Process of ionization of gases
Gases are dielectrics if they are in usual physical conditions. In this case gases consist generally of neutral atoms and molecules, and the charged particles (electrons, ions) which are available in some volume of gas only in insignificant quantity cannot form noticeable current. However the charged particles — ions can be formed of neutral molecules and atoms if for any reasons the number of electrons in them changes: this process carries the name of ionization. Ionized gas is the conductor.
Ionization happens under the influence of cosmic rays, x-ray and ultra-violet radiation, high temperature, electric field.
Experience shows that the listed ionizuyushchy factors in itself cannot cause significant growth in number of the charged particles in unit of volume especially as along with ionization there is the inverse process of formation of neutral molecules and atoms called by the recombination.
The conductivity of gas which resulted from external ionizuyushchy influence is called dependent. If the external ionizuyushchy factor ceases to work, then owing to the recombination the conductivity of gas disappears. The ionization of atoms and molecules of gas caused by their collision with fast-moving electrons has the greatest value. At such collision energy of the moving electron is partially or completely transferred to the neutral atom or the molecule.
At sufficient impact energy one or several electrons come off the neutral atom or the molecule, instead of the neutral atom or the molecule positively ionized atoms appear. Perhaps also coupling of the electron with the neutral atom or the molecule that leads to formation of the negatively ionized atom. Process of formation of ions at collision of neutral atoms and molecules with fast-moving electrons is called ionization by collision.
As a result of ionization the quantity of electrons increases, it leads to growth of number of collisions and, therefore, to the bigger increase in number of the charged particles.
In the ionized state gas is the conductor. The conductivity of gas maintained thanks to ionization by collision by action of external electric field is called the self-sustained discharge.
Distinguish several types of the self-sustained discharge in gas: silent, smoldering, spark, arc.
Silent discharge arises with rather big gas pressures (for example, atmospheric) when the field in discharge gap between electrodes very unevenly because of the small radius of curvature of electrodes.
Silent discharge is usually observed about electrodes in those places where the electric field intensity reaches some value called critical for this gas and is followed by the luminescence — the crown.
By transfer of electrical energy on high tension around wires of the line it is quite often possible to observe (especially during wet weather) silent (crown) discharge which does harm, causing, in particular, additional losses of energy.
Glow discharge. With low pressures (about 1 mm Hg.) in the long glass tube it is possible to receive the glow discharge if between the electrodes located at its ends, to put tension in several cells of volts. Different gases at the glow discharge give the luminescence of different color. Thanks to it lamps of the glow discharge are applied in the decorative purposes
The dependence of current in the lamp of the glow discharge from tension between electrodes (volt-ampere characteristic) nonlinear, and in some interval of change of current tension remains to constants (the site of BV in fig. 1). In this drawing the point And characteristics corresponds to ignition of the device, the point In — to the beginning of the electric arc. Gas-discharge devices of the glow discharge are used for voltage stabilization.
The spark discharge arises between cold electrodes at the big internal resistance of the power source. The gas ionization which began under the influence of electric field gains the avalanche character therefore the gas interval becomes carrying out, and between electrodes the spark slips. At the same time resistance of the gas interval sharply decreases.
On distance between electrodes at which there is the air breakdown it is possible to judge tension size between electrodes. On this basis for measurement of very high tension sphere gaps are applied.
At the big power of the power source the spark discharge can pass into arc, steadier self-sustained discharge in gas at atmospheric or elevated pressure.
Such discharge carries the name of the electric arc or Petrov's arch as it was for the first time observed in 1803 by professor V.V. Petrov. Characteristic of the electric arc is that it is followed by the dazzling luminescence and strong heating of electrodes (up to 3000 °C and more).
Light action of the electric arc is used for special lighting (searchlights, projecting cameras), and thermal action — for welding and melting of metals.
The electric arc arising at switching off of electrical units — the phenomenon undesirable as its thermal action destroys contacts of the disconnecting devices (breakers, contactors, switches). Therefore it is necessary to take special measures therefore the switching-off devices considerably become complicated, increase their sizes.Top