Entropia

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7-1

Chapter 7 ENTROPY
Entropy and the Increase of Entropy Principle

7-1C No. The

∫ δ Q represents the net heat transfer during a cycle, which could be positive.

7-2C No. A system may produce more (or less) work than it receives during a cycle. A steam power plant, for example, produces more work than it receives during a cycle, the difference being the net work output. 7-3C Theentropy change will be the same for both cases since entropy is a property and it has a fixed value at a fixed state. 7-4C No. In general, that integral will have a different value for different processes. However, it will have the same value for all reversible processes. 7-5C Yes. 7-6C That integral should be performed along a reversible path to determine the entropy change. 7-7C No. An isothermalprocess can be irreversible. Example: A system that involves paddle-wheel work while losing an equivalent amount of heat. 7-8C The value of this integral is always larger for reversible processes. 7-9C No. Because the entropy of the surrounding air increases even more during that process, making the total entropy change positive. 7-10C It is possible to create entropy, but it is not possible todestroy it. 7-11C If the system undergoes a reversible process, the entropy of the system cannot change without a heat transfer. Otherwise, the entropy must increase since there are no offsetting entropy changes associated with reservoirs exchanging heat with the system.

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators forcourse preparation. If you are a student using this Manual, you are using it without permission.

7-2

7-12C The claim that work will not change the entropy of a fluid passing through an adiabatic steady-flow system with a single inlet and outlet is true only if the process is also reversible. Since no real process is reversible, there will be an entropy increase in the fluid during theadiabatic process in devices such as pumps, compressors, and turbines. 7-13C Sometimes. 7-14C Never. 7-15C Always. 7-16C Increase. 7-17C Increases. 7-18C Decreases. 7-19C Sometimes. 7-20C Yes. This will happen when the system is losing heat, and the decrease in entropy as a result of this heat loss is equal to the increase in entropy as a result of irreversibilities. 7-21C They are heat transfer,irreversibilities, and entropy transport with mass. 7-22C Greater than.

7-23 A rigid tank contains an ideal gas that is being stirred by a paddle wheel. The temperature of the gas remains constant as a result of heat transfer out. The entropy change of the gas is to be determined. Assumptions The gas in the tank is given to be an ideal gas. Analysis The temperature and the specific volume of the gasremain constant during this process. Therefore, the initial and the final states of the gas are the same. Then s2 = s1 since entropy is a property. Therefore,
ΔS sys = 0

IDEAL GAS 40°C 200 kJ

Heat 30°C

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course preparation. If you are a student using this Manual, youare using it without permission.

7-3

7-24 Air is compressed steadily by a compressor. The air temperature is maintained constant by heat rejection to the surroundings. The rate of entropy change of air is to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas. 4 Theprocess involves no internal irreversibilities such as friction, and thus it is an isothermal, internally reversible process. Properties Noting that h = h(T) for ideal gases, we have h1 = h2 since T1 = T2 = 25°C. Analysis We take the compressor as the system. Noting that the enthalpy of air remains constant, the energy balance for this steady-flow system can be expressed in the rate form as
& & Ein...
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