chapter 22 Heat Engines, Entropy, and the Second Law of Thermodynamics 22.1 Heat Engines and the Second Law of Thermodynamics 22.2 Heat Pumps and Refrigerators 22.3 Reversible and Irreversible Processes 22.4 The Carnot Engine 22.5 Gasoline and Diesel Engines 22.6 Entropy 22.7 Entropy and the Second Law 22.8 Entropy on a Microscopic Scale The first law of thermodynamics, which we studied in Chapter 20, is a statement of conservation of energy and is a special-case reduction of Equation 8.2. This law states that a change in internal energy in a system can occur as a result of energy transfer by heat, by work, or by both. Although the first law of thermodynamics is very important, it makes no distinction between processes that occur spontaneously and those that do not. Only certain types of energy conversion and energy transfer processes actually take place in nature, however. The second law of thermodynamics, the major topic in this chapter, establishes which processes do and A Stirling engine from the early nineteenth century. Air is heated in the lower cylinder using an external source. As this happens, the air expands and pushes against a piston, causing it to move. The air is then cooled, allowing the cycle to begin again. This is one example of a heat engine, which we study in this chapter. (© SSPL/The Image Works) do not occur. The following are examples of 625 27819_22_c22_p625-656.indd 625 6/30/09 12:42:49 PM 626 CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics J-L Charmet/Science Photo Library/Photo Researchers, Inc. processes that do not violate the first law of thermodynamics if they proceed in either direction, but are observed in reality to proceed in only one direction: • When two objects at different temperatures are placed in thermal contact with each other, the net transfer of energy by heat is always from the warmer object to the cooler object, never from the cooler to the warmer. • A rubber ball dropped to the ground bounces several times and eventually comes to rest, but a ball lying on the ground never gathers internal energy from the ground and begins bouncing on its own. Lord Kelvin British physicist and mathematician (1824–1907) Born William Thomson in Belfast, Kelvin was the first to propose the use of an absolute scale of temperature. The Kelvin temperature scale is named in his honor. Kelvin’s work in thermodynamics led to the idea that energy cannot pass spontaneously from a colder object to a hotter object. • An oscillating pendulum eventually comes to rest because of collisions with air molecules and friction at the point of suspension. The mechanical energy of the system is converted to internal energy in the air, the pendulum, and the suspension; the reverse conversion of energy never occurs. All these processes are irreversible; that is, they are processes that occur naturally in one direction only. No irreversible process has ever been observed to run backward. If it were to do so, it would violate the second law of thermodynamics.1 22.1 Heat Engines and the Second Law © Andy Moore/Photolibrary/Jupiterimages of Thermodynamics Figure 22.1 A steam-driven locomotive obtains its energy by burning wood or coal. The generated energy vaporizes water into steam, which powers the locomotive. Modern locomotives use diesel fuel instead of wood or coal. Whether old-fashioned or modern, such locomotives can be modeled as heat engines, which extract energy from a burning fuel and convert a fraction of it to mechanical energy. 27819_22_c22_p625-656.indd 626 A heat engine is a device that takes in energy by heat2 and, operating in a cyclic process, expels a fraction of that energy by means of work. For instance, in a typical process by which a power plant produces electricity, a fuel such as coal is burned and the high-temperature gases produced are used to convert liquid water to steam. This steam is directed at the blades of a turbine, setting it into rotation. The mechanical energy associated with this rotation is used to drive an electric generator. Another device that can be modeled as a heat engine is the internal combustion engine in an automobile. This device uses energy from a burning fuel to perform work on pistons that results in the motion of the automobile. A heat engine carries some working substance through a cyclic process during which (1) the working substance absorbs energy by heat from a high-temperature energy reservoir, (2) work is done by the engine, and (3) energy is expelled by heat to a lower-temperature reservoir. As an example, consider the operation of a steam engine (Fig. 22.1), which uses water as the working substance. The water in a boiler absorbs energy from burning fuel and evaporates to steam, which then does work by expanding against a piston. After the steam cools and condenses, the liquid water produced returns to the boiler and the cycle repeats. It is useful to represent a heat engine schematically as in Active Figure 22.2. The engine absorbs a quantity of energy |Q h| from the hot reservoir. For the mathematical discussion of heat engines, we use absolute values to make all energy transfers 1Although a process occurring in the time-reversed sense has never been observed, it is possible for it to occur. As we shall see later in this chapter, however, the probability of such a process occurring is infinitesimally small. From this viewpoint, processes occur with a vastly greater probability in one direction than in the opposite direction. 