It is a frequent heat transfer mechanism in solids, although it also occurs inside stars, which are normally gaseous. However, the cores of stars that are advanced in their evolution are dense enough for this mechanism to be significant.
Mechanism of heat conduction
Heat transfer by conduction occurs through the exchange of energy between molecules, atoms, and electrons. While the exchange occurs, the more energetic particles give up part of their energy to the less energetic ones through the collisions that take place between them. example of heat transfer by conduction
For example, in a saucepan that is going to be put on the fire, all the particles of the material oscillate with a certain amplitude. The flame heats the ones closest to it, and they begin to vibrate faster, increasing their amplitude of oscillation and gaining energy. example of heat transfer by conduction
Part of this energy is transmitted to neighboring molecules, which in turn increase their amplitude of oscillation and also acquire energy progressively. And of these particles, part of the energy propagates towards the particles farthest from the flame.
Increases in the amplitude of oscillation of the particles translate into an increase in temperature, which can be felt with the hand, if you get close enough to the metal of the pan, including the handle or the handles, hence they are always coated of an insulator, so that they can be handled without burning.
However, the speed of the conduction process depends on the material, since some substances conduct much better than others. example of heat transfer by conduction
In this regard, metals are definitely excellent conductors of heat and electricity. They are better than wood and plastics, because their atoms have at least one free electron in the outermost shell, which can move through the material and carry energy with them.
But surprisingly, diamond is the best thermal conductor out there, although due to its price, there is no alternative but to settle for metals when it comes to practical applications.
New research indicates that a compound of boron and arsenic is possibly as good as diamond at efficiently dissipating heat.
Thermal conduction speed
To find out how fast heat propagates by conduction, consider a sheet of material of width L and lateral area A. The left face is in contact with a heat source (in red) at temperature T h , while the other face it is adjacent to a colder object, of temperature T c .
The heat Q flows between the faces, from the hottest side to the coldest side, in a time Δt. Experimentally it is found that the rate of change or speed with which the heat flows between the faces is proportional to: example of heat transfer by conduction
-The area A of the faces.
-The temperature difference ΔT between them.
And it is also inversely proportional to the thickness L of the plate. Mathematically it is expressed like this:
The constant of proportionality is called thermal conductivity k , therefore:
Thermal conductivity is a characteristic of the material. Regarding the units present, in the International System Q is measured in joules (J), Δt in seconds (s), therefore Q / Δt remains in J / s which is equivalent to watts (W). In this case, the units of thermal conductivity are W / m ∙ ºC, if the temperature is measured in degrees Celsius or W / m ∙ K using the absolute scale in kelvin.
Good thermal conductors have high k values , with metals and diamond standing out.
Since Q / Δt is power , if it is denoted by P we have:
Thermal conductivities of some materials
The following is the thermal conductivity of some known substances of frequent use, in units of the SI International System W / m ∙ K:
-Synthetic diamond : 2000
-Tungsten: 174 -Air
Examples of heat transfer by conduction
Heat transfer by conduction is present in many aspects of everyday life:
Kitchen utensils example of heat transfer by conduction
Pots, pans and, in general, kitchen utensils made of metal such as steel, have handles made of insulating material. This reduces the risk of burns when handling them while they are in contact with the flame or the contents are hot. example of heat transfer by conduction
Metal and wood example of heat transfer by conduction
When you hold a piece of wood in one hand and a piece of metal in the other, you immediately notice that it is cooler to the touch. Metals, as explained above, are good conductors of heat, so heat flows faster from the hand to the metal than from the hand to the wood. example of heat transfer by conduction
In this way, contact with the metal cools the hand of the person holding it faster, and consequently it feels colder than wood, which is not as good a conductor.
Blankets and covers example of heat transfer by conduction
New covers feel warmer than used covers, and that’s because new ones have more air inside the fibers and pores. The more air inside, the better a cover works, since air is a very good thermal insulator. example of heat transfer by conduction
Insulators for houses
In many parts of the world where it is very cold in winter, houses are protected with heat-insulating materials to keep the interior more comfortable. example of heat transfer by conduction
For example, there is fiberglass, which contains spaces with air inside, which works as a thermal insulator, preventing heat from escaping.
Heat pumps example of heat transfer by conduction
Heat pumps extract heat from machinery by conducting heat through metal conduits, from overheated parts to colder areas.
Thermal expansion in metals
When a metallic object is heated, the constituent particles oscillate with a greater amplitude and the consequence is that the dimensions of the object are increased.
Insulating material containers
Containers intended to keep food in good condition for longer are made of insulating material so that the heat from the outside does not decompose the food. example of heat transfer by conduction
The cross section of a copper block has an area of 20 cm 2 and a length of 50 cm. One side is at 0ºC and the other at 100ºC. Calculate the rate at which heat is transmitted.
Solution example of heat transfer by conduction
The equation derived previously will be used:
From the list of conductivities, that of copper is k = 400 W / m ∙ K, and although the temperatures given in the statement are in degrees Celsius, the interval ΔT is the same on both scales:
ΔT = 100 K
The length is L = 50 cm = 0.5m and the area is A = 20 cm 2 = 0.002 m 2 , it remains to substitute values in the equation:
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