed in it per unit time and the greater the speed they acquire under the influence of electric field forces.

It is clear that with increasing voltage applied to a given volume of gas, the electrical forces acting on electrons and ions increase. In this case, the speed of charged particles, and therefore the current in the gas, increases.

The change in current value depending on the voltage applied to the volume of gas is expressed graphically in the form of a curve called current-voltage characteristic.

Current-voltage characteristic for a gaseous dielectric

The current-voltage characteristic shows that in the region of weak electric fields, when the electric forces acting on charged particles are relatively small (region I on the graph), the current in the gas increases in proportion to the magnitude of the applied voltage. In this region, the current changes according to Ohm's law.

With a further increase in voltage (region II), the proportionality between current and voltage is violated. In this region, the conduction current does not depend on voltage. Here, energy is accumulated by charged gas particles - electrons and ions.

With a further increase in voltage (region III), the speed of charged particles increases sharply, as a result of which frequent collisions with neutral gas particles occur. During these elastic collisions, electrons and ions transfer part of their accumulated energy to neutral gas particles. As a result, electrons are separated from their atoms. In this case, new electrically charged particles are formed: free electrons and ions.

Due to the fact that flying charged particles collide with gas atoms and molecules very often, the formation of new electrically charged particles occurs very intensively. This process is called impact ionization of gas.

In the region of impact ionization (region III in the figure), the current in the gas increases rapidly with the slightest increase in voltage. The process of impact ionization in gaseous dielectrics is accompanied by a sharp decrease in the volumetric resistivity of the gas and an increase.

Naturally, gaseous dielectrics can be used at voltages lower than those values ​​at which the process of impact ionization occurs. In this case, gases are very good dielectrics, in which the specific volume resistance is very high (1020 ohmx cm), and the dielectric loss tangent is very small ( tan δ ≈ 10 -6 ). Therefore, gases, in particular air, are used as dielectrics in standard capacitors, gas-filled cables, etc.

In any insulating structure, air or some other gas is present to one degree or another as an insulating element. The wires of overhead lines (OHL), busbars of switchgear, terminals of transformers and various high-voltage devices are separated from each other by gaps, the only insulating medium in which is air.

Violation of the electrical strength of such structures can occur either through breakdown of the dielectric from which the insulators are made, or as a result of a discharge in the air or along the surface of the dielectric.

Unlike the breakdown of an insulator, which leads to its complete failure, a discharge along the surface is usually not accompanied by damage. Consequently, if the insulating structure is made in such a way that the overlap voltage on the surface or the discharge voltage in the air is less than the breakdown voltage of the insulators, then the actual electrical strength of such structures will be determined by the electrical strength of the air.

In the above cases, air is the natural gaseous medium in which the insulating structures are located. Along with this, air or other gas is often used as one of the main insulating materials when insulating cables, capacitors, transformers and other electrical devices.

To ensure reliable and trouble-free operation of insulating structures, it is necessary to know how the electrical strength of gas is affected by various factors, such as the shape and duration of the voltage, gas temperature and pressure, the nature of the electric field, etc.

In addition to the potential gradient, another quantity can be measured - the current in the atmosphere. Its density is low: through every square meter parallel to the earth's surface, about . Air is apparently not a perfect insulator; Because of this conductivity, a weak current flows all the time from heaven to earth, caused by the electric field we have described.

Why does the atmosphere have conductivity? Because in it, among the air molecules, there are ions, for example, oxygen molecules, sometimes equipped with an extra electron, and sometimes deprived of one of their own. These ions do not remain alone; Thanks to their electric field, they tend to gather other molecules near them. Each ion then becomes a small lump, which, together with other similar lumps, drifts into the field, slowly moving up or down, creating the current that we were talking about.

Where do ions come from? At first they thought that ions were created by the radioactivity of the Earth. (It was known that radiation from radioactive substances makes air conductive by ionizing air molecules.) Particles coming out of an atomic nucleus, say. -rays move so fast that they strip electrons from atoms, leaving behind a trail of ions. This view, of course, assumes that at higher altitudes the ionization would become less, because all the radioactivity - all traces of radium, uranium, sodium, etc. - is in the earth's dust.

Figure. 9.3. Measurement of air conductivity caused by the movement of ions.

