The phenomenon of convection and examples of convection
If you bring your hand closer to the included light bulbor place a palm over a hot plate, you can feel the movement of warm airflows. The same effect can be observed with a swing of a sheet of paper placed over an open flame. Both effects are explained by convection.
What is it?
The convection phenomenon is based on the expansion ofcold matter when in contact with hot masses. Under such circumstances, the heated substance loses its density and becomes lighter compared to the surrounding cold space. Most accurately, this characteristic of the phenomenon corresponds to the movement of heat fluxes when water is heated.
The movement of molecules in opposite directionsunder the influence of heating - this is exactly what convection is based on. Radiation, heat conduction are similar processes, but they relate primarily to the transfer of thermal energy in solids.
Bright examples of convection - moving warmair in the middle of the room with radiators, when heated flows move to the ceiling, and cold air descends to the very surface of the floor. That is why when the heating is switched on at the top of the room, the air is noticeably warmer than the lower part of the room.
Archimedes' law and thermal expansion of physical bodies
To understand what a naturalconvection, it is sufficient to consider the process by the example of the action of Archimedes' law and the phenomenon of expansion of bodies under the influence of thermal radiation. So, according to the law, an increase in temperature necessarily leads to an increase in the volume of liquid. The liquid heated from below in the tanks rises higher, and the moisture of greater density, respectively, moves lower. In the case of heating from above, more and less dense liquids will remain in their places, in which case no phenomenon will occur.
The emergence of the concept
The term "convection" was first proposed by the English scientist William Prut in 1834. It was used to describe the displacement of thermal masses in heated, moving liquids.
The first theoretical studies of the phenomenonconvection started only in 1916. In the course of the experiments it was established that the transition from diffusion to convection in liquids heated from below occurs when certain critical temperature values are reached. Later this value was defined as the "Roel number". It was so named after the researcher who was studying it. The results of the experiments made it possible to explain the movement of heat fluxes under the influence of Archimedes forces.
Types of convection
Convection is impossible when the solids are heated. It's all to blame for a strong mutual attraction with the vibration of their solid particles. As a result of heating the bodies of a solid structure, convection and radiation do not occur. Thermal conductivity replaces these phenomena in such bodies and facilitates the transfer of thermal energy.
A separate view is the so-calledcapillary convection. There is a process with temperature drops during the flow of liquid through the pipes. In natural conditions, the importance of such convection, along with the natural and forced, is immaterial. However, in space technology, capillary convection, radiation, and thermal conductivity of materials become very significant factors. Even the weakest convective movements in conditions of weightlessness lead to difficulties in the realization of certain technical problems.
Convection in layers of the earth's crust
The processes of convection are inextricably linked withnatural formation of gaseous substances in the thickness of the earth's crust. Consider the globe as a sphere consisting of several concentric layers. In the center is a massive hot core, which is a liquid mass of high density with iron, nickel, and other metals.
The surrounding layers for the earth's core arelithosphere and semi-liquid mantle. The upper layer of the globe is directly the earth's crust. The lithosphere is formed of individual plates that are in free motion, moving along the surface of the liquid mantle. During the uneven heating of various parts of the mantle and rocks, which differ in their different composition and density, convective currents are formed. It is under the influence of such flows that there is a natural transformation of the ocean floor and the movement of the bearing continents.
Differences of convection from heat conduction
Thermal conductivity should be understoodThe ability of physical bodies to transfer heat through the movement of atomic and molecular compounds. Metals are excellent conductors of heat, since their molecules are in inextricable contact with each other. On the contrary, gaseous and volatile substances act as poor heat conductors.
How does convection occur? The physics of the process is based on the transfer of heat due to the free movement of the mass of molecules of substances. In turn, the thermal conductivity consists solely in the transfer of energy between the constituent particles of the physical body. However, both processes are impossible without the presence of particles of matter.
Examples of the phenomenon
Located on the back of the refrigeratorThe grating plays the role of an element contributing to the removal of warm air generated in the compressor of the unit during gas compression. Lattice cooling is also based on convective mechanisms. It is for this reason that it is not recommended to clutter up the space behind the refrigerator. After all, only in this case cooling can take place without difficulties.
Other examples of convection can be seen by observingFor such a natural phenomenon as the movement of the wind. Warming over arid continents and cooling over terrain with more severe conditions, air flows begin to displace each other, which leads to their movement, as well as the movement of moisture and energy.
Convection tied the possibility of hovering birds andgliders. Less dense and warmer air masses with uneven heating at the surface of the Earth lead to the formation of ascending currents, which contributes to the process of soaring. To overcome the maximum distances without the expenditure of energy and energy, birds need the ability to find similar flows.
Good examples of convection are the formation of smoke inchimneys and volcanic craters. Moving the smoke up is based on its higher temperature and lower density compared to the environment. When cooling, the smoke gradually settles into the lower layers of the atmosphere. It is for this reason that industrial pipes, through which harmful substances are released into the atmosphere, are made as high as possible.
The most common examples of convection in nature and technology
Among the simplest, understandable examples that can be observed in nature, life and technology, it should be noted:
- the movement of air currents during the operation of domestic radiators;
- formation and movement of clouds;
- the process of wind, monsoon and breeze;
- displacement of tectonic earth plates;
- processes that lead to free gas generation.
Cooking food
Increasingly, the phenomenon of convection is realized inmodern household appliances, in particular in ovens. A gas cabinet with convection allows cooking different dishes simultaneously on separate levels at different temperatures. At the same time mixing of tastes and smells is completely excluded.
Air heating in a traditional ovenBased on the operation of a single burner, which leads to an uneven heat distribution. Due to the purposeful movement of hot air flows with the help of a specialized fan, the dishes in the convection oven are more juicy, better roasted. Such devices quickly heat up, which allows you to reduce the time required for cooking.
Naturally, for housewives who cook inoven only a few times a year, a household appliance with the function of convection can not be called a technique of first necessity. However, for those who can not live without culinary experiments, such a device will become simply irreplaceable in the kitchen.
We hope that the material presented was useful for you. All the best!
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