The examples of energy analysis in Chapter 2 show that, in order to obtain quantitative results from energy balances, we need to evaluate the thermodynamic properties of fluids at the relevant states of the processes. We used the ideal gas model with constant specific heat to calculate pressure, temperature, specific volume, and internal energy, an anticipation of what is explained and generalized in this chapter. In order to treat problems in which the working fluid can be a liquid, or a vapor, and in all cases in which the fluid does not obey the ideal gas law, we have to learn how we can obtain this information first from tables and charts, then using software. Before we master how to use these tools, we must learn in a more systematic way what thermodynamic properties and states of fluids are, understand the relations and transitions between the various possible states of matter, and how they can be visualized on appropriate diagrams, revealing their complex interaction.
Concepts of Property and State
Properties
A property is any characteristic or attribute of matter that can be quantitatively evaluated. Volume, mass, density, energy, charge, temperature, pressure, magnetic dipole moment, electric dipole moment, internal energy, momentum, surface tension, velocity, entropy, viscosity, and color are all properties. Work and heat are not properties because they are not something that matter has, but instead are amounts of energy transfer to or from matter.
States
The state of a system is its condition as described by the values of all of its properties. Usually we are content to describe a more limited aspect of the system. For example, the geometric state of a solid object can be described by the values of its various dimensions, which are the geometric properties. The dynamical state of a rigid body would be specified by its geometric state plus its three components of translational velocity and three angular rotation rates.
Intensive and extensive properties
An extensive property of a system is the sum of the values of that property for the parts of the system. Volume, mass, energy, charge, magnetic dipole moment, internal energy, momentum, and entropy are extensive properties; they depend on the extent of the system.