There are no absolute dielectrics in nature. The ordered movement of particles - carriers of an electric charge, that is, a current, can be caused in any medium, but special conditions are necessary for this. We will consider here how electrical phenomena occur in gases and how a gas can be turned from a very good dielectric into a very good conductor. We will be interested in the conditions under which it arises, as well as what features are characterized by electric current in gases.
Electrical properties of gases
A dielectric is a substance (medium) in which the concentration of particles - free carriers of an electric charge - does not reach any significant value, as a result of which the conductivity is negligible. All gases are good dielectrics. Their insulating properties are used universally. For example, in any switch, an open circuit occurs when the contacts are brought into a position such that an air gap forms 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 electric charges, distributed in some way in space. The gas molecule can be an electric dipole due to the peculiarities of its structure or 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 ordinary conditions, since the charges in them cancel each other out.
If an electric field is applied to the gas, the molecules will take a dipole orientation, occupying a spatial position that compensates for the effect of the field. The charged particles present in the gas under the influence of Coulomb forces will begin to move: positive ions in the direction of the cathode, negative ions and electrons towards the anode. However, if the field has insufficient potential, a single directed flow of charges does not arise, and we can speak more about individual currents so weak that they should be neglected. Gas behaves like a dielectric.
Thus, for the appearance of an electric current in gases, a large concentration of free charge carriers and the presence of a field are necessary.
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 are 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. Due to the detachment of an electron, positive ions arise, the attachment of an electron to a molecule leads to the formation of a negative ion. In addition, there are many free electrons in the ionized gas. 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. So, an external electron in a molecule, having received this energy, can leave the molecule. Mutual collisions of charged particles with neutral ones lead to the knocking out of new electrons, and the process assumes an avalanche-like character. The kinetic energy of the particles also increases, which greatly contributes to ionization.
Where does the energy spent on the excitation of electric current in gases come from? Ionization of gases has several sources of energy, according to which it is customary to name its types.
- Ionization by an electric field. In this case, the potential energy of the field is converted into the kinetic energy of the particles.
- Thermionization. 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 transmitted to electrons by electromagnetic radiation quanta - photons, if they have a sufficiently high frequency (ultraviolet, x-ray, gamma rays).
- Impact ionization is the result of the conversion of the kinetic energy of colliding particles into the energy of electron detachment. Along with thermal ionization, it serves as the main factor in the excitation of electric current in gases.
Each gas is characterized by a certain threshold value - the ionization energy necessary for the electron to break away from the molecule, breaking the potential barrier. This value for the first electron is from several volts to two tens of volts; to detach the next electron from the molecule, more energy is needed, and so on.
It should be borne in mind that simultaneously with ionization in a gas, the reverse process takes place - recombination, that is, the restoration of neutral molecules under the influence of Coulomb attraction forces.
Gas discharge and its types
So, the electric current in gases is due to the ordered movement of charged particles under the action of an electric field applied to them. The presence of such charges, in turn, is possible due to various ionization factors.
So, thermal ionization requires significant temperatures, but an open flame in connection with some chemical processes contributes to ionization. Even at a relatively low temperature in the presence of a flame, the appearance of an electric current in the gases is recorded, and experience with gas conductivity makes it easy to verify this. It is necessary to place the flame of a burner or a candle between the plates of a charged capacitor. The circuit previously opened due to the air gap in the condenser will close. The galvanometer included in 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 the stability of the discharge, the action of the ionizer must be constant, since due to constant recombination, the gas loses its electrically conductive properties. Some carriers of electric current in gases - ions - are neutralized at the electrodes, others - electrons - getting on the anode, are sent to the "plus" of the field source. If the ionizing factor ceases to act, the gas will immediately become an insulator 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 features of the passage of electric current through gases are described by a special dependence of the current strength on voltage - the current-voltage characteristic.
