Thermodynamics

A short overview on the rules of thermodynamics shall be given here, with special emphasis on their consequences for computer calculations. This part will not replace a textbook on thermodynamics, but shall help the reader to remember its rules and maybe present them in a more practically useful way, which facilitates the understanding of thermodynamic calculations.

Thermodynamics deals with energy and the transformation of energy in various ways. In thermodynamics all rules are deduced from three principal laws, two of which are based on axioms expressing everyday experiences. Even though these laws are very simple, they have many important consequences.

Thermodynamics can strictly be applied only to systems that are at equilibrium, i.e. in a state that does not change with time. As noted in the introduction, the thermodynamic models can be extrapolated outside the equilibrium state and provide essential information on thermodynamic quantities for simulations of time-dependent transformations of systems.

The equation of state

The concept of thermodynamic state must be introduced before beginning with the principal laws. This can be done by invoking the principle of the “equation of state.” This is connected with the introduction of temperature as a measurable quantity. If pressure–volume work is the only work considered, then one can state that in a homogeneous unary system the state is defined by two variables.