2We use heat as our model for energy transfer into a heat engine. Other methods of energy transfer are possible in the model of a heat engine, however. For example, the Earth’s atmosphere can be modeled as a heat engine in which the input energy transfer is by means of electromagnetic radiation from the Sun. The output of the atmospheric heat engine causes the wind structure in the atmosphere. 6/30/09 12:42:56 PM 22.1 | Heat Engines and the Second Law of Thermodynamics 627 by heat positive, and the direction of transfer is indicated with an explicit positive or negative sign. The engine does work Weng (so that negative work W 5 2Weng is done on the engine) and then gives up a quantity of energy |Q c | to the cold reservoir. Because the working substance goes through a cycle, its initial and final internal energies are equal: DE int 5 0. Hence, from the first law of thermodynamics, DE int 5 Q 1 W 5 Q 2 Weng 5 0, and the net work Weng done by a heat engine is equal to the net energy Q net transferred to it. As you can see from Active Figure 22.2, Q net 5 |Q h| 2 |Q c |; therefore, Weng 5 |Q h| 2 |Q c | (22.1) The thermal efficiency e of a heat engine is defined as the ratio of the net work done by the engine during one cycle to the energy input at the higher temperature during the cycle: e; Weng 0Qh0 5 0Qh0 2 0Qc0 0Qc0 512 0Qh0 0Qh0 (22.2) You can think of the efficiency as the ratio of what you gain (work) to what you give (energy transfer at the higher temperature). In practice, all heat engines expel only a fraction of the input energy Q h by mechanical work; consequently, their efficiency is always less than 100%. For example, a good automobile engine has an efficiency of about 20%, and diesel engines have efficiencies ranging from 35% to 40%. Equation 22.2 shows that a heat engine has 100% efficiency (e 5 1) only if |Q c | 5 0, that is, if no energy is expelled to the cold reservoir. In other words, a heat engine with perfect efficiency would have to expel all the input energy by work. Because efficiencies of real engines are well below 100%, the Kelvin–Planck form of the second law of thermodynamics states the following: It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the input of energy by heat from a reservoir and the performance of an equal amount of work. This statement of the second law means that during the operation of a heat engine, Weng can never be equal to |Q h| or, alternatively, that some energy |Q c | must be rejected to the environment. Every heat engine must have some energy exhaust. Figure 22.3 is a schematic diagram of the impossible “perfect” heat engine. W Thermal efficiency of a heat engine The engine does work Weng. Energy Q h enters the engine. Hot reservoir att Th Qh Weng Heat engine Energy Q c leaves the engine. Qc Cold reservoir at Tc ACTIVE FIGURE 22.2 Schematic representation of a heat engine. Quick Quiz 22.1 The energy input to an engine is 3.00 times greater than the work it performs. (i) What is its thermal efficiency? (a) 3.00 (b) 1.00 (c) 0.333 (d) impossible to determine (ii) What fraction of the energy input is expelled to the cold reservoir? (a) 0.333 (b) 0.667 (c) 1.00 (d) impossible to determine An impossible heat engine Hot reservoir att Th Qh Heat engine Cold reservoir at Tc 27819_22_c22_p625-656.indd 627 Weng Figure 22.3 Schematic diagram of a heat engine that takes in energy from a hot reservoir and does an equivalent amount of work. It is impossible to construct such a perfect engine. Pitfall Prevention 22.1 The First and Second Laws Notice the distinction between the first and second laws of thermodynamics. If a gas undergoes a one-time isothermal process, then DE int 5 Q 1 W 5 0 and W 5 2Q. Therefore, the first law allows all energy input by heat to be expelled by work. In a heat engine, however, in which a substance undergoes a cyclic process, only a portion of the energy input by heat can be expelled by work according to the second law. 6/30/09 12:42:58 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 628 The Efficiency of an Engine Ex a m pl e 22.1 An engine transfers 2.00 3 103 J of energy from a hot reservoir during a cycle and transfers 1.50 3 103 J as exhaust to a cold reservoir. (A) Find the efficiency of the engine. SOLUTION Conceptualize Review Active Figure 22.2; think about energy going into the engine from the hot reservoir and splitting, with part coming out by work and