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A message on the topic of electric current in gases. Electric current in gases: definition, features and interesting facts

1. Ionization, its essence and types.

The first condition for the existence of electric current is the presence of free charge carriers. In gases they arise as a result of ionization. Under the influence of ionization factors, an electron is separated from a neutral particle. The atom becomes a positive ion. Thus, 2 types of charge carriers arise: a positive ion and a free electron. If an electron joins a neutral atom, a negative ion appears, i.e. third type of charge carriers. Ionized gas is called a conductor of the third kind. There are 2 types of conductivity possible here: electronic and ionic. Simultaneously with the ionization processes, the reverse process occurs - recombination. To separate an electron from an atom, energy must be expended. If the energy is supplied from the outside, then the factors promoting ionization are called external (high temperature, ionizing radiation, ultraviolet radiation, strong magnetic fields). Depending on the ionization factors, it is called thermal ionization or photoionization. Ionization can also be caused by mechanical shock. Ionization factors are divided into natural and artificial. Natural is caused by radiation from the Sun and the radioactive background of the Earth. In addition to external ionization, there is internal ionization. It is divided into shock and step.

Impact ionization.

At a sufficiently high voltage, electrons accelerated by the field to high speeds themselves become a source of ionization. When such an electron hits a neutral atom, the electron is knocked out of the atom. This occurs when the energy of the electron causing ionization exceeds the ionization energy of the atom. The voltage between the electrodes must be sufficient for the electron to acquire the required energy. This voltage is called ionization voltage. It has its own meaning for everyone.

If the energy of a moving electron is less than necessary, then upon impact only the excitation of a neutral atom occurs. If a moving electron collides with a pre-excited atom, stepwise ionization occurs.

2. Non-self-sustaining gas discharge and its current-voltage characteristics.

Ionization leads to the fulfillment of the first condition for the existence of current, i.e. to the appearance of free charges. For a current to occur, the presence of an external force is necessary, which will force the charges to move in a direction, i.e. necessary electric field. Electric current in gases are accompanied by a number of phenomena: light, sound, the formation of ozone, nitrogen oxides. The set of phenomena accompanying the passage of current through gas - gas rank . The process of current flow itself is often referred to as a gas discharge.

A discharge is called non-self-sustaining if it exists only during the action of an external ionizer. In this case, after the termination of the external ionizer, no new charge carriers are formed, and the current stops. During a non-self-sustained discharge, the currents are small in magnitude, and there is no gas glow.

Independent gas discharge, its types and characteristics.

An independent gas discharge is a discharge that can exist after the action of the external ionizer ceases, i.e. due to impact ionization. In this case, light and sound phenomena are observed, and the current strength can increase significantly.

Types of self-discharge:

1. quiet discharge - follows directly after a non-self-sustaining one, the current strength does not exceed 1 mA, there are no sound or light phenomena. Used in physiotherapy, Geiger-Muller counters.

2. glow discharge. As the voltage increases, quiet turns into smoldering. It occurs at a certain voltage - ignition voltage. It depends on the type of gas. Neon has 60-80 V. It also depends on gas pressure. The glow discharge is accompanied by a glow; it is associated with recombination, which occurs with the release of energy. The color also depends on the type of gas. It is used in indicator lamps (neon, UV bactericidal, lighting, fluorescent).

3. arc discharge. The current strength is 10 - 100 A. Accompanied by an intense glow, the temperature in the gas-discharge gap reaches several thousand degrees. Ionization reaches almost 100%. 100% ionized gas - cold gas plasma. It has good conductivity. Used in high and ultra-high pressure mercury lamps.

4. A spark discharge is a type of arc discharge. This is a pulse-oscillatory discharge. In medicine, exposure to high-frequency vibrations is used. At high current densities, intense sound phenomena are observed.

5. corona discharge. This is a type of glow discharge. It is observed in places where there is a sharp change in the electric field strength. Here an avalanche of charges and a glow of gases appears - a corona.

It is formed by the directed movement of free electrons and that in this case no changes in the substance from which the conductor is made occur.

Such conductors in which the passage of electric current is not accompanied by chemical changes in their substance are called conductors of the first kind. These include all metals, coal and a number of other substances.

But there are also conductors of electric current in nature in which, during the passage of current, chemical phenomena. These conductors are called conductors of the second kind. These include mainly various solutions of acids, salts and alkalis in water.

