The age we live in can be called the age of electricity. The operation of computers, televisions, cars, satellites, artificial lighting devices - this is just a small part of the examples where it is used. One of the interesting and important processes for a person is an electric discharge. Let's take a closer look at what it is.
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.
The 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-mentionedlightning. 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 an electric discharge.
What is a discharge and what conditions are necessary for its existence?
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 break down this definition.
Firstly, 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 the 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);
- 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-sustained discharge turns intoself-supporting or self-supporting.
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
Let's list these species, 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: available in the gascations 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;
- transition of neutral gas molecules (atoms) from the 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, depending on the pressure of the gas.
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 atmospheric electrical discharge that appears as lightning. For its existence, not only the presence of high gas pressures (1 atm or more), but also huge stresses are necessary. Air is a fairly good dielectric (insulator). Its permeability ranges from 4 to 30 kV/cm, depending onthe presence of moisture and solid particles in it. These figures indicate that a minimum of 4,000,000 volts must be applied to each meter of air to produce a breakdown (spark)!
In nature, such conditions occur in cumulus clouds, when, as a result of friction between air masses, air convection and crystallization (condensation), the charges are redistributed in such a way that the lower layers of the clouds are charged negatively, and the upper layers positively. 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=110-6 s, I=10 000 A, U=109 B, then we get:
E=IUt=10 million J
The resulting figure is equivalent to the energy released by the explosion of 250 kg of dynamite.
Arc Discharge
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 is several hundred volts, which is due tohighly conductive medium;
- secondly, the arc discharge exists stably in time, unlike the spark.
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 striking 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 "St. Elmo's lights."
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 to human he alth
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 as not to stand out from the surrounding area in terms ofheight.
