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Electric discharge definition. Lightning. Normal glow discharge

28.03.2023

L E C T I A

in the discipline "Electronics and fire automatics" for cadets and students

specialty 030502.65 - "Forensic examination"

on topic number 1."Semiconductor, electronic, ion devices"

The topic of the lecture is "Indicating and photoelectric devices".

Indicator devices

Electric discharge in gases.

Gas-discharge (ionic) devices are called electrovacuum devices with an electric discharge in a gas or vapor. The gas in such devices is under reduced pressure. An electric discharge in a gas (in steam) is a set of phenomena that accompany the passage of an electric current through it. With such a discharge, several processes take place.

Excitation of atoms.

Under the impact of an electron, one of the electrons of the gas atom moves to a more distant orbit (to a higher energy level). Such an excited state of the atom lasts 10 -7 - 10 -8 seconds, after which the electron returns to its normal orbit, giving off the energy received during the impact in the form of radiation. The radiation is accompanied by the glow of a gas if the emitted rays belong to the visible part of the electromagnetic spectrum. In order for an atom to be excited, the striking electron must have a certain energy, the so-called excitation energy.

Ionization.

The ionization of atoms (or molecules) of a gas occurs when the energy of the impacting electron is greater than the excitation energy. As a result of ionization, an electron is knocked out of an atom. Consequently, there will be two free electrons in space, and the atom itself will turn into a positive ion. If these two electrons gain sufficient energy while moving in the accelerating field, each of them can ionize a new atom. There will be four free electrons, and three ions. There is an avalanche-like increase in the number of free electrons and ions.

Stepwise ionization is possible. From the impact of one electron, the atom goes into an excited state and, not having time to return to the normal state, is ionized by the impact of another electron. An increase in the number of charged particles in a gas due to ionization (free electrons and ions) is called gas electrization.

Recombination.

Along with ionization in a gas, the reverse process of neutralization of charges opposite in sign also takes place. Positive ions and electrons move chaotically in the gas, and as they approach each other, they can combine to form a neutral atom. This is facilitated by the mutual attraction of oppositely charged particles. The reduction of neutral atoms is called recombination. Since energy is expended on ionization, a positive ion and an electron together have an energy greater than that of a neutral atom. Therefore, recombination is accompanied by the emission of energy. Usually, this is observed gas glow.

When an electric discharge occurs in a gas, ionization prevails, with a decrease in its intensity, recombination. At a constant intensity of an electric discharge in a gas, a steady state is observed in which the number of free electrons (and positive ions) arising per unit time due to ionization is on average equal to the number of neutral atoms resulting from recombination. With the termination of the discharge, ionization disappears and, due to recombination, the neutral state of the gas is restored.

Recombination requires a certain period of time, so deionization is completed in 10 -5 - 10 -3 seconds. Thus, compared with electronic devices, gas discharge devices are much more inertial.

Types of electrical discharges in gases.

Distinguish between self-sustained and non-self-sustained discharges in a gas. Self-discharge is maintained under the action of only electrical voltage. A non-self-sustained discharge can exist provided that, in addition to the voltage, some additional factors act. They can be light radiation, radioactive radiation, thermionic emission of a hot electrode, etc.

T is dependent silent or silent discharge. The glow of the gas is usually imperceptible. It is practically not used in gas-discharge devices.

The independent ones include t flowing discharge. It is characterized by a glow of gas, reminiscent of the glow of smoldering coal. The discharge is maintained due to the electron emission of the cathode under the impact of ions. Glow discharge devices include zener diodes (gas-discharge voltage stabilizers), gas-light lamps, glow-discharge thyratrons, sign indicator lamps, and decatrons (gas-discharge counters).

arc discharge it can be both dependent and independent. The arc discharge is obtained at a current density much higher than in a glow discharge and is accompanied by an intense glow of the gas. Non-self-sustaining arc discharge devices include gastrons and hot-cathode thyratrons. Devices for independent arc discharge include mercury valves (excitrons) and ignitrons with a liquid mercury cathode, as well as gas dischargers.

spark discharge is similar to an arc discharge. It is a short-term pulsed electrical discharge. It is used in arresters that serve for short-term circuits of certain circuits.

high frequency discharge can occur in a gas under the action of an alternating electromagnetic field, even in the absence of conductive electrodes.

corona discharge is independent and is used in gas-discharge devices for voltage stabilization. It is observed in cases where one of the electrodes has a very small radius.

