The theory is restricted to closed thermodynamic systems. A closed thermodynamic system is a quantity of matter separated from its environment by a container. The system has a set of equilibrium states. These equilibrium states are the basic elements of the theory.
A transition is a change from one equilibrium state to another. The theory is about what transitions are possible and what energy exchanges occur between the system and its environment during transitions. During a transition a system may pass through non-equilibrium states. In such cases the theory deals only with the relation between the end states and with the total effect of the transition; it cannot deal with the non-equilibrium states between the end states.
The equilibrium states of a thermodynamic system are characterized by pairs of non-thermal macroscopic variables such as pressure and volume, magnetic field and magnetic dipole moment, etc. The product of each pair of non-thermal variables has the dimensions of energy. For simplicity in this presentation of the theory we consider systems in which the only non-thermal variables are pressure P and volume V, and the pair (P,V) denotes an equilibrium state of the system.
Unfortunately, pressure and volume alone do not always define the equilibrium state uniquely. For example, at a pressure of one atmosphere, one kilogram of liquid water has volume 1.00016 liters at 0°C, 1.00002 liters at 4°C, and 1.00016 liters again at 8°C. We shall suppose, however, that different equilibrium states with the same pressure and volume can be distinguished by observations in some other way.
A non-thermal variable which can be freely controlled, and whose change involves the performance of work on the system, is called an external parameter. A set of equilibrium states in which all the external parameters are constant is called an isometric set. In a pressure-volume system the external parameter is volume, and all the states in an isometric set have the same volume. When the walls of the system's container are rigid we can measure the system's volume V, which is independent of the properties of the environment.
The environment of a thermodynamic system may have a high pressure or a low pressure. When the walls are flexible or movable, so that the volume of the system can change in response to changes in the pressure of the environment, the system is in mechanical contact with the environment. A state that remains unchanged when the system is in mechanical contact with the environment is said to be in mechanical equilibrium with the environment, and the pressure of the system is the same as the pressure of the environment. A set of states all in mechanical equilibrium with the same environment is called an isobaric set because all the states in the set have the same pressure.
Volume and pressure are the basic mechanical properties of thermodynamic systems related to the movability of the walls of the container.
By R. H. B. Exell, 2001. King Mongkut's University of Technology Thonburi.
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