Thermodynamics / Edition 5

Chapter 1: Basic Concepts and Definitions

Thermodynamics is a branch of both physics and chemistry. Scientists in these fields have developed the basic principles that govern the physical and chemical behavior of matter with relationship to energy. In addition, basic relationships among the properties of matter which are influenced by energy interactions have evolved. On the other hand, engineers use this basic information as part of an engineering science to study and design energy systems. In practice, these studies also include the use of other engineering sciences such as heat and mass transfer, and fluid mechanics. This text assumes no formal background in the latter engineering sciences. However, the need for them in more complete engineering design studies will become apparent as we progress through the text.

This chapter reviews some of the basic concepts and definitions presented in earlier courses, and presents new ones that are important in our study of engineering thermodynamics. Since many of the topics are review items, the discussions frequently are brief.

The Nature of Thermodynamics

Thermodynamics is a science that includes the study of energy transformations and of the relationships among the physical properties of substances which are affected by these transformations. Engineering thermodynamics traditionally has involved the study of such diverse areas as stationary and mobile power-producing devices, refrigeration and air-conditioning processes, fluid expanders and compressors, jet engines and rockets, chemical processing as in oil refineries, and the combustion of hydrocarbon fuels (coal, oil, and natural gas). More recently, other areas of interest have evolved. For example, the use of passive and active solar energy units, including solar ponds, is undergoing tremendous growth. Commercial power production from fluids heated by geothermal sources beneath the ground is available on a growing scale. In addition, wind power systems continue to be developed and to be added to the electric power grid. Tidal power is under active investigation, as well as the use of the temperature difference between the surface and deeper layers of the oceans as a potential source of power production. This latter energy system is called ocean thermal energy conversion (OTEC). Study continues on a magnetohydrodynamic (MHD) power cycle which generates electricity by passing a high-temperature gas through a magnetic field. Other processes of interest include thermionic and photovoltaic devices, as well as biomedical applications.

Thermodynamic properties and energy relationships can be studied by two methods. Classical Thermodynamics involve studies which are under- taken without recourse to the nature of the individual particles which make up a substance and to their interactions. This is a macroscopic viewpoint toward matter, and it requires no hypothesis about the detailed structure of matter on the atomic scale. Consequently, the general laws of classical thermodynamics are based-on macroscopic measurements and are not subject to change as knowledge concerning the nature of matter is discovered.

A second method called statistical thermoodynamics is based on the statistical behavior of large groups of individual particles. This is a microscopic viewpoint of matter. It postulates that the values of macroscopic properties (such as pressure, temperature, and density, among others), which we measure directly or calculate from other measurements, merely reflect some sort of statistical average of the behavior of a tremendous number of particles. This theory has been helpful in the modern development of new, direct energy-conversion methods, such as thermionics and thermoelectries.

Five laws, or postulates, govern the study of energy transformations and the relationships among properties. Two of these--the first and second laws-deal with energy, directly or indirectly. Consequently, they are of fundamental importance in engineering studies of energy transformations and use. The remaining three statements-the zeroth law, the third law, and the state postulate--relate to thermodynamic properties. The first law of thermodynamics leads to the concept of energy and a conservation of energy principle. When energy is transferred from one region to another or changes form within a region of space, the total quantity of energy is constant. (In this text we do not consider nuclear transformations of mass to energy.)

The second law of thermodynamics has many ramifications with respect to engineering processes. One of these is that the first law deals with the quantity of energy, while the second law deals with the quality of energy. The idea of quality arises when one needs to optimize the conversion and transmission of energy. We find that the second law places restrictions on the transformation of some forms of energy to more "useful" types. The second law enables the engineer to measure the "degradation," or change in quality, of energy in quantitative terms. The second law also introduces an important thermodynamic property-entropy.

The use of energy by industrialized countries is an important factor in their continued growth. In addition, the desire of underdeveloped nations to improve their standards of living will lead to continuing studies of improving energy use throughout the world. A move must be made to cut wasteful use of energy in industry, in transportation, and in residential and commercial applications. With the increasing cost and decreasing supply of conventional fossil fuels in the future, it is imperative that engineers look seriously at increasing the efficiency of energy use. As an example several methods are under development for increasing the overall energy conversion efficiency for large electric power generation units Thus thermodynamics will continue to make a valuable contribution to the study of new energy systems as well as to the revitalization of older energy systems....