Let us consider the development of a gas discharge in the graph of the current – ​​voltage dependence. With increasing voltage to a certain value of U 1, the current increases in proportion to it, that is, Ohm's law is fulfilled. The kinetic energy increases, and consequently, the speed of charges in the gas, and this process is ahead of recombination. With voltage values ​​from U 1 to U 2, this relationship is violated; when reaching U 2, all charge carriers reach the electrodes, not having time to recombine. All free charges are involved, and a further increase in voltage does not lead to an increase in current strength. This nature of the movement of charges is called the saturation current. Thus, we can say that the electric current in gases is also due to the peculiarities of the behavior of the ionized gas in electric fields of different strengths.
When the potential difference at the electrodes reaches a certain value of U 3 , the voltage becomes sufficient so that the electric field causes 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 by the approximate formula v = 600 U i , where U i is the ionization potential). At this moment, gas breakdown and a substantial increase in current due to the internal ionization source occur. Therefore, this category is called independent.
The presence of an external ionizer in this case no longer plays a role in maintaining the electric current in gases. An independent discharge under different conditions and with different characteristics of the source of the electric field can have certain features. Such types of self-discharge are distinguished as smoldering, spark, arc and corona. We will consider how electric current behaves in gases, briefly for each of these types.
Glow discharge
In a rarefied gas , a potential difference from 100 (or even less) to 1000 volts is sufficient to initiate an independent discharge. Therefore, a glow discharge, characterized by a small current value (from 10 -5 A to 1 A), occurs at pressures of not more than a few millimeters of mercury.
In a tube with rarefied gas and cold electrodes, the emerging glow discharge looks like a thin luminous cord between the electrodes. If we continue pumping gas out of the tube, the cord will erode, 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 a discharge are localized precisely in the dark cathode space and in the region adjacent to it. Here the acceleration of charged gas particles takes place, 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 luminescence of the positive column is mainly associated with the emission of photons by excited gas molecules, and a certain color of luminescence is characteristic of various gases. A positive pole takes part in the formation of a glow discharge only as a portion of an electric circuit. If you bring the electrodes together, you can achieve the disappearance of the positive column, but the discharge will not stop. However, with a further reduction in the distance between the electrodes, a 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 causing an expansion on the cathode surface of the region that takes part in the discharge, remains unclear.
Spark discharge
Spark breakdown is pulsed. It occurs at pressures close to normal atmospheric, 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 bang, as well as the release of heat and bright glow. Avalanche ionization processes generate high pressures and temperatures in the spark channel up to 10 thousand degrees and above.
The brightest example of a natural spark discharge is lightning. The diameter of the main spark channel of lightning can range from a few centimeters to 4 m, and the channel length can reach 10 km. The magnitude of the current reaches 500 thousand amperes, and the potential difference between a thundercloud and the Earth's surface reaches a billion volts.
The longest lightning with a length of 321 km was observed in 2007 in Oklahoma, USA. The record holder for the duration was the lightning recorded in 2012 in the French Alps - it lasted over 7.7 seconds. With a lightning strike, the air can warm 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 large enough, a spark discharge develops in an arc.
Arc discharge
This type of self-discharge is characterized by a high current density and a low voltage (less than during a glow discharge). The breakdown distance is small due to the proximity of the electrodes. The discharge is initiated by the emission of an electron from the surface of the cathode (for metal atoms, the ionization potential is small compared to gas molecules). During the breakdown between the electrodes, conditions are created under which the gas conducts an electric current, and a spark discharge closes the circuit. If the power of the voltage source is large enough, spark discharges pass into a stable electric arc.
Ionization during an arc discharge reaches almost 100%, the current strength is very large 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. Electron bombardment of the anode leads to partial destruction: a depression forms on it - a crater with a temperature of about 4000 ° C. An increase in pressure entails an even greater increase in temperature.
When diluting the electrodes, the arc discharge remains stable up to a certain distance, which allows you to deal with it in those areas of electrical equipment where it is harmful due to the corrosion caused by it and burnout of the contacts. These are devices such as high-voltage and circuit breakers, contactors and others. One of the methods for controlling the arc that occurs when contacts are opened is the use of arcing chambers based on the principle of arc lengthening. Many other methods are also used: contact shunting, the use of materials with high ionization potential, and so on.