part by heat into the cold reservoir. Categorize This example involves evaluation of quantities from the equations introduced in this section, so we categorize it as a substitution problem. Find the efficiency of the engine from Equation 22.2: e512 0Qc0 0Qh0 512 1.50 3 103 J 2.00 3 103 J 5 0.250, or 25.0% (B) How much work does this engine do in one cycle? SOLUTION Find the work done by the engine by taking the difference between the input and output energies: Weng 5 |Q h | 2 |Q c | 5 2.00 3 103 J 2 1.50 3 103 J 5 5.0 3 102 J WHAT IF? Suppose you were asked for the power output of this engine. Do you have sufficient information to answer this question? Answer No, you do not have enough information. The power of an engine is the rate at which work is done by the engine. You know how much work is done per cycle, but you have no information about the time interval associated with one cycle. If you were told that the engine operates at 2 000 rpm (revolutions per minute), however, you could relate this rate to the period of rotation T of the mechanism of the engine. Assuming there is one thermodynamic cycle per revolution, the power is P5 Weng T 5 5.0 3 102 J 1 min a b 5 1.7 3 104 W 1 1 2 000 min 2 60 s 22.2 Heat Pumps and Refrigerators Work W is done on the heat pump. Energy Q h is expelled to the hot reservoir. Energy Q c is drawn from the cold reservoir. Hot reservoir at Th Qh Heat pump W Qc Cold reservoir at Tc ACTIVE FIGURE 22.4 Schematic representation of a heat pump. 27819_22_c22_p625-656.indd 628 In a heat engine, the direction of energy transfer is from the hot reservoir to the cold reservoir, which is the natural direction. The role of the heat engine is to process the energy from the hot reservoir so as to do useful work. What if we wanted to transfer energy from the cold reservoir to the hot reservoir? Because that is not the natural direction of energy transfer, we must put some energy into a device to be successful. Devices that perform this task are called heat pumps and refrigerators. For example, homes in summer are cooled using heat pumps called air conditioners. The air conditioner transfers energy from the cool room in the home to the warm air outside. In a refrigerator or a heat pump, the engine takes in energy |Q c | from a cold reservoir and expels energy |Q h| to a hot reservoir (Active Fig. 22.4), which can be accomplished only if work is done on the engine. From the first law, we know that the energy given up to the hot reservoir must equal the sum of the work done and the energy taken in from the cold reservoir. Therefore, the refrigerator or heat pump transfers energy from a colder body (for example, the contents of a kitchen refrigerator or the winter air outside a building) to a hotter body (the air in the kitchen or a room in the building). In practice, it is desirable to carry out this process with a minimum of work. If the process could be accomplished without doing any work, the refrigerator or heat pump would be “perfect” (Fig. 22.5). Again, the 6/30/09 12:43:01 PM 22.2 | Heat Pumps and Refrigerators existence of such a device would be in violation of the second law of thermodynamics, which in the form of the Clausius statement 3 states: It is impossible to construct a cyclical machine whose sole effect is to transfer energy continuously by heat from one object to another object at a higher temperature without the input of energy by work. In simpler terms, energy does not transfer spontaneously by heat from a cold object to a hot object. Work input is required to run a refrigerator. The Clausius and Kelvin–Planck statements of the second law of thermodynamics appear at first sight to be unrelated, but in fact they are equivalent in all respects. Although we do not prove so here, if either statement is false, so is the other.4 In practice, a heat pump includes a circulating fluid that passes through two sets of metal coils that can exchange energy with the surroundings. The fluid is cold and at low pressure when it is in the coils located in a cool environment, where it absorbs energy by heat. The resulting warm fluid is then compressed and enters the other coils as a hot, high-pressure fluid. There it releases its stored energy to the warm surroundings. In an air conditioner, energy is absorbed into the fluid in coils located in a building’s interior; after the fluid is compressed, energy leaves the fluid through coils located outdoors. In a refrigerator, the external coils are behind or underneath the unit (Fig. 22.6). The internal coils are in the walls of the refrigerator and absorb energy from the food. The effectiveness of a heat pump is described in terms of a number called the coefficient of performance (COP). The COP is similar to the thermal efficiency for a heat engine in that it is a ratio of what you gain (energy transferred to or from a reservoir) to what you give (work input). For a heat pump operating in the cooling mode, “what