If you pour water into a glass vessel and add a few drops of sulfuric acid (or some other acid or alkali), and then take two metal plates and connect conductors to them, lowering these plates into the vessel, and connect a current source to the other ends of the conductors through the switch and ammeter, then gas will be released from the solution, and it will continue continuously as long as the circuit is closed because acidified water is indeed a conductor. In addition, the plates will begin to become covered with gas bubbles. These bubbles will then break off the plates and come out.

When an electric current passes through a solution, chemical changes occur, resulting in the release of gas.

Conductors of the second kind are called electrolytes, and the phenomenon that occurs in an electrolyte when an electric current passes through it is.

Metal plates dipped into an electrolyte are called electrodes; one of them connected to positive pole current source is called the anode, and the other, connected to the negative pole, is the cathode.

What determines the passage of electric current in a liquid conductor? It turns out that in such solutions (electrolytes) molecules of acid (alkali, salt) under the action of a solvent (in in this case water) split into two components, and One particle of the molecule has a positive electrical charge, and the other has a negative one.

Molecular particles that have electric charge, are called ions. When an acid, salt or alkali is dissolved in water, a large number both positive and negative ions.

Now it should become clear why an electric current passed through the solution, because a voltage was created between the electrodes connected to the current source, in other words, one of them turned out to be positively charged and the other negatively. Under the influence of this potential difference, positive ions began to mix towards the negative electrode - the cathode, and negative ions - towards the anode.

Thus, the chaotic movement of ions became an ordered counter movement of negative ions in one direction and positive ones in the other. This process of charge transfer constitutes the flow of electric current through the electrolyte and occurs as long as there is a potential difference across the electrodes. With the disappearance of the potential difference, the current through the electrolyte stops, the ordered movement of ions is disrupted, and chaotic movement begins again.

As an example, let us consider the phenomenon of electrolysis when passing an electric current through a solution of copper sulfate CuSO4 with copper electrodes lowered into it.

The phenomenon of electrolysis when current passes through a solution of copper sulfate: C - vessel with electrolyte, B - current source, C - switch

Here there will also be a counter movement of ions to the electrodes. The positive ion will be the copper ion (Cu), and the negative ion will be the acid residue ion (SO4). Copper ions, upon contact with the cathode, will be discharged (attaching the missing electrons), i.e., converted into neutral molecules of pure copper, and deposited on the cathode in the form of a thin (molecular) layer.

Negative ions, having reached the anode, are also discharged (they give up excess electrons). But at the same time they enter into chemical reaction with the copper of the anode, as a result of which a copper molecule Cu is added to the acidic residue SO4 and a molecule of copper sulfate CuS O4 is formed, which is returned back to the electrolyte.

Since this chemical process leaks long time, then copper is deposited on the cathode, released from the electrolyte. In this case, the electrolyte, instead of the copper molecules that went to the cathode, receives new copper molecules due to the dissolution of the second electrode - the anode.

The same process occurs if zinc electrodes are used instead of copper ones, and the electrolyte is a solution of zinc sulfate Zn SO4. Zinc will also be transferred from the anode to the cathode.

Thus, difference between electric current in metals and liquid conductors lies in the fact that in metals the charge carriers are only free electrons, i.e., negative charges, whereas in electrolytes it is carried by oppositely charged particles of the substance - ions moving in opposite directions. Therefore they say that Electrolytes exhibit ionic conductivity.

Electrolysis phenomenon was discovered in 1837 by B. S. Jacobi, who carried out numerous experiments on research and improvement of chemical current sources. Jacobi found that one of the electrodes placed in a solution of copper sulfate became coated with copper when an electric current passed through it.

This phenomenon is called electroplating, is now extremely large practical application. One example of this is coating metal objects with a thin layer of other metals, i.e. nickel plating, gold plating, silver plating, etc.

Gases (including air) do not conduct electric current under normal conditions. For example, naked ones, being suspended parallel to each other, find themselves isolated from one another by a layer of air.

However, under the influence of high temperature, large potential differences and other reasons, gases, like liquid conductors, are ionized, i.e., particles of gas molecules appear in them in large quantities, which, being carriers of electricity, facilitate the passage of electric current through the gas.

But at the same time, the ionization of a gas differs from the ionization of a liquid conductor. If in a liquid a molecule disintegrates into two charged parts, then in gases, under the influence of ionization, electrons are always separated from each molecule and an ion remains in the form of a positively charged part of the molecule.

