What particles create electric current in gases. Introduction
Abstract on physics
on the topic of:
"Electric current in gases."
Electric current in gases.
1. Electric discharge in gases.
All gases in natural state do not conduct electric current. As can be seen from the following experience:
Let's take an electrometer with the 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 deflection of the electrometer needle, you need 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: heat the air between the disks with the flame of an alcohol lamp. Then the angle of deflection of the electrometer needle quickly decreases, i.e. the potential difference between the capacitor disks decreases - the capacitor is discharged. Consequently, the heated air between the disks has become a conductor, and an electric current is established in it.
The insulating properties of gases are explained by the fact that they have no free electrical charges: atoms and molecules of gases in their natural state are neutral.
2. Ionization of gases.
The experience described above shows that charged particles appear in gases under the influence of high temperature. They arise due to the detachment of one or more electrons from gas atoms, as a result of which a positive ion and electrons appear instead of a neutral atom. Some of the resulting 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 ionization of 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. Factors causing gas ionization are called ionizers. A quantitative characteristic of the ionization process is ionization intensity, measured by the number of pairs of charged particles of opposite sign arising in a unit volume of gas per unit time.
Ionization of an atom requires the expenditure of a certain energy - ionization energy. To ionize an atom (or molecule), it is necessary to do work against the interaction forces between the ejected electron and the remaining particles of the atom (or molecule). This work is called the ionization work A i. The amount of ionization work depends on the chemical nature of the gas and the energy state of the ejected electron in the atom or molecule.
After the ionizer stops working, 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 participate in thermal motion and therefore collide with each other. When a positive ion and an electron collide, they can reunite into a neutral atom. Similarly, when a positive and negative ion collide, the negative ion may give up its excess electron to the positive ion and both ions will become neutral atoms. This process of mutual neutralization of ions is called recombination of ions. 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 glow (recombination glow).
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 with sufficient kinetic energy, upon collision with a neutral atom, knocks out one or more atomic electrons from it, 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. The mechanism of electrical conductivity of gases.
The mechanism of conductivity of gases is similar to the mechanism of conductivity of solutions and melts of electrolytes. In the absence of an external field, charged particles, like neutral molecules, move chaotically. If ions and free electrons find themselves in an external electric field, then they begin to move in a direction and create an electric current in the gases.
Thus, the electric current in a gas represents a directed movement of positive ions towards the cathode, and negative ions and electrons towards the anode. The total current in the gas consists of two flows of charged particles: the flow going to the anode and the flow directed to the cathode.
Neutralization of charged particles occurs at the electrodes, as with 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 electrolyte solutions (melts) is that the negative charge when current passes through the gases is carried primarily not by negative ions, but by electrons, although conductivity due to negative ions can also play a role.
Thus, gases combine electronic conductivity, similar to that of metals, with ionic conductivity, similar to conductivity aqueous solutions and electrolyte melts.
4. Non-self-sustaining gas discharge.
The process of passing an electric current through a gas is called a gas discharge. If the electrical conductivity of a gas is created by external ionizers, then the electric current arising in it is called non-sustained gas discharge. With the cessation 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 current on voltage during a non-self-sustaining discharge in a gas. To plot the graph, a glass tube with two metal electrodes sealed into the glass was used. The chain is assembled as shown in the figure below.
At a certain voltage, a moment comes at which all the charged particles formed in the gas by the ionizer per second reach the electrodes during the same time. A further increase in voltage can no longer lead to an increase in the number of transferred ions. The current reaches saturation (horizontal section of graph 1).
5. Self-contained gas discharge.
An electrical discharge in a gas that persists after the external ionizer stops working 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 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 begin to 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 generated during the discharge process can become so large that an external ionizer will no longer be needed to maintain the discharge. Therefore, the ionizer can now be removed.
What are the reasons for the sharp increase in current 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 to the positive electrode - the anode, and the positive ion - to the cathode. On its way, the electron encounters ions and neutral atoms. In the intervals between two successive collisions, the electron's energy increases due to the work of the electric field forces.
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The greater the potential difference between the electrodes, the greater the electric field strength. The kinetic energy of the electron before the next collision is proportional to the field strength and the electron's mean free path: MV 2 /2=eEl. If the kinetic energy of an electron exceeds the work A i that must be done 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 appear (one that strikes the atom and one that is torn out of the atom). They, in turn, receive energy in the field and ionize oncoming atoms, etc. As a result, the number of charged particles quickly increases, and an electron avalanche occurs. The described process is called ionization by electron impact.