you gain” is energy removed from the cold reservoir. The most effective refrigerator or air conditioner is one that removes the greatest amount of energy from the cold reservoir in exchange for the least amount of work. Therefore, for these devices operating in the cooling mode, we define the COP in terms of |Q c |: COP 1 cooling mode 2 5 energy transferred at low temperature work done on heat pump 5 0Qc0 W (22.3) 629 An impossible heat pump Hot reservoir at Th Qh Qc Heat pump Qc Cold reservoir at Tc Figure 22.5 Schematic diagram of an impossible heat pump or refrigerator, that is, one that takes in energy from a cold reservoir and expels an equivalent amount of energy to a hot reservoir without the input of energy by work. The coils on the back of a refrigerator transfer energy by heat to the air. A good refrigerator should have a high COP, typically 5 or 6. In addition to cooling applications, heat pumps are becoming increasingly popular for heating purposes. The energy-absorbing coils for a heat pump are located outside a building, in contact with the air or buried in the ground. The other set of coils are in the building’s interior. The circulating fluid flowing through the coils absorbs energy from the outside and releases it to the interior of the building from the interior coils. In the heating mode, the COP of a heat pump is defined as the ratio of the energy transferred to the hot reservoir to the work required to transfer that energy: energy transferred at high temperature work done on heat pump 5 0 Qh 0 W (22.4) © Cengage Learning/Charles D. Winters COP 1 heating mode 2 5 If the outside temperature is 25°F (24°C) or higher, a typical value of the COP for a heat pump is about 4. That is, the amount of energy transferred to the building is about four times greater than the work done by the motor in the heat pump. As the outside temperature decreases, however, it becomes more difficult for the heat pump to extract sufficient energy from the air and so the COP decreases. Therefore, the use of heat pumps that extract energy from the air, although satisfactory in Figure 22.6 The back of a house3First 4 See expressed by Rudolf Clausius (1822–1888). an advanced textbook on thermodynamics for this proof. 27819_22_c22_p625-656.indd 629 hold refrigerator. The air surrounding the coils is the hot reservoir. 6/30/09 12:43:03 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 630 moderate climates, is not appropriate in areas where winter temperatures are very low. It is possible to use heat pumps in colder areas by burying the external coils deep in the ground. In that case, the energy is extracted from the ground, which tends to be warmer than the air in the winter. Quick Quiz 22.2 The energy entering an electric heater by electrical transmission can be converted to internal energy with an efficiency of 100%. By what factor does the cost of heating your home change when you replace your electric heating system with an electric heat pump that has a COP of 4.00? Assume the motor running the heat pump is 100% efficient. (a) 4.00 (b) 2.00 (c) 0.500 (d) 0.250 Ex a m pl e 22.2 Freezing Water A certain refrigerator has a COP of 5.00. When the refrigerator is running, its power input is 500 W. A sample of water of mass 500 g and temperature 20.0°C is placed in the freezer compartment. How long does it take to freeze the water to ice at 0°C? Assume all other parts of the refrigerator stay at the same temperature and there is no leakage of energy from the exterior, so the operation of the refrigerator results only in energy being extracted from the water. SOLUTION Conceptualize Energy leaves the water, reducing its temperature and then freezing it into ice. The time interval required for this entire process is related to the rate at which energy is withdrawn from the water, which, in turn, is related to the power input of the refrigerator. Categorize We categorize this example as one that combines our understanding of temperature changes and phase changes from Chapter 20 and our understanding of heat pumps from this chapter. W Dt Analyze Use the power rating of the refrigerator to find the time interval Dt required for the freezing process to occur: P5 Use Equation 22.3 to relate the work W done on the heat pump to the energy |Q c | extracted from the water: Dt 5 Use Equations 20.4 and 20.7 to substitute the amount of energy |Q c | that must be extracted from the water of mass m: Dt 5 Recognize that the amount of water that freezes is Dm 5 2m because all the water freezes: Dt 5 Subsitute numerical values: Dt 5 S Dt 5 W P |Q c | P 1 COP 2 0 mc DT 1 L f Dm 0 P 1 COP 2 0 m 1 c DT 2 L f 2 0 P 1 COP 2 0 1 0.500 kg 2 3 1 4 186 J/kg ? °C 2 1 220.0°C 2 2 3.33 3 105 J/kg 4 0 1 500 W 2 1 5.00 2 5 83.3 s Finalize In reality, the time interval for the water to freeze in a refrigerator is much longer than 83.3 s, which suggests that the assumptions of our model are not valid. Only a small part of the energy extracted from the refrigerator interior in a given time interval comes from the water. Energy must also be extracted from the container in which the water is placed, and energy that continuously leaks into the interior from the exterior must be extracted. 