Reversible process in thermodynamics

Question

How can we look at reversible processes in an intuitive manner?

How is it possible, that at any moment, it can be reversed by an infinitesimal change?

Answer 1

I sometimes explain it this way: imagine a staircase where each step is $h$ high. If $h$ is small then it is quite easy to take a single step up or down, i.e. the process is relatively easily reversed. However, as $h$ increases it starts to become increasingly difficult to go up (and hazardous to go down).

Even for small steps there is a difference between going up and down since you expend more energy going up (against gravity) than going down, so it's not truly reversible, but as $h$ decreases the difference becomes minuscule (but never 0).

Similarly, there is no truly reversible thermodynamic process but the more slowly you let a process proceed (and equilibrate) the more it resembles the reversible ideal process

Answer 2

For the process to be truly reversible, you need to be able to return both the system and the surroundings to their original states, without any significant change in anything else.

I find it useful to think of the surroundings as having a limited tool kit for accomplishing this:

1. A set of very small weights that can be added or removed from a massless piston at different elevations

2. A set of constant temperature baths featuring a broad sequence of slightly different temperatures.

If you are considering the reversible compression or expansion of a gas inside of a cylinder in the transition between two different equilibrium states, you can do this by adding or removing the weights at different elevations and contacting the cylinder with a sequence of baths at slightly higher- or slightly lower temperatures than the gas temperature at each stage of the transition. Then, if you wish to reverse the process, you just do the opposite sequence with the weights and the reservoirs.

Hope this helps.