There are no absolute dielectrics in nature. The ordered movement of particles - carriers of electric charge - that is, current, can be caused in any medium, but this requires special conditions. We will consider here how electrical phenomena proceed in gases and how a gas can be changed 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 characterize the 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 everywhere. For example, in any circuit breaker, the opening of the circuit occurs when the contacts are brought into such a position that an air gap forms between them. Wires in power linesare also isolated 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 space in some way. A gas molecule can be an electric dipole due to the peculiarities of its structure, or it can be polarized under the action 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 assume 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 one can rather speak of separate currents, so weak that they should be neglected. The gas behaves like a dielectric.
Thus, for the occurrence of an electric current in gases, a large 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 there is a significant amount of charged particles is called ionized. It is in such gases that an electric current is created.
The ionization process is associated with the violation of the neutrality of molecules. As a result of the detachment of an electron, positive ions appear, the attachment of an electron to a molecule leads to the formation of a negative ion. In addition, there are many free electrons in an ionized gas. Positive ions and especially electrons are the main charge carriers for electric current in gases.
Ionization occurs when a certain amount of energy is imparted to a particle. Thus, an external electron in the composition of 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 takes on an avalanche-like character. The kinetic energy of the particles also increases, which greatly promotes ionization.
Where does the energy used to excite 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 electric field. In this case, the potential energy of the field is converted into the kinetic energy of the particles.
- Thermoionization. An increase in temperature also leads to the formation of a large number of free charges.
- Photoionization. The essence of this process is that electrons are supplied with energy by electromagnetic radiation quanta - photons, if they have a sufficiently high frequency (ultraviolet, x-ray, gamma quanta).
- Impact ionization is the result of the conversion of the kinetic energy of colliding particles into the energy of electron separation. As well asthermal ionization, it serves as the main excitation factor in gases of electric current.
Each gas is characterized by a certain threshold value - the ionization energy required for an electron to break away from a molecule, overcoming a potential barrier. This value for the first electron ranges from several volts to two tens of volts; more energy is needed to remove the next electron from the molecule, 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 action of Coulomb forces of attraction.
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 due to 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 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 previously open due to the air gap in the capacitor will close. A galvanometer connected to the circuit will show the presence of current.
Electric current in gases is called a gas discharge. It must be borne in mind thatto maintain the stability of the discharge, the action of the ionizer must be constant, since due to the constant recombination, the gas loses its electrically conductive properties. Some carriers of electric current in gases - ions - are neutralized on the electrodes, others - electrons - falling on the anode, are sent to the "plus" of the field source. If the ionizing factor ceases to operate, the gas will immediately become a dielectric again, and the current will cease. Such a current, dependent on the action of an external ionizer, is called a non-self-sustained discharge.
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's consider the development of a gas discharge on the graph of the current-voltage dependence. When the voltage rises to a certain value U1, the current increases proportionally to it, that is, Ohm's law is fulfilled. The kinetic energy increases, and hence the velocity of charges in the gas, and this process is ahead of recombination. At voltage values from U1 to U2 this ratio is violated; when U2 is reached, all charge carriers reach the electrodes without having time to recombine. All free charges are involved, and a further increase in voltage does not lead to an increase in current. This nature of the movement of charges is called saturation current. Thus, we can say that the electric current in gases is also due to the behavior of the ionized gas in electric fields of various strengths.
When the potential difference across the electrodes reaches a certain value U3, the voltage becomes sufficient for the electric field to cause an avalanche-like gas ionization. The kinetic energy of free electrons is already enough for impact ionization of molecules. At the same time, their speed in most gases is about 2000 km/s and higher (it is calculated by the approximate formula v=600 Ui, where Ui is the ionization potential). At this moment, a gas breakdown occurs and a significant increase in current occurs due to an 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 electric current in gases. A self-sustained discharge under different conditions and with different characteristics of the electric field source can 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.
Glow Discharge
In a rarefied gas, a potential difference from 100 (and even less) to 1000 volts is enough to initiate an independent discharge. Therefore, a glow discharge, characterized by a low current strength (from 10-5 A to 1 A), occurs at pressures of no more than a few millimeters of mercury.
In a tube with a rarefied gas and cold electrodes, the emerging glow discharge looks like a thin luminous cord between the electrodes. If you continue pumping gas from the tube, you will observeblurring of the cord, and at pressures of tenths of millimeters of mercury, the glow fills the tube almost completely. The glow is absent near the cathode - in the so-called dark cathode space. The rest is called the positive column. In this case, the main processes that ensure the existence of the discharge are localized precisely in the dark cathode space and in the region 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. The electrons emitted by the cathode produce impact ionization of gas molecules, the emerging positive ions cause secondary emission from the cathode, and so on. The glow of the positive column is mainly due to the recoil 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 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, the glow discharge will not be able to exist.
It should be noted that for this type of electric current in gases, the physics of some processes has not yet been fully elucidated. For example, the nature of the forces that cause an expansion on the cathode surface of the region that takes part in the discharge remains unclear.
Spark discharge
Sparkbreakdown has an impulsive character. It occurs at pressures close to normal atmospheric, in cases where the power of the electric field source is not enough to maintain a stationary discharge. In this case, 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 is repeated.
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 heats up rapidly and undergoes thermal expansion, which causes acoustic waves. Therefore, the spark discharge is accompanied by crackling, as well as the release of heat and a bright glow. Avalanche ionization processes generate high pressures and temperatures up to 10 thousand degrees and more in the spark channel.
