The phenomenon of thermal conductivity is the transfer of energy in the form of heat in direct contact of two bodies without any exchange of matter or with its exchange. In this case, the energy passes from one body or region of the body having a higher temperature to the body or region with a lower temperature. The physical characteristic that determines the parameters of heat transfer is thermal conductivity. What is thermal conductivity, and how is it described in physics? These questions will be answered by this article.
General concept of thermal conductivity and its nature
If we answer in simple words the question of what is thermal conductivity in physics, it should be said that heat transfer between two bodies or different regions of the same body is a process of exchange of internal energy between the particles that make up the body (molecules, atoms, electrons and ions). Internal energy itself consists of two important parts: from kinetic and from potential energy.
What is the thermal conductivity in physics from the point of view of the nature of this quantity? At the microscopic level, the ability of materials to conduct heat depends on their microstructure. For example, for liquids and gases, this physical process occurs due to random collisions between molecules; in solids, the bulk of the heat transferred is due to the exchange of energy between free electrons (in metal systems) or phonons (non-metallic substances), which are mechanical vibrations of the crystal lattice .
The mathematical representation of thermal conductivity
We answer the question of what is thermal conductivity from a mathematical point of view. If we take a homogeneous body, then the amount of heat transferred through it in this direction will be proportional to the surface area perpendicular to the direction of heat transfer, the thermal conductivity of the material itself and the temperature difference at the ends of the body, and will also be inversely proportional to the thickness of the body.
As a result, the formula is obtained: Q / t = kA (T 2 -T 1 ) / x, here Q / t is the heat (energy) transmitted through the body during time t, k is the thermal conductivity of the material from which the body is made, A is the cross-sectional area of the body, T 2 -T 1 is the temperature difference at the ends of the body, with T 2 > T 1 , x is the thickness of the body through which heat Q is transmitted.
Methods of heat transfer
Considering the question of what is the thermal conductivity of materials, mention should be made of possible methods of heat transfer. Thermal energy can be transferred between different bodies using the following processes:
- conductivity - this process goes without the transfer of matter;
- convection - heat transfer is directly related to the movement of matter itself;
- radiation - heat transfer is due to electromagnetic radiation, that is, with the help of photons.
In order for heat to be transferred using the processes of conduction or convection, direct contact between different bodies is necessary, with the difference that in the process of conduction there is no macroscopic motion of matter, but in the process of convection this motion is present. Note that microscopic motion takes place in all heat transfer processes.
For ordinary temperatures of several tens of degrees Celsius, we can say that convection and conduction account for the bulk of the transferred heat, and the amount of energy transferred during the radiation process is insignificant. However, radiation begins to play a major role in the heat transfer process at temperatures of several hundreds and thousands of Kelvin, since the amount of energy Q transmitted by this method grows in proportion to the 4th power of the absolute temperature, i.e., ~ T 4 . For example, our sun loses most of its energy precisely due to radiation.
Thermal conductivity of solids
Since in solids each molecule or atom is in a certain position and cannot leave it, heat transfer using convection is impossible, and the only possible process is conductivity. With increasing body temperature, the kinetic energy of its constituent particles increases, and each molecule or atom begins to oscillate more intensively. This process leads to their collision with neighboring molecules or atoms, as a result of such collisions, kinetic energy is transferred from particle to particle until all particles of the body are covered by this process.
As a result of the described microscopic mechanism, when one end of a metal rod is heated, the temperature after a while is aligned throughout the rod.
Heat is not transferred equally in various solid materials. So, there are materials that have good thermal conductivity. They easily and quickly conduct heat through themselves. But there are also bad heat conductors or insulators through which heat practically does not pass.
Thermal conductivity for solids
The thermal conductivity coefficient for solids k has the following physical meaning: it indicates the amount of heat that passes per unit time through a unit surface area in a body of unit thickness and infinite length and width with a temperature difference at its ends equal to one degree. In the international SI system of units, the coefficient k is measured in J / (s * m * K).
This coefficient in solids depends on temperature, therefore it is customary to determine it at a temperature of 300 K in order to compare the ability to conduct heat with different materials.
Thermal conductivity coefficient for metals and non-metallic solid materials
All metals, without exception, are good conductors of heat, for the transfer of which electron gas is responsible for them. In turn, ionic and covalent materials, as well as materials having a fibrous structure, are good heat insulators, that is, they conduct heat poorly. To complete the disclosure of the question of what is thermal conductivity, it should be noted that this process requires the presence of a substance if it is carried out due to convection or conductivity, therefore, in vacuum, heat can only be transmitted due to electromagnetic radiation.
The list below shows the values of the thermal conductivity coefficients for some metals and non-metals in J / (s * m * K):
- steel - 47-58 depending on the grade of steel;
- aluminum - 209.3;
- bronze - 116-186;
- zinc - 106-140 depending on purity;
- copper - 372.1-385.2;
- brass - 81-116;
- gold - 308.2;
- silver - 406.1-418.7;
- rubber - 0.04-0.30;
- fiberglass - 0.03-0.07;
- brick - 0.80;
- tree - 0.13;
- glass - 0.6-1.0.
Thus, the thermal conductivity of metals is 2-3 orders of magnitude higher than the thermal conductivity for insulators, which are a clear example of the answer to the question of what is low thermal conductivity.
The value of thermal conductivity plays an important role in many industrial processes. In some processes, they strive to increase it by using good heat conductors and increasing the contact area, while in others they try to reduce thermal conductivity by reducing the contact area and using heat-insulating materials.
Convection in liquids and gases
Heat transfer in fluids is due to the convection process. This process involves the movement of matter molecules between zones with different temperatures, that is, during convection, mixing of a liquid or gas occurs. When fluid matter gives off heat, its molecules lose part of the kinetic energy, and matter becomes denser. On the contrary, when fluid matter heats up, its molecules increase their kinetic energy, their movement becomes more intense, respectively, the volume of matter increases, and the density decreases. That is why the cold layers of matter tend to fall down under the action of gravity, and the hot layers try to rise up. This process leads to the mixing of matter, contributing to the transfer of heat between its layers.
Thermal conductivity of some liquids
If you answer the question of what is the thermal conductivity of water, then it should be understood that it is due to the convection process. The thermal conductivity coefficient for it is 0.58 J / (s * m * K).
For other fluids, this value is listed below:
- ethyl alcohol - 0.17;
- acetone - 0.16;
- glycerol - 0.28.
That is, the thermal conductivity values for liquids are comparable to those for solid heat insulators.
Atmospheric convection
The importance of atmospheric convection is great, because thanks to it there are such phenomena as winds, cyclones, cloud formation, rain and others. All these processes obey the physical laws of thermodynamics.
Among the convection processes in the atmosphere, the most important is the water cycle. Here you should consider questions about what is the thermal conductivity and heat capacity of water. Under the heat capacity of water is meant a physical quantity that shows how much heat it is necessary to transfer 1 kg of water in order for its temperature to increase by one degree. It is equal to 4220 J.
The water cycle is as follows: the sun heats the waters of the oceans, and part of the water evaporates into the atmosphere. Due to the convection process, water vapor rises to a great height, cools, clouds and clouds form, which lead to the appearance of precipitation in the form of hail or rain.