Direct Energy, 2018a

8 THERMOELECTRICS 175
Symbol Name Value and Unit
k B Boltzmann constant 1.381 · 10 −23 J K
R Molar gas constant 8.314 J
mol·K
N a Avogadro constant 6.022 · 10 23 1
mol
Table 8.3: Values of the Boltzmann constant, the molar gas constant, and
the Avogadro constant.
Temperature T is an intensive property measured in either the SI units
of degrees Celsius or kelvins. By denition, we can relate the two units by
T [
◦ C ] = T [K]
− 273.15 (8.1)
[68]. We can also measure temperature in the non-SI unit of degrees Fahrenheit.
Temperature in degrees Celsius and temperature in degrees Fahrenheit
are related by
( T[
T [
◦ C ] = ◦ F ] − 32 )
. (8.2)
1.8
As with absolute pressure measurements, temperature in kelvins is said to
be measured on an absolute temperature scale because the lowest possible
temperature is given by zero kelvin. All temperatures are either absolute
zero or have positive values. We use the term temperature to describe
a property of a system. We use the term heat transfer to describe the
process of transferring energy from a hot to a cold object. Entropy S is
measured in units J K
, and it is an extensive property. Intuitively, entropy
is a measure of the lack of order or organization of a material. The atoms
in an amorphous material are less ordered than the atoms in a crystal of
the same composition, so the amorphous material has more entropy.
Some further denitions will be needed. The symbol N represents the
number of atoms or molecules of a substance. While it is not usually
considered a fundamental thermodynamic property, it is a useful property
of a sample. Sometimes it is specied in the units of mols instead of by the
number of atoms or molecules. The Avogadro constant
N a =6.022 · 10 23 1
(8.3)
mol
is a constant which is used to convert a number given to the number per
mol. The molar gas constant is
R =8.314
J
mol · K . (8.4)