The phenomenon of convection and examples of convection

If you bring your hand closer to the included light bulb or place your hand over the 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?

At the heart of the phenomenon of convection lies the expansion of a colder substance when it comes into 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 directions under the influence of heating 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.

Striking examples of convection are the movement of warm air in the middle of the room with radiators, 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 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 natural convection is, 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 origin 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 phenomenon of convection 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

There are several types of phenomena described by us - natural and forced convection. An example of moving flows of hot and cold air in the middle of the room is the best way to characterize the process of natural convection. As for the forced one, it can be observed while stirring the liquid with a spoon, a pump or a stirrer.

Convection is impossible with heating of solids. It's all to blame for a strong mutual attraction with the vibration of their solid particles. As a result of heating 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.

The so-called capillary convection is a separate type. 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 with the natural 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 are the lithosphere and semi-liquid mantle. The upper layer of the globe is directly the earth's crust. The lithosphere is formed from individual slabs, which are in free motion, moving along the surface of the liquid mantle. During the uneven heating of various sections 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.

The convection differs from the thermal conductivity

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, 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 exclusively in the transfer of energy between the constituent particles of the physical body. However, neither process is possible without the presence of particles of matter.

Examples of the phenomenon

The simplest and most accessible example of convection can be the process of operating an ordinary refrigerator. Circulation of refrigerated Freon gas through the pipes of the refrigerating chamber leads to a decrease in the temperature of the upper layers of the air. Accordingly, replacing the warmer streams, the cold ones drop down, thus cooling the products.

Located on the rear panel of the refrigerator, the grill acts as an element that facilitates 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 occur without difficulties.

Other examples of convection can be noticed by observing such a natural phenomenon as wind movement. 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.

The possibility of hovering birds and gliders is connected with convection. 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 such flows.

Good examples of convection are the formation of smoke in chimneys 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;
• Process of movement of wind, monsoons and breezes;
• Displacement of tectonic earth plates;
• Processes that lead to free gassing.

Cooking food

Increasingly, the phenomenon of convection is realized in modern 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.

Heating of air in a traditional oven is based on the operation of a single burner, which results in 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, which are cooked in the oven only a few times a year, a household appliance with convection function 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!