The phenomenon of convection and examples of convection

If I can get the hand to on the bulb or place the hand over a hot stove, you can feel the movement of warm air flow. The same effect can be observed with the fluctuation of the sheet of paper placed over an open flame. Both effects are explained by convection.

What is a?

The basis of the phenomenon of convection is the expansion of a colder substance in contact with the hot masses. In such circumstances, the heat a substance loses density and becomes lighter than the surrounding cold space. Most precisely this characteristic corresponds to the displacement of heat flows when heating the water.

The Movement of molecules in opposite directions under the influence of heat – this is what makes convection. Radiation, thermal conduction processes are similar, but relate primarily to the transfer of heat energy in solids.

Notable examples of convection-movement of warm air in the middle of the room with radiator, when heated flows move under the ceiling and cold air falls to the floor. Therefore, when the heating is on at the top of the room the air is noticeably warmer than the lower part of the room.

The Law of Archimedes and thermal expansion of solids

To understand what constitutes natural convection, it is sufficient to consider the process on the example of the law of Archimedes and of the phenomenon of expansion of solids by thermal radiation. So, according to the law, the increase in temperature necessarily leads to an increase in the volume of the liquid. The heated bottom liquid in the tanks rises above, and the moisture greater density, respectively, is moved below. In the case of heating from above more and less dense liquid will remain in their places, in this case, the phenomena will not occur.

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Notion

The term "convection" was proposed by the English scientist William Bar in 1834. It was used to describe the movement of thermal mass in a heated, moving fluid.

The First theoretical study of the phenomena of convection began only in 1916. During the experiments it was found that the transition from diffusion to convection in fluids heated from below occurs when reaching a certain critical temperature values. Later this value was defined as “the number of Role”. It was so named in honor of the researchers involved in its study. The results of experiments allowed to explain the movement of heat flows under the influence of the force of Archimedes.

Types of convection

There are several types of the described phenomena – natural and forced convection. An example of the flows of hot and cold air in the middle of the room could not be better characterize the process of natural convection. With regard to forced, it can be observed when stirring liquid with spoon, pump, or mixer.

Convection is impossible when heating solids. All because of a rather strong mutual attraction with the fluctuation of solid particles. As a result of heating of the bodies of the solid structure do not occur convection, radiation. The thermal conductivity replaces these phenomena in such bodies and facilitates the transfer of thermal energy.

A Separate kind is the so-called capillary convection. The process time and temperature changes during fluid motion in pipes. In natural conditions the value of this convection, together with natural and forced is utterly insignificant. However, in space technology capillary convection, radiation and conductivity of materials are very important factors. Even the weak convective motion in microgravity lead to the obstruction of the implementation of some technical problems.

Convection in the earth's crust layers

The Process of convection are inextricably linked to the natural formation of gaseous substances in the earth's crust. To consider the earth as the sphere consisting of several concentric layers. In the center is a massive hot core, which is a liquid mass of high density, iron content, Nickel and other metals.

The Surrounding layers to the earth's core are the lithosphere and the semi-liquid mantle. The top layer of the earth is a the earth's crust. The lithosphere is formed from separate plates that are in motion, moving on the surface of the liquid mantle. During uneven heating on different portions of the mantle rocks that vary in composition and density, the formation of convective currents. It was under the influence of such flows there is a natural transformation of the bed of the oceans and the movement of the load-bearing continents.

Differences from convection, conduction

Under the conductivity is understood as the ability of physical bodies to the transfer of heat through the movement of atomic and molecular compounds. Metals are excellent conductors of heat because their molecules are in close contact with eachother. In contrast, gaseous and volatile substances are bad conductors of heat.

How is convection? The physics of the process is based on heat transfer due to free motion of the mass of the molecules. In turn, the conductivity is solely in the transfer of energy between the constituent particles of the physical body. However, he and the other process is impossible without the presence of particles of matter.

Examples of phenomena

The Most simple and easy to understand example of convection would be the process of ordinary refrigerator. Circulation of the cooled gas refrigerant pipe of the refrigerating chamber reduces the temperature of the upper layers of air. Accordingly, being replaced by warmer streams, cool down, cooling the products.

Located on the rear panel of the refrigerator grille plays the role of element in facilitating the removal of warm air produced in the compressor Assembly during gas compression. The cooling grid is also based on convective mechanisms. For this reason, it is not recommended to clutter the space behind the refrigerator. It is only in this case, cooling can occur without difficulty.

Other examples of convection you can see, watching such natural phenomena as the movement of the wind. Hot, dry continents and cooling over the area of more severe conditions, air currents begin to crowd out each other, which leads to their movement and the movement of moisture and energy.

Convection is tied to 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 updrafts that contributes to the process of evaporation. To overcome the maximum distance without expenditure of energy and energy birds need to be able to find these threads.

Good examples of convection – the formation of smoke in chimneys and volcanic craters. The movement of smoke up based on its higher temperature and low density in comparison with the environment. During cooling the smoke gradually settles into the lower layers of the atmosphere. For this reason, industrial pipes, by which the emission of harmful substances into the atmosphere, are made with the most high.

The Most common examples of convection in nature and technology

Among the most simple, easy to understand examples that can be observed in nature, life and technology should be allocated:

• Movement of air currents during operation of household radiators;
• The formation and movement of clouds;
• The process of moving winds, monsoons and breezes;
• Offset of the earth's tectonic plates;
• The processes that lead to free gas.

Cooking

Increasingly, the phenomenon of convection is realized in the modern household appliances, in particular in an oven. Gas Cabinet with convection allows you to cook different dishes simultaneously on separate levels at different temperatures. It is completely eliminated mixing smells and tastes.

The Heating of the air in a traditional oven is based on the work of a single burner, which leads to uneven distribution of heat. Due to the targeted move hot air flow via a special fan of dishes in a convection oven are more juicy, better baked. Such devices heat up faster, which reduces the time required for cooking.

Of Course, for the Housewives that cook in the oven only a few times a year, household appliance with the convection feature cannot be called a technique Essentials. However, for those who can't live without culinary experiments, such a device will be simply irreplaceable in the kitchen.

I Hope that the material presented was useful for you. All the best!