Under normal conditions, the conductivity of insulators is very low. However, in sufficiently strong electric fields, the so-called breakdown of the insulator, or electric discharge, occurs. At the point of breakdown, the conductivity of the insulator sharply increases, and it depends in a complex way on the field strength, current, initial conditions, and many other factors.

Let's start with an electric discharge in a gas. The conductivity of a gas in weak fields is associated with the presence of a small number of ions and electrons in it, which arise due to the ionization of gas molecules under the action of cosmic rays, the radioactivity of the earth's crust, and, to a lesser extent, ultraviolet radiation from the sun. For example, at the surface of the sea, cosmic rays create about two pairs of ions per cubic centimeter per second. At the land surface, about five more pairs of ions are added to this due to the radioactivity of the earth's crust. The average concentration of all ions near the Earth's surface is about 100 s. For such a long time, all the electrons that have arisen as a result of ionization have time to form negative ions, "adhering" to oxygen molecules. Under normal conditions, an electron needs about 105 collisions for this, i.e., only s. This shows that, under normal conditions, the conductivity of a gas in weak fields is ionic. The real picture is even more complicated: conductivity is determined mainly by ion clusters containing dozens of gas atoms. The conductivity of air near the surface of the Earth while the conductivity of the best solid insulators (amber, fused quartz) is and for ordinary glass -

In a liquid, unlike a gas, the concentration of ions is determined not by external ionization, but by the dissociation of molecules due to their interaction with each other. This liquid is called an electrolyte. Dissociation is particularly facilitated if the liquid is a solution, so that the latter generally has a significant conductivity. So, for example, the conductivity of a solution of copper sulfate, which is still seven orders of magnitude less than that of copper. This is explained by the fact that the charge carriers in the electrolyte (as well as in the gas) are heavy ions, and the viscosity of the liquid is much higher than the viscosity of the electron gas in the metal.

Let us now return to the gas and consider its behavior in stronger fields. On fig. II 1.5 shows schematically the current-voltage characteristic of the gas gap. Weak field region

Rice. 111.5. Volt-ampere characteristic of the gas gap.

Rice. 111.6. Paschen curves for some gases.

corresponds to section a, where Ohm's law is valid. It is followed by the so-called plateau (the area where the current is practically independent of the field strength. In this area, the electric field pulls out all the electrons born (in the gap). In even stronger fields (section c), the current increases sharply, and a breakdown occurs. associated with the processes of secondary ionization, leading to an avalanche "multiplication" of electrons. Very simply, this process can be represented as follows. An electron knocked out of an atom during ionization is accelerated by an external field to such an energy (~ 10 eV) that it can itself ionize others atoms.

The electron avalanche in itself leads only to an increase in the conduction current in the gas gap (section c, see Fig. III.5). For the occurrence of an electric, or, more precisely, a self-sustaining discharge, the so-called feedback between the electrodes of the gas gap is also necessary. It is necessary that the electron avalanche moving towards the anode would somehow cause new avalanches from the cathode. One of the possible mechanisms for such feedback is the photoelectric effect from the cathode under the action of photons emitted by excited gas or anode atoms.

The discharge ignition conditions are characterized by the so-called Paschen curve (Fig. III.6), which connects three main quantities: the voltage across the discharge gap V, the length of the gap and the gas pressure. First of all, it turns out that the ignition of the discharge depends only on the product where is the length free path of an electron. It characterizes the rate of development of the electron avalanche.

The dependence of the discharge ignition voltage on has a characteristic minimum. The form of the Paschen curve can be easily explained qualitatively by considering the case of constant pressure. The development of an avalanche is determined by the field strength, so the ignition voltage increases approximately in proportion to the length of the gap. At very small values, however, the development of an electron avalanche is also difficult, since the electrons do not have time to collide with gas atoms in the gap. It is interesting to note that at voltages less than

minimum, the gap does not break through under any conditions.

At very high pressures (more precisely, large values), the discharge development mechanism changes significantly. Due to the short electron mean free path, the discharge is first localized in a small region of the gap near the site of primary ionization. Under these conditions, the discharge propagates mainly due to the photoionization of neighboring gas regions. Such a process is called a streamer.An example of a streamer discharge is lightning.