But ionization by electron impact alone cannot ensure the maintenance of an independent charge. Indeed, all the electrons generated in this way move towards the anode and, upon reaching the anode, “eliminate from the game.” To maintain the discharge, electrons must be emitted from the cathode (“emission” means “emission”). Electron emission can be due to several reasons.
Positive ions formed during collisions of electrons with neutral atoms, when moving towards the cathode, acquire high kinetic energy under the influence 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 high temperatures. This process is called thermionic emission. It can be thought of as the evaporation of electrons from a metal. In many solids, thermionic emission occurs at temperatures at which the evaporation of the substance itself is still small. Such substances are used to make cathodes.
During self-discharge, heating of the cathode can occur due to bombardment of it with positive ions. If the ion energy is not too high, then electrons are not knocked out from the cathode and electrons are emitted due to thermionic emission.
6. Various types of self-discharge and their technical applications.
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 discharge. Let's look at a few of them.
A. Glow discharge.
A glow discharge is observed in gases at low pressures of the order of several tens of millimeters of mercury or 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, sharply distant from him negative, or smoldering glow, which gradually moves 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 defines it optical properties and 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 sharp drop 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 movement of ions near the cathode. In the cathode dark space there is a strong acceleration of electrons and positive ions, knocking electrons out of the cathode. In the region of smoldering glow, electrons produce intense impact ionization of gas molecules and lose their energy. Here positive ions are formed, necessary to maintain the discharge. The electric field strength in this region is low. The glow is mainly caused by the recombination of ions and electrons. The extent of the cathode dark space is determined by the properties of the gas and the 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 the 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 potential is again observed, associated with the process of generating 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, when the distance between the electrodes of the tube decreases, the length of the positive column is reduced and it may disappear completely. 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 come 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, and to produce electron and ion beams. If a slit is made in the cathode, narrow ion beams, often called channel beams. Widely used phenomenon cathode sputtering, i.e. destruction of the cathode surface under the action of positive ions hitting it. Ultramicroscopic fragments of cathode material fly in all directions in straight lines and cover the surface of bodies (especially dielectrics) placed in the tube with a thin layer. In this way, mirrors are made for a number of devices, and a thin layer of metal is applied to selenium photocells.
B. Corona discharge.
Corona discharge occurs when normal pressure in a gas located in a highly inhomogeneous electric field (for example, near the tips or wires of high voltage lines). During a corona discharge, gas ionization and 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 bombarded with positive ions. If the anode is coronaed (positive corona), then the creation of electrons occurs due to photoionization of the gas near the anode. Corona is a harmful phenomenon accompanied by current leakage and loss of electrical energy. To reduce corona damage, 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 discharge.
At increased voltage, the corona discharge at the tip takes the form of light lines emanating from the tip and alternating in time. These lines, which have a number of kinks and bends, form a semblance of a brush, as a result of which such a discharge is called carpal .
A charged thundercloud induces electrical charges of the opposite sign on the surface of the Earth beneath it. A particularly large charge accumulates on the tips. Therefore, before or during a thunderstorm, tassel-like cones of light often flash on the points and sharp corners of highly raised objects. Since ancient times, this glow has been called the fires of St. Elmo.
Climbers especially often witness this phenomenon. Sometimes 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 taken into account when dealing with high voltage. If there are protruding parts or very thin wires, corona discharge may occur. This leads to 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 threads-channels that penetrate the discharge gap and disappear, replaced by new ones. Research has shown that 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 ionization by impact in the case of a spark discharge does not occur throughout the entire volume of gas, but through individual channels passing in those places in which the ion concentration accidentally turns out to be the highest. The spark discharge is accompanied by the release large quantity warmth, bright gas glow, crackling or thunder. All these phenomena are caused by electron and ion avalanches that occur in the 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 length of the lightning can reach several kilometers. The maximum current strength of a lightning pulse reaches tens and hundreds of thousands of amperes.
When the discharge gap is short, the spark discharge causes specific destruction of the anode, called erosion. This phenomenon was used in the electric spark 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, telephone lines). If a strong short-term current passes near a line, then voltages and currents are induced in the wires of this line, which can destroy the electrical installation and are 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, lengthens and breaks off.
