What is Thermal conductivity

Thermal conductivity

Thermal conductivity is a measure of the ability of a substance to conduct heat

It is also defined as it the quantity of heat passing through a quantity of material of unit thickness with unit heat flow area in a unit time when a unit temperature difference is maintained across the opposite faces of material.

• It is denoted by ' k '

Thermal conductivity formula

• If Q = Heat flow in unit time (in watts or J/s)
• A = Heat flow area (in m²)
• x = thickness (in m)
• T = temperature (in K)

• According to Fourier's law thermal conductivity k can be written as

         By using this formula thermal conductivity can be calculated

• The unit of thermal conductivity is W/(m²*K) or J/(m*s*K)
• Hence thermal conductivity depends on the nature of the material and its temperature

Variation of thermal conductivity with temperature

• Metals have highest the thermal conductivity which is rages from 2.3 to 420 W/m.K
• Solid has thermal conductivity higher than that of liquids
• The thermal conductivity of liquid ranges from 0.09 to 0.7 W/m.K
• Liquid has thermal conductivity is higher than gasses.
• The thermal conductivity of gases ranges from 0.006 to 0.6 W/m.K
• The thermal conductivity of the gases and liquids increase with an increase in temperature.
• The thermal conductivity of the metal decreases with an increase in temperature.
• 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))
• The material having low values of thermal conductivity (less than 0.20 W/ (m.K)) is called as a heat insulator example asbestos, glass wool, cork.
• For small temperature ranges, thermal conductivity may be constant
• But for large temperature ranges, thermal conductivity varies as follows

k = k₀ (1+ bT )

                            where

                                k₀ =is the thermal conductivity of the metal at 0^oC 

                                b = constant

                                T =temperature