Ionization of gases. Non-self-sustained and self-sustained gas discharge

LAB #2.5

"Study of a gas discharge using a thyratron"

Objective: to study the processes occurring in gases during non-self-sustained and self-sustained discharge in gases, to study the principle of operation of the thyratron, to build the current-voltage and starting characteristics of the thyratron.

THEORETICAL PART

Ionization of gases. Non-self-sustained and self-sustained gas discharge

Atoms and molecules of gases under normal everyday conditions are electrically neutral, i.e. do not contain free charge carriers, which means that, like a vacuum gap, they should not conduct electricity. In fact, gases always contain a certain amount of free electrons, positive and negative ions, and therefore, although poorly, they conduct electricity. current.

Free charge carriers in a gas are usually formed as a result of the ejection of electrons from the electron shell of gas atoms, i.e. as a result ionization gas. Gas ionization is the result of external energy impact: heating, particle bombardment (electrons, ions, etc.), electromagnetic radiation (ultraviolet, X-ray, radioactive, etc.). In this case, the gas located between the electrodes conducts an electric current, which is called gas discharge. Power ionizing factor ( ionizer) is the number of pairs of oppositely charged charge carriers resulting from ionization per unit volume of gas per unit time. Along with the ionization process, there is also a reverse process - recombination: the interaction of oppositely charged particles, as a result of which electrically neutral atoms or molecules appear and electromagnetic waves are emitted. If the electrical conductivity of the gas requires the presence of an external ionizer, then such a discharge is called dependent. If the applied electric field (EF) is sufficiently large, then the number of free charge carriers formed as a result of impact ionization due to the external field is sufficient to maintain an electric discharge. Such a discharge does not need an external ionizer and is called independent.

Let us consider the current-voltage characteristic (CVC) of a gas discharge in a gas located between the electrodes (Fig. 1).

With a non-self-sustained gas discharge in the region of weak electric fields (I), the number of charges formed as a result of ionization is equal to the number of charges recombining with each other. Due to this dynamic equilibrium, the concentration of free charge carriers in the gas remains practically constant and, as a result, Ohm's law (1):

where E is the electric field strength; n– concentration; j is the current density.

and ( ) are the mobility of positive and negative charge carriers, respectively;<υ > is the drift velocity of the directed motion of the charge.

In the region of high EC (II), saturation of the current in the gas (I) is observed, since all the carriers created by the ionizer participate in the directed drift, in the creation of the current.

With a further increase in the field (III), charge carriers (electrons and ions), moving at an accelerated rate, ionize neutral atoms and gas molecules ( impact ionization), resulting in the formation of additional charge carriers and the formation electronic avalanche(electrons are lighter than ions and are significantly accelerated in the EP) – the current density increases ( gas amplification). When the external ionizer is turned off, the gas discharge will stop due to recombination processes.

As a result of these processes, flows of electrons, ions and photons are formed, the number of particles grows like an avalanche, there is a sharp increase in current with practically no amplification of the electric field between the electrodes. Arises independent gas discharge. The transition from an inconsistent gas discharge to an independent one is called email breakdown, and the voltage between the electrodes , where d- the distance between the electrodes is called breakdown voltage.

For e-mail breakdown, it is necessary that the electrons, along their path, have time to gain kinetic energy that exceeds the ionization potential of gas molecules, and on the other hand, that positive ions, along their path, have time to acquire kinetic energy greater than the work function of the cathode material. Since the mean free path depends on the configuration of the electrodes, the distance between them d and the number of particles per unit volume (and, consequently, on the pressure), the ignition of a self-sustained discharge can be controlled by changing the distance between the electrodes d with their unchanged configuration, and changing the pressure P. If the work Pd turns out to be the same, other things being equal, then the nature of the observed breakdown should be the same. This conclusion was reflected in the experimental law e (1889) German. physics F. Pashen(1865–1947):

The ignition voltage of a gas discharge for a given value of the product of gas pressure and the distance between the electrodes Pd is a constant value characteristic of a given gas .

There are several types of self-discharge.

glow discharge occurs at low pressures. If a constant voltage of several hundred volts is applied to the electrodes soldered into a glass tube 30–50 cm long, gradually pumping air out of the tube, then at a pressure of 5.3–6.7 kPa a discharge occurs in the form of a luminous tortuous reddish cord coming from cathode to anode. With a further decrease in pressure, the filament thickens, and at a pressure of » 13 Pa, the discharge has the form shown schematically in Fig. 2.

