Heat engines and the second law of thermodynamics II. The second law of thermody

Question

Heat engines and the second law of thermodynamics II. The second law of thermodynamics Not all processes that satisfy the first law of thermodynamics can occur. The seco thermodynamics helps id ways. One expressi nd law of entify processes that do occur. It can be expressed in many equivalent on. known as the entropy inequality, saysAS-, (0.7-) where S-is property of the system called entropy. Entropy, like energy. is a function of state. The device discussed on the previous page is shown at right. 1. Is the quantity /Tu positive or negative? What does it A. 300 J equal? 2. Is the quantity Q /Ti positive or negative? What does it equal? it equal? 400 3. What does the entropy inequality say about the minimum value of AS AS Is your answer consistent with the idea that entropy is a state function and the assumption that the proposed device is cyclic? Explain 4. The Kelvin-Plank statement of the second law says, "No cyclic device can convert heat entirely into work." Is your answer from the previous question consistent with this statement? B.

Explanation / Answer

1) Qh/TH = 300 J/600K = 0.5 J/K

2) Ql/TL = 400J/400K = 1 J/K

3) delta S >= Qh/TH + Ql/TL = 1.5 J/K

4) Considering entropy a state function,

The entropy change of a cycle is always zero whether it is a reversible or an irreversible one, because entropy is a property (point function)

But in our cyclic process it is not. Hence it is inconsistent

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