27819_22_c22_p625-656.indd 630 6/30/09 12:43:06 PM 22.3 | Reversible and Irreversible Processes 631 22.3 Reversible and Irreversible Processes In the next section, we will discuss a theoretical heat engine that is the most efficient possible. To understand its nature, we must first examine the meaning of reversible and irreversible processes. In a reversible process, the system undergoing the process can be returned to its initial conditions along the same path on a PV diagram, and every point along this path is an equilibrium state. A process that does not satisfy these requirements is irreversible. All natural processes are known to be irreversible. Let’s examine the adiabatic free expansion of a gas, which was already discussed in Section 20.6, and show that it cannot be reversible. Consider a gas in a thermally insulated container as shown in Figure 22.7. A membrane separates the gas from a vacuum. When the membrane is punctured, the gas expands freely into the vacuum. As a result of the puncture, the system has changed because it occupies a greater volume after the expansion. Because the gas does not exert a force through a displacement, it does no work on the surroundings as it expands. In addition, no energy is transferred to or from the gas by heat because the container is insulated from its surroundings. Therefore, in this adiabatic process, the system has changed but the surroundings have not. For this process to be reversible, we must return the gas to its original volume and temperature without changing the surroundings. Imagine trying to reverse the process by compressing the gas to its original volume. To do so, we fit the container with a piston and use an engine to force the piston inward. During this process, the surroundings change because work is being done by an outside agent on the system. In addition, the system changes because the compression increases the temperature of the gas. The temperature of the gas can be lowered by allowing it to come into contact with an external energy reservoir. Although this step returns the gas to its original conditions, the surroundings are again affected because energy is being added to the surroundings from the gas. If this energy could be used to drive the engine that compressed the gas, the net energy transfer to the surroundings would be zero. In this way, the system and its surroundings could be returned to their initial conditions and we could identify the process as reversible. The Kelvin–Planck statement of the second law, however, specifies that the energy removed from the gas to return the temperature to its original value cannot be completely converted to mechanical energy in the form of the work done by the engine in compressing the gas. Therefore, we must conclude that the process is irreversible. We could also argue that the adiabatic free expansion is irreversible by relying on the portion of the definition of a reversible process that refers to equilibrium states. For example, during the sudden expansion, significant variations in pressure occur throughout the gas. Therefore, there is no well-defined value of the pressure for the entire system at any time between the initial and final states. In fact, the process cannot even be represented as a path on a PV diagram. The PV diagram for an adiabatic free expansion would show the initial and final conditions as points, but these points would not be connected by a path. Therefore, because the intermediate conditions between the initial and final states are not equilibrium states, the process is irreversible. Although all real processes are irreversible, some are almost reversible. If a real process occurs very slowly such that the system is always very nearly in an equilibrium state, the process can be approximated as being reversible. Suppose a gas is compressed isothermally in a piston–cylinder arrangement in which the gas is in thermal contact with an energy reservoir and we continuously transfer just enough energy from the gas to the reservoir to keep the temperature constant. For example, imagine that the gas is compressed very slowly by dropping grains of sand onto a frictionless piston as shown in Figure 22.8. As each grain lands on the piston and compresses the gas a small amount, the system deviates from an equilibrium state, but it is so close to one that it achieves a new equilibrium state in a relatively short time interval. Each grain added represents a change to a new equilibrium state, but 27819_22_c22_p625-656.indd 631 Pitfall Prevention 22.2 All Real Processes Are Irreversible The reversible process is an idealization; all real processes on the Earth are irreversible. Insulating wall Vacuum Membrane Gas at Ti Figure 22.7 Adiabatic free expansion of a gas. The gas is compressed slowly as individual grains of sand drop onto the piston. Energy reservoir Figure 22.8 A method for compressing a gas in a reversible isothermal process. 