Electric current in gases message. Electric current in gases: definition, features and interesting facts

In gases, there are non-self-sustaining and self-sustaining electrical discharges.

The phenomenon of the flow of electric current through a gas, observed only under the condition of any external influence on the gas, is called a non-self-sustained electric discharge. The process of detachment of an electron from an atom is called ionization of the atom. The minimum energy that must be expended to detach an electron from an atom is called the ionization energy. A partially or fully ionized gas, in which the densities of positive and negative charges are the same, is called plasma.

The carriers of electric current in non-self-sustained discharge are positive ions and negative electrons. The current-voltage characteristic is shown in fig. 54. In the field of OAB - a non-self-sustained discharge. In the BC region, the discharge becomes independent.

In self-discharge, one of the methods of ionization of atoms is electron impact ionization. Ionization by electron impact becomes possible when the electron acquires a kinetic energy W k at the mean free path A, sufficient to do the work of detaching the electron from the atom. Types of independent discharges in gases - spark, corona, arc and glow discharges.

spark discharge occurs between two electrodes charged with different charges and having a large potential difference. The voltage between oppositely charged bodies reaches up to 40,000 V. The spark discharge is short-term, its mechanism is electronic impact. Lightning is a type of spark discharge.

In highly inhomogeneous electric fields, formed, for example, between a tip and a plane or between a power line wire and the Earth's surface, a special form of self-sustained discharge in gases occurs, called corona discharge.

Electric arc discharge was discovered by the Russian scientist V.V. Petrov in 1802. When two electrodes made of coal come into contact at a voltage of 40-50 V, in some places there are areas of small cross section with high electrical resistance. These areas get very hot, emit electrons that ionize the atoms and molecules between the electrodes. The carriers of electric current in the arc are positively charged ions and electrons.

A discharge that occurs at reduced pressure is called glow discharge. As the pressure decreases, the mean free path of the electron increases, and during the time between collisions, it has time to acquire enough energy for ionization in electric field with less stress. The discharge is carried out by an electron-ion avalanche.

1. Ionization, its essence and types.

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

Impact ionization.

At a sufficiently high voltage, the electrons accelerated by the field to high speeds themselves become a source of ionization. When such an electron strikes a neutral atom, the electron is knocked out of the atom. This occurs when the energy of the electron causing the 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. Each has its own meaning.

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

2. Non-self-sustained gas discharge and its current-voltage characteristic.

Ionization leads to the fulfillment of the first condition for the existence of current, i.e. to the appearance of free charges. For the current to occur, an external force is required, which will make the charges move in a direction, i.e. an electric field is needed. An electric current in gases is 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 . Often, the process of passing current is called a gas discharge.

The 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 action of the external ionizer, no new charge carriers are formed, and the current stops. With 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 termination of the external ionizer, i.e. due to impact ionization. In this case, light and sound phenomena are observed, the current strength can increase significantly.

Types of self-discharge:

1. quiet discharge - follows directly after the non-self-sustained one, the current strength does not exceed 1 mA, there are no sound and light phenomena. It is used in physiotherapy, Geiger-Muller counters.

2. glow discharge. As the voltage increases, the 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 the gas pressure. The glow discharge is accompanied by a glow, it is associated with recombination, which goes with the release of energy. The color also depends on the type of gas. It is used in indicator lamps (neon, ultraviolet bactericidal, lighting, luminescent).

3. arc discharge. The current strength is 10 - 100 A. It is 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. She has good conductivity. It is used in mercury lamps of high and ultrahigh pressure.

4. Spark discharge is a kind of arc discharge. This is a pulse-oscillatory discharge. In medicine, the effect of high-frequency oscillations is used. At a high current density, intense sound phenomena are observed.

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

Physics abstract

on the topic:

"Electric current in gases".

Electric current in gases.

