If you bring your hand closer to a switched on electric lamp or place your palm over a hot stove, you can feel the movement of warm air currents. The same effect can be observed when a sheet of paper is oscillated over an open flame. Both effects are explained by convection.
What is it?
The phenomenon of convection is based on the expansion of a colder substance in contact with hot masses. In such circumstances, the heated substance loses its density and becomes lighter in comparison with the cold space surrounding it. Most accurately, this characteristic of the phenomenon corresponds to the movement of heat flows when water is heated.
The movement of molecules in opposite directions under the influence of heating is exactly what convection is based on. Radiation and thermal conductivity are similar processes, but they primarily concern the transfer of thermal energy in solids.
Vivid examples of convection - the movement of warm air in the middle of a room with heatingappliances, when heated streams move under the ceiling, and cold air descends to the very surface of the floor. That is why, when the heating is on, the air at the top of the room is noticeably warmer compared to the bottom of the room.
Archimedes' Law and thermal expansion of physical bodies
To understand what natural convection is, it is enough to consider the process using the example of the 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 containers rises higher, and moisture of higher density, respectively, moves lower. In the case of heating from above, more and less dense liquids will remain in their places, in which case the phenomenon will not occur.
The emergence of the concept
The term "convection" was first proposed by the English scientist William Prout back in 1834. It was used to describe the movement of thermal masses in heated, moving liquids.
The first theoretical studies of the phenomenon of convection started only in 1916. During the experiments, it was found 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 studied it. The results of the experiments made it possible to explain the movement of heat flows under the influence of the forces of Archimedes.
Types of convection
There are several types of the phenomenon we describe - natural and forced convection. An example of the movement of hot and cold air flows in the middle of a room is the best way to characterize the process of natural convection. As for forced, it can be observed when mixing the liquid with a spoon, pump or stirrer.
Convection is impossible when solids are heated. This is due to the rather strong mutual attraction during the vibration of their solid particles. As a result of heating of solid structure bodies, convection and radiation do not occur. Thermal conductivity replaces these phenomena in such bodies and contributes to the transfer of thermal energy.
The so-called capillary convection is a separate type. The process occurs when the temperature changes during the movement of fluid through the pipes. Under natural conditions, the significance of such convection, along with natural and forced convection, is extremely insignificant. However, in space technology, capillary convection, radiation, and thermal conductivity of materials become very significant factors. Even the weakest convective motions under weightless conditions make it difficult to implement some technical tasks.
Convection in the layers of the earth's crust
Convection processes are inextricably linked with the natural formation of gaseous substances in the thickness of the earth's crust. The globe can be considered as a sphere consisting of several concentric layers. At the very center is a massive hot core, which is a high-density liquid mass containing iron,nickel, as well as other metals.
The surrounding layers for the earth's core are the lithosphere and semi-liquid mantle. The top layer of the globe is directly the earth's crust. The lithosphere is formed from individual plates that are in free motion, moving along the surface of the liquid mantle. In the course of uneven heating of various parts of the mantle and rocks, which differ in different composition and density, convective flows are formed. It is under the influence of such flows that the natural transformation of the ocean floor and the movement of the bearing continents occur.
Differences between convection and heat conduction
Thermal conductivity should be understood as the ability of physical bodies to transfer heat through the movement of atomic and molecular compounds. Metals are excellent conductors of heat because their molecules are in close contact with each other. On the contrary, gaseous and volatile substances act as poor conductors of heat.
How does convection happen? 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, thermal conductivity consists solely in the transfer of energy between the constituent particles of a physical body. However, both processes are impossible without the presence of matter particles.
Examples of the phenomenon
The simplest and most understandable example of convection is the process of an ordinary refrigerator. Circulationcooled freon gas through the pipes of the refrigeration chamber leads to a decrease in the temperature of the upper layers of air. Accordingly, being replaced by warmer streams, cold ones sink down, thus cooling the products.
The grate located on the rear panel of the refrigerator plays the role of an element that facilitates the removal of warm air formed in the compressor of the unit during gas compression. Grid 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 occur without difficulty.
Other examples of convection can be seen by observing such a natural phenomenon as the movement of the wind. Warming over arid continents and cooling over harsher terrain, air currents begin to displace each other, causing them to move, as well as to move moisture and energy.
The possibility of soaring birds and gliders is tied to convection. Less dense and warmer air masses, with uneven heating near the Earth's surface, lead to the formation of ascending currents, which contributes to the soaring process. To overcome the maximum distances without the expenditure of strength and energy, birds need the ability to find such streams.
Good examples of convection are the formation of smoke in chimneys and volcanic craters. The upward movement of smoke is based on its higher temperature and lower density compared to its surroundings. As the smoke cools, it gradually settles into the lower layers of the atmosphere. Exactly because of this reasonindustrial 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, easy-to-understand examples that can be observed in nature, everyday life and technology, we should highlight:
- air flow during operation of household heating batteries;
- formation and movement of clouds;
- the process of movement of wind, monsoons and breezes;
- shift of tectonic earth plates;
- processes that lead to free gas formation.
Cooking
Increasingly, the phenomenon of convection is realized in modern household appliances, in particular in ovens. The gas cabinet with convection allows you to cook different dishes at the same time on separate levels at different temperatures. This completely eliminates the mixing of tastes and smells.
The traditional oven relies on a single burner to heat the air, resulting in uneven heat distribution. Due to the purposeful movement of hot air streams with the help of a specialized fan, dishes in a convection oven turn out to be juicier and better baked. Such devices heat up faster, which reduces the time required for cooking.
Of course, for housewives who cook in the oven only a few times a year, a household appliance withthe function of convection cannot be called a technique of first necessity. However, for those who cannot live without culinary experiments, such a device will become simply indispensable in the kitchen.
We hope that the material presented was useful to you. Good luck!
