- #1
johng23
- 294
- 1
In my thermo course, we made a distinction between quasi-static and reversible transitions (which apparently some don't make). Reversible means both system and surroundings need to be able to return to their initial state. Quasi-static only means that the system transitions to a new state by going through only equilibrium states. There is no friction or dissipative effects.
Intuitively, this makes sense but I'm not sure precisely how to differentiate an equilibrium state from a non-equilibrium state. The case I want to use to illustrate is an adiabatic container with rigid walls, with a paddle wheel going into it.
If I turn the paddle wheel, I can bring the system to any isometric state of higher energy by increasing the entropy. Is this a quasi-static process? Let's assume there is no friction between the paddle wheel and the wall. Then the general picture I have is that if I turn the paddle wheel slowly enough, the process is quasi-static since the distribution of gas molecules never appears macroscopically different than it otherwise would, even though you are gradually transferring energy to the system. But can I really reach an arbitrarily high energy this way? If I move the paddle wheel fast enough that there are pressure variations or heterogeneous velocity distributions in the container, at that point I am no longer going through equilibrium states. But why does it matter if energy is dissipated in these ways between molecules? I'm still transferring the same amount of energy to the system.
As another (maybe clearer) question, how do we avoid friction in a quasi-static process? If I slide two plates across each other, is it even conceptually possible to avoid friction? As I pull two atoms away from each other which are attracted by van der Waals forces or otherwise, don't they increase distance up to a point and then reach some threshold where they pull apart and start to oscillate? That's how it seems like it would happen in my mind.
Intuitively, this makes sense but I'm not sure precisely how to differentiate an equilibrium state from a non-equilibrium state. The case I want to use to illustrate is an adiabatic container with rigid walls, with a paddle wheel going into it.
If I turn the paddle wheel, I can bring the system to any isometric state of higher energy by increasing the entropy. Is this a quasi-static process? Let's assume there is no friction between the paddle wheel and the wall. Then the general picture I have is that if I turn the paddle wheel slowly enough, the process is quasi-static since the distribution of gas molecules never appears macroscopically different than it otherwise would, even though you are gradually transferring energy to the system. But can I really reach an arbitrarily high energy this way? If I move the paddle wheel fast enough that there are pressure variations or heterogeneous velocity distributions in the container, at that point I am no longer going through equilibrium states. But why does it matter if energy is dissipated in these ways between molecules? I'm still transferring the same amount of energy to the system.
As another (maybe clearer) question, how do we avoid friction in a quasi-static process? If I slide two plates across each other, is it even conceptually possible to avoid friction? As I pull two atoms away from each other which are attracted by van der Waals forces or otherwise, don't they increase distance up to a point and then reach some threshold where they pull apart and start to oscillate? That's how it seems like it would happen in my mind.