Basics of Thermal Conductivity


What is Thermal Conductivity?

The measure of a materials ability to conduct heat is called thermal conductivity. It is usually denoted by K, λ, k.

Thermal Conductivity formula

Thermal conductivity is calculated by using the following formula:
q = -k.T
Where,
T is the temperature gradient
q refers to thermal flux or the heat flux and
k is the thermal conductivity of the material.

Factors affecting Thermal Conductivity

Material - The type of material used in thermal conductivity affects the rate at which the energy flows between two points. The flow of energy is faster if the conductivity of the material is high. Some examples of materials with high conductivity are graphite, copper, diamonds, gold, and silver. The conductivity level of liquids and gases is low.

Temperature difference - Thermal conductivity may vary with temperature. The thermal conductivity of a material increases with an increase in temperature. Also, the flow of energy is increased.

Length - The length of the material affects the rate of flow of energy. The flow is faster when the length is short. The thermal conductivity will increase as the length increases but the pace will be slower than before.

Magnetic field - Maggi-Righi-Leduc effect is the change of thermal conductivity of material when placed inside a magnetic field. On applying magnetic field it develops an orthogonal temperature gradient.

Cross section - The type of cross-section viz, round, hollow-shaped, or C- shaped affects the thermal conductivity. It is experienced that the thermal diffusivity factor value of hollow and C-shaped carbon fibre-reinforced composites twice of round-type ones.

Thermal conductivity of common materials in a table form

Sl. no.
Material
Thermal conductivity
1.
Diamond
1000 W/m K
2.
Air (at 0°C)
0.024 W/m K
3.
Steel
50.2 W/m K
4.
Silver
406.0  W/m K
5.
Glass
0.8 W/m K
6.
Gold
314 W/m K

SI unit of Thermal Conductivity

The expression of thermal conductivity is in terms of the following - Length, time, temperature, mass.
SI unit is expressed as Wm-1K-1 or watts per meter-Kelvin.

Also, expressed as power/(length * temperature).

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