ism_chapter_19

ism_chapter_19 - Chapter 19 The Second Law of...

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1439 Chapter 19 The Second Law of Thermodynamics Conceptual Problems 1 Determine the Concept Friction reduces the efficiency of the engine. *2 Determine the Concept As described by the second law of thermodynamics, more heat must be transmitted to the outside world than is removed by a refrigerator or air conditioner. The heating coils on a refrigerator are inside the room–the refrigerator actually heats the room it is in. The heating coils on an air conditioner are outside, so the waste heat is vented to the outside. 3 Determine the Concept Increasing the temperature of the steam increases the Carnot efficiency, and generally increases the efficiency of any heat engine. 4 •• Determine the Concept To condense, water must lose heat. Because its entropy change is given by dS = dQ rev / T and dQ rev is negative, the entropy of the water decreases. correct. is ) ( c *5 Determine the Concept ( a ) Because the temperature changes during an adiabatic process, the internal energy of the system changes continuously during the process. ( b ) Both the pressure and volume change during an adiabatic process and hence work is done by the system. ( c ) Q = 0 during an adiabatic process. Therefore S = 0. correct. is ) ( c ( d ) Because the pressure and volume change during an adiabatic process, so does the temperature. 6 •• ( a ) False. The complete conversion of mechanical energy into heat is not prohibited by either the 1 st or 2 nd laws of thermodynamics and is common place in energy transformations. ( b ) True. This is the heat-engine statement of the 2 nd law of thermodynamics.
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Chapter 19 1440 ( c ) False. The efficiency of a heat engine is a function of the thermodynamic processes of its cycle. ( d ) False. With the input of sufficient energy, a heat pump can transfer a given quantity of heat from a cold reservoir to a hot reservoir. ( e ) False. The only restriction that the refrigerator statement of the 2 nd law places on the COP is that it can not be infinite. ( f ) True. The Carnot engine, as a consequence of its thermodynamic processes, is reversible. ( g ) False. The entropy of one system can decrease at the expense of one or more other systems. ( h ) True. This is one statement of the 2 nd law of thermodynamics. 7 •• Determine the Concept The two paths are shown on the PV diagram to the right. We can use the concept of a state function to choose from among the alternatives given as possible answers to the problem. ( a ) Because E int is a state function and the initial and final states are the same for the two paths and B int, A int, E E = . ( b ) and ( c ) S , like E int , is a state function and its change when the system moves from one state to another depends only on the system’s initial and final states. It is not dependent on the process by which the change occurs and B A S S = .
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ism_chapter_19 - Chapter 19 The Second Law of...

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