The clearest 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 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 321 km lightning was observed in 2007 in Oklahoma, USA. The record holder for the duration was lightning, recordedin 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 the temperature of the visible surface of the Sun.
In cases where the power of the source of the electric field is large enough, the spark discharge develops into an arc.
Arc Discharge
This type of self-discharge is characterized by high current density and low (less than glow discharge) voltage. The breakdown distance is small due to the 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 between the electrodes, conditions are created under which the gas conducts an electric current, and a spark discharge occurs, which closes the circuit. If the power of the voltage source is large enough, spark discharges turn into a stable electric arc.
Ionization during an arc discharge reaches almost 100%, the current strength is very high and can be 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. The bombardment of the anode with electrons leads to partial destruction: a recess is formed on it - a crater with a temperature of about 4000 °C. An increase in pressure causes an even greater increase in temperatures.
When spreading 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 and burnout of contacts caused by it. These are devices such as high-voltage and automatic switches, contactors and others. One of the methods to combat the arc that occurs when the contacts open is the use of arc chutes based on the principle of arc extension. Many other methods are also used: bridging contacts, using materials with a high ionization potential, and so on.
Corona discharge
The development of a corona discharge occurs at normal atmospheric pressure in sharply inhomogeneous fields near electrodes with a large curvature of the surface. 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 a corona electrode. Ionization processes and, accordingly, the glow of gas take place only near it.
A corona can form both on the cathode (negative corona) when bombarded with ions, and on the anode (positive) as a result of photoionization. The negative corona, in which the ionization process is directed away from the electrode as a result of thermal emission, 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 a corona discharge in natural conditions are St. Elmo's fires that occur on the tips of tall masts, treetops and so on. They are formed at a high voltage of the electricfields 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 the wires of power lines leads to significant losses of electricity. At a high voltage, a corona discharge can turn into an arc. It is fought 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 matter. On the whole, 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 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 vast majority of baryonic matter in the Universe is in the state of plasma. These are stars, part of interstellar matter, intergalactic gas. The Earth's ionosphere is also a rarefied, weakly ionized plasma.
The degree of ionization is an important characteristic of a plasma - its 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 more ionized the plasma, the higher its electrical conductivity. In addition, it is characterized by high mobility.
We see, therefore, that the gases that conduct electricity are withindischarge channels are nothing but plasma. Thus, glow and corona discharges are examples of cold plasma; a spark channel of lightning or an electric arc are examples of hot, almost completely ionized plasma.
Electric current in metals, liquids and gases - differences and similarities
Let's consider the features that characterize the 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 s alts and oxides. They are distinguished by electronic conductivity.
Conductors of the second kind are electrolytes, that is, liquid aqueous solutions of alkalis, acids and s alts. The passage of current is associated with a chemical change in the electrolyte - electrolysis. Ions of a substance dissolved in water, under the action of a potential difference, move in opposite directions: positive cations - to the cathode, negative anions - to the anode. The process is accompanied by gas evolution or deposition of a metal layer on the cathode. Conductors of the second kind are characterized by ionic conductivity.
As for the conductivity of gases, it is, firstly, temporary, and secondly, it has signs of similarities and differences with each of them. So, the 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 when combinedelectronic and ionic types of conductivity, the leading role belongs to electrons. Another difference between the 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 break down, but only lose electrons. Therefore, the gas discharge, like the current in metals, is not associated with chemical changes.
The physics of electric current in liquids and gases is also not the same. The conductivity of electrolytes as a whole obeys Ohm's law, but it is not observed during a gas discharge. The volt-ampere characteristic of gases has a much more complex character associated with the properties of plasma.
It is worth mentioning the general and distinctive features of electric current in gases and in vacuum. Vacuum is almost a 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 only need to be ionized. Charge carriers are brought into vacuum from matter. As a rule, this occurs in the process of electron emission, for example, when the cathode is heated (thermionic emission). But, as we have seen, emission also plays an important role in various types of gas discharges.
Use 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 that they bring in industry and in everyday life.
Glow discharge is used in electrical engineering(voltage stabilizers), in coating technology (cathode sputtering method based on the phenomenon of cathode corrosion). In electronics, it is used to produce ion and electron beams. A well-known field of application for glow discharges are fluorescent and so-called economical lamps and decorative neon and argon discharge tubes. In addition, glow discharges are used in gas lasers and in spectroscopy.
Spark discharge is used in fuses, in electroerosive methods of precision metal processing (spark cutting, drilling, and so on). But it is best known for its use in spark plugs of internal combustion engines and in household appliances (gas stoves).
Arc discharge, being first used in lighting technology back in 1876 (Yablochkov's candle - "Russian light"), still serves as a light source - for example, in projectors and powerful spotlights. In electrical engineering, the arc is used in mercury rectifiers. In addition, it is used in electric welding, metal cutting, industrial electric furnaces for steel and alloy smelting.
Corona discharge is used in electrostatic precipitators for ion gas cleaning, elementary particle counters, lightning rods, air conditioning systems. Corona discharge also works in copiers and laser printers, where it charges and discharges the photosensitive drum and transfers powder from the drum to paper.
Thus, gas discharges of all types find the mostwide application. Electric current in gases is successfully and effectively used in many areas of technology.
