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outlook office365.com Solve circled Problem s DME 256 Chapter 5 ENTROPY AND THE SECOND LAW The chapter concluded with the increase-in-entropy principle, which states that where ds, is the sum of the net entropy changes of the system and the sur- roundings. The equality sign applies to all reversible processes and the inequality applies to all irreversible processes. The increase-in-entropy prin- ple provides a means for analyzing processes on the basis of the second law. Those processes that result in a total entropy change that is positive are possible according to the second law. Those processes that suggest a decrease in the total entropy are impossible, and only totally reversible pro- cesses could produce no change in the total entropy. The implications of these observations are investigated and discussed in further detail in the fol- lowing chapter. PROBLEMS Explain the diference between a totally reversible process and an internally reversible process. Give an example of each. between the 5.2. Two heat engines, one reversible and the other irreversible, operate ofheat from same two energy reservoirs. Both engines absorb the same amount the high-temperature reservoir. Prove the following statements: a. The reversible engine produces more work than the irreversible engine. b. The irreversible engine rejects more heat to the low-temperature reservoir than the reversible engine does. irreversible process always leaves a 5-3. Comment on the following observation: An history that it has occurred, while a reversible process leaves no history of its 5-4. List the factors, such as heat transfer across afinite temperature dilerence, that can render a process irreversible. 5-5. Usinga kitchen refrigerator example acyclicdevice,show thatitdoesnou violate the Clausius statement of the second law. to a 5-6. Prove that a violation of the Causius statement of the second law leads violation of the Kelvin-Planck statement. heat engine, show that it 5-7. Using an automobile engine as an example of a cyclic does not violate the Kelvin-Plaack statement of the second law. rated 5.8 An automobile engine has a thermal efficiency of 20 percent. The fuel- mechanical power output of the engine is 75 kW. Determine the consumption rate in liters per hour of the engine ifthe fuel has a heat content of 4,8 x 10 kJ/kg and a density of 090 gom cbyertracting beat from besu 5.11. Wha 5-12. A hr 5-14. Sup 5-15. An 5-16, Shc 5-17. Ca law 5-18. Ca 5.19, AI 5.20. A

Solution

5-1

Internally reversible process means that there is no entropy generation within the system, that is, the process is frictionless.

A totally reversible process is the one that is both internally and externally reversible. Externally reversible process means that the heat interaction between the system and the surroundings is taking place through infinitesimally small temperature difference. Hence, increase in entropy of system due to heat gain exactly equals the decrease in entropy of surroundings due to heat loss, thus making the overall change in entropy zero.