As soon as the ionization of the gas stops, it will cease to be conductive, while the liquid always remains a conductor of electric current. Consequently, gas conductivity is a temporary phenomenon, depending on the action of external causes.

However, there is another one called arc discharge or simply an electric arc. The phenomenon of the electric arc was discovered at the beginning of the 19th century by the first Russian electrical engineer V.V. Petrov.

V.V. Petrov, performing numerous experiments, discovered that between the two charcoal connected to a current source, a continuous electrical discharge occurs through the air, accompanied by a bright light. In his writings, V.V. Petrov wrote that in this case “dark peace can be illuminated quite brightly.” This is how electric light was first obtained, which was practically applied by another Russian electrical engineer Pavel Nikolaevich Yablochkov.

The Yablochkov Candle, whose operation is based on the use of an electric arc, made a real revolution in electrical engineering in those days.

The arc discharge is still used as a light source today, for example in spotlights and projection devices. The high temperature of the arc discharge allows it to be used for. Currently, arc furnaces powered by current are very great strength, are used in a number of industries: for the smelting of steel, cast iron, ferroalloys, bronze, etc. And in 1882, N.N. Benardos first used an arc discharge for cutting and welding metal.

In gas-light tubes, fluorescent lamps, voltage stabilizers, the so-called glow gas discharge.

A spark discharge is used to measure large potential differences using a ball gap, the electrodes of which are two metal balls with a polished surface. The balls are moved apart and a measured potential difference is applied to them. Then the balls are brought closer together until a spark jumps between them. Knowing the diameter of the balls, the distance between them, pressure, temperature and air humidity, find the potential difference between the balls using special tables. This method can measure potential differences of the order of tens of thousands of volts with an accuracy of a few percent.

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 using the simplest experiments in 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 persists for a long time, quickly disappears when a lit match is inserted 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 electric discharge in a gas). Gas discharges are divided into two types: self-sustaining and non-self-sustaining.

Non-independent discharge. A discharge in a gas is called non-self-sustaining if an external source is required 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 attach to neutral atoms or molecules, turning them into negative ions.

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

Change in ion concentration over time due to constant source ionization and recombination processes can be described as follows. Let us assume that the ionization source creates positive ions and the same number of electrons per unit volume of gas per unit time. If there is no electric current in the gas and the departure of ions from the volume under consideration 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, i.e. proportional to . Therefore, the decrease in the number of ions per unit volume per unit time can be written in the form , where a is a constant value called the recombination coefficient.

If the introduced assumptions are valid, the balance equation for ions in a gas will be written in the form

We will not solve this differential equation in general view, but let's look at 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 when

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

After turning off the ionizer, the decrease in ion concentration is described by equation (1), in which you need to put as initial value concentrations

Rewriting this equation in the form after integration we get

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

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

Non-self-conductivity. The process of decrease in ion concentration after the ionizer stops working 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 reduce the electrical conductivity of the gas to zero in the absence of an ionizer.

To understand the laws 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 intensity 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 the 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 subject to 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. Thanks to the forces of resistance

are set on average constant speeds electrons and ions, proportional to the electric field strength E:

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

Electric current density, i.e., the charge transferred by electrons and ions per unit time through a unit area, is expressed through the concentration of electrons and ions, their charges and the speed of steady motion

Quasi-neutrality. Under ordinary 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 is satisfied

Current density during non-self-sustaining discharge. To obtain the law for the change in the concentration of current carriers over time during a non-self-sustaining discharge in a gas, it is necessary, along with the processes of ionization by an external source and recombination, to also take into account the escape of electrons and ions to the electrodes. The number of particles per unit time per electrode area from the volume is equal to. We obtain the rate of decrease in the concentration of such particles 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) allows us to find the dependence of the steady-state current density during a non-self-sustaining discharge on the applied voltage (or on the field strength E).

Two limiting cases are immediately visible.

Ohm's law. At low voltage, when in equation (9) the second term on the right side can be neglected, after which we obtain formulas (7) and 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), the first one (quadratic in terms of the terms on the right side) can be neglected. 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 talk about saturation current.

Both considered limiting cases can be studied without resorting to equation (9). However, in this way it is impossible to trace how, with increasing voltage, a transition occurs from Ohm’s law to a nonlinear dependence of current on voltage.