1. Electric discharge in gases.

All gases in their natural state do not conduct electricity. This can be seen from the following experience:

Let's take an electrometer with disks of a flat capacitor attached to it and charge it. At room temperature if the air is dry enough, the capacitor does not noticeably discharge - the position of the electrometer needle does not change. To notice a decrease in the angle of deviation of the electrometer needle, it is required long time. This shows that electricity in the air between the disks is very small. This experience shows that air is a poor conductor of electric current.

Let's modify the experiment: let's heat the air between the discs with the flame of an alcohol lamp. Then the angle of deflection of the electrometer pointer rapidly decreases, i.e. the potential difference between the disks of the capacitor decreases - the capacitor is discharged. Consequently, the heated air between the discs has become a conductor, and an electric current is established in it.

The insulating properties of gases are explained by the fact that there are no free electric charges in them: the atoms and molecules of gases in their natural state are neutral.

2. Ionization of gases.

The above experience shows that charged particles appear in gases under the influence of high temperature. They arise as a result of the splitting off of one or more electrons from gas atoms, as a result of which a positive ion and electrons appear instead of a neutral atom. Part of the formed electrons can be captured by other neutral atoms, and then more negative ions will appear. The breakdown of gas molecules into electrons and positive ions is called ionization of gases.

Heating a gas to a high temperature is not the only way to ionize gas molecules or atoms. Gas ionization can occur under the influence of various external interactions: strong heating of the gas, x-rays, a-, b- and g-rays arising from radioactive decay, cosmic rays, bombardment of gas molecules by fast moving electrons or ions. The factors that cause gas ionization are called ionizers. The quantitative characteristic of the ionization process is ionization intensity, measured by the number of pairs of charged particles opposite in sign that appear in a unit volume of gas per unit time.

The ionization of an atom requires the expenditure of a certain energy - the ionization energy. To ionize an atom (or molecule), it is necessary to do work against the forces of interaction between the ejected electron and the rest of the particles of the atom (or molecule). This work is called the work of ionization A i . The value of the ionization work depends on chemical nature gas and energy state of an ejected electron in an atom or molecule.

After the termination of the ionizer, the number of ions in the gas decreases over time and eventually the ions disappear altogether. The disappearance of ions is explained by the fact that ions and electrons are involved in thermal motion and therefore collide with each other. When a positive ion and an electron collide, they can reunite into a neutral atom. In the same way, when a positive and negative ion collides, the negative ion can give up its excess electron to the positive ion and both ions will turn into neutral atoms. This process of mutual neutralization of ions is called ion recombination. When a positive ion and an electron or two ions recombine, a certain energy is released, equal to the energy spent on ionization. Partially, it is emitted in the form of light, and therefore the recombination of ions is accompanied by luminescence (luminescence of recombination).

In the phenomena of electric discharge in gases, the ionization of atoms by electron impacts plays an important role. This process consists in the fact that a moving electron, which has sufficient kinetic energy, knocks out one or more atomic electrons, as a result of which the neutral atom turns into a positive ion, and new electrons appear in the gas (this will be discussed later).

The table below gives the ionization energies of some atoms.

3. Mechanism of electrical conductivity of gases.

The mechanism of gas conductivity is similar to the mechanism of conductivity of electrolyte solutions and melts. In the absence of an external field, charged particles, like neutral molecules, move randomly. If ions and free electrons find themselves in an external electric field, then they come into directed motion and create an electric current in gases.

Thus, the electric current in a gas is a directed movement of positive ions to the cathode, and negative ions and electrons to the anode. The total current in the gas is composed of two streams of charged particles: the stream going to the anode and the stream directed to the cathode.

Neutralization of charged particles occurs on the electrodes, as in the case of the passage of electric current through solutions and melts of electrolytes. However, in gases there is no release of substances on the electrodes, as is the case in electrolyte solutions. Gas ions, approaching the electrodes, give them their charges, turn into neutral molecules and diffuse back into the gas.

Another difference in the electrical conductivity of ionized gases and solutions (melts) of electrolytes is that the negative charge during the passage of current through gases is transferred mainly not by negative ions, but by electrons, although conductivity due to negative ions can also play a certain role.