6/30/09 12:43:07 PM 632 CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics the differences between states are so small that the entire process can be approximated as occurring through continuous equilibrium states. The process can be reversed by slowly removing grains from the piston. A general characteristic of a reversible process is that no dissipative effects (such as turbulence or friction) that convert mechanical energy to internal energy can be present. Such effects can be impossible to eliminate completely. Hence, it is not surprising that real processes in nature are irreversible. 22.4 The Carnot Engine Pitfall Prevention 22.3 Don’t Shop for a Carnot Engine The Carnot engine is an idealization; do not expect a Carnot engine to be developed for commercial use. We explore the Carnot engine only for theoretical considerations. In 1824, a French engineer named Sadi Carnot described a theoretical engine, now called a Carnot engine, that is of great importance from both practical and theoretical viewpoints. He showed that a heat engine operating in an ideal, reversible cycle—called a Carnot cycle—between two energy reservoirs is the most efficient engine possible. Such an ideal engine establishes an upper limit on the efficiencies of all other engines. That is, the net work done by a working substance taken through the Carnot cycle is the greatest amount of work possible for a given amount of energy supplied to the substance at the higher temperature. Carnot’s theorem can be stated as follows: J.-L. Charmet/Science Photo Library/ Photo Researchers, Inc. No real heat engine operating between two energy reservoirs can be more efficient than a Carnot engine operating between the same two reservoirs. Sadi Carnot French engineer (1796–1832) Carnot was the first to show the quantitative relationship between work and heat. In 1824, he published his only work, Reflections on the Motive Power of Heat, which reviewed the industrial, political, and economic importance of the steam engine. In it, he defined work as “weight lifted through a height.” 27819_22_c22_p625-656.indd 632 To prove the validity of this theorem, imagine two heat engines operating between the same energy reservoirs. One is a Carnot engine with efficiency e C , and the other is an engine with efficiency e, where we assume e . e C . Because the cycle in the Carnot engine is reversible, the engine can operate in reverse as a refrigerator. The more efficient engine is used to drive the Carnot engine as a Carnot refrigerator. The output by work of the more efficient engine is matched to the input by work of the Carnot refrigerator. For the combination of the engine and refrigerator, no exchange by work with the surroundings occurs. Because we have assumed the engine is more efficient than the refrigerator, the net result of the combination is a transfer of energy from the cold to the hot reservoir without work being done on the combination. According to the Clausius statement of the second law, this process is impossible. Hence, the assumption that e . e C must be false. All real engines are less efficient than the Carnot engine because they do not operate through a reversible cycle. The efficiency of a real engine is further reduced by such practical difficulties as friction and energy losses by conduction. To describe the Carnot cycle taking place between temperatures Tc and Th , let’s assume the working substance is an ideal gas contained in a cylinder fitted with a movable piston at one end. The cylinder’s walls and the piston are thermally nonconducting. Four stages of the Carnot cycle are shown in Active Figure 22.9, and the PV diagram for the cycle is shown in Active Figure 22.10. The Carnot cycle consists of two adiabatic processes and two isothermal processes, all reversible: 1. Process A S B (Active Fig. 22.9a) is an isothermal expansion at temperature Th . The gas is placed in thermal contact with an energy reservoir at temperature Th . During the expansion, the gas absorbs energy |Q h| from the reservoir through the base of the cylinder and does work WAB in raising the piston. 2. In process B S C (Active Fig. 22.9b), the base of the cylinder is replaced by a thermally nonconducting wall and the gas expands adiabatically; that is, no energy enters or leaves the system by heat. During the expansion, the temperature of the gas decreases from Th to Tc and the gas does work W BC in raising the piston. 