A vacuum is a space in which there is no substance. In applied physics and technology, it means an environment in which gas is contained under a pressure less than atmospheric. What are rarefied gases when they first learned about them?
Pages of history
The idea of emptiness has been the subject of controversy for many centuries. The rarefied gases tried to analyze the ancient Greek and Roman philosophers. Democritus, Lucretius, their students believed: if there were no free space between the atoms, their movement would be impossible.
Aristotle and his followers refuted this concept, in their opinion, there should be no "emptiness" in nature. In the Middle Ages in Europe, the idea of “fear of the void” became a priority, it was used for religious purposes.
The mechanics of ancient Greece when creating technical devices were based on rarefaction of air. For example, water pumps that functioned to create a vacuum over a piston appeared during the time of Aristotle.
The rarefied state of gas, air, has become the basis for the manufacture of reciprocating vacuum pumps, which are currently widely used in technology.
Their prototype was the famous piston syringe of Heron of Alexandria, created by him to draw pus.
The first vacuum chamber was developed in the mid-seventeenth century, and six years later, the German scientist Otto von Gerik managed to invent the first vacuum pump.
This piston cylinder easily pumped air from a sealed container, creating a vacuum there. This made it possible to study the main characteristics of the new state and analyze its operational properties.
Technical vacuum
In practice, the rarefied state of gas and air is called a technical vacuum. In large volumes it is impossible to obtain such an ideal state, since at a certain temperature the materials have a non-zero density of saturated vapors.
The reason for the impossibility of obtaining an ideal vacuum is also the passage of gaseous substances through the glass, metal walls of the vessels.
In small quantities, it is quite possible to obtain rarefied gases. As a measure of rarefaction, the mean free path of gas molecules that randomly collide, as well as the linear size of the vessel used, are used.
Technical vacuum can be considered gas in a pipeline or vessel with a pressure less than in the atmosphere. Low vacuum occurs when the atoms or molecules of the gas cease to collide with each other.
Between the high-vacuum pump and atmospheric air a fore vacuum is placed, which creates a preliminary vacuum. In the case of a subsequent decrease in the pressure chamber, an increase in the mean free path of gaseous particles is observed.
At pressure indices of 10 -9 Pa an ultrahigh vacuum is created. It is these rarefied gases that are used for experiments using a scanning tunneling microscope.
It is possible to obtain such a state in the pores of some crystals even at atmospheric pressure, since the pore diameter is much smaller than the mean free path of a free particle.
Vacuum Devices
The rarefied state of gas is actively used in devices called vacuum pumps. Getters are used to absorb gases and obtain a certain degree of vacuum. Vacuum technology also implies numerous devices that are necessary for monitoring and measuring this state, as well as for managing objects, carrying out various technological processes. The most complex technical devices that use rarefied gases are high vacuum pumps. For example, diffusion devices operate on the basis of the movement of residual gas molecules under the action of a working gas stream. Even in the case of an ideal vacuum, insignificant thermal radiation exists when the final temperature is reached. This explains the basic properties of rarefied gases, for example, the onset of thermal equilibrium after a certain time interval between the body and the walls of the vacuum chamber.
Sparse monatomic gas is an excellent thermal insulator. In it, the transfer of thermal energy is carried out only using radiation, thermal conductivity and convection are not observed. This property is used in Dewar vessels (thermoses), consisting of two containers, between which there is a vacuum.
Vacuum has been widely used in radio tubes, for example, magnetrons of picture tubes, microwave ovens.
Physical vacuum
In quantum physics, such a state means the ground (lower) energy state of a quantum field, which is characterized by zero values of quantum numbers.
In this state, the monatomic gas is not completely empty. According to quantum theory, virtual particles systematically appear and disappear in a physical vacuum, which causes zero field oscillations.
Theoretically, several different vacuums can exist simultaneously, which differ in energy density, as well as other physical characteristics. This idea became the basis in the inflationary theory of the huge explosion.
False vacuum
By it is meant the state of the field in quantum theory, which is not a state with minimal energy. It is stable over a certain period of time. There is a possibility of “tunneling” a false state into a true vacuum when the necessary values of the basic physical quantities are reached.