To test this theory, physicists flew up in balloons and measured ionization (Hess, 1912). It turned out that everything happens just the opposite - ionization per unit volume increases with height! (The device was similar to that shown in Fig. 9.3. Two plates were periodically charged to a potential of . Due to the conductivity of the air, they were slowly discharged; the rate of discharge was measured by an electrometer.) This incomprehensible result was the most stunning discovery in the entire history of atmospheric electricity. The discovery was so important that it required the creation of a new branch of science - cosmic ray physics. And atmospheric electricity itself remained among the less surprising phenomena. The ionization was apparently generated by something outside the Earth; the search for this unearthly source led to the discovery of cosmic rays. We will not talk about them now and will only say that they are the ones that support the supply of ions to the air. Although ions are constantly being carried away, cosmic particles, bursting from cosmic space, continually create new ions.

To be precise, we must note that, in addition to ions made up of molecules, there are other types of ions. Tiny clumps of soil, like extremely fine particles of dust, float in the air and become charged. They are sometimes called "nuclei". For example, when waves splash in the sea, small splashes fly into the air. When such a droplet evaporates, a small crystal remains floating in the air. These crystals can then attract charges and become ions; they are called "large ions".

Small ions, i.e. those created by cosmic rays, are the most mobile. Due to the fact that they are very small, they quickly fly through the air, at a speed of about , or . Large and heavy ions move much more slowly. It turns out that if there are a lot of “nuclei”, then they intercept charges from small ions. Then, since the "large ions" move very slowly in the field, the overall conductivity decreases. Therefore, the conductivity of the air is very changeable - it is very sensitive to its “clogging”. There is much more of this “litter” over land than over the sea, the wind raises dust from the ground, and people also pollute the air in every possible way. It is not surprising that from day to day, from moment to moment, from one place to another, conductivity near the earth's surface changes significantly. The electric field at each point above the earth's surface also changes, because the current flowing from top to bottom is approximately the same in different places, and changes in conductivity near the earth's surface lead to variations in the field.

Air conductivity, resulting from ion drift, also increases rapidly with altitude. This happens for two reasons. Firstly, the ionization of air by cosmic rays increases with altitude. Secondly, as the density of the air decreases, the free path of the ions increases, so that they can travel further in the electric field before colliding. As a result, at altitude the conductivity jumps sharply.

The density of electric current in the air itself is equal to only a few micro-microamperes per square meter, but there are a lot of such square meters on Earth. The entire electrical current reaching the earth's surface is approximately . This current is, of course, "positive" - ​​it transfers a positive charge to the Earth. So the result is a current in at a voltage of . Power!

With such a strong current, the negative charge of the Earth should soon disappear. In fact, it would only take about half an hour to discharge the entire Earth. But much more than half an hour has passed since the discovery of the electric field in the atmosphere. How does it hold up? How is the tension maintained? And between what and what is it? The Earth is on one electrode, and what is on the other? There are many such questions.

The earth is negatively charged, but the potential in the air is positive. At a high enough altitude, the conductivity is so great that the probability of horizontal voltage changes becomes zero. Air, at the time scale we are talking about now, actually turns into a conductor. This occurs at an altitude of about . This is not yet as high as what is called the "ionosphere", where there are a very large number of ions formed due to the photoelectric effect from the sun's rays. For our purposes, when discussing the properties of atmospheric electricity, we can assume that at approximately an altitude the air becomes sufficiently conductive and there exists a practically conductive sphere from which currents flow downward. The state of affairs is depicted in Fig. 9.4. The question is how the positive charge is maintained there. How does it pump back up? Since it flows to Earth, then it must somehow be pumped back? For a long time this was one of the main mysteries of atmospheric electricity.

Figure. 9.4. Typical characteristics of the electrical properties of a pure atmosphere.

Any information on this matter can provide a clue to the mystery, or at least tell us something about it. Here is one interesting phenomenon: if we measure the current (and it, as we know, is more stable than the potential gradient), say over the sea, and with careful observance of precautions, very carefully average everything and get rid of all errors, then we find that there remain There are still some daily variations. The average of many measurements over the oceans has a temporal variation approximately as shown in Fig. 9.5. The current varies by approximately ±15% and reaches its highest value at 7 pm London time. The strangest thing here is that, no matter where you measure the current - in the Atlantic, Pacific or Arctic Ocean - its peak hours are when the clock in London shows 7 pm! Throughout the world the current reaches its maximum at 19:00 London time and its minimum at 4:00 London time. In other words, the current depends on absolute terrestrial time, and not on local time at the observation point. In one respect this is not so strange after all; this is quite consistent with our idea that at the very top there is a very large horizontal conductivity, which excludes local changes in the potential difference between the Earth and the top. Any change in capacity must be worldwide, and so it is. So, now we know that the voltage “above” with a change in absolute earthly time either rises or falls by 15%.