Thus, gases combine electronic conductivity, similar to the conductivity of metals, with ionic conductivity, similar to the conductivity of aqueous solutions and electrolyte melts.

4. Non-self-sustained gas discharge.

The process of passing an electric current through a gas is called a gas discharge. If the electrical conductivity of the gas is created by external ionizers, then the electric current arising in it is called non-self-sustaining gas discharge. With the termination of the action of external ionizers, the non-self-sustained discharge ceases. A non-self-sustaining gas discharge is not accompanied by gas glow.

Below is a graph of the dependence of the current strength on the voltage for a non-self-sustained discharge in a gas. A glass tube with two metal electrodes soldered into the glass was used to plot the graph. The chain is assembled as shown in the figure below.

At a certain voltage, there comes a moment at which all the charged particles formed in the gas by the ionizer in a second reach the electrodes in the same time. A further increase in voltage can no longer lead to an increase in the number of transported ions. The current reaches saturation (horizontal section of graph 1).

5. Independent gas discharge.

An electric discharge in a gas that persists after the termination of the action of an external ionizer is called independent gas discharge. For its implementation, it is necessary that as a result of the discharge itself, free charges are continuously formed in the gas. The main source of their occurrence is the impact ionization of gas molecules.

If, after reaching saturation, we continue to increase the potential difference between the electrodes, then the current strength at a sufficiently high voltage will increase sharply (graph 2).

This means that additional ions appear in the gas, which are formed due to the action of the ionizer. The current strength can increase hundreds and thousands of times, and the number of charged particles that appear during the discharge can become so large that an external ionizer is no longer needed to maintain the discharge. Therefore, the ionizer can now be removed.

What are the reasons for the sharp increase in current strength at high voltages? Let us consider any pair of charged particles (a positive ion and an electron) formed due to the action of an external ionizer. The free electron that appears in this way begins to move towards the positive electrode - the anode, and the positive ion - towards the cathode. On its way, the electron meets ions and neutral atoms. In the intervals between two successive collisions, the energy of the electron increases due to the work of the electric field forces.

The greater the potential difference between the electrodes, the greater the electric field strength. The kinetic energy of an electron before the next collision is proportional to the field strength and the free path of the electron: MV 2 /2=eEl. If the kinetic energy of an electron exceeds the work A i that needs to be done in order to ionize a neutral atom (or molecule), i.e. MV 2 >A i , then when an electron collides with an atom (or molecule), it is ionized. As a result, instead of one electron, two electrons appear (attacking on the atom and torn out of the atom). They, in turn, receive energy in the field and ionize the oncoming atoms, etc. As a result, the number of charged particles increases rapidly, and an electron avalanche arises. The described process is called electron impact ionization.

But ionization by electron impact alone cannot ensure the maintenance of an independent charge. Indeed, after all, all the electrons that arise in this way move towards the anode and, upon reaching the anode, "drop out of the game." To maintain the discharge requires the emission of electrons from the cathode ("emission" means "emission"). The emission of an electron can be due to several reasons.

Positive ions formed during the collision of electrons with neutral atoms, when moving towards the cathode, acquire a large kinetic energy under the action of the field. When such fast ions hit the cathode, electrons are knocked out from the cathode surface.

In addition, the cathode can emit electrons when heated to a high temperature. This process is called thermionic emission. It can be considered as the evaporation of electrons from the metal. In many solids thermionic emission occurs at temperatures at which the evaporation of the substance itself is still small. Such substances are used for the manufacture of cathodes.

During self-discharge, the cathode can be heated by bombarding it with positive ions. If the energy of the ions is not too high, then there is no knocking out of electrons from the cathode and electrons are emitted due to thermionic emission.

6. Various types of self-discharge and their technical application.

Depending on the properties and state of the gas, the nature and location of the electrodes, as well as on the voltage applied to the electrodes, different kinds independent rank. Let's consider a few of them.

