Phenomena of electric current in gases. Introduction

Under normal conditions, gases are dielectrics, because. consist of neutral atoms and molecules, and they do not have a sufficient number of free charges. Gases become conductors only when they are somehow ionized. The process of ionization of gases consists in the fact that under the influence of any reasons one or more electrons are detached from the atom. As a result, instead of a neutral atom, positive ion and electron.

The breakdown of molecules into ions and electrons is called gas ionization.

Part of the formed electrons can be captured by other neutral atoms, and then appear negatively charged ions.

Thus, there are three types of charge carriers in an ionized gas: electrons, positive ions, and negative ones.

The separation of an electron from an atom requires the expenditure of a certain energy - ionization energy W i . The ionization energy depends on the chemical nature of the gas and the energy state of the electron in the atom. So, for the detachment of the first electron from the nitrogen atom, an energy of 14.5 eV is spent, and for the detachment of the second electron - 29.5 eV, for the detachment of the third - 47.4 eV.

The factors that cause gas ionization are called ionizers.

There are three types of ionization: thermal ionization, photoionization and impact ionization.

Thermal ionization occurs as a result of a collision of atoms or molecules of a gas at high temperature, if the kinetic energy of the relative motion of the colliding particles exceeds the binding energy of an electron in an atom.

Photoionization occurs under the influence of electromagnetic radiation (ultraviolet, x-ray or γ-radiation), when the energy necessary to detach an electron from an atom is transferred to it by a radiation quantum.

Ionization by electron impact(or impact ionization) is the formation of positively charged ions as a result of collisions of atoms or molecules with fast electrons with high kinetic energy.

The process of gas ionization is always accompanied by the opposite process of recovery of neutral molecules from oppositely charged ions due to their electrical attraction. This phenomenon is called recombination. During recombination, energy is released equal to the energy spent on ionization. This can cause, for example, gas glow.

If the action of the ionizer is unchanged, then dynamic equilibrium is established in the ionized gas, in which as many molecules are restored per unit time as they decay into ions. In this case, the concentration of charged particles in the ionized gas remains unchanged. If, however, the action of the ionizer is stopped, then recombination will begin to prevail over ionization, and the number of ions will rapidly decrease to almost zero. Consequently, the presence of charged particles in a gas is a temporary phenomenon (as long as the ionizer is in operation).

In the absence of an external field, charged particles move randomly.

gas discharge

When an ionized gas is placed in electric field electric forces begin to act on free charges, and they drift parallel to the lines of tension: electrons and negative ions - to the anode, positive ions - to the cathode (Fig. 1). At the electrodes, ions turn into neutral atoms by donating or accepting electrons, thereby completing the circuit. An electric current is generated in the gas.

Electric current in gases is the directed movement of ions and electrons.

Electric current in gases is called gas discharge.

The total current in the gas is composed of two streams of charged particles: the stream going to the cathode and the stream directed to the anode.

In gases, electronic conductivity, similar to the conductivity of metals, is combined with ionic conductivity, similar to the conductivity of aqueous solutions or electrolyte melts.

Thus, the conductivity of gases has ion-electronic character.

There are no absolute dielectrics in nature. The ordered movement of particles - carriers of electric charge - that is, current, can be caused in any medium, but this requires special conditions. We will consider here how electrical phenomena proceed in gases and how a gas can be changed from a very good dielectric into a very good conductor. We will be interested in the conditions under which it arises, and also in what features the electric current in gases is characterized.

Electrical properties of gases

A dielectric is a substance (medium) in which the concentration of particles - free carriers of an electric charge - does not reach any significant value, as a result of which the conductivity is negligible. All gases are good dielectrics. Their insulating properties are used everywhere. For example, in any circuit breaker, the opening of the circuit occurs when the contacts are brought into such a position that an air gap forms between them. Wires in power lines are also isolated from each other by an air layer.

The structural unit of any gas is a molecule. It consists of atomic nuclei and electronic clouds, that is, it is a collection electric charges distributed in some way in space. A gas molecule can be due to the peculiarities of its structure or be polarized under the action of an external electric field. The vast majority of the molecules that make up a gas are electrically neutral under normal conditions, since the charges in them cancel each other out.

If an electric field is applied to the gas, the molecules will assume a dipole orientation, occupying a spatial position that compensates for the effect of the field. The charged particles present in the gas under the influence of Coulomb forces will begin to move: positive ions - in the direction of the cathode, negative ions and electrons - towards the anode. However, if the field has insufficient potential, a single directed flow of charges does not arise, and one can rather speak of separate currents, so weak that they should be neglected. The gas behaves like a dielectric.

Thus, for the occurrence electric current in gases, a high concentration of free charge carriers and the presence of a field are required.

Ionization

The process of an avalanche-like increase in the number of free charges in a gas is called ionization. Accordingly, a gas in which there is a significant amount of charged particles is called ionized. It is in such gases that an electric current is created.

The ionization process is associated with a violation of the neutrality of molecules. As a result of the detachment of an electron, positive ions appear, the attachment of an electron to a molecule leads to the formation of a negative ion. In addition, there are many free electrons in an ionized gas. Positive ions and especially electrons are the main charge carriers for electric current in gases.

Ionization occurs when a certain amount of energy is imparted to a particle. Thus, an external electron in the composition of a molecule, having received this energy, can leave the molecule. Mutual collisions of charged particles with neutral ones lead to the knocking out of new electrons, and the process takes avalanche character. The kinetic energy of the particles also increases, which greatly promotes ionization.

Where does the energy expended on the excitation of electric current in gases come from? Ionization of gases has several sources of energy, according to which it is customary to name its types.