Figure. 9.3. Average daily variation of the atmospheric potential gradient in clear weather over the oceans.

General concepts

Compared to the electrical conductivity of conductors (see Section 2) and semiconductors (see Section 3), the electrical conductivity of dielectrics has a number of characteristic features.

All dielectrics, under the influence of a time-invariant voltage, pass some, albeit very insignificant, current, called leakage current (I), which consists of two components: volumetric current () and surface current () (Fig. 4.1).

Consequently, the total conductivity of the dielectric () is the sum of the volume () and surface () conductivities:

The reciprocal values ​​of the indicated conductivities are respectively called volume () and surface () resistances.

The next characteristic feature of the electrical conductivity of dielectrics is the gradual decrease in current over time (Fig. 4.2). When a dielectric is connected to a voltage that does not change over time, in the initial period of time a rapidly decreasing displacement current (I cm) flows in the circuit, the density of which is equal to:

This current decreases in a time of 10 13 ... 10 15 s on the order of the time constant () of the “source-sample” circuit. That is, to a first approximation, we can say that this current is determined by the charging of the geometric capacitance. However, the total current continues to change after this. This decline can last for several minutes or even hours and is due to redistribution of space charges , as well as the establishment of slow (mostly) and fast types of polarization. This falling part of the current is called absorption current ().

Over time, when the geometric capacitance is charged, i.e. all types of polarization will be established, a redistribution of space charges will occur, and an electric current that does not change over time will remain in the dielectric - through current (), which is due to surface and volumetric electrical conductivities:

When the specific resistance of dielectrics changes, the absorption current must be eliminated by keeping the sample under voltage for some time.

For a comparative assessment of various dielectrics in relation to their volumetric and surface electrical conductivity, the following values ​​are used: volumetric resistivity (), And specific surface resistance (). Based on specific and volumetric resistivity, it can be determined specific volume conductivity :

and in terms of specific surface resistance - specific surface conductivity :

The volume resistivity of a dielectric sample of arbitrary shape can be found from the expression:

where is the volume resistance of a sample of arbitrary shape, Ohm; – geometric parameter, m.

So, for a flat sample, for which (see Section 1), the resistivity is equal to:

where is the cross-sectional area of ​​the sample (the area of ​​the measuring electrode), m2; – sample thickness, m.

Volume conductivity () is measured in siemens per meter ().

Surface resistivity (in ohms) can be found from the expression:

, ………………..(4.6)

where is the surface resistance of the sample, Ohm; – length of electrodes, m; – distance between electrodes, m.

Specific surface conductivity is measured in siemens.

Electrical conductivity of gases

The electrical conductivity of gases is due to the presence of a certain amount of charged particles in them. Under normal conditions, the number of charged particles (gas ions or solid and liquid impurities in suspension) in 1 m 3 of atmospheric air does not exceed several tens of millions.

The origin of charge carriers in gases is explained by various factors:

· radioactive radiation of the Earth;

· radiation penetrating from outer space;

· radiation from the Sun;

· sometimes by thermal movement of molecules, etc.

When the energy of a bombarding particle is absorbed, the gas molecule loses an electron and becomes a positive ion. The electron released in this process “sticks” to the neutral molecule, forming a negative ion.

In some cases, the concentration of free charge carriers can reach very high values. This is usually due to photoionization of gas molecules. Such ionization can occur, for example, under the influence of ionizing radiation: X-rays and gamma rays, neutron fluxes, etc. Charged ions, as well as the surrounding gas molecules that do not have an electrical charge, undergo random thermal movements, and due to diffusion, the concentrations are equalized.

tration of ions in gas. When positive and negative ions meet, they recombine. In the stationary case, when the number of ions does not change over time, a dynamic equilibrium is established between the processes of generation and recombination of charged particles.

Let's calculate the specific conductivity of the gas. When an external electric field is applied, positive and negative ions, overcoming the frictional resistance of the gas, will move between the electrodes at speeds respectively:

where and are the mobility of positive and negative ions.

The relationship between the number of positive () and negative () ions present in 1 m 3 of gas and the number of ions recombining in 1 m 3 of gas in 1 s () can be represented as follows:

where is the recombination coefficient of gas ions, m 3 /s. For air, for example, m 3 / s.

In stationary case

,

So .