6/30/09 12:43:08 PM 22.4 | The Carnot Engine 633 ACTIVE FIGURE 22.9 ASB The gas undergoes an isothermal expansion. The Carnot cycle. The letters A, B, C, and D refer to the states of the gas shown in Active Figure 22.10. The arrows on the piston indicate the direction of its motion during each process. Qh Energy reservoir at Th a BSC The gas undergoes an adiabatic expansion. DSA The gas undergoes an adiabatic compression. Q0 Q0 Cycle Thermal insulation Thermal insulation d b CSD The gas undergoes an isothermal compression. Qc P Energy reservoir at Tc c A 3. In process C S D (Active Fig. 22.9c), the gas is placed in thermal contact with an energy reservoir at temperature Tc and is compressed isothermally at temperature Tc . During this time, the gas expels energy |Q c | to the reservoir and the work done by the piston on the gas is WCD . 4. In the final process D S A (Active Fig. 22.9d), the base of the cylinder is replaced by a nonconducting wall and the gas is compressed adiabatically. The temperature of the gas increases to Th , and the work done by the piston on the gas is W DA . The thermal efficiency of the engine is given by Equation 22.2: e512 0Qh0 5 B Weng Th C D Qc Tc V ACTIVE FIGURE 22.10 0Qc0 0Qh0 Tc Th Qh PV diagram for the Carnot cycle. The net work done Weng equals the net energy transferred into the Carnot engine in one cycle, |Q h | 2 |Q c |. In Example 22.3, we show that for a Carnot cycle, 0Qc0 The work done during the cycle equals the area enclosed by the path on the PV diagram. (22.5) Hence, the thermal efficiency of a Carnot engine is eC 5 1 2 27819_22_c22_p625-656.indd 633 Tc Th (22.6) W Efficiency of a Carnot engine 6/30/09 12:43:10 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 634 This result indicates that all Carnot engines operating between the same two temperatures have the same efficiency.5 Equation 22.6 can be applied to any working substance operating in a Carnot cycle between two energy reservoirs. According to this equation, the efficiency is zero if Tc 5 Th , as one would expect. The efficiency increases as Tc is lowered and Th is raised. The efficiency can be unity (100%), however, only if Tc 5 0 K. Such reservoirs are not available; therefore, the maximum efficiency is always less than 100%. In most practical cases, Tc is near room temperature, which is about 300 K. Therefore, one usually strives to increase the efficiency by raising Th . Theoretically, a Carnot-cycle heat engine run in reverse constitutes the most effective heat pump possible, and it determines the maximum COP for a given combination of hot and cold reservoir temperatures. Using Equations 22.1 and 22.4, we see that the maximum COP for a heat pump in its heating mode is COPC 1 heating mode 2 5 5 0Qh0 W 0Qh0 0Qh0 2 0Qc0 5 12 1 5 0Qc0 0Qh0 1 Tc 12 Th 5 Th Th 2 Tc The Carnot COP for a heat pump in the cooling mode is COPC 1 cooling mode 2 5 Tc Th 2 Tc As the difference between the temperatures of the two reservoirs approaches zero in this expression, the theoretical COP approaches infinity. In practice, the low temperature of the cooling coils and the high temperature at the compressor limit the COP to values below 10. Quick Quiz 22.3 Three engines operate between reservoirs separated in temperature by 300 K. The reservoir temperatures are as follows: Engine A: Th 5 1 000 K, Tc 5 700 K; Engine B: Th 5 800 K, Tc 5 500 K; Engine C: Th 5 600 K, Tc 5 300 K. Rank the engines in order of theoretically possible efficiency from highest to lowest. Ex a m pl e 22.3 Efficiency of the Carnot Engine Show that the ratio of energy transfers by heat in a Carnot engine is equal to the ratio of reservoir temperatures, as given by Equation 22.5. SOLUTION Conceptualize Make use of Active Figures 22.9 and 22.10 to help you visualize the processes in the Carnot cycle. Categorize Because of our understanding of the Carnot cycle, we can categorize the processes in the cycle as isothermal and adiabatic. 5For the processes in the Carnot cycle to be reversible, they must be carried out infinitesimally slowly. Therefore, although the Carnot engine is the most efficient engine possible, it has zero power output because it takes an infinite time interval to complete one cycle! For a real engine, the short time interval for each cycle results in the working substance reaching a high temperature lower than that of the hot reservoir and a low temperature higher than that of the cold reservoir. An engine undergoing a Carnot cycle between this narrower temperature range was analyzed by F. L. Curzon and B. Ahlborn (“Efficiency of a Carnot engine at maximum power output,” Am. J. Phys. 43(1), 22, 1975), who found that the efficiency at maximum power output depends only on the reservoir temperatures Tc and Th and is given by e C-A 5 1 2 (Tc/Th)1/2. The Curzon–Ahlborn efficiency e C-A provides a closer approximation to the efficiencies of real engines than does the Carnot efficiency. 