Ionization electric field. In this case, the potential energy of the field is converted into the kinetic energy of the particles.
Thermal ionization. An increase in temperature also leads to the formation of a large number of free charges.
Photoionization. The essence of this process is that quanta impart energy to electrons electromagnetic radiation- photons, if they have a sufficiently high frequency (ultraviolet, x-ray, gamma quanta).
Impact ionization is the result of the conversion of the kinetic energy of colliding particles into the energy of electron detachment. Along with thermal ionization, it serves as the main factor in the excitation of electric current in gases.

Each gas is characterized by a certain threshold value - the ionization energy necessary for an electron to break away from a molecule, overcoming a potential barrier. This value for the first electron ranges from several volts to two tens of volts; more energy is needed to detach the next electron from the molecule, and so on.

It should be borne in mind that simultaneously with ionization in the gas, the reverse process occurs - recombination, that is, the restoration of neutral molecules under the action of the Coulomb forces of attraction.

Gas discharge and its types

So, the electric current in gases is due to the ordered movement of charged particles under the action of an electric field applied to them. The presence of such charges, in turn, is possible due to various ionization factors.

Thus, thermal ionization requires significant temperatures, but an open flame in connection with some chemical processes contributes to ionization. Even at a relatively low temperature in the presence of a flame, the appearance of an electric current in gases is recorded, and experiment with gas conductivity makes it easy to verify this. It is necessary to place the flame of a burner or candle between the plates of a charged capacitor. The circuit previously open due to the air gap in the capacitor will close. A galvanometer connected to the circuit will show the presence of current.

Electric current in gases is called a gas discharge. It must be borne in mind that in order to maintain the stability of the discharge, the action of the ionizer must be constant, since due to constant recombination, the gas loses its electrically conductive properties. Some carriers of electric current in gases - ions - are neutralized on the electrodes, others - electrons - getting to the anode, are sent to the "plus" of the field source. If the ionizing factor ceases to operate, the gas will immediately become a dielectric again, and the current will cease. Such a current, dependent on the action of an external ionizer, is called a non-self-sustaining discharge.

Features of the passage of electric current through gases are described by a special dependence of the current strength on voltage - the current-voltage characteristic.

Let us consider the development of a gas discharge on the graph of the current-voltage dependence. When the voltage rises to a certain value U 1, the current increases in proportion to it, that is, Ohm's law is fulfilled. The kinetic energy increases, and hence the velocity of charges in the gas, and this process is ahead of recombination. At voltage values from U 1 to U 2, this relationship is violated; when U 2 is reached, all charge carriers reach the electrodes without having time to recombine. All free charges are involved, and a further increase in voltage does not lead to an increase in current. This nature of the movement of charges is called saturation current. Thus, we can say that the electric current in gases is also due to the peculiarities of the behavior of an ionized gas in electric fields of various strengths.

When the potential difference across the electrodes reaches certain value U 3 , the voltage becomes sufficient for the electric field to cause an avalanche-like ionization of the gas. The kinetic energy of free electrons is already enough for impact ionization of molecules. At the same time, their speed in most gases is about 2000 km / s and higher (it is calculated by the approximate formula v=600 U i , where U i is the ionization potential). At this moment, a gas breakdown occurs and a significant increase in current occurs due to an internal ionization source. Therefore, such a discharge is called independent.

The presence of an external ionizer in this case no longer plays a role in maintaining an electric current in the gases. Self discharge in different conditions and with different characteristics of the source of the electric field, it can have certain features. There are such types of self-discharge as glow, spark, arc and corona. We will look at how electric current behaves in gases, briefly for each of these types.

A potential difference from 100 (and even less) to 1000 volts is enough to initiate a self-discharge. Therefore, a glow discharge, characterized by a low current strength (from 10 -5 A to 1 A), occurs at pressures of no more than a few millimeters of mercury.

In a tube with a rarefied gas and cold electrodes, the emerging glow discharge looks like a thin luminous cord between the electrodes. If we continue pumping the gas out of the tube, the filament will be washed out, and at pressures of tenths of millimeters of mercury, the glow fills the tube almost completely. The glow is absent near the cathode - in the so-called dark cathode space. The rest is called the positive column. In this case, the main processes that ensure the existence of the discharge are localized precisely in the dark cathode space and in the region adjacent to it. Here, charged gas particles are accelerated, knocking out electrons from the cathode.

In a glow discharge, the cause of ionization is electron emission from the cathode. The electrons emitted by the cathode produce impact ionization of gas molecules, the emerging positive ions cause secondary emission from the cathode, and so on. The glow of the positive column is mainly due to the recoil of photons by excited gas molecules, and different gases are characterized by a glow of a certain color. The positive column takes part in the formation of a glow discharge only as a section of the electrical circuit. If you bring the electrodes closer together, you can achieve the disappearance of the positive column, but the discharge will not stop. However, with a further reduction in the distance between the electrodes, the glow discharge cannot exist.

It should be noted that for of this type electric current in gases, the physics of some processes has not yet been fully elucidated. For example, the nature of the forces causing an increase in the current to expand the area on the cathode surface that takes part in the discharge remains unclear.

spark discharge

Spark breakdown has a pulsed character. It occurs at pressures close to normal atmospheric, in cases where the power of the electric field source is not enough to maintain a stationary discharge. In this case, the field strength is high and can reach 3 MV/m. The phenomenon is characterized by a sharp increase in the discharge electric current in the gas, at the same time the voltage drops extremely quickly, and the discharge stops. Then the potential difference increases again, and the whole process is repeated.

With this type of discharge, short-term spark channels are formed, the growth of which can begin from any point between the electrodes. This is due to the fact that impact ionization occurs randomly in places where this moment the largest concentration of ions. Near the spark channel, the gas heats up rapidly and undergoes thermal expansion, which causes acoustic waves. Therefore, the spark discharge is accompanied by crackling, as well as the release of heat and a bright glow. Avalanche ionization processes generate high pressures and temperatures up to 10,000 degrees and more in the spark channel.

