University of Puerto Rico partment of Mechanical Engineering Mayaguez, PR 00680

Question

University of Puerto Rico partment of Mechanical Engineering Mayaguez, PR 00680 Name I.D January 08, 2018 INME 4045/020 - -Partial Exam tab Open book examination. You can use your own book and class notes. Individually, do all the required work for a 100-point score. 1. Answer the following true or false. (50 points total) L.l The entropy of a fixed amount of an incompressible substance increases in every process in which the ap ift temperature decreases (Check one) True_ False 1.2 The change of entropy of a closed system is the same for every process between twos specific states because it is independent of the process. True False 1.3 The entropy of a fixed amount of an ideal gas increases in every isothermal compression. True False If false, prove this mathematically. 1.4 The second law of thermodynamics states that the change in entropy of a reservoir at high temperatur must be greater than zero or equal to zero. True False_ If false, prove this statement mathematically

Explanation / Answer

Answer:

1.1 The entropy of a fixed amount of an incompressible substance increases in every process in which temperature decreases. => Answer(False)

Reason: The entropy change for an incompressible substance, s2 - s1 = c ln(T2 / T1), implies that s and T are directly proportional.

1.2 Answer => True

Reason: Since entropy is a property, the change in entropy of a system in going from one state to another is the same for all processes, both internally reversible and irreversible, between these two states.

1.3 Answer=>False

Reason: We know that the entropy change in an isothermal process is given by:
s = -R ln(p2 / p1) = R ln(v2 / v1) implies that entropy can either increase (expansion) or decrease (compression).

1.4 Answer=> True.

Reason: The second law says that the entropy change must be equal to or greater than zero. This corresponds to the statement that heat must flow from the higher temperature source to the lower temperature source.

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