Physics · Ch 11 — Thermodynamics
Introduction
Introduction
11.1 Introduction
Thermodynamics is the branch of physics that studies the laws governing thermal energy — the conversion of work into heat and heat into work. You already know from daily life that rubbing your palms together makes them feel warmer. That is work being converted into heat. In a steam engine, the opposite happens: the heat of steam pushes pistons, which turn the wheels of a train — heat is converted into useful work.
Before we can study these processes properly, we need precise definitions of heat, temperature, work, and related ideas. Historically, this took a long time to get right.
The Caloric Theory of Heat
For many years, heat was thought to be a fine, invisible fluid called caloric that filled the pores of a substance. According to this picture, when a hot body touches a cold body, the caloric fluid flows from the hotter body to the colder one — much like water flowing through a horizontal pipe connecting two tanks at different levels. The flow continues until the water levels (or, in the heat case, the temperatures) become equal.
This sounds plausible, but it was eventually proven wrong.
Count Rumford's Experiment (1798)
Benjamin Thompson (Count Rumford) performed a crucial experiment while boring a brass cannon. He observed that the drilling generated a tremendous amount of heat — enough to boil water. More importantly, the amount of heat produced depended only on the work done by the horses turning the drill, not on how sharp the drill was.
If the caloric theory were correct, a sharper drill would scoop out more caloric fluid from the pores of the brass, producing more heat. But that did not happen. The only natural explanation was that heat is not a fluid at all — it is a form of energy. The experiment demonstrated the conversion of energy from one form to another: from mechanical work into heat.
The caloric theory predicted that a sharper drill would produce more heat. Rumford's experiment showed the opposite — heat depended only on the work done. This was the death blow for the caloric theory.
Thermodynamics as a Macroscopic Science
Thermodynamics is a macroscopic science. It deals with bulk systems and does not concern itself with the molecular constitution of matter. In fact, its laws were formulated in the nineteenth century, before the molecular picture of matter was firmly established.
A thermodynamic description uses only a few macroscopic variables — quantities that common sense suggests and that can usually be measured directly. For a gas, for example, these variables are pressure, volume, temperature, mass, and composition. These are quantities we can feel with our senses and measure with instruments.
Contrast this with a microscopic description. In the kinetic theory of gases, you would need to specify the coordinates and velocities of the huge number of molecules in the gas — an impossibly detailed task. Even the kinetic theory, which is less detailed than a full molecular description, still involves the distribution of molecular velocities. Thermodynamics avoids all of that entirely.
Thermodynamics was developed before scientists even knew about atoms and molecules. Its laws are based entirely on bulk, measurable quantities — and they remain valid regardless of the underlying molecular structure.
Thermodynamics vs. Mechanics
It is important to distinguish thermodynamics from mechanics. In mechanics, we study the motion of particles or bodies under forces and torques. Thermodynamics is not concerned with the motion of the system as a whole. It is concerned with the internal macroscopic state of the body.
Consider a bullet fired from a gun. What changes is the mechanical state of the bullet — specifically, its kinetic energy. Its temperature does not change. Now suppose the bullet pierces a piece of wood and stops. The kinetic energy of the bullet gets converted into heat, raising the temperature of the bullet and the surrounding wood.
Temperature is related to the energy of the internal disordered motion of the bullet's molecules, not to the motion of the bullet as a whole. A bullet flying through the air has high kinetic energy as a whole object, but its internal molecular motion (and hence its temperature) is unchanged. When it stops, that organized kinetic energy is dissipated into the random, disordered motion of its molecules — and that is what we feel as heat.
Temperature is a measure of the average kinetic energy of the random, disordered motion of molecules inside a body. It has nothing to do with the motion of the body as a whole.