The most striking example of a natural spark discharge is lightning. The diameter of the main lightning spark channel can range from a few centimeters to 4 m, and the channel length can reach 10 km. The magnitude of the current reaches 500 thousand amperes, and the potential difference between a thundercloud and the Earth's surface reaches a billion volts.

The longest lightning with a length of 321 km was observed in 2007 in Oklahoma, USA. The record holder for the duration was lightning, recorded in 2012 in the French Alps - it lasted over 7.7 seconds. When struck by lightning, the air can heat up to 30 thousand degrees, which is 6 times higher than the temperature of the visible surface of the Sun.

In cases where the power of the source of the electric field is large enough, the spark discharge develops into an arc discharge.

This type of self-sustained discharge is characterized by high current density and low (less than glow discharge) voltage. The breakdown distance is small due to the proximity of the electrodes. The discharge is initiated by the emission of an electron from the cathode surface (for metal atoms, the ionization potential is small compared to gas molecules). During a breakdown between the electrodes, conditions are created under which the gas conducts an electric current, and a spark discharge occurs, which closes the circuit. If the power of the voltage source is large enough, spark discharges turn into a stable electric arc.

Ionization during an arc discharge reaches almost 100%, the current strength is very high and can range from 10 to 100 amperes. At atmospheric pressure, the arc is capable of heating up to 5-6 thousand degrees, and the cathode - up to 3 thousand degrees, which leads to intense thermionic emission from its surface. The bombardment of the anode with electrons leads to partial destruction: a recess is formed on it - a crater with a temperature of about 4000 ° C. An increase in pressure causes an even greater increase in temperature.

When diluting the electrodes, the arc discharge remains stable up to a certain distance, which makes it possible to deal with it in those parts of electrical equipment where it is harmful due to the corrosion and burnout of contacts caused by it. These are devices such as high voltage and circuit breakers, contactors and others. One of the methods to combat the arc that occurs when the contacts open is the use of arc chutes based on the principle of arc extension. Many other methods are also used: shunting contacts, using materials with a high ionization potential, and so on.

The development of a corona discharge occurs at normal atmospheric pressure in sharply inhomogeneous fields for electrodes with a large curvature of the surface. These can be spiers, masts, wires, various elements of electrical equipment that have complex shape and even human hair. Such an electrode is called a corona electrode. Ionization processes and, accordingly, the glow of the gas take place only near it.

The corona can be formed both on the cathode (negative corona) when it is bombarded with ions, and on the anode (positive) as a result of photoionization. The negative corona, in which the ionization process is directed away from the electrode as a result of thermal emission, is characterized by an even glow. In the positive corona, streamers can be observed - luminous lines of a broken configuration that can turn into spark channels.

An example of a corona discharge in natural conditions are those arising on the tips of tall masts, treetops, and so on. They are formed at a high electric field strength in the atmosphere, often before a thunderstorm or during a snowstorm. In addition, they were fixed on the skin of aircraft that fell into a cloud of volcanic ash.

Corona discharge on the wires of power lines leads to significant losses of electricity. At a high voltage, a corona discharge can turn into an arc. They fight with him different ways, for example, by increasing the radius of curvature of the conductors.

Electric current in gases and plasma

A fully or partially ionized gas is called plasma and is considered the fourth state of matter. On the whole, plasma is electrically neutral, since the total charge of its constituent particles zero. This distinguishes it from other systems of charged particles, such as, for example, electron beams.

Under natural conditions, plasma is formed, as a rule, at high temperatures due to the collision of gas atoms at high speeds. The vast majority of baryonic matter in the Universe is in the state of plasma. These are stars, part of interstellar matter, intergalactic gas. The earth's ionosphere is also a rarefied, weakly ionized plasma.

The degree of ionization is an important characteristic of a plasma; its conductive properties depend on it. The degree of ionization is defined as the ratio of the number of ionized atoms to the total number of atoms per unit volume. The more ionized the plasma, the higher its electrical conductivity. In addition, it has high mobility.

We see, therefore, that the gases that conduct electricity within the discharge channel are nothing but plasma. Thus, glow and corona discharges are examples of cold plasma; a lightning spark channel or an electric arc are examples of a hot, almost completely ionized plasma.

Electric current in metals, liquids and gases - differences and similarities

Let us consider the features that characterize the gas discharge in comparison with the properties of the current in other media.

In metals, current is the directed movement of free electrons that does not entail chemical changes. Conductors of this type are called conductors of the first kind; these include, in addition to metals and alloys, coal, some salts and oxides. They are distinguished by electronic conductivity.

Conductors of the second kind are electrolytes, that is, liquid aqueous solutions of alkalis, acids and salts. The passage of current is associated with a chemical change in the electrolyte - electrolysis. Ions of a substance dissolved in water, under the action of a potential difference, move in opposite directions: positive cations - to the cathode, negative anions - to the anode. The process is accompanied by gas evolution or deposition of a metal layer on the cathode. Conductors of the second kind are characterized by ionic conductivity.

As for the conductivity of gases, it is, firstly, temporary, and secondly, it has signs of similarity and difference with each of them. So, the electric current in both electrolytes and gases is a drift of oppositely charged particles directed towards opposite electrodes. However, while electrolytes are characterized by purely ionic conductivity, in a gas discharge with a combination of electronic and ionic types of conductivity, the leading role belongs to electrons. Another difference between the electric current in liquids and gases is the nature of ionization. In an electrolyte, the molecules of a dissolved compound dissociate in water, but in a gas, the molecules do not break down, but only lose electrons. Therefore, the gas discharge, like the current in metals, is not associated with chemical changes.

The current in liquids and gases is also not the same. The conductivity of electrolytes as a whole obeys Ohm's law, but it is not observed during a gas discharge. The volt-ampere characteristic of gases has a much more complex character associated with the properties of the plasma.