If the field strength (E) is very small, so that the flowing current does not change the concentration of ions in the gas, the current density can be determined from the expression:

Taking into account that , we obtain an expression for the specific conductivity of the gas:

. (4.9)

The specific conductivity of air in weak fields is about 10 -15 S/m.

From formula (4.8) it is clear that at low values ​​of the external electric field strength, when , , and can be considered constant, the current density in the gas is directly proportional to the applied field strength, i.e. under these conditions, Ohm's law is observed (Fig. 4.3, section 0A). However, with a further increase in the applied field strength due to an increase in the drift speed of ions, the probability of their recombination decreases, and basically all ions will rush towards the electrodes. This is the saturation current (section AB).

For air with a distance between electrodes of 0.01 m, saturation is achieved at a field strength of 0.5 V/m. The saturation current density in air (under normal conditions) is very small and reaches 10 -14 A/m 2.

Section 0AB is called the region of non-self-conductivity, since electrical conductivity (concentration of free charge carriers) is determined by the power of external ionizers.

The value of specific air resistance () is about 10 18 Ohm∙m. With a further increase in the field strength V/m (Fig. 4.3, section BC), a significant increase in current density occurs due to the processes impact ionization molecules by electrons in a strong electric field until the breakdown of the gas gap. Aircraft section is called area of ​​independent electrical conductivity .

Electrical conductivity of liquids

The electrical conductivity of liquids is due to ions formed during the dissociation of molecules of the liquid itself or its impurities. Due to the increase in the energy of the chaotic thermal movement of molecules, the degree of ionization and concentration of ions increases with increasing temperature according to an exponential law:

, (4.10)

where W is the dissociation energy. Hence the specific conductivity is:

where n is the charge of the ion; and are the mobility of positive and negative ions, respectively; A is a constant.

The logarithm of liquid conductivity decreases linearly with increasing reciprocal absolute temperature 1/T (Fig.

The specific resistance of liquids is:

, (4.12)

where B is a constant.

According to a similar law, the viscosity of liquids changes (). The dependence of liquids is explained by both a change and a change in the temperature dissociation of molecules.

The dissociation of molecules occurs more easily in polar liquids than in non-polar ones. Due to the fact that the dissociation energy of polar liquids is much lower than that of non-polar liquids, their specific conductivity is significantly higher. So, for highly polar liquids (distilled water, ethyl alcohol, acetone), for weakly polar (sovol, castor oil), for non-polar (benzene, transformer oil) Ohm∙m. In nonpolar liquids, the molecules of the main substance practically do not dissociate into ions, and their electrical conductivity is due to impurities of especially polar substances.

In liquids (and gases) with impurities it is sometimes observed molion conductivity , characteristic of colloidal systems , which are a close mixture of two phases of substances; moreover, one phase in the form of small particles (droplets, grains, dust particles, etc.) is evenly suspended in the other. Of the colloidal systems, they are most often found in electrical insulating technology. emulsions (both phases are liquids) and suspensions (dispersed phase – solid, dispersion medium – liquid). hundred

the potency of emulsions and suspensions, i.e. their ability to persist for a long time without the dispersed phase settling to the bottom of the vessel (or floating to the surface) due to the difference in the densities of both phases is explained by the presence of electrical charges on the surface of the particles of the dispersed phase (with the same charge, the particles repel each other). Such charged particles of the dispersed phase are called molions . When an electric field is applied to a colloidal system, the molions begin to move, which is expressed as electrophoresis .

Examples of the practical use of electrophoresis are the coating of metal objects with rubber and resins from their suspensions, the dehydration of various materials in an electric field, etc. Unlike electrolysis, the formation of new substances is not observed during electrophoresis, but only the relative concentration of the dispersed phase in different parts of the volume of the substance changes. Molion electrical conductivity is inherent in liquid varnishes and compounds, moistened oils, etc. Its contribution to conductivity, like the contribution of ionic electrical conductivity, depends on the viscosity of the liquid.

Electrical conductivity of solid dielectrics

The electrical conductivity of dielectrics, unlike the electrical conductivity of semiconductors, is most often not electronic, but ionic in nature. This is due to the fact that the band gap in dielectrics is that only a tiny number of electrons can be separated from their atoms due to thermal motion. Ions often turn out to be weakly bound at lattice sites, and the energy W required for their disruption is comparable to kT. For example, in a NaCl crystal eV, and the energy of separation of a sodium ion is eV. Therefore, despite the lower mobility of ions () compared to the mobility of electrons (), ionic conductivity turns out to be greater than electronic conductivity due to a significantly higher concentration of free ions:

. (4.13)

Charge carriers in dielectrics are usually small ions, whose mobility is higher:

· protons in hydrogen-containing compounds (in polymers, crystals like KH 2 PO 4 and others with hydrogen bonds);

· sodium ions (in NaCl and sodium-containing glass), etc.

