If you bring your hand closer to the included light bulb 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 placed above an open flame oscillates. Both effects are explained by convection.
What is it like?
The convection phenomenon 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 surrounding cold space. Most accurately, this characteristic of the phenomenon corresponds to the movement of heat fluxes when heating water.
The movement of molecules in opposite directions under the influence of heating is exactly what convection is based on. Radiation, thermal conductivity are similar processes, however, they relate primarily to the transfer of thermal energy in solids.
Bright examples of convection are the movement of warm air in the middle of a room with heating devices, when heated streams move under the ceiling, and cold air descends to the floor surface itself. That is why with the heating turned on at the top of the room, the air is noticeably warmer compared to the lower part 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 action 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 in the bottom of the tank rises higher, and moisture of higher density, respectively, moves lower. In the case of heating from above, more or 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 Prut in 1834. It was used to describe the movement of thermal masses in heated, moving fluids.
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 bottom-heated fluids occurs when some critical temperature values are reached. Later, this value was defined as the “Roel number”. It was so named in honor of the researcher who studied it. The results of the experiments made it possible to explain the displacement of heat flows under the influence of the forces of Archimedes.
Types of convection
There are several types of the phenomenon described by us - natural and forced convection. An example of the movement of flows of hot and cold air in the middle of a room perfectly characterizes the process of natural convection. As for the forced, it can be observed when mixing the liquid with a spoon, pump or stirrer.
Convection is not possible when heating solids. The reason for this is a sufficiently strong mutual attraction during the oscillation of their solid particles. As a result of heating the bodies of a solid structure, convection, radiation do not occur. Thermal conductivity replaces these phenomena in such bodies and promotes the transfer of thermal energy.
A separate type is the so-called capillary convection. The process occurs with temperature changes during the movement of the fluid through the pipes. Under natural conditions, the importance of such convection along with natural and forced is extremely insignificant. However, in space technology, capillary convection, radiation and thermal conductivity of materials become very significant factors. Even the weakest convective movements in zero gravity 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 crust. The globe can be considered as a sphere consisting of several concentric layers. In the center is a massive hot core, which is a high-density liquid mass containing 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 plates, which are in free motion, moving along the surface of the liquid mantle. In the course of uneven heating of various sections of the mantle and rocks, which differ in different composition and density, the formation of convective flows. It is under the influence of such flows that the natural transformation of the bed of the oceans and the movement of bearing continents arise.
Differences between convection and 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 heat conductors, since their molecules are in inextricable contact with each other. In contrast, gaseous and volatile substances act as poor heat conductors.
How does convection occur? The physics of the process is based on heat transfer 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 the physical body. However, the one and the other process is impossible without the presence of particles of matter.
Examples of the phenomenon
The simplest and most understandable example of convection can be the operation of an ordinary refrigerator. The circulation of chilled freon gas through the pipes of the refrigerating chamber leads to a decrease in the temperature of the upper air layers. Accordingly, being replaced by warmer streams, cold ones go down, thus cooling products.
The grill located on the rear panel of the refrigerator plays the role of an element that contributes to the removal of warm air formed in the compressor of the unit during gas compression. Lattice cooling is also based on convective mechanisms. For this reason, it is not recommended to clutter up the space behind the refrigerator. Indeed, only in this case, cooling can occur without difficulty.
Other examples of convection can be seen by observing a natural phenomenon such as wind movement. Heated over arid continents and cooled over terrain with more severe conditions, air currents begin to displace each other, which leads to their movement, as well as the movement of moisture and energy.
The convection is bound up with the possibility of soaring birds and gliders. Less dense and warmer air masses with uneven heating at the Earth's surface lead to the formation of ascending flows, which contributes to the process of soaring. To overcome maximum distances without the expenditure of energy and energy, birds need the ability to find such flows.
Good examples of convection are smoke formation in chimneys and volcanic craters. Smoke upward movement is based on its higher temperature and lower density compared to the environment. During cooling, smoke gradually settles into the lower atmosphere. For this reason, 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, most understandable examples that can be observed in nature, life and technology, it should be noted:
- the movement of air flow during operation of household heating batteries;
- cloud formation and movement;
- the process of movement of wind, monsoon and breezes;
- displacement of tectonic earth plates;
- processes that lead to free gas formation.
Cooking food
Increasingly, the phenomenon of convection is realized in modern household appliances, in particular in ovens. A gas cabinet with convection allows you to cook different dishes at the same time at separate levels at different temperatures. This completely eliminates the mixing of tastes and smells.
Air heating in a traditional oven is based on the operation of a single burner, which leads to an uneven distribution of heat. Due to the purposeful movement of hot air streams with the help of a specialized fan, the dishes in the convection oven are more juicy, better baked. Such devices heat up faster, which reduces the time required for cooking.
Naturally, for housewives who cook in the oven only a few times a year, a household appliance with a convection function cannot be called an essential technique. However, for those who cannot live without culinary experiments, such a device will become simply indispensable in the kitchen.
We hope the material presented was useful to you. All the best!