A thin luminous layer is attached directly to the cathode 1 - cathode film , followed by 2 - cathode dark space , passing further into the luminous layer 3 – smoldering glow , which has a sharp boundary on the cathode side, gradually disappearing on the anode side. Layers 1-3 form the cathode part of the glow discharge. Follows the smoldering glow faraday dark space 4. The rest of the tube is filled with luminous gas - positive post - 5.

The potential varies unevenly along the tube (see Fig. 2). Almost the entire voltage drop occurs in the first sections of the discharge, including the dark cathode space.

The main processes necessary to maintain the discharge occur in its cathode part:

1) positive ions, accelerated by the cathodic potential drop, bombard the cathode and knock out electrons from it;

2) the electrons are accelerated in the cathode part and gain sufficient energy and ionize the gas molecules. Many electrons and positive ions are formed. In the smoldering region, intense recombination of electrons and ions takes place, energy is released, part of which goes to additional ionization. The electrons that have penetrated into the Faraday dark space gradually accumulate energy, so that the conditions necessary for the existence of the plasma arise (a high degree of gas ionization). The positive column is a gas-discharge plasma. It acts as a conductor connecting the anode to the cathode parts. The glow of the positive column is caused mainly by transitions of excited molecules to the ground state. Molecules of different gases emit radiation of different wavelengths during such transitions. Therefore, the glow of the column has a color characteristic of each gas. This is used to make luminous tubes. Neon tubes give a red glow, argon tubes give a bluish-green.

arc discharge observed at normal and elevated pressures. In this case, the current reaches tens and hundreds of amperes, and the voltage across the gas gap drops to several tens of volts. Such a discharge can be obtained from a low voltage source if the electrodes are first brought together until they touch. At the point of contact, the electrodes are strongly heated due to the Joule heat, and after they are removed from each other, the cathode becomes a source of electrons due to thermionic emission. The main processes supporting the discharge are thermionic emission from the cathode and thermal ionization of molecules due to the high temperature of the gas in the interelectrode gap. Almost the entire interelectrode space is filled with high-temperature plasma. It serves as a conductor through which the electrons emitted by the cathode reach the anode. The plasma temperature is ~6000 K. The high temperature of the cathode is maintained by bombarding it with positive ions. In turn, the anode, under the action of fast electrons incident on it from the gas gap, heats up more strongly and can even melt, and a recess is formed on its surface - a crater - the brightest place of the arc. Electric arc was first received in 1802. Russian physicist V. Petrov (1761–1834), who used two pieces of coal as electrodes. Hot carbon electrodes gave a dazzling glow, and between them a bright column of luminous gas appeared - an electric arc. The arc discharge is used as a source of bright light in projector spotlights, as well as for cutting and welding metals. There is an arc discharge with a cold cathode. Electrons appear due to field emission from the cathode, the gas temperature is low. The ionization of molecules occurs due to electron impacts. A gas-discharge plasma appears between the cathode and anode.

spark discharge occurs between two electrodes at a high electric field strength between them . A spark jumps between the electrodes, having the form of a brightly luminous channel, connecting both electrodes. The gas near the spark is heated to a high temperature, a pressure difference occurs, which leads to the appearance of sound waves, a characteristic crack.

The appearance of a spark is preceded by the formation of electron avalanches in the gas. The ancestor of each avalanche is an electron accelerating in a strong electric field and producing the ionization of molecules. The resulting electrons, in turn, accelerate and produce the next ionization, an avalanche increase in the number of electrons occurs - avalanche.

The resulting positive ions do not play a significant role, because they are immobile. Electron avalanches intersect and form a conducting channel streamer, along which electrons rush from the cathode to the anode - there is breakdown.

Lightning is an example of a powerful spark discharge. Different parts of a thundercloud carry charges of different signs ("-" is facing the Earth). Therefore, if the clouds approach each other with oppositely charged parts, a spark breakdown occurs between them. The potential difference between the charged cloud and the Earth is ~10 8 V.

Spark discharge is used to initiate explosions and combustion processes (candles in internal combustion engines), to register charged particles in spark counters, to treat metal surfaces, etc.

Corona (coronary) discharge occurs between electrodes that have different curvature (one of the electrodes is a thin wire or a point). In a corona discharge, ionization and excitation of molecules occur not in the entire interelectrode space, but near the tip, where the intensity is high and exceeds E breakdown. In this part, the gas glows, the glow has the form of a corona surrounding the electrode.

