Equation of State Second Principle of Thermodynamics

43 The complete characterization of a thermodynamic system is said to describe the state of a system (here a continuum). This description is specified, in general, by several thermodynamic and kinematic state variables. A change in time of those state variables constitutes a thermodynamic process. Usually state variables are not all independent, and functional relationships exist among them through equations of state. Any state variable which may be expressed as a single valued function of a set of other state variables is known as a state function.

44 The first principle of thermodynamics can be regarded as an expression of the interconvertibility of heat and work, maintaining an energy balance. It places no restriction on the direction of the process. In classical mechanics, kinetic and potential energy can be easily transformed from one to the other in the absence of friction or other dissipative mechanism.

45 The first principle leaves unanswered the question of the extent to which conversion process is reversible or irreversible. If thermal processes are involved (friction) dissipative processes are irreversible processes, and it will be up to the second principle of thermodynamics to put limits on the direction of such processes.

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