Thermal Conductivity Definition, Explanation

Thermal conductivity is a measure of the ability of a substance to conduct heat. The larger the value of k, the higher will be the amount of heat conducted.

Thermal conductivity is the quantity of heat passing through a quantity of material of unit thickness with unit heat flow area in a unit time when the unit temperature difference is maintained across the opposite forces of material. 

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If Q is measured in watts (W=J/s), A in m2, x in m, and T in K, then the units of k are W/(m-K) in the SI system. 

Thermal conductivity formula

Q = - kA (dT/dx) 

k = -Q • dx/A • dT, W  • m/(m2 • K) 

= -Q • dx/A • dT, 

Thermal conductivity Unit

W/(m • K)  = J/(s.m.K) 

Thermal conductivity depends upon the nature of the material and its temperature. Thermal conductivities than for gases.

Thermal conductivity of Gases

In general, the thermal conductivity of gases ranges from 0.006 to 0.6 W/(m•K) while that of liquids ranges from 0.09 to 0.7 W/(m•K).

Thermal conductivity of Metals

The thermal conductivity of metals varies from 2.3 to 420 W/(m•K). The materials having higher values of thermal conductivity are referred to as good conductors of heat, e.g., metals. The best conductor of heat is silver [k = 420 W/(m•K)] followed by red copper [k=395 W/(m•K)], gold [k = 302 W/(m•K)] and aluminium [k = 210 W/(m•K)]. 

Thermal conductivity of glass

The materials having low values of thermal conductivity (less than 0.20 W/(m•K) are called as and used as heat insulators to minimize the rate of heat flow. e.g. asbestos, glass wool, cork, etc.

Relation between thermal conductivity and temperature

For small temperature ranges, thermal conductivity may be taken as constant but for large temperature ranges, it varies linearly with temperature, and the variation of the thermal conductivity with temperature is given by the relationship. 

k = a + bT

Where, 

a and b are empirical constants and T is the temperature in K. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane