There’s something basic about thermo that continues to elude me: what exactly does the reversibility criterion buy us?
I am trying to get my head around maximum caliber, which isn’t about heat engines but because it is another one of Jaynes’ ideas there is a lot of discussion about the statistical mechanics intuitions and why they do or do not apply. One entry in the macroscopic prediction paper rejects reversibility on the grounds that knowledge of the microstates may not be available, only that of the macrostate.
The reversibility of transformations here mostly seems in service of the engine being an ideal and general example for reasoning purposes; is that correct, or does it provide some other benefit I am missing?
To the best of my current understanding, (microscopic) reversibility is crucial to get something which looks like classical thermodynamics—i.e. second law, thermal efficiency limit, etc. Without reversibility, we could still apply similar reasoning and get analogous results, but there would be extra steps and the end result would look qualitatively different. Roughly speaking, we’d need to separate out the steps which reduce the number of microstates from the steps which move around our uncertainty about the microstate.
So, the assumption here is in service of reproducing classical thermodynamics, which is in turn a way to test that I’m setting things up right before moving on to more general applications.
Reversible/Carnot cycles in heat engines are a theoretical model that describe a system with perfect efficiency within each of the cycles. The Carnot heat engine is a model used in Thermodynamics 1 to introduce heat engines to students. The point of this is to allow students to focus on the four constituent cycles of the heat engine without worrying about tracking inefficiencies. It is, of course, impossible to design a heat engine that is operating at perfect efficiency with perfect reversibility.
Your are correct in your assumption that the Carnot cycle is just a distillation of the core principles of heat engines. Because of this, the Carnot model also helps at a higher level by helping students understand that:
The efficiency of a reversible heat engine is always greater than that of an irreversible one
Any reversible heat engines operating on the same two reservoirs have the same efficiency
The violation of either of these statements violates the second law, since order cannot be restored to a system, the only possible movement is an increase in disorder and subsequent lower of efficiency.
There’s something basic about thermo that continues to elude me: what exactly does the reversibility criterion buy us?
I am trying to get my head around maximum caliber, which isn’t about heat engines but because it is another one of Jaynes’ ideas there is a lot of discussion about the statistical mechanics intuitions and why they do or do not apply. One entry in the macroscopic prediction paper rejects reversibility on the grounds that knowledge of the microstates may not be available, only that of the macrostate.
The reversibility of transformations here mostly seems in service of the engine being an ideal and general example for reasoning purposes; is that correct, or does it provide some other benefit I am missing?
To the best of my current understanding, (microscopic) reversibility is crucial to get something which looks like classical thermodynamics—i.e. second law, thermal efficiency limit, etc. Without reversibility, we could still apply similar reasoning and get analogous results, but there would be extra steps and the end result would look qualitatively different. Roughly speaking, we’d need to separate out the steps which reduce the number of microstates from the steps which move around our uncertainty about the microstate.
So, the assumption here is in service of reproducing classical thermodynamics, which is in turn a way to test that I’m setting things up right before moving on to more general applications.
Reversible/Carnot cycles in heat engines are a theoretical model that describe a system with perfect efficiency within each of the cycles. The Carnot heat engine is a model used in Thermodynamics 1 to introduce heat engines to students. The point of this is to allow students to focus on the four constituent cycles of the heat engine without worrying about tracking inefficiencies. It is, of course, impossible to design a heat engine that is operating at perfect efficiency with perfect reversibility.
Your are correct in your assumption that the Carnot cycle is just a distillation of the core principles of heat engines. Because of this, the Carnot model also helps at a higher level by helping students understand that:
The efficiency of a reversible heat engine is always greater than that of an irreversible one
Any reversible heat engines operating on the same two reservoirs have the same efficiency
The violation of either of these statements violates the second law, since order cannot be restored to a system, the only possible movement is an increase in disorder and subsequent lower of efficiency.