Plasma and its properties

Plasma is called a strongly ionized gas, in which the concentration of positive and negative charges is almost the same. Distinguish high temperature plasma , which occurs at ultrahigh temperatures, and gas-discharge plasma arising from gas discharge.

Plasma has the following properties:

A high degree of ionization, in the limit - complete ionization (all electrons are separated from the nuclei);

The concentration of positive and negative particles in plasma is practically the same;

high electrical conductivity;

glow;

Strong interaction with electric and magnetic fields;

Oscillations of electrons in plasma with a high frequency (>10 8 Hz), causing a general vibration of the plasma;

Simultaneous interaction of a huge number of particles.

The process of occurrence and formation of avalanches considered above due to impact ionization does not lose the character of a non-self-sustained discharge, since in the event of termination of the external ionizer, the discharge quickly disappears.

However, the emergence and formation of an avalanche of charges is not limited to the process of impact ionization. With a further, relatively small increase in voltage, at the electrodes of the gas-discharge gap, positive ions acquire more energy and, hitting the cathode, knock out electrons from it, occurs secondary electron emission . The free electrons that have arisen on their way to the anode produce impact ionization of gas molecules. Positive ions on their way to the cathode in electric fields themselves ionize gas molecules.

If each electron ejected from the cathode is capable of being accelerated and producing impact ionization of gas molecules, then the discharge will be maintained even after the action of the external ionizer ceases. The voltage at which an independent discharge develops is called closing voltage.

Based on what has been said, independent discharge we will call such a gas discharge in which current carriers arise as a result of those processes in the gas that are due to the voltage applied to the gas. Those. this discharge continues even after the termination of the ionizer.

When the interelectrode gap is covered by a completely conducting gas-discharge plasma, it breakdown . The voltage at which the breakdown of the interelectrode gap occurs is called breakdown voltage. And the corresponding electric field strength is called breakdown tension.

Let us consider the conditions for the emergence and maintenance of an independent discharge.

At high voltages between the electrodes of the gas gap, the current increases greatly. This is due to the fact that the electrons arising under the action of an external ionizer, strongly accelerated by an electric field, collide with neutral gas molecules and ionize them. As a result of this, secondary electrons and positive ions(process 1, figure 8.4). Positive ions move towards the cathode and electrons move towards the anode. The secondary electrons again ionize the gas molecules, and, consequently, the total number of electrons and ions will increase as the electrons move towards the anode like an avalanche. This is the reason for the increase in electric current. The described process is called impact ionization.

However, impact ionization under the action of electrons is not sufficient to maintain the discharge when the external ionizer is removed. For this, it is necessary that the electron avalanches be “reproducible”, i.e. so that new electrons appear in the gas under the influence of some processes. These are the following processes:

• positive ions accelerated by an electric field, hitting the cathode, knock out electrons from it (process 2);
• positive ions, colliding with gas molecules, transfer them to an excited state; the transition of such molecules to the ground state is accompanied by the emission of photons (process 3);
• a photon absorbed by a neutral molecule ionizes it, the process of photon ionization of molecules occurs (process 4);
• knocking out electrons from the cathode under the action of photons (process 5);
• finally, at significant voltages between the electrodes of the gas gap, a moment comes when positive ions, which have a shorter mean free path than electrons, acquire energy sufficient to ionize gas molecules (process 6), and ion avalanches rush to the negative plate. When, in addition to electron avalanches, there are also ion avalanches, the current increases almost without increasing the voltage.

The process of passing email. current through the gas called. gas discharge.

There are 2 types of discharges: independent and non-independent.

If the electrical conductivity of the gas is created. external ionizers, then el. the current in it is called. nesamost. gas discharge. V

Consider. email scheme, comp. from a capacitor, a galvanometer, a voltmeter and a current source.

Between the plates of a flat capacitor is air at atmospheric pressure and room t. If a U equal to several hundred volts is applied to the capacitor, and the ionizer does not work, then the current galvanometer does not register, however, as soon as the space between the plates begins to penetrate. flow of UV rays, the galvanometer will start registering. current. If the current source is turned off, the flow of current through the circuit will stop, this current is a non-self-sustained discharge.

j = γ*E - Ohm's law for el. current in gases.