Mention should also be made of the general distinguishing features electric current in gases and in vacuum. Vacuum is an almost perfect dielectric. "Almost" - because in vacuum, despite the absence (more precisely, an extremely low concentration) of free charge carriers, a current is also possible. But potential carriers are already present in the gas, they only need to be ionized. Charge carriers are brought into vacuum from matter. As a rule, this occurs in the process of electron emission, for example, when the cathode is heated (thermionic emission). But also in various types In gas discharges, emission, as we have seen, plays an important role.

The use of gas discharges in technology

O harmful effects certain categories have already been briefly discussed above. Now let's pay attention to the benefits that they bring in industry and in everyday life.

Glow discharge is used in electrical engineering (voltage stabilizers), in coating technology (cathode sputtering method based on the phenomenon of cathode corrosion). In electronics, it is used to produce ion and electron beams. A well-known area of application for glow discharges are fluorescent and so-called economical lamps and decorative neon and argon discharge tubes. In addition, the glow discharge is used in and in spectroscopy.

Spark discharge is used in fuses, in electroerosive methods of precision metal processing (spark cutting, drilling, and so on). But it is best known for its use in spark plugs of internal combustion engines and in household appliances(gas stoves).

The arc discharge, being first used in lighting technology as early as 1876 (Yablochkov's candle - "Russian light"), still serves as a light source - for example, in projectors and powerful spotlights. In electrical engineering, the arc is used in mercury rectifiers. In addition, it is used in electric welding, metal cutting, industrial electric furnaces for steel and alloy smelting.

Corona discharge finds application in electrostatic precipitators for ion gas purification, in meters elementary particles, in lightning rods, in air conditioning systems. Corona discharge also works in copiers and laser printers, where it charges and discharges a photosensitive drum and transfers powder from the drum to paper.

Thus, gas discharges of all types are widely used. Electric current in gases is successfully and effectively used in many areas of technology.

Under normal conditions, gases do not conduct electricity because their molecules are electrically neutral. For example, dry air is a good insulator, as we could verify with the help of the simplest experiments on electrostatics. However, air and other gases become conductors of electric current if ions are created in them in one way or another.

Rice. 100. Air becomes a conductor of electric current if it is ionized

The simplest experiment illustrating the conductivity of air during its ionization by a flame is shown in Fig. 100: The charge on the plates, which remains for a long time, quickly disappears when a lit match is introduced into the space between the plates.

Gas discharge. The process of passing an electric current through a gas is usually called a gas discharge (or an electric discharge in a gas). Gas discharges are divided into two types: independent and non-self-sustaining.

Non-self-sufficient category. A discharge in a gas is called non-self-sustaining if an external source is needed to maintain it.

ionization. Ions in a gas can arise under the influence of high temperatures, X-ray and ultraviolet radiation, radioactivity, cosmic rays, etc. In all these cases, one or more electrons are released from electron shell atom or molecule. As a result, positive ions and free electrons appear in the gas. The released electrons can join neutral atoms or molecules, turning them into negative ions.

Ionization and recombination. Along with the processes of ionization in the gas, reverse recombination processes also occur: connecting with each other, positive and negative ions or positive ions and electrons form neutral molecules or atoms.

The change in the ion concentration with time, due to a constant source of ionization and recombination processes, can be described as follows. Let us assume that the ionization source creates positive ions per unit volume of gas per unit time and the same number of electrons. If there is no electric current in the gas and the escape of ions from the considered volume due to diffusion can be neglected, then the only mechanism for reducing the ion concentration will be recombination.

Recombination occurs when a positive ion meets an electron. The number of such meetings is proportional to both the number of ions and the number of free electrons, that is, proportional to . Therefore, the decrease in the number of ions per unit volume per unit time can be written as , where a is a constant value called the recombination coefficient.

Under the validity of the introduced assumptions, the balance equation for ions in a gas can be written in the form

We will not solve this differential equation in general view, and consider some interesting special cases.

First of all, we note that the processes of ionization and recombination after some time should compensate each other and a constant concentration will be established in the gas, it can be seen that at

The stationary ion concentration is the greater, the more powerful the ionization source and the smaller the recombination coefficient a.

After turning off the ionizer, the decrease in the ion concentration is described by equation (1), in which it is necessary to take as the initial value of the concentration

Rewriting this equation in the form after integration, we obtain

The graph of this function is shown in Fig. 101. It is a hyperbola whose asymptotes are the time axis and the vertical line. Of course, physical meaning has only a section of the hyperbola corresponding to the values. Note the slow nature of the decrease in concentration with time in comparison with the processes of exponential decay that are often encountered in physics, which are realized when the rate of decrease of a quantity is proportional to the first power of the instantaneous value of this quantity.

Rice. 101. The decrease in the concentration of ions in the gas after turning off the ionization source

Non-self conduction. The process of decreasing the concentration of ions after the termination of the action of the ionizer is significantly accelerated if the gas is in an external electric field. By pulling electrons and ions onto the electrodes, the electric field can very quickly nullify the electrical conductivity of the gas in the absence of an ionizer.

To understand the regularities of a non-self-sustaining discharge, let us consider for simplicity the case when the current in a gas ionized by an external source flows between two flat electrodes parallel to each other. In this case, the ions and electrons are in a uniform electric field of strength E, equal to the ratio of the voltage applied to the electrodes to the distance between them.

Mobility of electrons and ions. With a constant applied voltage, a certain constant current strength 1 is established in the circuit. This means that electrons and ions in an ionized gas move at constant speeds. To explain this fact, we must assume that in addition to the constant accelerating force of the electric field, moving ions and electrons are affected by resistance forces that increase with increasing speed. These forces describe the average effect of collisions of electrons and ions with neutral atoms and gas molecules. Through the forces of resistance

average constant velocities of electrons and ions are established, proportional to the strength E of the electric field:

The coefficients of proportionality are called the electron and ion mobilities. The mobilities of ions and electrons have different meanings and depend on the type of gas, its density, temperature, etc.

