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Friday, July 19, 2019

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 0o
                                b = constant
                                T =temperature

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