Corona discharge
The development of a corona discharge occurs at normal atmospheric pressure in sharply inhomogeneous fields at electrodes with a large surface curvature. It can be spiers, masts, wires, various elements of electrical equipment having a complex shape, and even human hair. Such an electrode is called a corona electrode. Ionization processes and, accordingly, gas luminescence take place only near it.
The corona can be formed both on the cathode (negative corona) during ion bombardment and on the anode (positive) as a result of photoionization. The negative corona, in which the ionization process as a result of thermal emission is directed from the electrode, is characterized by a steady glow. Streamers can be observed in the positive corona — luminous lines of a broken configuration that can turn into spark channels.
An example of a corona discharge in natural conditions is the lights of St. Elmo, arising on the tips of high masts, treetops and so on. They are formed with a high electric field in the atmosphere, often before a thunderstorm or during a snowstorm. In addition, they were fixed on the skin of aircraft that fell into a cloud of volcanic ash.
Corona discharge on power lines leads to significant energy losses. At high voltage, the corona discharge can go into an arc discharge. The fight against him is carried out in various ways, for example, by increasing the radius of curvature of the conductors.
Electric current in gases and plasma
Fully or partially ionized gas is called plasma and is considered the fourth state of aggregation of matter. In general, the plasma is electrically neutral, since the total charge of its constituent particles is zero. This distinguishes it from other systems of charged particles, such as, for example, electron beams.
Under natural conditions, a plasma is formed, as a rule, at high temperatures due to the collision of gas atoms at high speeds. The vast majority of baryonic matter in the Universe is in a state of plasma. These are stars, part of interstellar matter, intergalactic gas. The terrestrial ionosphere is also a rarefied weakly ionized plasma.
The degree of ionization is an important characteristic of a plasma — conductive properties depend on it. The degree of ionization is defined as the ratio of the number of ionized atoms to the total number of atoms per unit volume. The stronger the ionized plasma, the higher its electrical conductivity. In addition, it is characterized by high mobility.
Thus, we see that the gases conducting electric current within the discharge channel are nothing but plasma. So, 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.
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The physics of electric current in liquids and gases is not the same. Conductivity of electrolytes in general is subject to Ohm's law, but it is not observed in a gas discharge. The current – ​​voltage characteristic of gases has a much more complex character associated with plasma properties.
Mention should 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, current is also possible. But potential carriers are already present in the gas, they only need to be ionized. Carriers are introduced into the vacuum from the substance. As a rule, this occurs in the process of electron emission, for example, when the cathode is heated (thermionic emission). But also in various types of gas discharges, emission, as we have seen, plays an important role.
The use of gas discharges in technology
In brief, the harmful effects of various discharges have already been discussed above. Now let's pay attention to the benefits that they bring in industry and in everyday life.
A glow discharge is used in electrical engineering (voltage stabilizers), in the coating technology (cathodic sputtering method based on the phenomenon of cathode corrosion). In electronics, it is used to produce ion and electron beams. A widely known field of application for glow discharges are fluorescent and so-called economical lamps and decorative neon and argon gas discharge tubes. In addition, a glow discharge is used in gas lasers and in spectroscopy.
The spark discharge is used in fuses, in electroerosion methods for the precise processing of metals (spark cutting, drilling, and so on). But it is best known due to the use of internal combustion engines in spark plugs and in household appliances (gas stoves).
The arc discharge, being first used in lighting technology as far back as 1876 (Yablochkov's candle is “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, in metal cutting, in industrial electric furnaces for smelting steel and alloys.
Corona discharge is used in electrostatic precipitators for ion cleaning of gases, in particle counters, in lightning rods, and in air conditioning systems. The corona discharge also works in copiers and laser printers, where it charges and discharges the photosensitive drum and transfers the powder from the drum to paper.
Thus, gas discharges of all types are most widely used. Electric current in gases is successfully and effectively used in many areas of technology.