One of the interesting applications of the streamer discharge is the so-called streamer chamber, in which traces of charged particles can be observed. A strong electric field is created in the chamber for a very short time. A charged particle that has passed through the chamber just before the field is switched on produces gas ionization along its trajectory, and the free electrons formed in this case serve as the centers for the appearance of streamers. The glow of streamers makes it possible to observe the tracks of charged particles (Fig. III.7). Due to the impulsive nature of the field, the dimensions of the streamers remain small, which ensures a high degree of trajectory localization (of the order of 0.3 mm).

At very low pressures, i.e., in a high vacuum, the breakdown of the gap is determined almost exclusively by processes at the electrodes. The discharge develops due to autoelectronic (“cold”) electron emission from micropoints on the cathode surface, when the field strength on them rises to V/cm. The emission current, the density of which reaches enormous values at the tip, heats and evaporates the tip, and a strong electric field tears off and drags small pieces of the cathode to the anode. The latter cause evaporation of the anode material, and the resulting ions bombard, in turn, the cathode, heating it and causing thermionic emission.

Rice. 111.7. Photographs of electron and positron tracks in a stringer chamber (a) and a discharge (b) in a liquid (hexane). Electric field strength 700 kV/cm, exposure time 5ns.

The described mechanism of development of a discharge in a high vacuum makes it possible to understand the effect of "training" the vacuum gap, which is important in practical terms. Training is carried out by repeated breakdown of the gap at a low discharge power and leads to melting of the tips on the cathode.

It is also quite natural that the electrical strength of the gap increases significantly at a very short duration of high voltage or at its high frequency. So, for example, at with the vacuum gap withstands the field around, while at with this value drops to and further does not depend on .

The phenomenon of static electricity is usually observed in dielectrics. If the chemical bond in the dielectric is ionic, then due to the imperfection of the structure of the substance, the number of positive and negative ions per unit volume of the substance is not the same. This means that almost any dielectric body with an ionic bond initially has an electric charge, around which there is an electrostatic field.

In real conditions, this charge is usually compensated by charges from the environment, which are deposited on the surface of the dielectric. As a result, there is no electrostatic field around such a body.

If the chemical bond in the dielectric is covalent, then the dielectric can have a nonzero electric dipole moment and, as a result, creates an electrostatic field around itself. Under real conditions, compensating charges are deposited from the environment on the surface of such a dielectric, so that the electric field around such a body becomes zero.

The mechanical interaction of bodies can lead to the removal of compensating charges from the corresponding surfaces and the appearance of an electric field in the surrounding space, which can interfere with the inputs of electrical devices. This electric field can in some cases lead to a breakdown of the dielectric (for example, air).

The discharges associated with this breakdown form electromagnetic pulses in space, which also transmit interference.

The total internal resistance of the source is from 1 to 30 kOhm.

The total inductance of the discharge path is 0.3 - 1.5 μH.

Capacitance ranges from 100 to 300 pF.

Maximum voltage up to 15 kV.

The maximum discharge pulse current is up to 30 A.

Current slew rate from 2 to 35 A/ns.

An approximate form of a current pulse during a discharge of electricity:

Approximate current pulse shape Spectral characteristic:

when discharging electricity:

Classification of interference sources

Distinguish functional sources and non-functional.

Functional sources are radio and television transmitters that propagate electromagnetic waves into the environment in order to transmit information. This group includes all devices that emit electromagnetic waves not for the purpose of communication, but for the performance of their technical function, for example, a high-frequency generator for industrial or medical use, microwave radio control devices.

Non-functional sources include automotive ignition devices, fluorescent lamps, welding equipment, relay and protection coils, rectifiers, contact and proximity switches, wire lines and electrical components, intercoms, atmospheric discharges, corona discharges in lines, switching processes, discharges of static electricity, rapidly changing currents and voltages in high voltage laboratories.

There are also broadband and narrowband interference sources.

Broadband is interference with a wide frequency spectrum, and narrowband is narrow.

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electrical discharge

An electrical discharge is a complex process of formation of a conductive channel when the applied electric field reaches a critical value. As a result of the discharge, various types of plasma are formed. Any discharge begins with the formation of an electron avalanche. An electron avalanche is a process of increasing the number of primary electrons due to ionization.

