So there’s something about entropy that I just can’t wrap my head around. So: we saw in class that when undergoing a reversible process, the entropy change of the universe (so of the system + the environment) is 0. And we saw that to calculate the entropy change of a system between two states, we forget about the original process, then we design a reversible path that links the 2 states, and we calculate the entropy change along that path (and this entropy change of the system can be positive or negative). And then, if we want to calculate the entropy change of the universe, we imagine that our reversible process is driven by a Heat Engine or Heat Pump, and we calculate the change of entropy of the environment, which we then add to the the change of entropy of the system.
==> Now, what I don’t understand is the following: when undergoing an irreversible process, the entropy of the universe increases. How is that possible? Since the whole point of calculating entropy is forgetting about the original process and designing a reversible path, and we saw at the very beginning that along a reversible path the entropy change of the universe is equal to 0.
I know there’s something that I’ve misunderstood somewhere, but I don’t know what and this thing has been driving mad for some time.
I hope someone will have the time to answer this!