The electric current density, i.e., the charge carried by electrons and ions per unit time through a unit area, is expressed in terms of the concentration of electrons and ions, their charges and the speed of steady motion

Quasi-neutrality. Under normal conditions, an ionized gas as a whole is electrically neutral, or, as they say, quasi-neutral, because in small volumes containing a relatively small number of electrons and ions, the condition of electrical neutrality may be violated. This means that the relation

Current density at non-self-sustained discharge. In order to obtain the law of change in the concentration of current carriers with time during a non-self-sustained discharge in a gas, it is necessary, along with the processes of ionization by an external source and recombination, to take into account also the escape of electrons and ions to the electrodes. The number of particles leaving per unit time per area electrode from the volume is equal to The rate of decrease in the concentration of such particles, we get by dividing this number by the volume of gas between the electrodes. Therefore, the balance equation instead of (1) in the presence of current will be written in the form

To establish the regime, when from (8) we obtain

Equation (9) makes it possible to find the dependence of the steady-state current density in a non-self-sustained discharge on the applied voltage (or on the field strength E).

Two limiting cases are visible directly.

Ohm's law. At low voltage, when in equation (9) we can neglect the second term on the right side, after which we obtain formulas (7), we have

The current density is proportional to the strength of the applied electric field. Thus, for a non-self-sustaining gas discharge in weak electric fields, Ohm's law is satisfied.

Saturation current. At a low concentration of electrons and ions in equation (9), we can neglect the first one (quadratic in terms of the terms on the right side. In this approximation, the current density vector is directed along the electric field strength, and its modulus

does not depend on the applied voltage. This result is valid for strong electric fields. In this case, we speak of saturation current.

Both considered limiting cases can be investigated without referring to equation (9). However, in this way it is impossible to trace how, as the voltage increases, the transition from Ohm's law to a nonlinear dependence of current on voltage occurs.

In the first limiting case, when the current is very small, the main mechanism for removing electrons and ions from the discharge region is recombination. Therefore, for the stationary concentration, expression (2) can be used, which, when (7) is taken into account, immediately gives formula (10). In the second limiting case, on the contrary, recombination is neglected. In a strong electric field, electrons and ions do not have time to noticeably recombine during the time of flight from one electrode to another if their concentration is sufficiently low. Then all the electrons and ions generated by the external source reach the electrodes and the total current density is equal to It is proportional to the length of the ionization chamber, since the total number of electrons and ions produced by the ionizer is proportional to I.

Experimental study of gas discharge. The conclusions of the theory of non-self-sustaining gas discharge are confirmed by experiments. To study a discharge in a gas, it is convenient to use a glass tube with two metal electrodes. The electrical circuit of such an installation is shown in fig. 102. Mobility

electrons and ions strongly depend on the gas pressure (inversely proportional to pressure), so it is convenient to carry out experiments at reduced pressure.

On fig. 103 shows the dependence of the current I in the tube on the voltage applied to the electrodes of the tube. Ionization in the tube can be created, for example, by X-ray or ultraviolet rays or with a weak radioactive drug. It is only essential that the external ion source remains unchanged.

Rice. 102. Diagram of an installation for studying a gas discharge

Rice. 103. Experimental current-voltage characteristic of a gas discharge

In the section, the current strength is non-linearly dependent on the voltage. Starting from point B, the current reaches saturation and remains constant for some distance. All this is consistent with theoretical predictions.

Self rank. However, at point C, the current begins to increase again, at first slowly, and then very sharply. This means that a new, internal source of ions has appeared in the gas. If we now remove the external source, then the discharge in the gas does not stop, i.e., it passes from a non-self-sustaining discharge into an independent one. With a self-discharge, the formation of new electrons and ions occurs as a result of internal processes in the gas itself.

Ionization by electron impact. The increase in current during the transition from a non-self-sustained discharge to an independent one occurs like an avalanche and is called the electrical breakdown of the gas. The voltage at which breakdown occurs is called the ignition voltage. It depends on the type of gas and on the product of the gas pressure and the distance between the electrodes.

The processes in the gas responsible for the avalanche-like increase in the current strength with increasing applied voltage are associated with the ionization of neutral atoms or molecules of the gas by free electrons accelerated by the electric field to a sufficient

big energies. The kinetic energy of an electron before the next collision with a neutral atom or molecule is proportional to the electric field strength E and the free path of the electron X:

If this energy is sufficient to ionize a neutral atom or molecule, i.e., exceeds the work of ionization

then when an electron collides with an atom or molecule, they are ionized. As a result, two electrons appear instead of one. They, in turn, are accelerated by an electric field and ionize the atoms or molecules encountered on their way, etc. The process develops like an avalanche and is called an electron avalanche. The described ionization mechanism is called electron impact ionization.

An experimental proof that the ionization of neutral gas atoms occurs mainly due to the impacts of electrons, and not of positive ions, was given by J. Townsend. He took an ionization chamber in the form of a cylindrical capacitor, the internal electrode of which was a thin metal thread stretched along the axis of the cylinder. In such a chamber, the accelerating electric field is highly inhomogeneous, and the main role in ionization is played by particles that enter the region of the strongest field near the filament. Experience shows that for the same voltage between the electrodes, the discharge current is greater when the positive potential is applied to the filament and not to the outer cylinder. It is in this case that all free electrons that create current necessarily pass through the region of the strongest field.

Emission of electrons from the cathode. A self-sustained discharge can be stationary only if new free electrons constantly appear in the gas, since all the electrons that appear in the avalanche reach the anode and are eliminated from the game. New electrons are knocked out of the cathode by positive ions, which, when moving towards the cathode, are also accelerated by the electric field and acquire sufficient energy for this.

