Thermal conductivity and resistivity
Thermal conductivity is the quantity of heat that passes in unit time through unit area of a plate, when its opposite faces are subject to a temperature/thickness gradient of one degree per meter.
Units
In the SI system of units, thermal conductivity is measured in watts per meter-kelvin, (W·m-1·K-1) where a
Thermal conductivity should not be confused with thermal conductance, which is explained below.
Examples
In general thermal conductivity tracks electrical conductivity, metals being good thermal conductors. There are exceptions, the most outstanding is that of diamond which has a high thermal conductivity, between 1000 and 2600 W·m-1·K-1, while the electrical conductivity is low.
Thermal conductivity of other common materials:
- Silver 430 W·m-1·K-1
- Copper 390
- Gold 320
- Aluminium 236
- Platinum 70
- Quartz 8
- Glass 1
- Water 0.6
- Wool 0.05
- Expanded polystyrene ("Styrofoam") 0.03
Thermal conductivity changes with temperature. For most materials it decreases slightly as the temperature rises.
Since diamond has such a high thermal conductivity, natural blue diamond much higher still, one may test gems to determine if they are genuine diamonds using a thermal conductance tester, one of the instruments of gemology. Diamonds of any size are notably cool to the touch because of their high thermal conductivity, perhaps the origin of the term "ice."
Related terms
The reciprocal of thermal conductivity is thermal resistivity, measured in kelvin-meters per watt (K·m·W-1).
When dealing with a known amount of material, its thermal conductance and the reciprocal property, thermal resistance, can be described. Unfortunately there are differing definitions for these terms.
First definition
To physicists [1], thermal conductance is the quantity of heat that passes in unit time through a plate of particular area and thickness when its opposite faces differ in temperature by one degree. For a plate of thermal conductivity λ, area A and thickness T this is λA/T, measured in W·K-1. This matches the relationship between electrical conductivity (A·m-1·V-1) and electrical conductance (A·V-1).
There is also a measure sometimes known as thermal transmittance: the quantity of heat that passes in unit time through unit area of a plate of particular thickness when its opposite faces differ in temperature by one degree. The reciprocal is thermal insulance. In summary:
- thermal conductance = λA/T, measured in W·K-1
- thermal resistance = T/λA, measured in K·W-1
- thermal transmittance = λ/T, measured in W·K-1·m-2
- thermal insulance = T/λ, measured in K·m2·W-1.
Thermal transmittance is also known as thermal admittance or heat transfer coefficient. But thermal admittance may mean other things (e.g. [2], page 5).
Second definition
To the building trade [3], thermal resistance or R-value means what is described above as thermal insulance, and thermal conductance means the reciprocal. The term K-value is used as a synonym for thermal conductivity, and U-value for thermal tansmittance. Thermal transmittance is still defined as above, and so has the same units as thermal conductance in this definition.