Finally, the electric spark is used to measure large potential differences using ball arrester, 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.
D. Arc discharge.
The arc discharge was discovered by V.V. Petrov in 1802. This discharge is one of the forms of gas discharge, carried out at a high current density and a relatively low voltage between the electrodes (of the order of several tens of volts). The main cause of the arc discharge is the intense emission of thermionic electrons from the 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 you reduce the resistance of the external circuit and increase the arc discharge current, then the conductivity of the gas gap will increase so much that the voltage between the electrodes decreases. Therefore, they say that an arc discharge has a falling current-voltage characteristic. At atmospheric pressure The cathode temperature reaches 3000 °C. Electrons bombard the anode, creating a depression (crater) in it and heating it. The temperature of the crater is about 4000 °C, and at high air pressures it reaches 6000-7000 °C. The gas temperature in the arc discharge channel reaches 5000-6000 °C, so intense thermal ionization occurs in it.
In some cases, an arc discharge is observed at a relatively low cathode temperature (for example, in a mercury arc lamp).
In 1876, P. N. Yablochkov was the first to use an electric arc as a light source. In the “Yablochkov candle” the coals were arranged 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 an electric arc formed between the coals. As the coals burned, the insulating layer evaporated.
The arc discharge is still used as a light source today, for example in spotlights and projection devices.
Heat arc discharge allows it to be used for the construction of an arc furnace. Currently, arc furnaces powered by current are very great strength, are used in a number of industries: for smelting steel, cast iron, ferroalloys, bronze, producing calcium carbide, nitric oxide, etc.
In 1882, N. N. Benardos first used an arc discharge for cutting and welding metal. A discharge between a stationary carbon electrode and the metal heats the junction of two metal sheets (or plates) and welds them. Benardos used the same method to cut metal plates and make holes in them. In 1888, N. G. Slavyanov improved this welding method, replacing the carbon electrode with a metal one.
The arc discharge has found application in a mercury rectifier, which converts alternating electric current into direct current.
E. Plasma.
Plasma is a partially or fully ionized gas in which the densities of positive and negative charges are almost equal. Thus, plasma as a whole is an electrically neutral system.
A 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 a fraction of a percent), partially ionized (a is on the order of several percent) and completely ionized (a is close to 100%). Weakly ionized plasma in 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.
Average energies various types particles that make up plasma can differ significantly from one another. Therefore, plasma cannot be characterized by a single temperature value T; differentiate electron temperature T e, ion temperature T i (or ion temperatures if the plasma contains ions of several types) 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!!! (Ti<=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 their high mobility, charged plasma particles easily move under the influence of electric and magnetic fields. Therefore, any violation of the electrical neutrality of individual areas of the plasma caused by the accumulation of particles of the same charge sign is quickly eliminated. The resulting electric fields move the charged particles until electrical neutrality is restored and the electric field becomes zero. Unlike a neutral gas, between the molecules of which there are short-range forces, Coulomb forces act between the charged particles of plasma, which decrease relatively slowly with distance. Each particle interacts with a large number of surrounding particles at once. Due to this, along with chaotic thermal motion, plasma particles can participate in a variety of ordered movements. Various types of oscillations and waves are easily excited in plasma.
Plasma conductivity increases as the degree of ionization increases. At high temperatures, fully ionized plasma approaches superconductors in its conductivity.
Low-temperature plasma is used in gas-discharge light sources - in luminous tubes for advertising signs, in fluorescent lamps. Gas-discharge lamps are used in many devices, for example, in gas lasers - quantum light sources.
High-temperature plasma is used in magnetohydrodynamic generators.
Recently, a new device was created - a plasmatron. 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 grades: textbook. for in-depth study of physics/G. Y. Myakishev, A. Z. Sinyakov, B. A. Slobodskov. – 2nd edition – M.: Bustard, 1998. – 480 p.
2) Physics course (in three volumes). T. II. Electricity and magnetism. Textbook manual for colleges./Detlaf A.A., Yavorsky B.M., Milkovskaya L.B. Ed. 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.: Education, 1986.
Topics of the Unified State Examination 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 present. They appear as a result of the ionizing effects of radiation from radioactive substances that make up the earth's crust, ultraviolet and X-ray radiation from the Sun, as well as cosmic rays - streams of high-energy particles penetrating into the Earth's atmosphere from outer space. Subsequently, 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 the electrical circuit in your room.
