Summary of the basic laws

It is useful to summarize, in outline, the argument of the present chapter. The state of a system was regarded, at the beginning, as being determined by the two variables, pressure and volume, whose nature was taken as being understood. The experimental knowledge embodied in the zeroth law was then used to show the existence of a certain function of the pressure and volume, 6=0 p, F),J which determines whether or not two bodies are in thermal equilibrium. This function was called the temperature, which thereby becomes an extra variable of state. We can invert the relationship and write F = V(0, p) or p=p(0y F). 

It was then shown, as a result of Joule s experiments, that the total work done by a body in an adiabatic process depends only on 

Finally, we discussed the fact that certain changes are impossible under adiabatic conditions. The possibility or impossibility of the change A- B depends on the characteristics of the states A and B. There was thus shown to be a new function of state, the entropy, such that if S Sji the change is a possible one, within an adiabatic enclosure. The same empirical basis allowed also of the definition of a thermodynamic temperature, T, which is independent of the properties of any particular substance. 

in brief, the whole of the fundamental part of thermodynamics may be regarded as the discovery of the quantities T, U and 8, Their importance lies precisely in the fact that they are functions of state.f That is to say, they form exact differentials and their changes are independent of the path which is taken between assigned initial and final states. It would not be possible to develop an adequate thermodynamics on the basis of heat and work only, because their magnitudes depend on the details of the path. 

From a rather different point of view, the importance of 17 is that it is a quantity which remains constant in an isolated system. On 

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