2.16 Temperature
In the conservation of energy Eq. (2.51 ), the mechanical kinetic energy, power ﬂux and sources can be calculated from , and from the momentum Eq. (2.19 ). Heat sources can contribute to , e.g. from thermal radiation, chemical reactions etc.
That leaves the heat ﬂux term which represents conduction of heat. It is commonly modelled by Fourier’s law^{14} which states is proportional to the negative gradient of temperature , i.e.

(2.54) 
Temperature scale
The heat ﬂux Eq. (2.54) requires temperature to be deﬁned and measurable. Measurement requires a scale. Empirical scales correlate temperature with a measured physical property of a working substance, e.g. EMF at a junction of two metal alloys. Empirical scales have the drawbacks of: being dependent on the working substance; and, not actually deﬁning temperature.
Instead, the thermodynamic scale deﬁnes temperature as a measure of the average kinetic energy of random motions of particle constituents of matter. It provides an absolute measure of temperature that is independent of the choice of working substance and includes a zero point^{16}. It must be measured in units with a zero point, such as the SI unit Kelvin, .
Substitution of our model Eq. (2.54) into Eq. (2.51 ) yields the term . It is logical that this is a Laplacian term since it represents diﬀusion which is associated with random motions of submicroscopic particles, as we we established in Sec. 2.14 .
Ideal gas
The behaviour of many gases under typical working conditions is captured by the ideal gas equation of state

(2.55) 
The ideal gas equation originates from classical thermodynamics as a combination of empirical laws^{17}. Later, it was derived from ﬁrst principles from both statistical thermodynamics and kinetic theory, with temperature representing average kinetic energy.
The derivations assume that molecules have no volume, undergo purely elastic collisions and there are no intermolecular forces.
A scale of temperature deﬁned by the ideal gas equation of state is exactly equivalent to the thermodynamic temperature scale.