The cathode can emit electrons not only as a result of ion bombardment, but also independently, when it is heated to a high temperature. This process is called thermionic emission, it can be considered as a kind of evaporation of electrons from the metal. Usually it occurs at such temperatures, when the evaporation of the cathode material itself is still small. In the case of a self-sustained gas discharge, the cathode is usually heated without

filament, as in vacuum tubes, but due to the release of heat when bombarded with positive ions. Therefore, the cathode emits electrons even when the energy of the ions is insufficient to knock out electrons.

A self-sustained discharge in a gas occurs not only as a result of a transition from a non-self-sustained discharge with increasing voltage and moving away external source ionization, but also with the direct application of a voltage exceeding the ignition threshold voltage. The theory shows that the smallest amount of ions, which are always present in a neutral gas, if only because of the natural radioactive background, is sufficient to ignite the discharge.

Depending on the properties and pressure of the gas, the configuration of the electrodes, and the voltage applied to the electrodes, various types of self-discharge are possible.

Smoldering discharge. At low pressures(tenths and hundredths of a millimeter of mercury) a glow discharge is observed in the tube. To ignite a glow discharge, a voltage of several hundred or even tens of volts is sufficient. Four characteristic regions can be distinguished in the glow discharge. These are the dark cathode space, the smoldering (or negative) glow, the Faraday dark space, and the luminous positive column that occupies most of the space between the anode and cathode.

The first three regions are located near the cathode. It is here that a sharp drop in the potential occurs, associated with a large concentration of positive ions at the border of the cathode dark space and the smoldering glow. Electrons accelerated in the region of the cathode dark space produce intense impact ionization in the glow region. The smoldering glow is due to the recombination of ions and electrons into neutral atoms or molecules. The positive column of the discharge is characterized by a slight drop in potential and a glow caused by the return of excited atoms or molecules of the gas to the ground state.

Corona discharge. At relatively high pressures in the gas (of the order of atmospheric pressure), near the pointed sections of the conductor, where the electric field is highly inhomogeneous, a discharge is observed, the luminous region of which resembles a corona. Corona discharge sometimes occurs in vivo on treetops, ship masts, etc. ("St. Elmo's fires"). Corona discharge has to be considered in high voltage engineering, when this discharge occurs around the wires of high-voltage power lines and leads to power losses. Useful practical use corona discharge is found in electrostatic precipitators for cleaning industrial gases from impurities of solid and liquid particles.

With an increase in the voltage between the electrodes, the corona discharge turns into a spark with a complete breakdown of the gap between

electrodes. It has the form of a beam of bright zigzag branching channels, instantly penetrating the discharge gap and whimsically replacing each other. The spark discharge is accompanied by the release of a large amount of heat, a bright bluish-white glow and strong crackling. It can be observed between the balls of the electrophore machine. An example of a giant spark discharge is natural lightning, where the current strength reaches 5-105 A, and the potential difference is 109 V.

Since the spark discharge occurs at atmospheric (and higher) pressure, the ignition voltage is very high: in dry air, with a distance between the electrodes of 1 cm, it is about 30 kV.

Electric arc. Practically specific important view self-gas discharge is an electric arc. When two carbon or metal electrodes come into contact at the point of their contact, a large number of heat due to high contact resistance. As a result, thermionic emission begins, and when the electrodes are moved apart between them, a brightly luminous arc arises from a highly ionized, well-conducting gas. The current strength even in a small arc reaches several amperes, and in a large arc - several hundred amperes at a voltage of about 50 V. The electric arc is widely used in technology as a powerful light source, in electric furnaces and for electric welding. a weak retarding field with a voltage of about 0.5 V. This field prevents slow electrons from reaching the anode. The electrons are emitted by the cathode K heated by electric current.

On fig. 105 shows the dependence of the current in the anode circuit on the accelerating voltage obtained in these experiments. This dependence has a nonmonotonic character with maxima at voltages multiple of 4.9 V.

Discreteness of atomic energy levels. This dependence of current on voltage can be explained only by the presence of discrete stationary states in mercury atoms. If the atom had no discrete stationary states, i.e., its internal energy could take any value, then inelastic collisions, accompanied by an increase in the internal energy of the atom, could occur at any electron energy. If there are discrete states, then collisions of electrons with atoms can only be elastic, as long as the energy of the electrons is insufficient to transfer the atom from the ground state to the lowest excited state.

During elastic collisions, the kinetic energy of electrons practically does not change, since the mass of an electron is much less than the mass of a mercury atom. Under these conditions, the number of electrons reaching the anode increases monotonically with increasing voltage. When the accelerating voltage reaches 4.9 V, the collisions of electrons with atoms become inelastic. The internal energy of the atoms increases abruptly, and the electron loses almost all of its kinetic energy as a result of the collision.

The retarding field also does not allow slow electrons to reach the anode, and the current decreases sharply. It does not vanish only because some of the electrons reach the grid without experiencing inelastic collisions. The second and subsequent maxima of the current strength are obtained because at voltages that are multiples of 4.9 V, the electrons on their way to the grid can experience several inelastic collisions with mercury atoms.

So, the electron acquires the energy necessary for inelastic collision only after passing through a potential difference of 4.9 V. This means that the internal energy of mercury atoms cannot change by an amount less than eV, which proves the discreteness of the energy spectrum of an atom. The validity of this conclusion is also confirmed by the fact that at a voltage of 4.9 V the discharge begins to glow: excited atoms during spontaneous

transitions to the ground state emit visible light, the frequency of which coincides with that calculated by the formula

In the classical experiments of Frank and Hertz, the electron impact method determined not only the excitation potentials, but also the ionization potentials of a number of atoms.

Give an example of an electrostatic experiment that shows that dry air is a good insulator.

Where is the insulating properties of air used in engineering?

What is a non-self-sustaining gas discharge? Under what conditions does it run?