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, we can use expression (2), which, taking into account (7), 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 recombine noticeably during the 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 glass tube with two metal electrodes. The electrical diagram of such an installation is shown in Fig. 102. Mobility

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

In Fig. Figure 103 shows the dependence of the current strength 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-rays or ultraviolet rays, or using a weak radioactive drug. It is only essential that the external source of ions remains unchanged. The linear section of the OA current-voltage characteristic corresponds to the range of applicability of Ohm's law.

Rice. 102. Installation diagram for studying gas discharge

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

In a section, the current strength depends nonlinearly on voltage. Starting from point B, the current reaches saturation and remains constant over a certain area. All this corresponds to theoretical predictions.

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

Electron impact ionization. The increase in current during the transition from a non-self-sustaining discharge to a self-sustaining one occurs like an avalanche and is called 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 gas pressure and the distance between the electrodes.

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

high 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 electron mean free path 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, instead of one electron, two appear. They, in turn, are accelerated by the electric field and ionize atoms or molecules encountered along their path, etc. The process develops like an avalanche and is called an electron avalanche. The described ionization mechanism is called electron impact ionization.

Experimental proof that the ionization of neutral gas atoms occurs mainly due to the impacts of electrons, rather than 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 fall into the region of the strongest field near the filament. Experience shows that at the same voltage between the electrodes, the discharge current is greater when a positive potential is applied to the filament rather than to the outer cylinder. It is in this case that all free electrons creating a current necessarily pass through the region of the strongest field.

Emission of electrons from the cathode. A self-sustaining discharge can be stationary only if new free electrons constantly appear in the gas, since all electrons arising 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 bombardment by ions, but also independently when heated to a high temperature. This process is called thermionic emission, and can be considered as a kind of evaporation of electrons from a metal. Usually it occurs at temperatures when the evaporation of the cathode material itself is still small. In the case of a self-sustained gas discharge, the cathode usually does not heat up

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

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

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.

Glow 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. In the glow discharge there are four characteristic areas. These are the cathode dark space, the glow (or negative) glow, the Faraday dark space, and the glowing positive column, which 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 potential occurs, associated with a high concentration of positive ions at the boundary 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 region of the smoldering glow. The glow is caused by the recombination of ions and electrons into neutral atoms or molecules. A positive discharge column is characterized by a slight drop in potential and a glow caused by the return of excited atoms or gas molecules to the ground state.

Corona discharge. At relatively high pressures in the gas (on the order of atmospheric pressure) near 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 natural conditions on treetops, ship masts, etc. (“St. Elmo’s fire”). Corona discharge has to be taken into account in high voltage technology, when this discharge occurs around the wires of high-voltage power lines and leads to losses of electricity. Corona discharge finds useful practical application in electric precipitators for purifying industrial gases from impurities of solid and liquid particles.

As the voltage between the electrodes increases, the corona discharge turns into a spark discharge with complete breakdown of the gap between

electrodes. It looks like a bunch of bright zigzag branching channels, instantly piercing the discharge gap and whimsically replacing each other. A 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 reaches 5-105 A and the potential difference reaches 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. Specific practically important look An independent gas discharge is an electric arc. When two carbon or metal electrodes come into contact at the point of contact, a large amount of heat is released due to the high contact resistance. As a result, thermionic emission begins and when the electrodes move apart, a brightly glowing arc of highly ionized, highly conductive gas appears between them. 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. Electrons are emitted from the cathode K, which is heated by an electric current.

In Fig. Figure 105 shows the dependence of the current in the anode circuit on the accelerating voltage obtained in these experiments. This dependence is non-monotonic 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 did not have discrete stationary states, i.e., its internal energy could take on any values, 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 one.

During elastic collisions, the kinetic energy of electrons practically does not change, since the mass of the electron is much less than the mass of the 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, electron-atom collisions 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 pass to the anode and the current strength decreases sharply. It does not vanish only because some electrons reach the grid without experiencing inelastic collisions. The second and subsequent current maxima are obtained because at voltages that are multiples of 4.9 V, electrons on the way to the grid can experience several inelastic collisions with mercury atoms.

So, the electron acquires the energy necessary for an 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 the 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 with 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, not only the excitation potentials, but also the ionization potentials of a number of atoms were determined by the electron impact method.

Give an example of an experiment in electrostatics from which we can conclude that dry air is a good insulator.

Where are the insulating properties of air used in technology?

