The century in which we live can be called the time of electricity. The work of computers, televisions, cars, satellites, artificial lighting devices is just a small part of the examples where it is used. One of the interesting and important processes for humans is electrical discharge. Let us consider in more detail what it is.
A Brief History of the Study of Electricity
When did a person get acquainted with electricity? It is difficult to answer this question, since it was posed in an incorrect way, because the most striking natural phenomenon is lightning, known since time immemorial.
A meaningful study of electrical processes began only at the end of the first half of the 18th century. It should be noted that Charles Coulomb, who studied the force of interaction of charged particles, Georg Ohm, mathematically describing the parameters of the current in a closed circuit, and Benjamin Franklin, who conducted many experiments studying the nature of the aforementioned lightning, made a serious contribution to the person's ideas about electricity. 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 of the physics of electricity .
The achievements of all these scientists have created a solid foundation for the study and understanding of complex electrical processes, one of which is electrical discharge.
What is a discharge and what conditions are necessary for its existence?
An electric current discharge is a physical process that is characterized by the presence of a flow of charged particles between two spatial regions having different potentials in a gaseous medium. We analyze this definition.
First, when they talk about discharge, they always mean gas. Discharges in liquids and solids can also occur (breakdown of a solid capacitor), however, the process of studying this phenomenon is easier to consider in a less dense medium. Moreover, it is gas discharges that are often observed and are of great importance for human life.
Secondly, as stated in the definition of an electric discharge, it arises only if 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 provides directional movement of the charge. If it exceeds a certain threshold value, then a 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 fact of their presence is indisputable. This is due to the existence of such sources of ionization of neutral atoms and molecules, such 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 Electric Discharges
We give a list of these species, and then describe each of them in more detail. So, it is customary to divide all discharges in gaseous media into the following:
- smoldering;
- spark;
- arc;
- crown.
Physically, they differ from each other only in power (current density) and, as a consequence, 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). A glow discharge is observed in cathode tubes, which 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 in the gas begin to move rapidly, reaching the cathode, they hit it, transmitting a pulse and knocking out the 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 case of sufficient energy of the cations bombarding the cathode, and a certain number of them, which depends on the potential difference on the electrodes and the gas pressure in the tube.
The glow discharge glows. Radiation of electromagnetic waves is caused by two parallel processes:
- recombination of electron-cation pairs, accompanied by the release of energy;
- the transition of neutral molecules (atoms) of a gas from an excited state to the ground.
Typical characteristics of this type of discharge are small currents (several milliamps) and small stationary voltages (100-400 V), however, the threshold voltage is several thousand volts, which depends on the gas pressure.
Examples of glow discharge are fluorescent and neon lamps. In nature, this type can be attributed to the northern lights (the movement of ion flows in the Earth's magnetic field).
Spark discharge
This is a typical type of atmospheric electrical discharge, which manifests itself in the form of lightning. For its existence, it is necessary not only the presence of high gas pressures (1 atm or more), but also huge stresses. Air is a fairly good dielectric (insulator). Its permeability ranges from 4 to 30 kV / cm, which depends 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 per meter of air must be applied!
In nature, such conditions arise in cumulus clouds when, as a result of friction between air masses, air convection and crystallization (condensation), redistribution of charges occurs in such a way that the lower layers of the clouds are negatively charged, and the upper ones are positive. 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 electric discharge that lasts for a split 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 the following data are taken into account: the process develops over 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 during the explosion of 250 kg of dynamite.
Arc discharge
Like spark, it occurs when there is sufficient pressure in the gas. Its characteristics are almost completely similar to the spark, but there are also differences:
- firstly, currents reach ten thousand amperes, but the voltage is several hundred volts, which is associated with high conductivity of the medium;
- secondly, the arc discharge exists stably in time, in contrast to the spark.
The transition to this type of discharge is carried out by a gradual increase in voltage. The discharge is supported by thermionic emission from the cathode. A striking example is the welding arc.
Corona discharge
This type of electric 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 lights of St. Elmo."
A corona discharge arises around objects having a very strong electric field strength. Such conditions are created near sharp objects (masts of ships, buildings with gabled roofs). When the body has some static charge, then the field strength at its ends leads to ionization of the surrounding air. Arisen ions begin their drift to the source of the field. These weak currents, causing similar processes, as in the case of a glow discharge, lead to the appearance of a glow.
Danger of discharges to human health
Corona and glow discharges are not particularly dangerous for humans, since they are characterized by low currents (milliamps). The other two of the above categories 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 as not to stand out from the surrounding area in terms of height.