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, energy passes from one body or area of the body with a higher temperature to a body or area 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? This article will answer these questions.
General concept of thermal conductivity and its nature
If you answer in simple terms the question of what thermal conductivity is in physics, then it should be said that heat transfer between two bodies or different areas of the same body is a process of internal energy exchange between the particles that make up the body (molecules, atoms, electrons and ions). Internal energy itself consists of two important parts: kinetic energy and potential energy.
What is thermal conductivity in physics from the point of view of the nature of thisvalues? On a 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 chaotic collisions between molecules; in solids, the main share of the transferred heat falls on the exchange of energy between free electrons (in metallic systems) or phonons (non-metallic substances), which are mechanical vibrations of the crystal lattice.
Mathematical representation of thermal conductivity
Let's answer the question of what thermal conductivity is, from a mathematical point of view. If we take a homogeneous body, then the amount of heat transferred through it in a given 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.
The result is the formula: Q/t=kA(T2-T1)/x, here Q/t - heat (energy) transferred through the body in time t, k - coefficient of thermal conductivity of the material from which the considered body is made, A - cross-sectional area of the body, T2-T 1 - temperature difference at the ends of the body, with T2>T1, x - thickness of the body through which heat Q is transferred.
Methods of transferring thermal energy
Considering the question of what is the thermal conductivity of materials, we should mention the possible methods of heat transfer. Thermal energy can be transferred between different bodies usingfollowing processes:
- conductivity - this process goes without matter transfer;
- convection - heat transfer is directly related to the movement of matter itself;
- radiation - heat transfer is carried out 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 movement of matter, but in the process of convection this movement is present. Note that microscopic motion takes place in all heat transfer processes.
For normal temperatures of several tens of degrees Celsius, it can be said that convection and conduction account for the bulk of the heat transferred, and the amount of energy transferred in the radiation process is negligible. However, radiation begins to play a major role in the heat transfer process at temperatures of several hundred and thousands of Kelvin, since the amount of energy Q transferred in this way increases in proportion to the 4th power of absolute temperature, that is, ∼ T4. For example, our sun loses most of its energy through radiation.
Thermal conductivity of solids
Since in solids each molecule or atom is in a certain position and cannot leave it, the transfer of heat by convection is impossible, and the only possible process isconductivity. With an increase in body temperature, the kinetic energy of its constituent particles increases, and each molecule or atom begins to oscillate more intensely. 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 evens out over the entire rod after a while.
Heat does not transfer equally in different solid materials. So, there are materials that have good thermal conductivity. They easily and quickly conduct heat through themselves. But there are also poor heat conductors or insulators through which little or no heat can pass.
Coefficient of 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 any body of unit thickness and infinite length and width with a temperature difference at its ends equal to one degree. In the international system of units SI, the coefficient k is measured in J/(smK).
This coefficient in solids depends on temperature, so it is customary to determine it at a temperature of 300 K in order to compare the ability to conduct heatvarious materials.
Thermal conductivity coefficient for metals and non-metallic hard materials
All metals, without exception, are good conductors of heat, for the transfer of which they are responsible for the electron gas. In turn, ionic and covalent materials, as well as materials with a fibrous structure, are good heat insulators, that is, they conduct heat poorly. To complete the disclosure of the question of what thermal conductivity is, it should be noted that this process requires the obligatory presence of matter if it is carried out due to convection or conduction, therefore, in a vacuum, heat can only be transferred due to electromagnetic radiation.
The list below shows the values of thermal conductivity coefficients for some metals and non-metals in J/(smK):
- steel - 47-58 depending on steel grade;
- 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 values for insulators, which are a prime example of the answer to the question of what low thermal conductivity is.
The value of thermal conductivity plays an important role in manyindustrial processes. In some processes, they seek 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
The transfer of heat in fluids is carried out by the process of convection. This process involves the movement of molecules of a substance between zones with different temperatures, that is, during convection, a liquid or gas is mixed. When fluid matter releases heat, its molecules lose some of their kinetic energy and the matter becomes denser. On the contrary, when fluid matter is heated, 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 influence of gravity, and the hot layers try to rise up. This process results in the mixing of matter, facilitating the transfer of heat between its layers.
The thermal conductivity of some liquids
If you answer the question of what is the thermal conductivity of water, it should be understood that it is due to the convection process. The thermal conductivity coefficient for it is 0.58 J/(smK).
For other liquids, this value is listed below:
- ethyl alcohol - 0.17;
- acetone - 0, 16;
- glycerol - 0, 28.
That is, the valuesthermal conductivities for liquids are comparable to those for solid heat insulators.
Convection in the atmosphere
Atmospheric convection is important because it causes phenomena such as winds, cyclones, cloud formation, rain, and others. All these processes obey the physical laws of thermodynamics.
Among the processes of convection in the atmosphere, the most important is the water cycle. Here we should consider the questions of what is the thermal conductivity and heat capacity of water. The heat capacity of water is understood as a physical quantity showing how much heat must be transferred to 1 kg of water so that its temperature increases by one degree. It is equal to 4220 J.
The water cycle is carried out as follows: the sun heats the waters of the oceans, and part of the water evaporates into the atmosphere. Due to the process of convection, water vapor rises to a great height, cools, clouds and clouds form, which lead to precipitation in the form of hail or rain.