A. Smoldering discharge.

A glow discharge is observed in gases at low pressures of the order of several tens of millimeters of mercury and less. If we consider a tube with a glow discharge, we can see that the main parts of a glow discharge are cathode Dark Space, far away from him negative or smoldering glow, which gradually passes into the area faraday dark space. These three regions form the cathode part of the discharge, followed by the main luminous part of the discharge, which determines its optical properties and is called positive column.

The main role in maintaining the glow discharge is played by the first two regions of its cathode part. characteristic feature This type of discharge is a sharp drop in the potential near the cathode, which is associated with a high concentration of positive ions at the boundary of regions I and II, due to the relatively low speed of ions at the cathode. In the cathode dark space, there is a strong acceleration of electrons and positive ions, knocking out electrons from the cathode. In the region of glowing glow, electrons produce intense impact ionization of gas molecules and lose their energy. Here, positive ions are formed, which are necessary to maintain the discharge. The electric field strength in this region is low. The smoldering glow is mainly caused by the recombination of ions and electrons. The length of the cathode dark space is determined by the properties of the gas and cathode material.

In the region of the positive column, the concentration of electrons and ions is approximately the same and very high, which causes a high electrical conductivity of the positive column and a slight drop in potential in it. The glow of the positive column is determined by the glow of excited gas molecules. Near the anode, a relatively sharp change in the potential is again observed, which is associated with the process of generation of positive ions. In some cases, the positive column breaks up into separate luminous areas - strata, separated by dark spaces.

The positive column does not play a significant role in maintaining the glow discharge; therefore, as the distance between the electrodes of the tube decreases, the length of the positive column decreases and it may disappear altogether. The situation is different with the length of the cathode dark space, which does not change when the electrodes approach each other. If the electrodes are so close that the distance between them becomes less than the length of the cathode dark space, then the glow discharge in the gas will stop. Experiments show that, other things being equal, the length d of the cathode dark space is inversely proportional to the gas pressure. Consequently, at sufficiently low pressures, electrons knocked out of the cathode by positive ions pass through the gas almost without collisions with its molecules, forming electronic, or cathode rays .

Glow discharge is used in gas-light tubes, fluorescent lamps, voltage stabilizers, to obtain electron and ion beams. If a slit is made in the cathode, then narrow ion beams pass through it into the space behind the cathode, often called channel rays. widely used phenomenon cathode sputtering, i.e. destruction of the cathode surface under the action of positive ions hitting it. Ultramicroscopic fragments of the cathode material fly in all directions along straight lines and cover the surface of bodies (especially dielectrics) placed in a tube with a thin layer. In this way, mirrors are made for a number of devices, applied thin layer metal on selenium photocells.

b. Corona discharge.

A corona discharge occurs at normal pressure in a gas in a highly inhomogeneous electric field (for example, near spikes or wires of high voltage lines). In a corona discharge, gas ionization and its glow occur only near the corona electrodes. In the case of cathode corona (negative corona), electrons that cause impact ionization of gas molecules are knocked out of the cathode when it is bombarded with positive ions. If the anode is corona (positive corona), then the birth of electrons occurs due to the photoionization of the gas near the anode. Corona is a harmful phenomenon, accompanied by current leakage and loss electrical energy. To reduce corona, the radius of curvature of the conductors is increased, and their surface is made as smooth as possible. At a sufficiently high voltage between the electrodes, the corona discharge turns into a spark.

At an increased voltage, the corona discharge on the tip takes the form of light lines emanating from the tip and alternating in time. These lines, having a series of kinks and bends, form a kind of brush, as a result of which such a discharge is called carpal .

A charged thundercloud induces on the Earth's surface beneath it electric charges opposite sign. A particularly large charge accumulates on the tips. Therefore, before a thunderstorm or during a thunderstorm, cones of light like brushes often flare up on the points and sharp corners of highly raised objects. Since ancient times, this glow has been called the fires of St. Elmo.

Especially often climbers become witnesses of this phenomenon. Sometimes even not only metal objects, but also the ends of the hair on the head are decorated with small luminous tassels.

Corona discharge has to be considered when dealing with high voltage. If there are protruding parts or very thin wires, corona discharge can start. This results in power leakage. The higher the voltage of the high-voltage line, the thicker the wires should be.

