Thermodynamics

402 | ThermodynamicsPROBLEMS*Entropy and the Increase of Entropy Principle7–1C Does the temperature in the Clausius inequality relationhave to be absolute temperature? Why?7–2C Does a cycle for which dQ 0 violate the Clausiusinequality? Why?7–3C Is a quantity whose cyclic integral is zero necessarilya property?7–4C Does the cyclic integral of heat have to be zero (i.e.,does a system have to reject as much heat as it receives tocomplete a cycle)? Explain.7–5C Does the cyclic integral of work have to be zero (i.e.,does a system have to produce as much work as it consumesto complete a cycle)? Explain.7–6C A system undergoes a process between two fixedstates first in a reversible manner and then in an irreversiblemanner. For which case is the entropy change greater? Why?7–7C Is the value of the integral 12 dQ/T the same for allprocesses between states 1 and 2? Explain.7–8C Is the value of the integral 12 dQ/T the same for allreversible processes between states 1 and 2? Why?7–9C To determine the entropy change for an irreversibleprocess between states 1 and 2, should the integral 12 dQ/Tbe performed along the actual process path or an imaginaryreversible path? Explain.7–10C Is an isothermal process necessarily internallyreversible? Explain your answer with an example.7–11C How do the values of the integral 12 dQ/T comparefor a reversible and irreversible process between the sameend states?7–12C The entropy of a hot baked potato decreases as itcools. Is this a violation of the increase of entropy principle?Explain.7–13C Is it possible to create entropy? Is it possible todestroy it?7–14C A piston–cylinder device contains helium gas. Duringa reversible, isothermal process, the entropy of the heliumwill (never, sometimes, always) increase.*Problems designated by a “C” are concept questions, and studentsare encouraged to answer them all. Problems designated by an “E”are in English units, and the SI users can ignore them. Problemswith a CD-EES icon are solved using EES, and complete solutionstogether with parametric studies are included on the enclosed DVD.Problems with a computer-EES icon are comprehensive in nature,and are intended to be solved with a computer, preferably using theEES software that accompanies this text.7–15C A piston–cylinder device contains nitrogen gas.During a reversible, adiabatic process, the entropy of thenitrogen will (never, sometimes, always) increase.7–16C A piston–cylinder device contains superheatedsteam. During an actual adiabatic process, the entropy of thesteam will (never, sometimes, always) increase.7–17C The entropy of steam will (increase, decrease,remain the same) as it flows through an actual adiabatic turbine.7–18C The entropy of the working fluid of the ideal Carnotcycle (increases, decreases, remains the same) during theisothermal heat addition process.7–19C The entropy of the working fluid of the ideal Carnotcycle (increases, decreases, remains the same) during theisothermal heat rejection process.7–20C During a heat transfer process, the entropy of a system(always, sometimes, never) increases.7–21C Is it possible for the entropy change of a closed systemto be zero during an irreversible process? Explain.7–22C What three different mechanisms can cause theentropy of a control volume to change?7–23C Steam is accelerated as it flows through an actualadiabatic nozzle. The entropy of the steam at the nozzle exitwill be (greater than, equal to, less than) the entropy at thenozzle inlet.7–24 A rigid tank contains an ideal gas at 40°C that is beingstirred by a paddle wheel. The paddle wheel does 200 kJ ofwork on the ideal gas. It is observed that the temperature ofthe ideal gas remains constant during this process as a resultof heat transfer between the system and the surroundings at30°C. Determine the entropy change of the ideal gas.Heat30°CIDEAL GAS40°CFIGURE P7–24200 kJ7–25 Air is compressed by a 12-kW compressor from P 1 toP 2 . The air temperature is maintained constant at 25°C duringthis process as a result of heat transfer to the surroundingmedium at 10°C. Determine the rate of entropy change of theair. State the assumptions made in solving this problem.Answer: 0.0403 kW/K