Rice. 1 key
It is possible, however, to create conditions under which an electric current appears in the gas gap. Let's consider the following experience.
Let's charge the plates of the air capacitor and connect them to a sensitive galvanometer (Fig. 2, left). At room temperature and not too humid air, the galvanometer will not show any noticeable current: our air gap, as we said, is not a conductor of electricity.
Rice. 2. The appearance of current in the air
Now let’s bring a burner or candle flame into the gap between the capacitor plates (Fig. 2, right). The current appears! Why?
Free charges in gas
The occurrence of an electric current between the plates of the condenser means that in the air under the influence of a flame appeared free charges. Which ones exactly?
Experience shows that electric current in gases is the ordered movement of charged particles three types. This electrons, positive ions And negative ions.
Let's figure out how these charges can appear in the gas.
As the temperature of a gas increases, the thermal vibrations of its particles - molecules or atoms - become more intense. The collision of particles against each other reaches such force that it begins 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 equal to , then this means that the original gas particles have broken up into positive ions and electrons.
The degree of gas ionization depends on temperature and increases sharply with temperature. 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 (a 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 ionization of a gas is called ionizer.
Thus, ionization does not occur on its own, but under the influence of an ionizer.
At the same time, the reverse process occurs - recombination, that is, the reunification 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 influence of electrical forces, they meet and are able 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 increased (for example, by increasing the temperature), then the dynamic equilibrium will shift to side of ionization, and the concentration of charged particles in the gas will increase. On the contrary, if you turn off the ionizer, recombination will begin to predominate, 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 type of charge come from - negative ions? It’s very simple: an electron can hit a neutral atom and attach itself to it! This process is shown in Fig. 5 .
Rice. 5. The appearance of a negative ion
The negative ions thus formed will participate in the creation of current along with positive ions and electrons.
Non-self-sustaining discharge
If there is no external electric field, then free charges undergo 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-sustaining discharge
In 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 counter-movement of charged particles: positive ions - to the negative electrode (cathode), electrons and negative ions - to the positive electrode (anode).
Electrons, hitting the positive anode, are directed through the circuit to the “plus” of the current source. Negative ions give up an extra electron to the anode and, becoming neutral particles, return to the gas; the electron given to the anode also rushes to the “plus” of the source. Positive ions, arriving at the cathode, take electrons from there; the resulting deficit of electrons at the cathode is immediately compensated by their delivery there from the “minus” 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 described process shown in Fig. 6, called non-self-discharge in gas. Why dependent? Therefore, to maintain it, constant operation of the ionizer is necessary. 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.
Current-voltage characteristics of gas discharge
The dependence of the current 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. Current-voltage characteristics of gas discharge
At zero voltage, the current strength is naturally zero: charged particles perform only thermal motion, there is no ordered movement between the electrodes.
When the voltage is low, the current is also low. The fact is that not all charged particles are destined to reach the electrodes: some positive ions and electrons find each other and recombine during their movement.
As the voltage increases, free charges develop faster and faster, and the less chance a positive ion and electron have to meet and recombine. Therefore, an increasing part of the charged particles reaches the electrodes, and the current increases (section ).
At a certain voltage value (point), the speed of charge movement 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 happen up to a certain point.
Self discharge
After passing the point, the current strength increases sharply with increasing voltage - the independent category. 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 the electric field, increasing their kinetic energy. And so, when the voltage becomes large enough (that same 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 (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 collide with new atoms, now ionizing them and generating new electrons. As a result of the resulting 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 disappears. You can simply remove it. Free charged particles are now generated as a result internal processes occurring in the gas - that is 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 have only one free electron in the gas, and the electron avalanche described above will begin. And there will always be at least one free electron!
Let us remember once again that in gas, even under normal conditions, there is a certain “natural” amount of free charges, due to ionizing radioactive radiation from 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 high voltages - they will generate an avalanche of new particles, giving rise to an independent discharge. It will happen, as they say, breakdown gas gap.
The field strength required for the breakdown of 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 the voltage 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.
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 chemical phenomena occur during the passage of current. 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 the positive pole of the current source, is called anode, and the other, connected to the negative pole, is called a cathode.