C. Spark discharge.

The spark discharge has the appearance of bright zigzag branching filaments-channels that penetrate the discharge gap and disappear, being replaced by new ones. Studies have shown that the spark discharge channels begin to grow sometimes from the positive electrode, sometimes from the negative, and sometimes from some point between the electrodes. This is explained by the fact that impact ionization in the case of a spark discharge occurs not over the entire volume of gas, but through individual channels passing in those places where the ion concentration accidentally turned out to be the highest. The spark discharge is accompanied by the release a large number warmth, bright glow of gas, crackling or thunder. All these phenomena are caused by electron and ion avalanches that occur in spark channels and lead to a huge increase in pressure, reaching 10 7 ¸10 8 Pa, and an increase in temperature up to 10,000 °C.

A typical example of a spark discharge is lightning. The main lightning channel has a diameter of 10 to 25 cm, and the lightning length can reach several kilometers. Max Strength The current of the lightning pulse reaches tens and hundreds of thousands of amperes.

With a small length of the discharge gap, the spark discharge causes a specific destruction of the anode, called erosion. This phenomenon was used in the electrospark method of cutting, drilling and other types of precision metal processing.

The spark gap is used as a surge protector in electrical transmission lines (for example, in telephone lines). If a strong short-term current passes near the line, then voltages and currents are induced in the wires of this line, which can destroy electrical installation and dangerous to human life. To avoid this, special fuses are used, consisting of two curved electrodes, one of which is connected to the line and the other is grounded. If the potential of the line relative to the ground increases greatly, then a spark discharge occurs between the electrodes, which, together with the air heated by it, rises up, lengthens and breaks.

Finally, an electric spark is used to measure large potential differences using ball gap, whose electrodes 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 together until a spark jumps between them. Knowing the diameter of the balls, the distance between them, the pressure, temperature and humidity of the air, they find the potential difference between the balls according to special tables. This method can be used to measure, to within a few percent, potential differences of the order of tens of thousands of volts.

D. Arc discharge.

The arc discharge was discovered by V. V. Petrov in 1802. This discharge is one of the forms of gas discharge, which occurs at a high current density and a relatively low voltage between the electrodes (on the order of several tens of volts). The main cause of the arc discharge is the intense emission of thermoelectrons by a hot cathode. These electrons are accelerating electric field and produce impact ionization of gas molecules, due to which electrical resistance the gas gap between the electrodes is relatively small. If we reduce the resistance of the external circuit, increase the current of the arc discharge, then the conductivity of the gas gap will increase so much that the voltage between the electrodes decreases. Therefore, the arc discharge is said to have a falling current-voltage characteristic. At atmospheric pressure, the cathode temperature reaches 3000 °C. Electrons, bombarding the anode, create a recess (crater) in it and heat it. The temperature of the crater is about 4000 °C, and at high air pressures it reaches 6000-7000 °C. The temperature of the gas in the arc discharge channel reaches 5000-6000 °C, so intense thermal ionization occurs in it.

In a number of cases, an arc discharge is also observed at a relatively low cathode temperature (for example, in a mercury arc lamp).

In 1876, P. N. Yablochkov first used an electric arc as a light source. In the "Yablochkov candle", the coals were arranged in parallel and separated by a curved layer, and their ends were connected by a conductive "ignition bridge". When the current was turned on, the ignition bridge burned out and formed between the coals electric arc. As the coals burned, the insulating layer evaporated.

The arc discharge is used as a source of light even today, for example, in searchlights and projectors.

The high temperature of the arc discharge makes it possible to use it for the construction of an arc furnace. At present, arc furnaces powered by a very high current are used in a number of industries: for the smelting of steel, cast iron, ferroalloys, bronze, the production of calcium carbide, nitrogen oxide, etc.