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

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 not make 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, indefinitely?

In your opinion, why are opposite signs chosen in the definitions of mobilities 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 a 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, each electrode receives charges of only one sign. How does this fit with the fact that in all areas series circuit Is the current the same?

Why do electrons, and not positive ions, play the largest role in the ionization of gas in a discharge due to collisions?

Describe characteristic features various types independent gas discharge.

Why do the results of Frank and Hertz's experiments indicate discreteness of atomic energy levels?

Describe physical processes happening in gas discharge tube in the experiments of Frank and Hertz, with increasing accelerating voltage.

Electric current in gases in normal conditions impossible. That is, at atmospheric humidity, pressure and temperature there are no charge carriers in the gas. This property of gas, particularly air, is used in overhead transmission lines and relay switches to provide electrical insulation.

But under certain conditions, a current can be observed in gases. Let's conduct an experiment. For it we need an air capacitor electrometer and connecting wires. First, let's connect the electrometer to the capacitor. Then we impart a charge to the capacitor plates. The electrometer will show the presence of this same charge. The air capacitor will hold a charge for some time. That is, there will be no current between its plates. This suggests that the air between the plates of the capacitor has dielectric properties.

Figure 1 - Charged capacitor connected to an electrometer

Next, bring a candle flame into the gap between the plates. In this case, we will see that the electrometer will show a decrease in the charge on the capacitor plates. That is, current flows in the gap between the plates. Why is this happening?

Figure 2 - Inserting a candle into the gap between the plates of a charged capacitor

Under normal conditions, gas molecules are electrically neutral. And they are not able to provide current. But as the temperature rises, the so-called ionization of the gas occurs, and it becomes a conductor. Positive and negative ions appear in the gas.

In order for an electron to be removed from a gas atom, work must be done against Coulomb forces. This requires energy. The atom receives this energy with increasing temperature. Since the kinetic energy of thermal motion is directly proportional to the temperature of the gas. Then, with its increase, molecules and atoms receive enough energy so that upon collision electrons are torn away from the atoms. Such an atom becomes a positive ion. The detached electron can attach itself to another atom and it will become a negative ion.

As a result, positive and negative ions, as well as electrons, appear in the gap between the plates. They all begin to move under the influence of the field created by the charges on the capacitor plates. Positive ions move towards the cathode. Negative ions and electrons tend to the anode. Thus, current is provided in the air gap.

The dependence of current on voltage does not obey Ohm's law in all areas. In the first section, this is true: with increasing voltage, the number of ions increases and, consequently, the current. Further, in the second section, saturation occurs, that is, with increasing voltage, the current does not increase. Because the concentration of ions is maximum and new ones simply appear out of nowhere.

Figure 3 - current-voltage characteristic of the air gap

In the third section, an increase in current is again observed with increasing voltage. This section is called a self-discharge. That is, to maintain current in the gas, third-party ionizers are no longer needed. This happens due to the fact that electrons at high voltage receive sufficient energy to knock other electrons out of atoms on their own. These electrons, in turn, knock out others, and so on. The process is proceeding like an avalanche. And the main conductivity in the gas is provided by electrons.

Under normal conditions, gases are dielectrics, because are made up of neutral atoms and molecules and do not have enough free charges. Gases only become conductors when they are ionized in some way. The process of ionization of gases involves the removal of one or more electrons from an atom for some reason. As a result, instead of a neutral atom, positive ion And electron.

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

Some of the resulting electrons can be captured by other neutral atoms, and then negatively charged ions.

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

Removing an electron from an atom requires the expenditure of a certain amount of 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. Thus, to remove the first electron from a nitrogen atom, the energy required is 14.5 eV, to remove the second electron - 29.5 eV, and to remove the third - 47.4 eV.

Factors causing gas ionization are called ionizers.

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

    Thermal ionization occurs as a result of the collision of atoms or molecules of a gas during high temperature, if the kinetic energy of the relative motion of 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 required to separate an electron from an atom is transferred to it by a radiation quantum.

    Electron impact ionization(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 reduction 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 to glow.

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

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

Gas discharge

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

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

Electric current in gases is called gas discharge.

The total current in the gas consists of two flows of charged particles: the flow going to the cathode and the flow directed to the anode.

Gases combine electronic conductivity, similar to the conductivity of metals, with ionic conductivity, similar to the conductivity of aqueous solutions or electrolyte melts.

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