It should be noted that the number of dissociated (plucked) ions () with a change in temperature changes according to an exponential law:

, (4.14)

where is the total number of ions of the i-th type; – dissociation energy of the i-th type ion; kT – thermal energy.

The specific electrical conductivity of solid dielectrics, like semiconductors, increases with increasing temperature according to an exponential law:

However, the dependence is often caused not only by an exponential increase in carrier concentration (Fig. 4.5, b)

but also by increased mobility:

µ~exp(-W n /kT),

where W n is the energy of movement of the ion, which determines its transition from one equilibrium state to another). This is due to the fact that the drift mobility of ions is small and occurs by jumping from trap to trap, separated by a potential barrier W n (the so-called “hopping” electrical conductivity). The probability of such thermal jumps is directly proportional to exp(-W n /kT) (Fig. 4.5, a).

Typically, a dielectric contains several types of charge carriers. For example, in addition to ions of the main substance, there may be weakly bound impurity ions. In this case, the specific conductivity is the sum of the intrinsic conductivity with the activation energy (W) and the impurity conductivity with the activation energy (W np):

; (4.16)

,

where is a coefficient combining constants ( – charge of the i-th carrier; – concentration of the i-th carrier; – mobility of the i-th carrier); W i is the activation energy.

In a wide temperature range, the dependence of the logarithm of specific conductivity (γ) on the reciprocal value of absolute temperature (T) should consist of two straight sections with different values ​​of the angle of inclination to the abscissa axis (Fig. 4.6). At a temperature above the break point A, the electrical conductivity is determined mainly by its own defects - this is the region high temperature , or own electrical conductivity . Below the break, in the area low temperature , or impurity electrical conductivity , the dependence is flatter.

In contrast to the difficult to reproduce low-temperature region of electrical conductivity, which is determined mainly by the nature and concentration of impurities, the value of intrinsic conductivity does not depend on specific conductivity and does not depend on impurities, is well reproducible and is a physical parameter of a given compound.

The temperature at which the inflection point is observed strongly depends on the degree of purity and perfection of the material. As the content of impurities and defects increases, the impurity conductivity increases and turns out to be significant at higher temperatures (Fig. 4.6). From the slopes of the plots of the dependence lines, one can determine the activation energy of charge carriers and their nature.

Ionic electrical conductivity is accompanied by the transfer of matter: positive ions move towards the cathode, and negative ions towards the anode. Electrolysis is especially pronounced at elevated temperatures, when ρ is small, and when high constant voltages are applied. Based on the substance released on the electrodes, the nature of the charge carriers can be determined. In dielectrics with purely ionic conductivity, Faraday's law is strictly observed - the law of proportionality between the amount of electricity passed and the amount of substances released.

Some dielectrics (for example, other titanium-containing ceramic materials) exhibit electronic or hole electrical conductivity. However, the carriers are often electrons not of the main substance, but of impurities and defects. In titanium-containing ceramics, during high-temperature synthesis, oxygen vacancies appear in significant quantities, giving up weakly bound electrons or holes. The observed electrical conductivity depends on them.

Solid porous dielectrics, in the presence of moisture in them, even in insignificant quantities, sharply increase their electrical conductivity (Figure 4.7). In the AB section of the curve, the resistance value decreases as a result of a change in the degree of dissociation of water molecules and dielectric molecules in an aqueous solution into ions. The BC section is caused by drying processes, and in the SD section, the dissociation of dielectric molecules into ions occurs.

We considered the electrical conductivity of solid dielectrics at relatively low electric field strengths. At sufficiently high electric field strengths, an electronic component of electrical conductivity appears in dielectrics, which rapidly increases with increasing electric field strength, and therefore a violation of Ohm’s law is observed. At electric field strengths V/m, i.e. close to breakdown field strengths, the dependence of electrical conductivity on the field strength obeys Poole’s law:

, (4.17)

For a number of dielectrics, Frenkel's law turns out to be more accurate:

, (4.18)

where is electrical conductivity in weak electric fields; – nonlinearity coefficients characterizing the properties of the dielectric; E – electric field strength.