The follwoing sub-questions are independent of each other. (a) A system undergoe

Question

The follwoing sub-questions are independent of each other.

        (a) A system undergoes a process between two fixed states, first in a reversible manner and then in an irreversible manner. For which case is the entropy change of the system greater? Why?

        (b) If an isolated system is stable, then what is its entropy? Why?

        (c) When a cup of coffee cools down, it's entropy increases. On the contrary, if the coffee is heated, it's entropy increases. Does this violate the second law of thermodynamics? Why?

Explanation / Answer

(a)

Entropy change is greater for the irreversible manner because entropy must always increase in this case.

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(b) Along an irreversible path, the entropy of an isolated system increases, and along a reversible path, the entropy cannot change

In the state of equilibrium the entropy of an isolated system is the maximum possible for the given system, i.e. for an equilibrium isolated system, entropy chage is zero.

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(c) No. It does not violate the 2nd law of thermodynamics. In the cooling process, it absorbs heat from surroundings and in heating process, it deliver heat to surroundings.

In this situation, the gain in entropy due the melting of the ice exceeds the decrease in entropy due to the temperature decrease of the coffee. Therefore, there is a net increase in the entropy of the contents of your cup . Therfore, it doesn't violate 2nd law of thermodynamics.

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