Thermodynamics arose along with the development of steam engines. In fact, it was only a "theoretical study of steam engines" at the beginning. It focuses on the process of converting heat or energy into work. When this conversion occurs (heat drives the engine), Clausius realizes that the heat is not actually lost, but is transferred from the hotter object to the colder object, and does some work on the way. As the French engineer Nicolas Sadie Cano has repeatedly pointed out, this is like a waterwheel: water flows from a high place to a low place. The amount of water does not increase or decrease, but the water flows to the lower place. Did work. Carnot also imagined heat as such a substance, namely thermal mass. The ability of a thermodynamic system to do work does not depend on the heat itself, but on the temperature difference between cold and heat. Putting a hot stone in cold water can do work (for example, the steam produced can drive a turbine), but the total heat of the entire system of stone and water remains unchanged, and eventually the stone and water will gradually reach the same temperature. On the contrary, no matter how much energy is contained in a closed system, as long as all the objects in the system have the same temperature, it cannot do external work.
What Clausius wanted to measure was the degree to which this energy was unavailable (unusable for work). He came up with the word entropy (entropy), which is derived from the Greek word "transformation" (τροπή, tropē). *His British colleagues quickly realized the main points, but thought that Clausius had turned his attention on the negative side. James Clark Maxwell suggested in his book "Theory of Heat" that it would be "more convenient" to reverse the meaning of entropy ("the part of energy that can be converted into mechanical work"). In this way:
Note: * Clausius deliberately made the structure of the term entropy and energy similar because he believed that these two quantities are closely related in physical importance.
Translator's Note
When the pressure and temperature of the system become uniform, the "negative entropy" is exhausted.
But within a few years, Maxwell came to a 180-degree turn and decided to follow Clausius' usage. He rewrote the book and added a slightly awkward footnote: In previous editions of the book, I thought that Clausius defined the concept of "entropy" he introduced as something that could not be converted into work. That part of the energy is inappropriate. I then redefine it as usable energy, but this leads to serious confusion in the terminology of thermodynamics. Therefore, in this edition, I have tried to follow Clausius’ original definition to use the term “entropy”.
But the question here is not only whether to look at it from the positive side or the negative side, but also at a more subtle level. Maxwell previously regarded entropy as a category of energy that can be used to do work. But now after reconsidering, he realizes that what thermodynamics needs is a whole new measurement. Therefore, entropy is not a kind of energy, nor is it the amount of energy, but, as Clausius said, the degree of unavailability of energy. Although it sounds abstract, it is a measurable quantity like temperature, volume or pressure.
——Excerpted notes from "A Brief History of Information"