- #36
Darwin123
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You are wrong because the proper definition of reversible is not ΔS=0. The physical definition of ΔS is not clear in that equation. In order for the definition to be valid, one would have to state in words what ΔS means.Gavroy said:okay, i think i did not understand this. in this case, we have a change in entropy, why do you call this process reversible? i thought that a proper definition of reversible is ΔS=0 and nothing else, where am i wrong?
Here is the proper definition of reversible. A reversible process is defined as a process where entropy is not created. If ΔS is defined as the entropy that is created in a reversible process, then ΔS=0. However, if ΔS is defined as the entropy that is transferred to a body in a reversible process, then ΔS can take on any real value at all.
The symbol, ΔS, can be used to designate two entirely different types of process. First, ΔS can be the change of entropy due to the creation of entropy. Second, ΔS can be the change of entropy due to entropy transport. If you think the symbol ΔS always means the creation of entropy, then you are wrong.
A process is reversible when entropy isn't created inside a body. However, it is still possible for the entropy in a body to change because entropy is transported into (ΔS>0) or out of it (ΔS<0). When describing a reversible process, it is customary to let ΔS mean the change due to entropy transport.
A process is adiabatic when entropy isn't transported into or out of a body. However, it is still possible for entropy to change due to the creation of entropy. When describing an adiabatic process, it is customary to let ΔS be the change due to entropy creation.
A process can be both irreversible and adiabatic. In which case, the change in entropy of a body has two components. First, there is the change in entropy due to entropy transport. That can be positive or negative. Second, there is the change in entropy due to entropy creation. That is always positive. It is helpful in such a case to separately identify each component.
In a typical textbook problem, one is asked to calculate ΔS for a Carnot engine that is doing work. The Carnot cycle is by definition reversible. Therefore, there is no creation of entropy. However, entropy is transferred from the hot reservoir to the cold reservoir. By custom, ΔS is the change in entropy of the cold reservoir due to the transport.
The Carnot engine works by the spontaneous transfer of entropy from the hot reservoir to the cold reservoir. If ΔS is the change in entropy of the cold reservoir due to entropy transfer, then the Carnot engine causes ΔS>0. If ΔS is the creation of entropy by the Carnot engine, then ΔS=0. If ΔS is the change in entropy of the hot reservoir due to entropy transfer, then the Carnot engine causes ΔS<0. Note there are three "correct" answers that vary with the definition of ΔS.
The Carnot refrigerator works by the forced transfer of entropy from the cold reservoir to the hot reservoir. If ΔS is the change in entropy of the cold reservoir due to entropy transfer, then the Carnot engine causes ΔS<0.
You are wrong if you think that the symbol ΔS means the same thing to every reader every time it is written down. Sometimes ΔS stands for the creation of entropy, sometimes ΔS stands for the transfer of entropy, and sometimes ΔS stands for some combination of creation and transfer. ΔS has no meaning unless the user states in words what ΔS means. No proper definition can use the symbol ΔS unless it is defined elsewhere in words.
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