With a sufficiently strong e. field in the gas begins the process of self-ionization, due to which the current can exist in the absence of an external ionizer. This kind of current is called an independent gas discharge. The processes of self-ionization in general terms are as follows. In nature. conv. A gas always contains a small amount of free electrons and ions. They are created by such natures. ionizers, like space. rays, radiation of radioactive substances, soda in soil and water. Fairly strong email. the field can accelerate these particles to such speeds at which their kinetic energy exceeds the ionization energy when electrons and ions collide on the way to the electrodes with neutrons. molecules will ionize those molecules. arr. upon collision, new secondary electrons and ions also accelerate. field and in turn ionize new neutrons. molecules. The described self-ionization of gases is called impact polishing. Free electrons cause impact ionization already at E=10 3 V/m. Ions, on the other hand, can cause impact ionization only at E=10 5 V/m. This difference is due to a number of reasons, in particular, the fact that for electrons the mean free path is much longer than for ions. Therefore, ions acquire the energy necessary for impact ionization at a lower field strength than ions. However, even at not too strong “+” fields, ions play an important role in self-ionization. The fact is that the energy of these ions is approx. enough to knock electrons out of metals. Therefore, the ions dispersed by the “+” field, hitting the metal cathode of the field source, knock out the electrons from the cathode. These knocked-out electrons field and produce impact ionization of molecules. Ions and electrons, the energy of which is insufficient for impact ionization, can nevertheless lead them into excitation when colliding with molecules. state, that is, to cause some energy changes in the email. shells of neutral atoms and molecules. Excit. an atom or molecule after some time goes into a normal state, while it emits a photon. The emission of photons is manifested in the glow of gases. In addition, a photon, absorb. any of the gas molecules can ionize it, this kind of ionization is called photonionization. Some of the photons hit the cathode, they can knock electrons out of it, which then cause impact ionization of the neutron. molecules.

As a result of impact and photon ionization and knocking out of electrons from the “+” code by ions by photons, the number of photons and electrons in the entire volume of the gas increases sharply (avalanche-like) and an external ionizer is not needed for the existence of a current in the gas, and the discharge becomes independent. CVC of the gas discharge is as follows.

Gases, unlike metals and electrolytes, consist of electrically neutral atoms and molecules and under normal conditions do not contain free current carriers (electrons and ions). So gases under normal conditions are dielectrics.

Carriers of electric current in gases can arise only in the process of ionization of gases, i.e. during the formation of ions in a gas.

Ionization process gases occurs under the influence of external influences (external ionizers): strong heating, ultraviolet and X-rays.

The process of gas ionization is in the fact that under the action of ionizers one or more electrons are detached from the atoms. As a result, instead of a neutral atom, a positive ion and an electron arise. Some of the electrons formed can then be captured by other neutral atoms, and then negatively charged ions appear. The detachment of an electron from an atom requires the expenditure of a certain energy - the ionization energy Wi, which is measured by the work against the force of attraction of the electron by the atomic nucleus: Wi = eUi, where e is the electron charge, Ui is the ionization potential for a given substance.

The ionization energy depends on the chemical nature of the gas and the energy state of the electron in the atom

Electrons and positive ions that have arisen during the action of the ionizer cannot exist separately for a long time and, in collisions, again form neutral atoms or molecules.. This phenomenon is called recombination(opposite of ionization). Therefore, after the termination of the ionizer, the electric current in the gas disappears.

If during ionization energy is needed to detach an electron from an atom, then during recombination this energy is released mostly in the form of light radiation. At a sufficient intensity of recombination, the electric current in gases is accompanied by a noticeable glow.

With the continuous action of the ionizer and the absence of an electric field in the gas, a moving equilibrium is established between the ionization of molecules and the recombination of ions, characterized by a certain concentration of ions.

Mechanism of electrical conductivity of gases.

When an ionized gas is placed in an electric field, electric forces act on free charges and they drift parallel to the lines of tension: electrons and negative ions - to the anode, positive ions - to the cathode. At the electrodes, ions turn into neutral atoms by donating or accepting electrons, thereby completing the circuit. An electric current is generated in the gas. Electric current in gases is called gas discharge. In this way, conductivity of gases has an ion-electronic character.

gas discharge is of two types:

1. Independent gas discharge.

2. dependent gas discharge.

dependent if it is created under the influence of any external factors.

