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thermodynamics equilibrium state vs. steady state???

Guanchu Cheng's picture

I am not clear about two concepts: thermodynamics equilibrium state and steady state. It's easy to conceive of the following different two osmosis processes regarding thermodynamics equilibrium state and steady state, respecitvely.

1. two reserviors of different solute concentration are separated by membrane that only allows passage of solvent but not allows transport of solute. There is no head difference initially. As a result, osmosis arises across membrane from the side of low solute concentration to the side of high solute concentration. Gradually it produces a bulid-up in head difference, namely induced hydrostatic pressure as evidenced by the increase of solution level on the side of high solute concentration. Obviously osmosis ceases when the solution level no longer rises up. At this moment, chemical potentials of solvents on either side of membrane are identical, namely a thermodynamics equilibrium established.   

2. remain the same process as case 1 is except for always keeping the originally constant difference in solute concentration on either side of membrane. So after a while required for steady state flow, osmosis with constant velocity keeps on going all the time, namely a steady state established.     

So my main concerns are:

For case 1, I am not doubtful at all that it comes to an establishment in thermodynamics equilibrium at the end of osmosis. But for case 2, if from the thermodynamics standpoint, can we referr it to as a thermodynamics equilibrium or completely opposite to as a non-equilibirum in thermodynamics?  




It is best to begin with the definitions of the two concepts and then I will discuss what I believe your issues are.

Thermodynamic equilibrium is when a system is in thermal equilibrium, mechanical equilibrium, radiative equilibrium and chemical equilibrium. Equilibrium here means a state of balance. I.e. not net flow of matter or energy, no phase changes and no unbalanced potentials within the system. 

Steady state is when a property (or properties) of a system are unchanging in time, and so includes flow of fluid through a tube with constant velocity.


Now you have the definitions look again at the examples and consider if they satisfy the definitions. In case 1) there is clearly no variation in the height of the two fluids after a very long time, this implies mechanical equilibrium, assuming the system is isothermal then it is in thermal equilibrium after a long time. [Ignore radiative equilibrium and just assume it holds]. This leaves the chemical equilibrium, if you can monitor the chemical concentrations on each side if they are constant then a chemical equilibrium has been reached. That is the system is in thermodynamic equilibrium and further the chemical potential balances the mechanical potential.

In case 2) I am not sure that I fully understand this case, but the concern you raise at the end is resolved by the definitions. For thermodynamic equilibrium there must be no flow of matter or energy, if this occurs it is not in thermodynamic equilibrium at all. Take the flow through a pipe at constant velocity, clearly there is flow of matter and so it is not in thermodynamic equilibrium, however since the velocity does not change with time it is in a steady state.


Hope this helps.


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