Entropy for reversible and irreversible cycles.

[corona7w]

I am confused about the entropy change for reversible and irreversible cycles. I know entropy is a state function, so for cycles, the entropy change within the system should be 0, since the process ends up in the same state as the beginning. So does this mean that the entropy change for the system is 0 for both reversible and irreversible cycles?

 

[lanedance]

look at this thread

https://www.physicsforums.com/showthread.php?t=203797

 

[xxChrisxx]

Entropy can be thought of as a measure of how reversable the system is. So an ideal system would be reversable and has an entropy change of 0.

Irreversable cycles lose something to the surroundings, so you can take the ideal cycle and apply real world components to it. Those components will never be 100% efficient meaning you cannot travel back along the line (reverse the process) and get that energy back from the surroundings.

Entropy has to be one of the most mind bending concepts to grasp at first (sometimes I still get confised)

 

[Andrew Mason]

I am confused about the entropy change for reversible and irreversible cycles. I know entropy is a state function, so for cycles, the entropy change within the system should be 0, since the process ends up in the same state as the beginning. So does this mean that the entropy change for the system is 0 for both reversible and irreversible cycles?

Yes. The change in entropy of a system is the integral of dQ/T on a reversible path between two states. If there is no change in the state of the system, there can be no change in entropy. The difference between reversible and irreversible processes is in the entropy change of the surroundings.

AM

 

[lanedance]

but Andrew wouldn't you say the entropy of an isolated system can increase without interacting with its surroundings... (basically 2nd law of thermo)

 

[Andrew Mason]

but Andrew wouldn't you say the entropy of an isolated system can increase without interacting with its surroundings... (basically 2nd law of thermo)

Of course. But it will necessarily end up in a different state.

AM