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 0oC
b = constant
T =temperature
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