Consider a flat slot with a distance between the electrodes d, to which the voltage V is applied. The electric field strength in the gap will be. It can be imagined that one electron was formed near the cathode. This electron starts moving towards the anode, ionizing the gas on its way, i.e. producing secondary electrons, forming an avalanche. The avalanche develops in time and space because the secondary electrons also begin to move towards the anode.

Figure 1. - Electron avalanche

The ionization process is conveniently described not by the ionization coefficient, but by the Townsen's ionization coefficient?, which shows the number of electrons produced per unit length

where n e is the initial electron density, or

Townsen's ionization coefficient is related to the ionization coefficient as follows.

Where? i - ionization frequency with respect to one electron;

D is the electron drift velocity;

E - electron mobility;

K i () - ionization coefficient.

Taking into account that the avalanche starts moving at room temperature and the electron mobility is inversely proportional to the pressure, it is convenient to write α as, which depends on the value.

According to the definition?, each primary electron generates positive ions in the gap. Electrons can be lost through recombination and attachment to electronegative molecules such as oxygen. At this stage, we neglect these losses. All positive ions born in the gap move towards the cathode and create on it? secondary electrons, where? is the ion-electron emission coefficient, which depends on the cathode material, surface condition, gas type. Typical values? in electrical discharges 0.01-0.1. In the same ratio? includes secondary emission of electrons due to photons and metastable atoms and molecules. In order for the gap current to be self-sustaining, it is necessary that Now the discharge condition can be written as

Let us calculate the critical value of the electric field for the discharge to occur. Based on expressions (1.3, 1.4), we can write

where p is the pressure.

Parameters A and B are given in Table 1.1.

Combining (1.4) and (1.5) we obtain a formula for calculating the electric field.

Table 1.1 - Parameters A and B

The base of the natural logarithm.

As a result, when a critical value of the electric field is applied between the metal electrodes, a conductive channel appears, through which a large current passes, because the critical voltage is high enough and the channel resistance is low. As a result, a strong heating of the gas occurs, which is undesirable in many plasma-chemical processes.

electric discharge ionization streamer

Figure 2 - Mechanism of streamer formation

To eliminate this spark discharge, a barrier discharge mechanism has been developed.

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A Brief History of the Study of Electricity

When did man get acquainted with electricity? It is difficult to answer this question, because it was put in an incorrect way, because the most striking natural phenomenon is lightning, known from time immemorial.

A meaningful study of electrical processes began only at the end of the first half of the 18th century. Here it should be noted a serious contribution to the ideas of man about electricity by Charles Coulomb, who studied the force of interaction of charged particles, George Ohm, who mathematically described the parameters of the current in a closed circuit, and Benjamin Franklin, who conducted many experiments studying the nature of the above-mentioned lightning. In addition to them, scientists such as Luigi Galvani (the study of nerve impulses, the invention of the first "battery") and Michael Faraday (the study of current in electrolytes) played a large role in the development.

The achievements of all these scientists have created a solid foundation for the study and understanding of complex electrical processes, one of which is an electric discharge.

What is a discharge and what conditions are necessary for its existence?

The discharge of electric current is a physical process, which is characterized by the presence of a flow of charged particles between two spatial regions having different potentials in a gaseous medium. Let's analyze this definition.

First, when people talk about discharge, they always mean gas. Discharges in liquids and solids can also occur (breakdown of a solid capacitor), but the process of studying this phenomenon is easier to consider in a less dense medium. Moreover, it is discharges in gases that are often observed and are of great importance for human life.

Secondly, as stated in the definition of an electric discharge, it occurs only when two important conditions are met:

when there is a potential difference (electric field strength);
the presence of charge carriers (free ions and electrons).

The potential difference ensures the directed movement of the charge. If it exceeds a certain threshold value, then the non-self-sustaining discharge becomes self-sustaining or self-sustaining.

As for free charge carriers, they are always present in any gas. Their concentration, of course, depends on a number of external factors and the properties of the gas itself, but the very fact of their presence is indisputable. This is due to the existence of such sources of ionization of neutral atoms and molecules as ultraviolet rays from the Sun, cosmic radiation and the natural radiation of our planet.

The relationship between the potential difference and the carrier concentration determines the nature of the discharge.

