In the transfer processes we seek the relationships between fluxes and field intensities in terms of field properties, physical properties of the transfer media, and the dimensions of space and time. Thermodynamics deals with energy quantities which are transferred during the processes - work and heat. Its principles and laws apply to all fields of engineering. This chapter sets forth some fundamental concepts necessary for subsequent study of the transfer processes, unifying the definitions and symbols of thermodynamics and the rate processes.
In its broadest sense the science of thermodynamics considers thee conversion and transfer of energy. Classical, or macroscopic, thermodynamics is based upon man's observations. Its laws were developed inductively. No observable violations have occurred. Media are viewer from a continuum standpoint. Probabilistic, or microscopic, thermodynamics is based upon the interactions of molecules and the probability of their behaving in accordance with a set of laws which are identical to those developed in the classical approach. The two approaches are complementary in that the microscopic viewpoint describes fundamental behavior while the macroscopic viewpoint guarantees repeatability.
Equilibrium. Thermodynamics is based upon an equilibrium condition or a series of equilibrium states. Equilibrium is that state which is characterized by no change. In the preceding chapter we noted that change occurs when the field intensity - any field intensity - varies throughout a region. Therefore, for equilibrium the intensity of all fields must be identical; no potential gradient can exist. System and control volume. A thermodynamic system is a fixed quantity of matter. It does not vary in mass or identity. Everything outside the system is termed the surroundings. The system and surroundings are separated by boundaries. Consider, for example, filling an automobile gasoline tank from a large tank truck. We may define the system as that amount of gasoline which will be transferred into the smaller tank.
The thermodynamics problem then becomes that of determining what happens to the gasoline between the initial equilibrium state and the final equilibrium; it is a "book-keeping process" of tabulating observable quantities initially and finally.
An alternative method of solving the same problem involves focusing attention on a fixed region in space, say the automobile tank. The fixed region is the control surface (analogous to the system boundary) and observing the gasoline as it crosses.
All thermodynamic problems can be solved by using one of these two concepts, control volume or system. We shall use whichever is more convenient in any given problem. In some cases it will be more feasible to think in terms of a deformable control volume, typified by a balloon. At this point the student should ponder the analogy between the sulerian method of describing field properties and the thermodynamic concept of the control volume.
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