What is Thermal Conductivity?
The word thermal is associated with heat. Thermal conductivity is a property that corresponds to the ability of any material to pass (or conduct) heat through it. Like convention and radiation, it is another method of heat transfer that takes place in various materials.
Some general denotions of thermal conductivity are k, λ. Its unit is watts per kelvin meter.
Thermal resistivity which is the ability of a material to resist the transfer of heat is known to be the reciprocal of thermal conductivity.
Thermal conductivity and Temperature
Thermal conductivity is the thermodynamic property of the material. One thing you must note here that two bodies in contact with different temperature will undergo heat transfer. And the transfer of heat always takes place from a body with high temperature to a body with low temperature.
The transfer of heat between two bodies that are in direct contact with each other is known as heat conduction.
Whenever the word thermal is considered then people might get confused between heat and temperature. So, first, we have to understand, how temperature is different from the heat?
In some cases, the words heat and temperature are used interchangeably. However, their interchangeable use is not correct because the two hold different meanings. Heat basically defines the energy, symbolized as Q and measured in joules or calorie. While temperature defines the measure of the amount of energy that molecules of a substance exhibit. It is associated with the degree of hotness, symbolized as T, having scaling units Kelvin and Celsius.
We are familiar with the fact that molecules within various substances exhibit different mechanism due to which materials gain energy. In gaseous materials, the molecules are dispersed and are separated by a large distance, and molecular motion causes kinetic energy. This energy is the result of the collision of gaseous molecules.
But this is not the case with solids, because here the molecules are in close proximity with each other thus, do not undergo free motion like gaseous molecules do. Thus, the mechanism by which heat transfer takes place in gases does not apply to solids. In solids, the vibration of the nucleus leads to the production and transfer of energy.
Sometimes the energy is transferred when electrons move from a lower orbit to a higher one. Generally, in electrical conductors like metals, electrons are weakly bonded with the nucleus, thus undergo easy drifting thereby resulting in the transfer of energy. Hence, it is said that the energy transferring ability by such motion of electrons is quite effective.
This concludes that the materials which are brilliant electrical conductors are brilliant thermal conductors as well. While the ones that are bad electrical conductors are also bad thermal conductors.
Thermal conductivity of selected materials
- Silver: 429 W/m K
- Copper: 399 W/m K
- Gold: 317 W/m K
- Aluminium: 237 W/ m K
- Steel: 60.5 W/ m K
- Glass: 0.82 W/ m K
- Plastic: 0.3 W/ m K
- Water: 0.613 W/m K
- Air: 0.0263 W/ m K
For example, a plastic chair and metal vessel are kept in sunlight for around an hour. After 1 hour when you touch both of them simultaneously, you notice that the temperature of a metallic container is more than the temperature of the plastic chair. The reason behind this is that both of them exhibit different degree of thermal conduction.
Measurement of Thermal Conductivity
Thermal conductivity can be measured in various ways. This technique to be applied depends on the material, its thermal properties along with the medium temperature.
We know that it is defined as the conduction heat transfer per unit cross-sectional are per unit temperature gradient. Thus, is given as:
k = Q*L/A (ΔT)
: Q is heat flow,
L denotes material length,
A is its surface area,
ΔT is the temperature gradient
This property shows significant variation from material to material as pure metals possess high thermal conductivity whereas non-metals possess low thermal conductivity.
There are two techniques to measure thermal conductivity, namely, steady-state technique and transient technique.
Steady-state technique: This technique is used in all those conditions where the material temperature remains constant with time. The invariable temperature condition makes the analysis quite simple. However, such experimental analysis requires a properly equipped setup.
Some examples of this technique are Searle’s bar method, Lee’s disc method, etc.
Transient technique: This technique shows suitability in those conditions where the temperature of the material do change over a period of time. Here while heating the material, measurements are taken. It offers a fast measuring process. However, after measurement, the mathematical analysis is quite difficult in this case. Generally, needle probes are used to carry out this method.
Examples of this method are the plane source method, laser source method, etc.