Let's talk about what heat transfer is. This term refers to the process of energy transfer in matter. It is characterized by a complex mechanism, described by the heat equation.
Varieties of heat transfer
How is heat transfer classified? Heat conduction, convection, radiation are the three modes of energy transfer that exist in nature.
Each of them has its own distinctive characteristics, features, applications in technology.
Thermal conductivity
The amount of heat is understood as the sum of the kinetic energy of molecules. When they collide, they are able to transfer part of their heat to cold particles. Thermal conductivity is maximally manifested in solids, less typical for liquids, absolutely not typical for gaseous substances.
As an example that confirms the ability of solids to transfer heat from one area to another, consider the following experiment.
If you fix metal buttons on a steel wire, then bring the end of the wire to a burning spirit lamp, gradually the buttons will begin to fall off from it. When heated, the molecules begin to move at a faster speed, more oftencollide with each other. It is these particles that give their energy and heat to colder regions. If liquids and gases do not provide a sufficiently rapid outflow of heat, this leads to a sharp increase in the temperature gradient in the hot region.
Heat radiation
Answering the question of what type of heat transfer is accompanied by energy transfer, it is necessary to note this particular method. Radiant transfer involves the transfer of energy by electromagnetic radiation. This variant is observed at a temperature of 4000 K and is described by the heat conduction equation. The absorption coefficient depends on the chemical composition, temperature, density of a certain gas.
Heat transfer of air has a certain limit, with an increase in energy flow, an increase in the temperature gradient occurs, an increase in the absorption coefficient. After the value of the temperature gradient exceeds the adiabatic gradient, convection will occur.
What is heat transfer? This is the physical process of transferring energy from a hot object to a cold one by direct contact or through a partition that separates the materials.
If bodies of the same system have different temperatures, then the process of energy transfer occurs until thermodynamic equilibrium is established between them.
Heat transfer features
What is heat transfer? What are the features of this phenomenon? You can't stop it completely, you can onlyreduce its speed? Is heat transfer used in nature and technology? It is heat transfer that accompanies and characterizes many natural phenomena: the evolution of planets and stars, meteorological processes on the surface of our planet. For example, together with mass exchange, the process of heat transfer allows you to analyze evaporative cooling, drying, diffusion. It is carried out between two carriers of thermal energy through a solid wall, which acts as an interface between bodies.
Heat transfer in nature and technology is a way of characterizing the state of an individual body, analyzing the properties of a thermodynamic system.
Fourier's Law
It is called the law of heat conduction, because it links the total power of heat loss, temperature difference with the cross-sectional area of the parallelepiped, its length, and also with the coefficient of thermal conductivity. For example, for a vacuum, this indicator is almost zero. The reason for this phenomenon is the minimum concentration of material particles in a vacuum that can carry heat. Despite this feature, in vacuum there is a variant of energy transfer by radiation. Consider the use of heat transfer on the basis of a thermos. Its walls are made double in order to increase the reflection process. Air is pumped out between them, while reducing heat loss.
Convection
Answering the question of what heat transfer is, consider the process of heat transfer in liquidsor in gases by spontaneous or forced mixing. In the case of forced convection, the movement of matter is caused by the action of external forces: fan blades, pump. A similar option is used in situations where natural convection is not effective.
A natural process is observed in those cases when, with uneven heating, the lower layers of the substance are heated. Their density decreases, they rise up. The upper layers, on the contrary, cool down, become heavier, and sink down. Further, the process is repeated several times, and during mixing, self-organization into the structure of vortices is observed, a regular lattice is formed from the convection cells.
Due to natural convection, clouds form, precipitation falls, and tectonic plates move. It is by convection that granules are formed on the Sun.
Proper use of heat transfer ensures minimum heat loss, maximum consumption.
The essence of convection
To explain convection, you can use the law of Archimedes, as well as thermal expansion of solids and liquids. As the temperature rises, the volume of the liquid increases and the density decreases. Under the influence of the Archimedes force, a lighter (heated) liquid tends upward, and cold (dense) layers fall down, gradually warming up.
If the liquid is heated from above, the warm liquid remains in its original position, so no convection is observed. This is how the cycle worksfluid, which is accompanied by the transfer of energy from warm areas to cold places. In gases, convection occurs according to a similar mechanism.
From a thermodynamic point of view, convection is considered as a variant of heat transfer, in which the transfer of internal energy occurs by separate flows of substances heated unevenly. A similar phenomenon occurs in nature and in everyday life. For example, heating radiators are installed at a minimum height from the floor, near the windowsill.
Cold air is warmed up by the battery, then gradually rises up, where it mixes with cold air masses descending from the window. Convection leads to the establishment of a uniform temperature in the room.
Among the common examples of atmospheric convection are winds: monsoons, breezes. The air that heats up over some fragments of the Earth cools over others, as a result of which it circulates, moisture and energy are transferred.
Features of natural convection
It is influenced by several factors at once. For example, the rate of natural convection is affected by the daily movement of the Earth, sea currents, and surface topography. It is convection that is the basis for the exit from volcano craters and smoke pipes, the formation of mountains, the soaring of various birds.
In closing
Thermal radiation is an electromagnetic process with a continuous spectrum, which is emitted by matter, occurs due to internal energy. In order to carry out calculations of thermal radiation, inPhysics uses the blackbody model. Describe thermal radiation using the Stefan-Boltzmann law. The radiation power of such a body is directly proportional to the surface area and temperature of the body, taken to the fourth power.
Thermal conductivity is possible in any bodies that have a non-uniform temperature distribution. The essence of the phenomenon is the change in the kinetic energy of molecules and atoms, which determines the temperature of the body. In some cases, thermal conductivity is considered the quantitative ability of a certain substance to conduct heat.
Large-scale processes of thermal energy exchange are not limited to the heating of the earth's surface by solar radiation.
Severe convection currents in the earth's atmosphere are characterized by changes in weather conditions throughout the planet. With temperature differences in the atmosphere between the polar and equatorial regions, convection flows arise: jet streams, trade winds, cold and warm fronts.
The transfer of heat from the earth's core to the surface causes volcanic eruptions, the emergence of geysers. In many regions, geothermal energy is used to generate electricity, heat residential and industrial premises.
It is heat that becomes an obligatory participant in many production technologies. For example, the processing and smelting of metals, the manufacture of food, oil refining, the operation of engines - all this is carried out only in the presence of thermal energy.
