Thermal Conductivity Units Examples Related Terms

Thermal conductivity should not be confused with thermal conductance, which is explained below.

In old literature you may find the following unit for thermal conductivity: cal/(mhC). This stands for calories/(meter/hour/degrees centigrade). by multiplying this value with 0.001161 (=4.18/3600) it is converted to the SI unit.

2 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. CuSil is a brand name for an alloy of 72% silver and 28% copper, that has better properties than silver alone.

Thermal conductivity changes with temperature. For most materials it decreases slightly as the temperature rises.

Material	Thermal Conductivity W·m^-1·K^-1
Diamond	1000-2600
CuSil	>430
Silver	430
Copper	390
Brass	111
GoldFor alternative meanings, see gold (disambiguation Gold is a chemical element in the periodic table that has the symbol Au ( L. aurum and atomic number 79. A soft, shiny, yellow, heavy, malleable, ductile (trivalent and univalent) transition metal, gold d	320
AluminiumAluminium (or aluminum in North American English) is the chemical element in the periodic table with the symbol Al and atomic number 13. A silvery and ductile member of the poor metal group of elements, aluminium is found primarily as the ore bauxite and	236
PlatinumPlatinum is also a certification by the RIAA and other world recording industries, see: RIAA certification Platinum is a chemical element in the periodic table that has the symbol Pt and atomic number 78. A heavy, malleable, ductile, precious, gray-white	70
QuartzFor other uses of this word, see Quartz (disambiguation). Milk quartz rock Quartz is the most abundant mineral on Earth (about 12% vol. It has a hexagonal crystal structure made of trigonal-crystallized silica (silicon dioxide, SiO), with a hardness of 7	8
GlassFor eyeglasses, see spectacles The physics definition of a glass is a uniform amorphous solid material, usually produced when a suitably viscous molten material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form.	1
WaterThis article describes water from a scientific and technical perspective. See the water article for its importance for life and mankind. Water is liquid at standard temperature and pressure. It has the chemical formula H O, meaning that one molecule of wa	0.6
Wool	0.05
Expanded polystyrene ("Styrofoam")	0.03
Silica aerogel	0.00001

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."

3 Related terms

The reciprocal of thermal conductivity is thermal resistivity, measured in kelvin- metres 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.

3.1 First definition (general)

For general scientific use, 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 known as heat transfer coefficient: 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
heat transfer coefficient = λ/T, measured in W·K^-1·m^-2
- thermal insulance = T/λ, measured in K·m²·W^-1.

The heat transfer coefficient is also known as thermal admittance. But thermal admittance may mean other things.

3.2 Second definition (buildings)

When dealing with buildings, thermal resistance or R value means what is described above as thermal insulance, and thermal conductance means the reciprocal. For materials in series, these thermal resistances (unlike conductances) can simply be added to give a thermal resistance for the whole.

A third term, thermal transmittance, incoporates the thermal conductance of a structure along with heat transfer due to convection and radiation. It is measured in the same units as thermal conductance and is sometimes known as the composite thermal conductance. The term U value is another synonym.

The term K value is a synonym for thermal conductivity.

In summary, for a plate of thermal conductivity λ, area A and thickness T:

thermal conductance = λ/T, measured in W·K^-1·m^-2
thermal resistance (R value) = T/λ, measured in K·m²·W^-1.
thermal transmittance (U value)= 1/( Σ(T/λ)) + convection + radiation, measured in W·K^-1·m^-2

4 See Also

5 External links

Chemical properties Thermodynamics Physical quantity

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1 Units