D. James Benton - Thermodynamics: Theory & Practice: The science of energy and power.

Thermodynamics:
Theory & Practice

The science of energy and power.

by D. James Benton

Copyright 2016 by D. James Benton, all rights reserved.

Foreword

...Thermodynamics is the branch of physical science that deals with energy in its various forms, including heat and work. Energy is a property of systems that can be stored and transferred to other systems. Energy is not the "ability to perform work." This is a misnomer. A system may contain considerable energy without having the capacity to perform any work. Thermodynamics is the key to understanding how things work. In this text we will explore classical (or macroscopic) as well as statistical (microscopic) thermodynamics, properties, processes, and heat engines. A variety of applications will be presented and all software is included.

...This book is not intended to be a textbook on thermodynamics or to replace any of the excellent textbooks that are already available. I hope you will find this to be a helpful companion to such texts. I cover several topics (e.g., microscopic point of view, speed distributions, and probability) that are never covered in textbooks on classical (i.e., macroscopic) thermodynamics and are often reserved for graduate courses. I consider these topics to be essential to understanding the whole of thermodynamics and many practical applications in particular. I hope this presentation will inspire you to dig deeper into this fascinating field.

All of the examples contained in this book,
(as well as a lot of free programs) are available at

http://www.dudleybenton.altervista.org/software/index.html

Chapter 1. Systems, Heat, and Work

...After energy, systems are the most important concept in thermodynamics and are essential even to understanding energy in its various forms. A system is an abstract group of one or more objects, separated from everything else by a boundary. Although a system boundary may correspond to a physical boundary, such as a tank, this is not a requirement. A system boundary has no mass and occupies no space (i.e., has no thickness).

...Furthermore, a system boundary may move, objects and energy may pass through it, and it may not be physically possible to construct a corresponding physical boundary. An example of this would be the oxygen molecules in a room. It is not possibleand may not even be desirableto separate the oxygen molecules from the nitrogen and other gases in the room, but this doesn't preclude us from considering a system containing only the oxygen molecules.

...Even in the case of something as simple as a tank, it may be advantageous to consider the contents and the tank separately. We can imagine any shape or form of system boundary and even multiple overlapping system boundaries. There are an infinite number of such selections. All selections for a system boundary are not equal. Some selections are far more useful than others. There is quite an art to the selection of a system boundary. Once we have selected a system boundary, it is essential that we handle it properly and consistently.

...There are three basic types of system boundaries: 1) open, 2) closed, and 3) isolated. An open system can exchange mass and energy with its surroundings. A closed system can exchange energy, but not mass with its surroundings. An isolated system cannot exchange mass or energy with its surroundings. Closed and isolated systems are often confused and there is much useless debate over this confusion.[1]

...Heat is that transient form of energy that crosses a system boundary by virtue of a temperature gradient. Work is that transient form of energy that crosses a system boundary by virtue of a force. Systems do not contain heat or work and neither can be stored. Only energy is stored. All other forms of energy that cross a system boundary apart from mass can be classified as either heat or work. For example, electricity is a form of work. Electrons flow because of a force, not a temperature. Systems do not contain temperature. Temperaturewhen applicablemay be a measure of the energy of a system.

...Because energy is a property of objectsthat is, objects can contain energywhen mass crosses a system boundary (i.e., an open system), energy is transferred from the system to its surroundings (outflow) or from the surroundings to a system (inflow). It is essential to recognize that the surroundings include everything that isn't inside the system boundary. For convenience, the surroundings may be separated into the immediate and the distant or ultimate. The three types of systems are illustrated in the following figure:

...For an ideal closed system (e.g., a cylinder fitted with a frictionless piston that doesn't leak), the work is equal to the integral of force with respect to distance. For a piston, force is equal to pressure times area, so that:

(1.1)

...The work per unit mass (i.e., W/m ) is given the symbol w . Specific volume is defined as volume per unit mass (i.e., V/m ) and is given the symbol v . These two definitions can be substituted into Equation 1.1 to obtain:

(1.2)

...For a continuously flowing open system, power is equal to the integral of force times velocity or pressure differential times volumetric flow rate. The volumetric flow rate is equal to the specific volume times the mass flow rate; thus we have the following equation for the ideal power of a continuously flowing open system: