20.1. Thermodynamics http://www.ck12.org
20.1 Thermodynamics
This chapter is a short introduction to the basics of thermodynamics: state variables and their measurements, as
well as the empirical gas laws. Thermodynamics is a rich and complicated science, and the chapters that follow
attempt to only outline its tenets. This chapter presents some thermodynamic phenomena in a manner that reflects
how they were first discovered — through observation of experiments on various gases. The next chapter links
this empirical understanding with statistical mechanics and kinetic theory and discusses the practical applications of
thermodynamics.
The Thermodynamic Approach to Describing Systems
In kinematics, once the initial conditions of a system are set — we are given the masses and positions of objects in
question as well as the forces acting on them, we can theoretically obtain all future information about the system.
By applying Newton’s Laws, we can determine the positions and velocities of the objects at any point in time.
However, once we are talking about systems that consist of trillions of individual particles in constant motion, such
a description becomes inadequate. In this case, instead of tracking the velocities and positions of each individual
particle, we track severalparameters, orstate variables: aggregate quantities that sufficiently describe the system
in question. The state variables we will use in our study of thermodynamics include pressure, denoted by the letter
P, volume (V), and temperature (T).
For example, it can be shown that the temperature of a substance (whether gas, solid, or liquid) is related to the
internal motion (and therefore, kinetic energy) of the molecules or atoms that constitute it. In a gas, the molecules
or atoms might be flying around freely, while in a solid they can be thought of as trillions of masses connected by
springs. Keeping track of such complicated motions on such a large scale is impossible, so we use the concept of
temperature to obtain a significant amount of information about these motions in a single number.
Two Roads to Thermodynamics
Upon formulating his law of universal gravitation, Newton remarked that:
I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign
hypotheses.
In other words, Newton realized that his theory of gravity was descriptive: it could predict when and where an object
would fall, but could not explain why this happened.
Thermodynamics started in the same manner. Specifically, early physicists and chemists performed experiments on
various gases and found certain empirical relationships between the parameters defined above (pressure, temperature,
volume) that seemed to hold universally. The values of such parameters were measured using items such as pressure
gauges and thermometers. This is the basis for theempirical gas laws, which are the main substance of this
chapter. In reality, the laws they discovered were not always exact for all gases, but always seemed close to simple
mathematical representations. To account for this discrepancy, scientists created the theoretical construct known as
anideal gas. We will define this concept later, but for now an ideal gas can be thought of one that always exactly
obeys the laws listed later in the chapter.