Doing my best ...
by Barney68 (2019-09-22 22:00:40)

In reply to: Work with me, please.  posted by The Flash


Entropy is a very simple concept that is very complex in application. For a fuller understanding of it, I'd suggest two of the courses in the Teaching Company's Great Courses series: "Thermodynamics: Four Laws that Move the Universe," and "Mysteries of Modern Physics: Time." They explain it better than I can.

That said ...

1. No, entropy is not merely a descriptor. It is a real thermodynamic function with a real equation (δqrev/T = ΔS) that comes up in many ways. I learned it as an undergrad in the context of efficiency: no system can ever be 100% efficient, there are always losses, entropy always increases; that's the second law of thermodynamics. It's a mathematical measure of inefficiency.

2. If the system is totally static, entropy remains constant. That's a theoretical situation, not a real one. Any change in a real, that is an imperfect, system results in an increase in entropy.

An example: the kilogram used to be referenced to a chunk of platinum held in a vault in France. You might think that was a constant, but it was not because every time someone picked it up to put it on a balance to certify a copy, a few atoms would come off due to friction and be lost to the reference. That loss of atoms was an increase in entropy.

3. The entropy of the system is a measure of how disordered the system is; the change in entropy is a measure of a loss of order.

Imagine a disk brake on a car. The disk and the pad are ordered systems, the car has kinetic energy. When the driver wants to stop, the brake is applied and energy dissipated as heat, the disk and pad wear as part of the process and eventually have to be replaced. That's an intentional increase in entropy for the overall system because the driver uses that overall increase in disorder (heat lost to the environment, damage to the disk and pad) to achieve the desired end state of stopping the car. This example shows why using electrical braking to recharge the battery improves efficiency: disorder is still increased, but not as much.

Does that answer your question?