What determines the passage of electric current in a liquid conductor? It turns out that in such solutions (electrolytes), acid (alkali, salt) molecules under the influence of a solvent (in this case water) break down into two components, and One particle of the molecule has a positive electrical charge, and the other has a negative one.
The particles of a molecule that have an electrical 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, a voltage 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 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 in contact with the cathode will be discharged (attaching the missing electrons), i.e., they will turn into neutral molecules of pure copper, and will be 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 a 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 takes a long time, 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 taken 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 finding extremely wide 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.
Once the ionization of the gas stops, it will cease to be conductive, while a 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, through numerous experiments, discovered that between two charcoals connected to a current source, a continuous electric discharge occurs through the air, accompanied by 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 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, 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.
There are no absolute dielectrics in nature. The ordered movement of particles - carriers of electric charge - that is, current, can be caused in any environment, but this requires special conditions. We will look here at how electrical phenomena occur in gases and how a gas can be transformed from a very good dielectric into a very good conductor. We will be interested in the conditions under which electric current in gases occurs, as well as in what features it is characterized.
Electrical properties of gases
A dielectric is a substance (medium) in which the concentration of particles - free carriers of 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 switch, the circuit opens when the contacts are brought into such a position that an air gap is formed between them. Wires in power lines are also insulated from each other by an air layer.
The structural unit of any gas is a molecule. It consists of atomic nuclei and electron clouds, that is, it is a collection of electrical charges distributed in some way in space. Due to the peculiarities of its structure, a gas molecule can be polarized under the influence 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 a gas, the molecules will take on 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 - towards 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 individual currents, so weak that they should be neglected. Gas behaves like a dielectric.
Thus, for the occurrence of 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 a significant amount of charged particles is present 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 removal of an electron, positive ions appear; the addition of an electron to a molecule leads to the formation of a negative ion. In addition, ionized gas contains many free electrons. Positive ions and especially electrons are the main charge carriers during electric current in gases.
Ionization occurs when a certain amount of energy is imparted to a particle. Thus, the outer electron in the 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 on an avalanche-like character. The kinetic energy of the particles also increases, which greatly promotes ionization.
Where does the energy expended to excite electric current in gases come from? Ionization of gases has several energy sources, according to which its types are usually named.
- Ionization by electric field. In this case, the potential energy of the field is converted into kinetic energy of 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 energy is imparted to electrons by quanta of electromagnetic radiation - photons, if they have a sufficiently high frequency (ultraviolet, x-rays, gamma quanta).
- Impact ionization results from the conversion of the kinetic energy of colliding particles into the energy of electron separation. 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 the molecule, overcoming the potential barrier. This value for the first electron ranges from several volts to two tens of volts; To remove the next electron from a molecule, more energy is needed, and so on.
It should be taken into account that simultaneously with ionization in the gas, the reverse process occurs - recombination, that is, the restoration of neutral molecules under the influence of Coulomb attractive forces.
Gas discharge and its types
So, the electric current in gases is caused by the ordered movement of charged particles under the influence 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 certain chemical processes promotes 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 that was previously open due to the air gap in the capacitor will close. A galvanometer connected to the circuit will indicate the presence of current.
Electric current in gases is called gas discharge. It must be borne in mind that in order to maintain discharge stability, 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 at the electrodes, others - electrons - when they reach the anode, they are directed to the “plus” of the field source. If the ionizing factor ceases to act, the gas will immediately become a dielectric again and the current will stop. Such a current, dependent on the action of an external ionizer, is called a non-self-sustaining discharge.
The peculiarities of the passage of electric current through gases are described by a special dependence of the current 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 increases to a certain value U 1, the current increases in proportion to it, that is, Ohm's law is satisfied. The kinetic energy increases, and therefore the speed of charges in the gas, and this process outstrips recombination. At voltage values from U 1 to U 2, this relationship is violated; when U2 is reached, all charge carriers reach the electrodes without having time to recombine. All free charges are used, and a further increase in voltage does not lead to an increase in current. This type of 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 ionized gas in electric fields of various strengths.
When the potential difference across the electrodes reaches a 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. Their speed in most gases is about 2000 km/s and higher (it is calculated using the approximate formula v=600 Ui, where Ui is the ionization potential). At this moment, gas breakdown occurs and a significant increase in current occurs due to the 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. A self-sustained discharge under different conditions and with different characteristics of the electric field source may 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 of 100 (or even less) to 1000 volts is sufficient to initiate a self-discharge. Therefore, a glow discharge, characterized by a low current value (from 10 -5 A to 1 A), occurs at pressures of no more than a few millimeters of mercury.
