Thermodynamics

Author: H.U. Fuchs
Publisher:
Publish Date: 1999-11-01
Features: Excerpt:
PROLOGUE
A Unified View of Physical Processes
Heraclides (550-480)
Everything flows. Water and air flow on the surface of the Earth, where they create the multitude of phenomena we know from everyday life. Windsc impart their motion to the water of the oceans, and in a faraway place, this motion can be picked up again through the action of the waves. These processes are maintained by the radiation pouring out from the surface of the sun; light flows from there through space, and some of it is intercepted and absorbed by our planet. Both in nature and in machines, heat is produced and transported from place to place. In electrical machines, we make electricity flow in an imitation of its flow in the atmosphere, and in reactors, chemical substances flow while at the same time undergoing change. Today, we even see life as governed by flow processes. We shall take this observation as the starting point of our investigation of natural and man-made phenomena. It leads to one of the most general descriptions of nature we know today. There are few physical quantities which can flow into and out of systems, which can be absorbed and emitted, and which can be produced and destroyed. Electrical charge is transported in electrical processes, and mass and substance follow gravitational and chemical phenomena, respectively. In continuum mechanics, motion is seen as the exchange of linear and angular momentum. Thermal physics is the science of the transport and production of heat. One of the great advantages of this description of nature is that it relates the different phenomena, which leads to an economical and unified view of physical processes. It turns out that classical continuum physics is a precise method of expressing this point of view for macroscopic systems (see Section P.5). In this chapter, we shall present some examples of introductory physics, most of which you should be familiar with. We shall use as the main tool the images and the language found in continuum physics. In this way, we hope to prepare the ground for the approach to thermodynamics which you will find in this book. Note, however, that this chapter is a condensed overview, not a text. After reading Section P.1 you may want to venture directly into the main body of the book, in which case you might wish to return to this chapter later on. Either way, we believe you will find it advantageous to draw comparisons between different fields of physics as often as possible during your journey through thermodynamics. What is this unified approach to physics? In short, it is based on an analogy with continuum physics. First, we have to agree on which physical quantities we are going to use as the fundamental or primitive ones; on their basis other quantities are defined, and laws are expressed with their help. Second, there are the fundamental laws of balance of the quantities which are exchanged in processes, such as momentum, charge, or amount of substance; we call these quantities substance-like. Third, we need particular laws governing the behavior of, or distinguishing between, different bodies; these laws are called constitutive relations. Last but not least, we need a means of relating different types of physical phenomena. The tool which permits us to do this is energy. We use the energy principle, i.e., the law which expresses our belief that there is a conserved quantity which appears in all phenomena, and which has a particular relationship with each of the types of processes. To introduce the elements of the above principles, we shall begin with a comparison of the flow of water and electrical charge.
P.1 The Flow of Water and Charge
We all are familiar with the flow of water in simple settings, such as the filling or the discharging of containers through pipes (Figure 1). By looking at a special example we will be able to identify the elements of a physical theory which allow us to calculate such things as the current of water through a pipe, the pressure at various points in the fluid, and the time required to discharge a container. The analysis will also tell us that the system and the processes it may undergo are very similar to what we know from electricity. By comparing hydraulic and electrical systems we shall learn about the power of analogies between different fields of physics.
P.1.1 Physical Quantities
Our first question must be which physical quantities we can use as the basis for a quantitative description of the flow of water into and out of containers. We shall have to do the same for the electrical system. The choice of fundamental or primitive quantities is not unique. We simply have to begin somewhere, in some way. We certainly need a measure of the amount of water in a container. There are several possible choices. The simplest of these is the volume of the water. Another that comes to mind quickly is the mass of the water. Finally, chemists might be inclined to measure the amount of water on the basis of its amount of substance (Section P.2.7).