Explain why the rate of decrease in concentration due to recombination is proportional to the square of the concentration of electrons and ions. Why can these concentrations be considered the same?

Why does it make no sense for the law of decreasing concentration expressed by formula (3) to introduce the concept of characteristic time, which is widely used for exponentially decaying processes, although in both cases the processes continue, generally speaking, for an infinitely long time?

Why do you think opposite signs are chosen in the definitions of mobility in formulas (4) for electrons and ions?

How does the current strength in a non-self-sustaining gas discharge depend on the applied voltage? Why does the transition from Ohm's law to saturation current occur with increasing voltage?

Electric current in a gas is carried out by both electrons and ions. However, charges of only one sign come to each of the electrodes. How does this agree with the fact that in all sections of a series circuit the current strength is the same?

Why do electrons rather than positive ions play the greatest role in gas ionization in a discharge due to collisions?

Describe characteristics various kinds independent gas discharge.

Why do the results of the experiments of Frank and Hertz testify to the discreteness of the energy levels of atoms?

Describe physical processes occurring in the gas-discharge tube in the experiments of Frank and Hertz, with an increase in the accelerating voltage.

Themes USE codifier : carriers of free electric charges in gases.

Under ordinary conditions, gases consist of electrically neutral atoms or molecules; There are almost no free charges in gases. Therefore gases are dielectrics- electric current does not pass through them.

We said "almost none", because in fact, in gases and, in particular, in the air, there is always a certain amount of free charged particles. They appear as a result of the ionizing effect of radiation of radioactive substances that make up the earth's crust, ultraviolet and X-rays the Sun, as well as cosmic rays - streams of high-energy particles penetrating the Earth's atmosphere from outer space. Later we will return to this fact and discuss its importance, but for now we will only note that under normal conditions the conductivity of gases, caused by the “natural” amount of free charges, is negligible and can be ignored.

The action of switches in electrical circuits is based on the insulating properties of the air gap (Fig. 1). For example, a small air gap in a light switch is enough to open an electrical circuit in your room.

Rice. 1 key

It is possible, however, to create such conditions under which an electric current will appear in the gas gap. Let's consider the following experience.

We charge the plates of the air capacitor and connect them to a sensitive galvanometer (Fig. 2, left). At room temperature and in not too humid air, the galvanometer will not show a noticeable current: our air gap, as we said, is not a conductor of electricity.

Rice. 2. The occurrence of current in the air

Now let's bring the flame of a burner or a candle into the gap between the plates of the capacitor (Fig. 2, on the right). Current appears! Why?

Free charges in a gas

The occurrence of an electric current between the condenser plates means that in the air, under the influence of a flame, free charges. What exactly?

Experience shows that electric current in gases is an ordered movement of charged particles. three types. This is electrons, positive ions and negative ions.

Let's see how these charges can appear in a gas.

As the temperature of the gas increases, the thermal vibrations of its particles - molecules or atoms - become more intense. The impacts of particles against each other reach such a force that ionization- decay of neutral particles into electrons and positive ions (Fig. 3).

Rice. 3. Ionization

Degree of ionization is the ratio of the number of decayed gas particles to the total initial number of particles. For example, if the degree of ionization is , then this means that the original gas particles have decayed into positive ions and electrons.

The degree of gas ionization depends on temperature and increases sharply with its increase. For hydrogen, for example, at a temperature below the degree of ionization does not exceed , and at a temperature above the degree of ionization is close to (that is, hydrogen is almost completely ionized (partially or completely ionized gas is called plasma)).

In addition to high temperature, there are other factors that cause gas ionization.

We have already mentioned them in passing: these are radioactive radiation, ultraviolet, x-rays and gamma rays, cosmic particles. Any such factor that causes the ionization of a gas is called ionizer.

Thus, ionization does not occur by itself, but under the influence of an ionizer.

At the same time, the reverse process recombination, that is, the reunion of an electron and a positive ion into a neutral particle (Fig. 4).

Rice. 4. Recombination

The reason for recombination is simple: it is the Coulomb attraction of oppositely charged electrons and ions. Rushing towards each other under the action of electrical forces, they meet and get the opportunity to form a neutral atom (or molecule - depending on the type of gas).

At a constant intensity of the ionizer action, a dynamic equilibrium is established: the average number of particles decaying per unit time is equal to the average number of recombining particles (in other words, the ionization rate is equal to the recombination rate). If the ionizer action is strengthened (for example, the temperature is increased), then the dynamic equilibrium will shift to direction of ionization, and the concentration of charged particles in the gas will increase. On the contrary, if you turn off the ionizer, then recombination will begin to prevail, and free charges will gradually disappear completely.

So, positive ions and electrons appear in the gas as a result of ionization. Where does the third kind of charges come from - negative ions? Very simple: an electron can fly into a neutral atom and join it! This process is shown in Fig. 5 .

Rice. 5. The appearance of a negative ion

The negative ions formed in this way will participate in the creation of the current along with positive ions and electrons.

Non-self discharge

If there is no external electric field, then free charges perform chaotic thermal motion along with neutral gas particles. But when an electric field is applied, the ordered movement of charged particles begins - electric current in gas.

Rice. 6. Non-self-sustained discharge

On fig. 6 we see three types of charged particles arising in the gas gap under the action of an ionizer: positive ions, negative ions and electrons. An electric current in a gas is formed as a result of the oncoming movement of charged particles: positive ions - to the negative electrode (cathode), electrons and negative ions - to the positive electrode (anode).

Electrons, falling on the positive anode, are sent along the circuit to the "plus" of the current source. Negative ions donate an extra electron to the anode and, having become neutral particles, return to the gas; the electron given to the anode also rushes to the “plus” of the source. Positive ions, coming to the cathode, take electrons from there; the resulting shortage of electrons at the cathode is immediately compensated by their delivery there from the “minus” of the source. As a result of these processes, an ordered movement of electrons occurs in the external circuit. This is the electric current recorded by the galvanometer.

