State variables and characteristic functions of a phase

The number and nature of variables, used to study the state or thermodynamic modifications of a phase, are chosen by the operator. This limits the number of variables in order to not complicate the problem. 

The third colurtm in the table provides an expression of the corresponding work in the forms YAX, X and Y, the extensive variable and intensive variable of the couple, respectively. 

Note that with volume being the intensive variable, opposed pressiue must respect the form of the pressure work forces. 

When dealing with the thermodynamic modeling of a transformation, it is best to first choose the physical phenomena involved. From Table 1.1, the conjugate variable couples used are deduced. 

For example, in the case of a steam engine machine in which the phenomena involved are heating and changes in the state of water, the following thermomechanical couples ate used temperature-entropy (T, S) and opposed pressure-volume (-P, V). To study a chemical transformation in condensed phases at atmospheric pressure, incapable of varying or even influencing the phenomenon, the couple temperature-entropy is chosen and, for each component, the couple quantity of matter and chemical potential. 

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