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4.1
Average rating: 4.1
Total ratings received: 953.
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 or long-term.
Electric current concept
During a lightning discharge, an electric current can occur, which is called short-term. And to maintain current for a long time, the presence of an electric field and free electric charge carriers is necessary.
An electric field is created by differently charged bodies. The current strength is the ratio of the charge transferred through the cross-section of a conductor over a time interval to this time interval. It is measured in Amperes.
Rice. 1. Current formula
Current-voltage characteristics of gas discharge
Dependence of the current through the gas gap on the voltage between the anode and cathode (the so-called current-voltage characteristic of the 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 everything
charged particles formed under the action of the ionizer reach the electrodes, and
the current reaches saturation
- namely, the current strength ceases to change with increasing voltage. This will happen up to a certain point.
Self-sustaining and non-self-sustaining gas discharges
Discharges in gas can be independent or non-self-sustaining. 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 stops working, then the current stops.
Self-discharge of electric current in gases exists even after the cessation of the external ionizer. Independent discharges in physics are divided into quiet, glow, arc, spark, corona.
- Quiet is 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.
- Glow - if you increase the voltage in a quiet discharge, it goes to the next level - into a glow discharge. In this case, a glow appears, which is accompanied by recombination. Recombination is the reverse process of ionization, the meeting of an electron and a positive ion. Used in bactericidal and lighting lamps.
Rice. 2. Glow discharge
- Arc - the current strength ranges from 10 A to 100 A. Ionization is almost 100%. This type of discharge occurs, for example, when operating a welding machine.
Rice. 3. Arc discharge
- Spark - 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 suddenly.
Free charges in gas
The occurrence of an electric current between the plates of the condenser means that free charges
. Which ones exactly?
Experience shows that electric current in gases is the ordered movement of charged particles of three types
.
These are 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 with each other reaches such force that ionization
— disintegration 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 an 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.
Plasma concept
Plasma is a fully or partially ionized gas in which the density of oppositely charged particles is approximately the same. To determine the degree of ionization (α), the following formula is used: α = Ni / N. Here Ni represents the number of ionized atoms and N is the total number of particles.
An example of a weakly ionized plasma is the Earth's ionosphere. Stars, including the Sun, are densely ionized. Plasma has a number of unique properties, which makes it necessary to consider it as a special state of substances, such as, for example, liquid.
Today it is difficult to imagine human civilization without electricity. With its help, people light and heat their houses, send messages, etc. The use of electric current in gases is diverse. For example, gas electric current is used for lighting rooms, in welding, in metallurgy, etc. If the movement of plasma is controlled, it can be used as a working fluid. So, a few years ago, plasma TVs were very popular.
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 physics of electric 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.
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 surface of the cathode (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 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.
Application of gas discharges
Spark discharge is used in technology in the ignition system of internal combustion engines. The ignition coil produces a voltage of 12-15 thousand volts. This is enough for a spark to occur between the electrodes of the spark plug to ignite the combustible mixture.
A type of spark discharge is lightning.
Arc discharge is used as powerful light sources (spotlights), in electric melting furnaces, for electric welding, and for ultraviolet emitters.
Glow discharge is used in advertising gas-discharge tubes, fluorescent lamps, and digital indicators.
In nature, the glow of rarefied gases is observed in the form of aurora.
Corona discharge is used in electric precipitators to purify gases from impurities of solid particles, and in the operation of a lightning rod. In power lines it leads to electricity leakage.
In nature, a “crown” sometimes appears under the influence of atmospheric electricity on tree branches, the tops of lightning rods, and ship masts (St. Elmo’s Fire).
Ionization process
Under standard conditions, gaseous substances are dielectrics. This is explained by the absence in their structure of a large number of free particles with different charges. A gas can become electrically conductive only if it is ionized. This phenomenon is the splitting of molecules into positively and negatively charged particles.
Ionization is possible only under the influence of external factors. The factors influencing this process are called ionizers. Electrons deprived of atomic bonds can be captured by particles with a neutral charge, resulting in the formation of positive ions. In an electrical gas that has undergone ionization, electrons, positive and negative ions are present as charge carriers. There are 3 types of ionization:
- Thermo. Observed when particles of gaseous substances collide at high temperatures. Their kinetic energy of motion must exceed the molecular bond of electrons in atoms.
- Photo. This process occurs under the influence of electromagnetic radiation. The energy required to separate electrons is transferred to molecules by radiation quanta.
- Percussion. Charged particles are created by the collision of neutral particles with fast-moving electrons. At the same time, they must have a high kinetic energy.
It is also necessary to consider another phenomenon that occurs in ionized gases - recombination. Its essence comes down to the recovery of neutral ions from differently charged particles. The process is accompanied by the release of a certain amount of energy, the indicator of which corresponds to the value spent on ionization.
As a result, various phenomena may appear, such as glow. This suggests that the occurrence of electric current in gases is due to the ordered movement of particles with certain charges. This phenomenon is observed only under the influence of an external field. We can say that gas and electricity, under certain conditions, are completely compatible concepts.
Corona discharge
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 a corona discharge in natural conditions is St. Elmo's fire, which occurs at the tips of tall 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.