Space
Arguing over what rarefied gas means, it is necessary to dwell on the concept of “cosmic vacuum”. It can be considered close to the physical vacuum, but existing in interstellar space. Planets, their natural satellites, many stars have certain attractive forces that hold the atmosphere at a certain distance. As you move away from the surface of a stellar object, the density of the rarefied gas changes.
For example, there is the Karman line, which is considered a common definition with outer space of the planet’s border. Behind it, the isotropic gas pressure sharply decreases in comparison with solar radiation and the dynamic pressure of the solar wind, so it is difficult to interpret the pressure of a rarefied gas.
In outer space there are many photons, relic neutrinos, which are difficult to detect.
Measurement Features
The degree of vacuum is usually determined by the amount of substance that remains in the system. The main characteristic of measuring this state is absolute pressure, in addition, the chemical composition of the gas and its temperature are taken into account.
An important parameter for vacuum is the mean free path of the gases remaining in the system. There is a division of the vacuum into certain ranges in accordance with the technology that is necessary for measurements: false, technical, physical.
Vacuum forming
This is the manufacture of products from modern thermoplastic materials in a hot form by exposure to low air pressure or vacuum.
Vacuum molding is considered a drawing method, as a result of which the sheet plastic located above the matrix is heated to a certain temperature value. Next, the sheet repeats the shape of the matrix, this is due to the creation of a vacuum between it and the plastic.
Vacuum devices
They are devices that are designed to create, amplify, and also convert electromagnetic energy. In such a device, air is removed from the working space, and an impermeable shell is used to protect it from the environment. Examples of such devices are electronic vacuum devices where electrons are suitable in a vacuum. Incandescent lamps can also be considered electrovacuum devices.
Gases at low pressures
A gas is called rarefied if its density is negligible, and the mean free path of the molecules is comparable to the size of the vessel in which the gas is located. In such a state, a decrease in the number of electrons is proportional to the density of the gas.
In the case of a highly rarefied gas, internal friction is practically absent. Instead, external friction of the moving gas against the walls appears, which is explained by the change in the momentum of the molecules when they collide with the vessel. In such a situation, there is a direct proportionality between the particle velocity and the gas density.
In the case of low vacuum, frequent collisions between gas particles in full are observed, which are accompanied by a stable exchange of thermal energy. This explains the phenomenon of transfer (diffusion, thermal conductivity), is actively used in modern technology.
Production of rarefied gases
The scientific study and development of vacuum devices began in the mid-seventeenth century. In 1643, the Italian Torricelli was able to determine the value of atmospheric pressure, and after the invention of O. Guericke of a mechanical piston pump with a special water seal, a real opportunity appeared for conducting numerous studies of the characteristics of the discharged gas. At the same time, the possibilities of the effect of vacuum on living things were investigated. The experiments carried out in vacuum with an electric discharge, contributed to the discovery of the negative electron, x-ray radiation.
Thanks to the heat-insulating ability of the vacuum, it became possible to explain the methods of heat transfer, to use theoretical information for the development of modern cryogenic technology.
Vacuum application
In 1873, the first electrovacuum device was invented. It became an incandescent lamp created by the Russian physicist Lodygin. Since that time, the practical use of vacuum technology has expanded, new methods of obtaining, as well as studying this state have appeared.
For a short period of time, various types of vacuum pumps were created:
- rotational;
- cryosorption;
- molecular;
- diffusion.
At the beginning of the twentieth century, Academician Lebedev was able to improve the scientific foundations of the vacuum industry. Until the middle of the last century, scientists did not allow the possibility of obtaining a pressure of less than 10-6 Pa.
Currently, vacuum systems are being created all-metal to avoid leakage. Vacuum cryogenic pumps are used not only in research laboratories, but also in various industries.
For example, after the development of special pumping facilities that do not pollute the used object, new prospects for the use of vacuum equipment have appeared. In chemistry, such systems are actively used for qualitative and quantitative analysis of the properties of pure substances, separation of the mixture into components, and analysis of the rate of various processes.