Gas discharge (gas conductivity) is called independent, if it is created in a gas under the influence of the electric field itself, which exists between the electrodes (anode and cathode).

Non-self-sustained gas discharge

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

On fig. shows 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 picture.

1. When a potential difference is applied, an electric current arises in the tube.

2. With a small potential difference, not all of the formed ions reach the electrodes.

3. As the potential difference (voltage) between the electrodes of the tube increases, the proportion of charged particles reaching the electrodes increases. This also increases the current in the circuit.

4. At a certain voltage, there comes a point at which all the charged particles formed in the gas by the ionizer in a second reach the electrodes in the same time. In this case, there is no further increase in current. This maximum current is called saturation current .

5. If the action of the ionizer is stopped, then the current in the circuit will also stop, i.e. gas discharge, since there are no other sources of ions. If you remove the external ionizer, then no new ions are formed, and those that exist will reach the electrode or recombine.

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 (see Fig. and Graph 2).

Consequently, an additional source of ion formation appears in the gas. 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.

The dominant role begins to play the potential difference between the cathode and anode. 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 mean free path of the electron: meV2/2=eEl. If the kinetic energy of an electron exceeds the work Ai that must be done in order to ionize a neutral atom (or molecule), i.e. meV2/2>Ai, then when an electron collides with an atom (or molecule), it ionizes ( impact ionization). As a result, instead of one electron, two electrons appear (attacking on the atom and torn out of the atom). Electrons detached from molecules as a result of ionization, in turn, under the action of the field, can obtain energy sufficient for ionization. As a result, the concentration of ions, and with it the electrical conductivity of the gas, increases greatly. If you remove the external ionizer, the discharge will not stop. Since such a discharge does not need an external ionizer to maintain it, it is called an independent gas discharge.

But the presence of only ionization by electron impact does not yet lead to an independent discharge. For the existence of a self-sustained discharge, it is necessary that other processes occur in the gas, producing new electrons instead of those that have gone to the anode.. Such processes can be secondary emission of electrons from the cathode(knocking electrons out of the cathode by positive ions dispersed in an electric field), the cathode can emit electrons when heated to a high temperature. This process is called thermionic emission and etc.

Types of self-discharge:

1. spark discharge

Examples of a spark discharge are sparks that occur when combing hair, when a capacitor is discharged.

spark discharge, often observed in nature, is lightning. Lightning is a discharge between two charged clouds or between a cloud and the earth. Charge carriers in clouds are charged water droplets or snowflakes.

A spark discharge is accompanied by the release of a large amount of heat, a bright glow of gas, crackling or thunder.

1. arc discharge.

arc discharge can be observed under the following conditions: if, after ignition of the spark discharge, the resistance of the circuit is gradually reduced, then the current in the spark will increase. When the resistance of the circuit becomes small enough, a new form of gas discharge, called an arc, will occur. In this case, the current strength increases sharply, reaching tens and hundreds of amperes, and the voltage across the discharge gap decreases to several tens of volts. This shows that new processes arise in the discharge, giving the gas a very high electrical conductivity.

The electric arc is a powerful light source and is widely used in projection, spotlight and other lighting installations. Due to the high temperature, the arc is widely used for welding and cutting metals. The high temperature of the arc is also used in the construction of electric arc furnaces, which play an important role in modern electrometallurgy.

1. glow discharge

glow discharge observed at low gas pressures (about 0.1 mm Hg). If a constant voltage of several hundred volts is applied to the electrodes soldered into a glass tube and then the air is gradually pumped out of the tube, then the following phenomenon is observed: when the gas pressure decreases, at some point a discharge appears in the tube, which has the form of a luminous cord connecting the anode and cathode tubes (Fig. 1). With a further decrease in pressure, this filament expands and fills the entire cross section of the tube, and the glow near the cathode weakens. Near the cathode, the first dark space 1 is formed, to which the ion glowing layer 2 (glow glow) is adjacent, which has a sharp boundary on the cathode side and gradually disappears on the anode side. Behind the smoldering glow, there is again a dark gap 3, called the faraday or second dark space. Behind it lies a luminous region 4 extending to the anode, or a positive column.

Of particular importance in a glow discharge are only two of its parts - cathode dark space 1 and glow glow 2, in which the main processes that maintain the discharge occur. The electrons that ionize the gas are produced by photoemission from the cathode and collisions of positive ions with the cathode of the tube.