The process described in Fig. 6 is called non-self-sustained discharge in gas. Why dependent? Therefore, to maintain it, it is necessary permanent action ionizer. Let's remove the ionizer - and the current will stop, since the mechanism that ensures the appearance of free charges in the gas gap will disappear. The space between the anode and cathode will again become an insulator.

Volt-ampere characteristic of gas discharge

The dependence of the current strength through the gas gap on the voltage between the anode and cathode (the so-called current-voltage characteristic of gas discharge) is shown in Fig. 7.

Rice. 7. Volt-ampere characteristic of gas discharge

At zero voltage, the current strength, of course, is equal to zero: charged particles perform only thermal movement, there is no ordered movement between the electrodes.

With a small voltage, the current strength is also small. The fact is that not all charged particles are destined to get to the electrodes: some of the positive ions and electrons find each other and recombine in the process of their movement.

As the voltage increases, free charges develop more and more speed, and the less chance a positive ion and an electron have to meet and recombine. Therefore, an increasing part of the charged particles reaches the electrodes, and the current strength increases (section ).

At a certain voltage value (point ), the charge velocity becomes so high that recombination does not have time to occur at all. From now on all charged particles formed under the action of the ionizer reach the electrodes, and current reaches saturation- Namely, the current strength ceases to change with increasing voltage. This will continue up to a certain point.

self-discharge

After passing the point, the current strength increases sharply with increasing voltage - begins independent discharge. Now we will figure out what it is.

Charged gas particles move from collision to collision; in the intervals between collisions, they are accelerated by an electric field, increasing their kinetic energy. And now, when the voltage becomes large enough (that very point), the electrons during their free path reach such energies that when they collide with neutral atoms, they ionize them! (Using the laws of conservation of momentum and energy, it can be shown that it is electrons (and not ions) accelerated by an electric field that have the maximum ability to ionize atoms.)

The so-called electron impact ionization. Electrons knocked out of ionized atoms are also accelerated by the electric field and hit new atoms, ionizing them now and generating new electrons. As a result of the emerging electron avalanche, the number of ionized atoms rapidly increases, as a result of which the current strength also increases rapidly.

The number of free charges becomes so large that the need for an external ionizer is eliminated. It can be simply removed. Free charged particles are now spawned as a result of internal processes occurring in the gas - that's why the discharge is called independent.

If the gas gap is under high voltage, then no ionizer is needed for self-discharge. It is enough to find only one free electron in the gas, and the above-described electron avalanche will begin. And there will always be at least one free electron!

Let us recall once again that in a gas, even under normal conditions, there is a certain “natural” amount of free charges, due to the ionizing radioactive radiation of the earth's crust, high-frequency radiation from the Sun, and cosmic rays. We have seen that at low voltages the conductivity of the gas caused by these free charges is negligible, but now - at a high voltage - they will give rise to an avalanche of new particles, giving rise to an independent discharge. It will happen as they say breakdown gas gap.

The field strength required to break down dry air is approximately kV/cm. In other words, in order for a spark to jump between the electrodes separated by a centimeter of air, a kilovolt voltage must be applied to them. Imagine what voltage is needed to break through several kilometers of air! But it is precisely such breakdowns that occur during a thunderstorm - these are lightning well known to you.

This is a short summary.

Work on the full version continues

Lecture2 1

Current in gases

1. General Provisions

Definition: The phenomenon of the passage of electric current in gases is called gas discharge.

The behavior of gases is highly dependent on its parameters, such as temperature and pressure, and these parameters change quite easily. Therefore, the flow of electric current in gases is more complex than in metals or in a vacuum.

Gases do not obey Ohm's law.

2. Ionization and recombination

A gas under normal conditions consists of practically neutral molecules, therefore, it is an extremely poor conductor of electric current. However, under external influences, an electron can come off the atom and a positively charged ion appears. In addition, an electron can join a neutral atom and form a negatively charged ion. Thus, it is possible to obtain an ionized gas, i.e. plasma.

External influences include heating, irradiation with energetic photons, bombardment by other particles, and strong fields, i.e. the same conditions that are necessary for elemental emission.

An electron in an atom is in a potential well, and in order to escape from there, it is necessary to impart additional energy to the atom, which is called the ionization energy.

Substance	Ionization energy, eV
hydrogen atom	13,59
Hydrogen molecule	15,43
Helium	24,58
oxygen atom	13,614
oxygen molecule	12,06

Along with the phenomenon of ionization, the phenomenon of recombination is also observed, i.e. the union of an electron and a positive ion to form a neutral atom. This process occurs with the release of energy equal to the ionization energy. This energy can be used for radiation or heating. Local heating of the gas leads to a local change in pressure. Which in turn leads to sound waves. Thus, the gas discharge is accompanied by light, thermal and noise effects.

3. CVC of a gas discharge.

At the initial stages, the action of an external ionizer is necessary.

In the BAW section, the current exists under the action of an external ionizer and quickly reaches saturation when all ionized particles participate in the current generation. If you remove the external ionizer, the current stops.

This type of discharge is called a non-self-sustaining gas discharge. When you try to increase the voltage in the gas, an avalanche of electrons appears, and the current increases at a practically constant voltage, which is called the ignition voltage (BC).

From this moment on, the discharge becomes independent and there is no need for an external ionizer. The number of ions can become so large that the resistance of the interelectrode gap decreases and, accordingly, the voltage (SD) drops.

Then, in the interelectrode gap, the region of current passage begins to narrow, and the resistance increases, and, consequently, the voltage (DE) increases.

When you try to increase the voltage, the gas becomes fully ionized. The resistance and voltage drops to zero, and the current rises many times over. It turns out an arc discharge (EF).

CVC shows that the gas does not obey Ohm's law at all.

4. Processes in gas