Reversible Processes Explained: Entropy, Time, & More

[Silviu]

Can someone explain to me what a reversible process means, because I am not sure I really understand. Intuitively it should be a process that should work both ways, but I am not sure I understand how it is related to entropy (the change of entropy should be 0, but I am not sure I understand which entropy: system, surroundings, both?). And I am not sure I understand why does it take an infinite amount of time. Thank you!

 

[Muhammad Waleed Khan]

You may think of a reversible process as one that is occurring against a very small amount of opposing force. I'm assuming you're referring to a reversible process in regards to thermodynamics, so a reversible process would be a process where your system and surroundings can be brought back to their initial state. By the way entropy change is not 0 for a reversible process, it is so for a reversible,adiabatic process as dQrev is 0 in this case. Remember that entropy in not a conserved quantity, in reality it is always changing.

 

[zwierz]

In classical mechanics a solution $x(t)$ of equations of motion is reversible if $x(c -t)$ is also a solution to equations of motion $\forall c\in\mathbb{R}$.

 

[Mapes]

You may think of a reversible process as one that is occurring against a very small amount of opposing force.

It might be more precise to say a very small difference in the amount of opposing force, or, better still, a nearly equal opposing force. Zero opposing force would maximize reversibility because it would maximize the gradient driving the reaction.

 

[Chestermiller]

Can someone explain to me what a reversible process means, because I am not sure I really understand. Intuitively it should be a process that should work both ways, but I am not sure I understand how it is related to entropy (the change of entropy should be 0, but I am not sure I understand which entropy: system, surroundings, both?). And I am not sure I understand why does it take an infinite amount of time. Thank you!

Moran et al, in Fundamentals of Engineering Thermodynamics, discuss what they call Internally Reversible Processes and Overall Reversible Processes.

Internally Reversible Process: In an internally reversible process, a system is made to pass through a continuous sequence of thermodynamic equilibrium states. This is accomplished by doing work on the system boundary very slowly and transferring heat to the system at its boundary very slowly. So, at each point of the process, the system is only slightly removed from being at thermodynamic equilibrium. The main factors causing entropy to be generated in an irreversible process are viscous dissipation of mechanical energy resulting from rapid deformation, and rapid conductive heat transfer within the system resulting from large temperature gradients. In an internally reversible process, these sources of entropy generation are excluded from occurring. The slowness of the heat transfer and of the work cause the internally reversible process to take place over a long period of time. The reversible path that the system experiences in an internally reversible process can be the exact same path that it experiences in an Overall Reversible Process. However, even if the system is experiencing an internally reversible process path, this does not necessarily mean that the surroundings are also experiencing an internally reversible process path. For example, if we use our arm to cause a gas to be compressed within a cylinder along an internally reversible path for the gas (our arm is the surroundings), our arm will certainly not undergo a reversible path. There are all sorts of irreversible processes occurring within our muscles that prevent this. So the surroundings may not be experiencing an internally reversible path of their own.

Overall Reversible Process:
In an Overall Reversible Process, both the system and the surroundings are experiencing (complementary) internally reversible processes. In this case, the entropy change for the system plus the entropy change for the surroundings is equal to zero. In order to achieve this, we must make sure that the surroundings are handled very carefully. For example, if the surroundings are doing work on the system, this can be accomplished by adding tiny weights to a piston very gradually at a sequence of piston elevations. Or, heat can be transferred from the surroundings to the system by using a gradual sequence of constant temperature reservoirs at slightly different temperatures. Another key feature of an overall reversible process is that both the system and the surroundings can be returned to virtually their original states by running the process in reverse. This would not be the case if the system experienced an internally reversible process, but not the surroundings. And, it would not be the case if neither the system nor the surroundings experienced an internally reversible process.