In 1882, N. N. Benardos first used an arc discharge for cutting and welding metal. The discharge between the fixed carbon electrode and the metal heats the junction of the two metal sheets(or plates) and welds them. Benardos used the same method for cutting metal plates and getting holes in them. In 1888, N. G. Slavyanov improved this welding method by replacing the carbon electrode with a metal one.

The arc discharge has found application in a mercury rectifier, which converts an alternating electric current into a direct current.

E. Plasma.

Plasma is a partially or fully ionized gas in which the densities of positive and negative charges are almost the same. Thus, plasma as a whole is an electrically neutral system.

The quantitative characteristic of plasma is the degree of ionization. The degree of plasma ionization a is the ratio of the volume concentration of charged particles to the total volume concentration of particles. Depending on the degree of ionization, plasma is divided into weakly ionized(a is fractions of a percent), partially ionized (a of the order of a few percent) and fully ionized (a is close to 100%). Weakly ionized plasma natural conditions are the upper layers of the atmosphere - the ionosphere. The sun, hot stars, and some interstellar clouds are fully ionized plasma that forms at high temperatures.

Medium energies various types the particles that make up the plasma can differ significantly from one another. Therefore, plasma cannot be characterized by a single value of temperature T; a distinction is made between the electron temperature T e, the ion temperature T i (or ion temperatures, if there are several kinds of ions in the plasma) and the temperature of neutral atoms T a (neutral component). Such a plasma is called non-isothermal, in contrast to isothermal plasma, in which the temperatures of all components are the same.

Plasma is also divided into high-temperature (T i »10 6 -10 8 K and more) and low-temperature!!! (T i<=10 5 К). Это условное разделение связано с особой влажностью высокотемпературной плазмы в связи с проблемой осуществления управляемого термоядерного синтеза.

Plasma has a number of specific properties, which allows us to consider it as a special fourth state of matter.

Due to the high mobility of charged plasma particles, they easily move under the influence of electric and magnetic fields. Therefore, any violation of the electrical neutrality of individual regions of the plasma, caused by the accumulation of particles of the same charge sign, is quickly eliminated. The resulting electric fields move charged particles until electrical neutrality is restored and the electric field becomes zero. In contrast to a neutral gas, where short-range forces exist between the molecules, Coulomb forces act between charged plasma particles, decreasing relatively slowly with distance. Each particle interacts immediately with a large number of surrounding particles. Due to this, along with chaotic thermal motion, plasma particles can participate in various ordered motions. Various types of oscillations and waves are easily excited in a plasma.

The plasma conductivity increases as the degree of ionization increases. At high temperatures, a fully ionized plasma approaches superconductors in its conductivity.

Low-temperature plasma is used in gas-discharge light sources - in luminous tubes for advertising inscriptions, in fluorescent lamps. A gas discharge lamp is used in many devices, for example, in gas lasers - quantum light sources.

High-temperature plasma is used in magnetohydrodynamic generators.

A new device, the plasma torch, has recently been created. The plasma torch creates powerful jets of dense low-temperature plasma, which are widely used in various fields of technology: for cutting and welding metals, drilling wells in hard rocks, etc.

List of used literature:

1) Physics: Electrodynamics. 10-11 cells: textbook. for in-depth study of physics / G. Ya. Myakishev, A. Z. Sinyakov, B. A. Slobodskov. - 2nd edition - M.: Drofa, 1998. - 480 p.

2) Physics course (in three volumes). T. II. electricity and magnetism. Proc. manual for technical colleges. / Detlaf A.A., Yavorsky B. M., Milkovskaya L. B. Izd. 4th, revised. - M.: Higher School, 1977. - 375 p.

3) Electricity./E. G. Kalashnikov. Ed. "Science", Moscow, 1977.

4) Physics./B. B. Bukhovtsev, Yu. L. Klimontovich, G. Ya. Myakishev. 3rd edition, revised. – M.: Enlightenment, 1986.

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 do not occur.

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

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

When an electric current passes through the solution, chemical changes occur, as a result of which gas is released.

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

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

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

The particles of a molecule that have an electric charge are called ions. When an acid, salt or alkali is dissolved in water, a large number of both positive and negative ions appear in the solution.