In a tube with rarefied gas and cold electrodes, the glow discharge that forms looks like a thin glowing cord between the electrodes. If you continue pumping gas from the tube, the cord will be washed out, and at pressures of tenths of a millimeter of mercury, the glow fills the tube almost completely. There is no glow near the cathode - in the so-called dark cathode space. The rest is called the positive column. In this case, the main processes ensuring the existence of the discharge are localized precisely in the dark cathode space and in the area adjacent to it. Here, charged gas particles are accelerated, knocking electrons out of the cathode.
In a glow discharge, the cause of ionization is electron emission from the cathode. Electrons emitted by the cathode produce impact ionization of gas molecules, the resulting positive ions cause secondary emission from the cathode, and so on. The glow of a positive column is mainly due to the release 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, you can make the positive column disappear, 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 this type of electric current in gases, the physics of some processes has not yet been fully clarified. For example, the nature of the forces that cause an expansion of the region on the cathode surface that takes part in the discharge as the current increases remains unclear.
Spark discharge
Spark breakdown has a pulsed nature. It occurs at pressures close to normal atmospheric pressure, in cases where the power of the electric field source is insufficient to maintain a stationary discharge. 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 repeats.
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 the largest number of ions is currently concentrated. Near the spark channel, the gas quickly heats up and experiences thermal expansion, causing acoustic waves. Therefore, a spark discharge is accompanied by a crackling sound, as well as the release of heat and a bright glow. Avalanche ionization processes generate high pressures and temperatures in the spark channel of up to 10 thousand degrees and above.
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 length of the channel can reach 10 km. The current strength reaches 500 thousand amperes, and the potential difference between a thundercloud and the surface of the Earth reaches a billion volts.
The longest lightning strike, 321 km long, was observed in 2007 in Oklahoma, USA. The record holder for the longest 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 electric field source is sufficiently high, the spark discharge develops into an arc discharge.
This type of self-discharge is characterized by a high current density and low (less than a glow discharge) voltage. The breakdown distance is short due to the close 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, conditions are created between the electrodes under which the gas conducts an electric current, and a spark discharge occurs, closing the circuit. If the power of the voltage source is high enough, the spark discharges turn into a stable electric arc.
Ionization during an arc discharge reaches almost 100%, the current is very high and can range from 10 to 100 amperes. At atmospheric pressure, the arc can heat up to 5-6 thousand degrees, and the cathode - up to 3 thousand degrees, which leads to intense thermionic emission from its surface. Bombardment of the anode with electrons leads to partial destruction: a depression is formed on it - a crater with a temperature of about 4000 °C. An increase in pressure entails an even greater increase in temperatures.
When the electrodes are separated, the arc discharge remains stable up to a certain distance, which makes it possible to combat it in those areas of electrical equipment where it is harmful due to the corrosion and burnout of contacts it causes. These are devices such as high-voltage and circuit breakers, contactors and others. One of the methods of combating arcs that occur when contacts open is the use of arc suppression chambers based on the principle of arc elongation. Many other methods are also used: bypassing contacts, using materials with high ionization potential, and so on.
The development of a corona discharge occurs at normal atmospheric pressure in sharply inhomogeneous fields near electrodes with a large surface curvature. These can be spiers, masts, wires, various elements of electrical equipment that have a complex shape, and even human hair. Such an electrode is called corona electrode. Ionization processes and, accordingly, gas glow take place only near it.
A corona can form both on the cathode (negative corona) when it is bombarded with ions, and on the anode (positive corona) as a result of photoionization. The negative corona, in which the ionization process as a consequence of thermal emission is directed away from the electrode, 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 corona discharge in natural conditions is those occurring at the tips of high masts, treetops, and so on. They are formed at high electric field strength in the atmosphere, often before a thunderstorm or during a blizzard. In addition, they were recorded on the skin of aircraft caught in a cloud of volcanic ash.
Corona discharge on power line wires leads to significant losses of electricity. At high voltages, a corona discharge can turn into an arc discharge. It is combated in various 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. In general, plasma is electrically neutral, since the total charge of its constituent particles is zero. This distinguishes it from other charged particle systems, such as 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 overwhelming majority of baryonic matter in the Universe is in the state of plasma. These are stars, part of the interstellar matter, intergalactic gas. The earth's ionosphere is also a rarefied, weakly ionized plasma.
