Thermodynamically irreversible processes

loom91

Hi,

I was wondering something. Exactly what is meant by a thermodynamically reversible/irreversible process? What are their relation to spontaneous processes? These concepts seem to be fundamental to the understanding of the second law, but textbooks (including the industry standard Atkins Physical Chemistry) seem to gloss them over or give a very superfluous explanation. Thanks a lot!

Molu

f(x)

A reversible process is one which, at any point of time, can be reversed to attain the exact state as in the beginning. Obviously, for this to happen, we would need infinite intermediate steps, and thus infinite time.
In practice, no process is exactly reversible.
Rather, all spontaneous processes are irreversible in nature.

Andrew Mason

A reversible process is a process whose direction can be reversed by an infinitessimal change in conditions.

For example, a reversible expansion of a gas would be one that by an infinitiessimal increase in external pressure results in compression.

A reversible flow of heat would occur with an infinitessimal temperature difference. By increasing the temperature of the colder reservoir an infinitessimal amount, the flow of heat can be reversed.

AM

loom91

All right, I get that. But this seems a very formal definition. What is its significance? Why do we care whether a process is reversible?

Thanks.

Molu

FredGarvin

We know that every real process is irreversible in that they would require the input of more work to get the process back to its initial starting point.

The reversible process is usually easier to use to examine a system. That will usually give you a benchmark of what the absolute "best" performance/output you can ever get. In engineering, the first step is usually to look at the reversible process and see if the 100% efficient process would meet your requirements. If it does, then you start to look at the irreversibilities and make things more complicated.

sirgus

why is that Fred?