At present, glow discharge tubes find practical application as a light source - gas discharge lamps..

1. corona discharge

corona discharge observed at relatively high gas pressures (for example, at atmospheric pressure) in a sharply inhomogeneous electric field. To obtain a significant inhomogeneity of the field, the electrodes must have sharply different surfaces, i.e. one electrode - a very large surface, and the other - a very small one. So, for example, a corona discharge can be easily obtained by placing a thin wire inside a metal cylinder, the radius of which is much larger than the radius of the wire.

Corona discharge is used in engineering for the construction of electrostatic precipitators designed to purify industrial gases from solid and liquid impurities.

Corona discharge can occur on thin live wires. The occurrence of a corona discharge on the tips of the conductors explains the action of a lightning rod that protects buildings and transmission lines from lightning strikes.

The light emission of fluorescent lamps, gas-discharge lamps of street lighting is used; an electric arc is used in a film projection apparatus; mercury-quartz lamp has found application in clinics and hospitals.

1. 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. Degree of ionization plasma is called the ratio of the volume concentration of charged particles to the total volume concentration of particles. Depending on the degree of ionization, plasma is subdivided into weakly ionized(is a fraction of a percent), partially ionized(on the order of a few percent) and fully ionized(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.

Plasma cannot be characterized by a single temperature value T; distinguish electronic temperature Those, ion temperature Ti (or ion temperatures, if there are several kinds of ions in the plasma) and temperature of neutral atoms T (neutral component). Such a plasma is called non-isothermal, Unlike isothermal plasma where the temperatures of all components are the same.

Plasma also splits into high temperature(Ti 106-108 K and more) and low-temperature(Ti<=105 К).

The plasma conductivity increases as the degree of ionization increases. At a high temperature, a fully ionized plasma in its conductivity approaches superconductors (substances that, when cooled below a certain critical temperature, Ts electric resistance drops to zero).

Low temperature plasma it is used in gas-discharge light sources - in luminous tubes of 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 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.

An electric current is a flow that is caused by the ordered movement of electrically charged particles. The movement of charges is taken as the direction of the electric current. Electric current can be short-term and long-term.

The concept of electric current

During a lightning discharge, an electric current can occur, which is called short-term. And to maintain the current for a long time, it is necessary to have an electric field and free electric charge carriers.

An electric field is created by bodies charged differently. The current strength is the ratio of the charge transferred through the cross section of the conductor in a time interval to this time interval. It is measured in amperes.

Rice. 1. Current formula

Electric current in gases

Gas molecules do not conduct electricity under normal conditions. They are insulators (dielectrics). However, if environmental conditions are changed, gases can become conductors of electricity. As a result of ionization (during heating or under the action of radioactive radiation), an electric current arises in gases, which is often replaced by the term "electric discharge".

Self-sustained and non-self-sustained gas discharges

Discharges in gas can be self-sustaining and non-self-sustaining. The current begins to exist when free charges appear. Non-self-sustaining discharges exist as long as an external force acts on it, that is, an external ionizer. That is, if the external ionizer ceases to operate, then the current stops.

An independent discharge of electric current in gases exists even after the termination of the external ionizer. Independent discharges in physics are divided into quiet, smoldering, arc, spark, corona.

• Quiet - the weakest of the independent discharges. The current strength in it is very small (no more than 1 mA). It is not accompanied by sound or light phenomena.
• Smoldering - if you increase the voltage in a quiet discharge, it goes to the next level - to a glow discharge. In this case, a glow appears, which is accompanied by recombination. Recombination - the reverse ionization process, the meeting of an electron and a positive ion. It is used in bactericidal and lighting lamps.

Rice. 2. Glow discharge

• Arc - the current strength ranges from 10 A to 100 A. In this case, ionization is almost 100%. This type of discharge occurs, for example, during the operation of a welding machine.

Rice. 3. Arc discharge

• sparkling - can be considered one of the types of arc discharge. During such a discharge, a certain amount of electricity flows in a very short time.
• corona discharge – ionization of molecules occurs near electrodes with small radii of curvature. This type of charge occurs when the electric field strength changes dramatically.

What have we learned?

By themselves, the atoms and molecules of a gas are neutral. They are charged when exposed to the outside. Speaking briefly about the electric current in gases, it is a directed movement of particles (positive ions to the cathode and negative ions to the anode). It is also important that when the gas is ionized, its conductive properties improve.

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