The degree of ionization is an important characteristic of plasma - its conducting 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 is characterized by high mobility.
We see, therefore, that gases that conduct electric current within the discharge channel are nothing more than plasma. Thus, glow and corona discharges are examples of cold plasma; a lightning spark channel or an electric arc are examples of hot, almost completely ionized plasma.
Electric current in metals, liquids and gases - differences and similarities
Let us consider the features that characterize a gas discharge in comparison with the properties of current in other media.
In metals, current is the directed movement of free electrons, which 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 type 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 influence of a potential difference, move in opposite directions: positive cations - to the cathode, negative anions - to the anode. The process is accompanied by the release of gas or the deposition of a metal layer on the cathode. Conductors of the second type 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. Thus, 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 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 collapse, but only lose electrons. Therefore, a gas discharge, like a current in metals, is not associated with chemical changes.
The current in liquids and gases is also different. The conductivity of electrolytes generally obeys Ohm's law, but during a gas discharge it is not observed. The current-voltage characteristic of gases is much more complex, associated with the properties of plasma.
Mention should also be made of the general and distinctive features of electric current in gases and in vacuum. Vacuum is an almost perfect dielectric. “Almost” - because in a 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 just need to be ionized. Charge carriers are introduced into the vacuum from the substance. As a rule, this occurs through the process of electron emission, for example when the cathode is heated (thermionic emission). But in various types of gas discharges, emission, as we have seen, plays an important role.
Application of gas discharges in technology
The harmful effects of certain discharges have already been briefly discussed above. Now let's pay attention to the benefits they bring in industry and in everyday life.
Glow discharge is used in electrical engineering (voltage stabilizers) and in coating technology (cathode sputtering method, based on the phenomenon of cathode corrosion). In electronics it is used to produce ion and electron beams. Widely known areas of application of glow discharge are fluorescent and so-called energy-efficient lamps and decorative neon and argon gas discharge tubes. In addition, glow discharge is used in spectroscopy.
Spark discharge is used in fuses and in electrical discharge methods for precision metal processing (spark cutting, drilling, and so on). But it is best known for its use in spark plugs for internal combustion engines and in household appliances (gas stoves).
The arc discharge, having been first used in lighting technology back in 1876 (Yablochkov candle - “Russian light”), still serves as a light source - for example, in projection devices and powerful searchlights. In electrical engineering, the arc is used in mercury rectifiers. In addition, it is used in electric welding, metal cutting, and industrial electric furnaces for smelting steel and alloys.
Corona discharge is used in electric precipitators for ion gas purification, in particle counters, in lightning rods, and in air conditioning systems. Corona discharge also works in photocopiers 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 find the widest application. Electric current in gases is successfully and effectively used in many fields 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 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 the electron shell of an 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.
The change in ion concentration over 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 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 form, but will 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 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 take as the initial concentration value
Rewriting this equation in the form after integration we get
The graph of this function is shown in Fig. 101. It is a hyperbola, the asymptotes of which are the time axis and the vertical straight line. Of course, only the section of the hyperbola corresponding to the values has a physical meaning. Note the slow nature of the decrease in concentration with time in comparison with the exponential decay processes 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
On average, constant velocities of electrons and ions are established, proportional to the electric field strength E:
The proportionality coefficients are called the electron and ion mobilities. The mobilities of ions and electrons have different values 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 a 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 a 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, internal source of ions has appeared in the gas. 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 of 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 by the electric field to sufficiently
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. Four characteristic regions can be distinguished in a glow discharge. 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 naturally 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 strength 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. A specific practically important type of independent gas discharge is an electric arc. When two carbon or metal electrodes come into contact at the point of their 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 has a non-monotonic character with maxima at voltages that are multiples 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, they 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 is this consistent with the fact that the current strength is the same in all parts of a series circuit?
Why do electrons, and not positive ions, play the largest role in the ionization of gas in a discharge due to collisions?
Describe the characteristic features of various types of independent gas discharge.
Why do the results of Frank and Hertz's experiments indicate discreteness of atomic energy levels?
Describe the physical processes occurring in the gas-discharge tube in the experiments of Frank and Hertz with increasing accelerating voltage.