Now it should become clear why an electric current passed through the solution, because between the electrodes connected to the current source, it was created, 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 move towards the negative electrode - the cathode, and negative ions - towards the anode.

Thus, the chaotic movement of ions has become an ordered counter-movement of negative ions in one direction and positive ones in the other. This charge transfer process 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 orderly movement of ions is disturbed, and chaotic movement sets in again.

As an example, consider the phenomenon of electrolysis when an electric current is passed through a solution of copper sulphate CuSO4 with copper electrodes lowered into it.

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

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

Negative ions, having reached the anode, are also discharged (give away excess electrons). But at the same time, they enter into a chemical reaction with the copper of the anode, as a result of which a molecule of copper Cu is attached 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 takes a long time, copper is deposited on the cathode, which is released from the electrolyte. In this case, instead of the copper molecules that have gone to the cathode, the electrolyte receives new copper molecules due to the dissolution of the second electrode - the anode.

The same process occurs if zinc electrodes are taken instead of copper ones, and the electrolyte is a solution of zinc sulfate ZnSO4. 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 only free electrons, i.e., negative charges, are charge carriers, while in electrolytes it is carried by oppositely charged particles of matter - ions moving in opposite directions. Therefore they say that electrolytes have ionic conductivity.

The phenomenon of electrolysis was discovered in 1837 by B. S. Jacobi, who carried out numerous experiments on the study and improvement of chemical current sources. Jacobi found that one of the electrodes placed in a solution of copper sulphate, when an electric current passes through it, is covered with copper.

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

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

However, under the influence of high temperature, a large potential difference, and other reasons, gases, like liquid conductors, ionize, i.e., particles of gas molecules appear in them in large numbers, which, being carriers of electricity, contribute to 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 breaks up into two charged parts, then in gases, under the action of ionization, electrons are always separated from each molecule and an ion remains in the form of a positively charged part of the molecule.

One has only to stop the ionization of the gas, as it ceases to be conductive, while the liquid always remains a conductor of electric current. Consequently, the conductivity of a gas is a temporary phenomenon, depending on the action of external factors.

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

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

"Yablochkov's Candle", whose work is based on the use of an electric arc, made a real revolution in electrical engineering in those days.

The arc discharge is used as a source of light even today, for example, in searchlights and projectors. The high temperature of the arc discharge allows it to be used for . At present, arc furnaces powered by a very high current 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, to obtain electron and ion beams, the so-called glow gas discharge.

A spark discharge is used to measure large potential differences using a spherical spark 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 together until a spark jumps between them. Knowing the diameter of the balls, the distance between them, the pressure, temperature and humidity of the air, they find the potential difference between the balls according to special tables. This method can be used to measure, to within a few percent, potential differences of the order of tens of thousands of volts.

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

But under certain conditions, a current can be observed in gases. Let's do an experiment. For him, we need an air capacitor electrometer and connecting wires. First, let's connect the electrometer to the capacitor. Then we will report the charge to the capacitor plates. The electrometer will show the presence of this very charge. The air capacitor will store a charge for a while. 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, we introduce a candle flame into the gap between the plates. At the same time, we will see that the electrometer will show a decrease in 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 with an increase in temperature, 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 break away from a gas atom, it is necessary to do work against the Coulomb forces. This requires energy. The atom gains this energy as the temperature increases. Since the kinetic energy of thermal motion is directly proportional to the temperature of the gas. Then, with its increase, the molecules and atoms receive enough energy so that electrons break off from the atoms when they collide. Such an atom becomes a positive ion. The detached electron can cling to another atom, then it will become a negative ion.

As a result, positive and negative ions, as well as electrons, appear in the gap between the plates. All of them begin to move under the action 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, a 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 so with an increase in voltage, the number of ions increases and, consequently, the current. Further, saturation occurs in the second section, that is, with an increase in voltage, the current does not increase. Because the concentration of ions is maximum and new ones appear simply from nowhere.

Figure 3 - current-voltage characteristic of the air gap

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