Types of electrical discharges

Here is a list of these types, and then we will characterize each of them in more detail. So, all discharges in gaseous media are usually divided into the following:

smoldering;
spark;
arc;
crown.

Physically, they differ from each other only in power (current density) and, as a result, in temperature, as well as in the nature of their manifestation in time. In all cases, we are talking about the transfer of a positive charge (cations) to the cathode (low potential region) and a negative charge (anions, electrons) to the anode (high potential zone).

glow discharge

For its existence, it is necessary to create low gas pressures (hundreds and thousands of times less than atmospheric pressure). A glow discharge is observed in cathode tubes that are filled with some kind of gas (for example, Ne, Ar, Kr, and others). The application of voltage to the electrodes of the tube leads to the activation of the following process: the cations present in the gas begin to move rapidly, reaching the cathode, they hit it, transferring momentum and knocking out electrons. The latter, in the presence of sufficient kinetic energy, can lead to the ionization of neutral gas molecules. The described process will be self-sustaining only in the case of sufficient energy of the cations bombarding the cathode and a certain amount of them, which depends on the potential difference at the electrodes and the gas pressure in the tube.

Glow discharge glows. The emission of electromagnetic waves is due to two parallel processes:

recombination of electron-cation pairs, accompanied by energy release;
the transition of neutral molecules (atoms) of a gas from an excited state to the ground state.

Typical characteristics of this type of discharge are small currents (a few milliamps) and small stationary voltages (100-400 V), but the threshold voltage is several thousand volts, which depends on the gas pressure.

Examples of glow discharges are fluorescent and neon lamps. In nature, northern lights (the movement of ion flows in the Earth's magnetic field) can be attributed to this type.

spark discharge

This is a typical type of discharge, which manifests itself in For its existence, not only the presence of high gas pressures (1 atm or more), but also huge voltages are necessary. Air is a fairly good dielectric (insulator). Its permeability ranges from 4 to 30 kV/cm, depending on the presence of moisture and solid particles in it. These figures indicate that in order to obtain a breakdown (spark), a minimum of 4,000,000 volts must be applied to each meter of air!

In nature, such conditions arise in cumulus clouds, when, as a result of friction between air masses, air convection and crystallization (condensation), charges are redistributed in such a way that the lower layers of the clouds are charged negatively, and the upper layers are positively charged. The potential difference gradually accumulates, when its value begins to exceed the insulating capabilities of air (several million volts per meter), then lightning occurs - an electrical discharge that lasts for a fraction of a second. The current strength in it reaches 10-40 thousand amperes, and the plasma temperature in the channel rises to 20,000 K.

The minimum energy that is released during the lightning process can be calculated if we take into account the following data: the process develops during t=1*10 -6 s, I = 10,000 A, U = 10 9 V, then we get:

E = I*U*t = 10 million J

The resulting figure is equivalent to the energy that is released in the explosion of 250 kg of dynamite.

As well as spark, it occurs when there is sufficient pressure in the gas. Its characteristics are almost completely similar to spark, but there are differences:

firstly, the currents reach ten thousand amperes, but the voltage in this case is several hundred volts, which is associated with the high conductivity of the medium;
secondly, the arc discharge exists stably in time, in contrast to the spark discharge.

The transition to this type of discharge is carried out by a gradual increase in voltage. The discharge is maintained due to thermionic emission from the cathode. A prime example of this is the welding arc.

corona discharge

This type of electrical discharge in gases was often observed by sailors who traveled to the New World discovered by Columbus. They called the bluish glow at the ends of the masts "the fires of St. Elmo."

A corona discharge occurs around objects that have a very strong electric field. Such conditions are created near sharp objects (masts of ships, buildings with gabled roofs). When a body has some static charge, then the field strength at its ends leads to ionization of the surrounding air. The resulting ions begin their drift towards the source of the field. These weak currents, which cause similar processes as in the case of a glow discharge, lead to the appearance of a glow.

Danger of discharges for human health

Corona and glow discharges do not pose a particular danger to humans, since they are characterized by low currents (milliamps). The other two of the above discharges are deadly in case of direct contact with them.

If a person observes the approach of lightning, then he must turn off all electrical appliances (including mobile phones), and also position himself so that he does not stand out from the surrounding area in terms of height.

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