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A quasistatic process is a process where all intermediate states are in equilibrium. A reversible process is a process where no entropy (of the universe) is generated (and thus can be reversed to its original state). Every reversible process is quasistatic but not every quasistatic process is reversible. However, can the difference also be explained with the following statements:

  1. For every quasistatic process the system is in internal equilibrium (with itself).
  2. For every reversible process the system is in both internal equilibrium and in equilibrium with the surroundings.

Examples: If a gas expands by suddenly removing a force on a piston, there is a difference in pressure inside the gas, hence it's not in internal equilibrium so it's not quasistatic (and also not reversible).

A gas is expanded by removing the force on a piston, however due to friction this expansion happens slowly. The gas is in internal equilibrium, but the pressure inside the gas is not equal to the pressure of the surroundings, so this process is quasistatic but not reversible.

The force exerted on a piston is very slowly decreased in such a way that the process is isothermal (due to the heatflow from the surroundings). This process is quasistatic and also reversible (the pressure in the gas is equal to the "pressure" exerted by the surroundings and the temperature of the gas is equal to the temperature of the surroundings).

Are my statements correct, or can you come up with a counter example?

Roger V.
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A gas is expanded by removing the force on a piston, however due to friction this expansion happens slowly. The gas is in internal equilibrium, but the pressure inside the gas is not equal to the pressure of the surroundings, so this process is quasistatic but not reversible.

What really makes a process reversible is not whether it may spontaneously reverse itself, but whether the system would return to its initial thermodynamic state, if the process were reversed (i.e., no entropy increase). If a process is quasistatic, i.e., the system always remains in equilibrium, no entropy increase occurs (more precisely, this increase is negligible), and therefore the process is reversible.

The "piston with friction" experiment is unworkable. Its key feature is that friction is supposed to make the process irreversible, by converting the work done by the gas into heat. IMHO this doesn't work: the two forces acting on the piston are that due to the pressure of the gas and the friction. Quasistatic nature of the process implies that the former is only marginally greater than the latter. As the piston moves the two forces equilibrate and the piston stops. Since their difference was marginally small to start with, this means that nothing happens (displacement of the piston is negligible). To actually have a quasistatic process we need to keep the piston moving by pulling it or, if there is no friction, to apply a force to compensate the gas pressure, so that the piston moves very slowly. In other words, in both cases the energy of the gas is changed by the external work, and we can return it to the initial state by reversing this external action, and going successively through the same equilibrium states.

Roger V.
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