Thread: necessity of entropy

I know that entropy is important to explain phenemena in our world. But in a state which reaches almost thermal equilibrium or a state where particles strongly correlate each other by Coulombic force, is entropy an essential thermodynamical quantity?

Yes and no. Entropy isn't really a definable quantity if everything is the same (as in a system in thermal equilibrium). We don't (can't really, not in any meaningful sense) define entropy. Instead, we define the change in entropy. I guess you could define the entropy of a system compared to absolute 0, but it's about as arbitrary as anything else.

So your system in thermal equilibrium has entropy, but it doesn't actually mean anything until you can compare it to some other system. You can, however, talk about the free energy in the system (it'd be 0).

If you're confused by entropy, think of it as energy poop. Some process ate useful work and pooped out entropy. There might be another process which can eat that poop and use it for useful work (like a dung beetle), but the original process can't eat it's own poop. Talking about entropy in absolute terms is silly because what's entropy to one process is useful work to another. But each process robs useful work from the system, making it increasingly poopy.