27819_22_c22_p625-656.indd 634 6/30/09 12:43:10 PM 22.4 | The Carnot Engine 635 22.3 cont. Analyze For the isothermal expansion (process A S B in Active Fig. 22.9), find the energy transfer by heat from the hot reservoir using Equation 20.14 and the first law of thermodynamics: 0 Q h 0 5 0 DE int 2 WAB 0 5 0 0 2 WAB 0 5 nRTh ln VB VA In a similar manner, find the energy transfer to the cold reservoir during the isothermal compression C S D: 0 Q c 0 5 0 DE int 2 WCD 0 5 0 0 2 WCD 0 5 nRTc ln VC VD Divide the second expression by the first: (1) Apply Equation 21.20 to the adiabatic processes B S C and D S A: ThV Bg21 5 TcVCg21 ThVAg21 5 TcV D g21 Divide the first equation by the second: a 0Qh0 5 Tc ln 1 VC /VD 2 Th ln 1 VB /VA 2 VC g21 VB g21 5a b b VA VD (2) 0Qc0 Substitute Equation (2) into Equation (1): 0Qc0 0Qh0 VB VC 5 VA VD 5 Tc ln 1 VC /VD 2 Tc ln 1 VC /VD 2 Tc 5 5 Th ln 1 VB /VA 2 Th ln 1 VC /VD 2 Th Finalize This last equation is Equation 22.5, the one we set out to prove. Ex a m pl e 22.4 The Steam Engine A steam engine has a boiler that operates at 500 K. The energy from the burning fuel changes water to steam, and this steam then drives a piston. The cold reservoir’s temperature is that of the outside air, approximately 300 K. What is the maximum thermal efficiency of this steam engine? SOLUTION Conceptualize In a steam engine, the gas pushing on the piston in Active Figure 22.9 is steam. A real steam engine does not operate in a Carnot cycle, but, to find the maximum possible efficiency, imagine a Carnot steam engine. Categorize We calculate an efficiency using Equation 22.6, so we categorize this example as a substitution problem. Substitute the reservoir temperatures into Equation 22.6: eC 5 1 2 Tc 300 K 512 5 0.400 Th 500 K or 40.0% This result is the highest theoretical efficiency of the engine. In practice, the efficiency is considerably lower. WHAT IF? Suppose we wished to increase the theoretical efficiency of this engine. This increase can be achieved by raising Th by DT or by decreasing Tc by the same DT. Which would be more effective? Answer A given DT would have a larger fractional effect on a smaller temperature, so you would expect a larger change in efficiency if you alter Tc by DT. Let’s test that numerically. Raising Th by 50 K, corresponding to Th 5 550 K, would give a maximum efficiency of eC 5 1 2 Tc 300 K 512 5 0.455 Th 550 K continued 27819_22_c22_p625-656.indd 635 6/30/09 12:43:11 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 636 22.4 cont. Decreasing Tc by 50 K, corresponding to Tc 5 250 K, would give a maximum efficiency of eC 5 1 2 Tc 250 K 512 5 0.500 Th 500 K Although changing Tc is mathematically more effective, often changing Th is practically more feasible. 22.5 Gasoline and Diesel Engines In a gasoline engine, six processes occur in each cycle; they are illustrated in Active Figure 22.11. In this discussion, let’s consider the interior of the cylinder above the piston to be the system that is taken through repeated cycles in the engine’s operation. For a given cycle, the piston moves up and down twice, which represents a four-stroke cycle consisting of two upstrokes and two downstrokes. The processes in the cycle can be approximated by the Otto cycle shown in the PV diagram in Active Figure 22.12. In the following discussion, refer to Active Figure 22.11 for the pictorial representation of the strokes and Active Figure 22.12 for the significance on the PV diagram of the letter designations below: 1. During the intake stroke (Active Fig. 22.11a and O S A in Active Figure 22.12), the piston moves downward and a gaseous mixture of air and fuel is drawn into the cylinder at atmospheric pressure. That is the energy input part of the cycle: energy enters the system (the interior of the cylinder) by matter transfer as potential energy stored in the fuel. In this process, the volume increases from V2 to V1. This apparent backward numbering is The intake valve opens, and the air– fuel mixture enters as the piston moves down. The piston moves up and compresses the mixture. The spark plug fires and ignites the mixture. The hot gas pushes the piston downward. The exhaust valve opens, and the residual gas escapes. The piston moves up and pushes the remaining gas out. Spark plug Air and fuel Exhaust Piston Intake Compression Spark Power Release Exhaust a b c d e f ACTIVE FIGURE 22.11 The four-stroke cycle of a conventional gasoline engine. The arrows on the piston indicate the direction of its motion during each process. 27819_22_c22_p625-656.indd 636 6/30/09 12:43:14 PM 22.5 | Gasoline and Diesel Engines 2. 3. 4. 5. 6. based on the compression stroke (process 2 below), in which the air–fuel mixture is compressed from V1 to V2. During the compression stroke (Active Fig. 22.11b and A S B in Active Fig. 22.12), the piston moves upward, the air–fuel mixture is compressed adiabatically from volume V1 to volume V2, and the temperature increases from TA to TB . The work done on the gas is positive, and its value is equal to the negative of the area under the curve AB in Active Figure 22.12. Combustion occurs when the spark plug fires (Active Fig. 22.11c and B S C in Active Fig. 22.12). That is not one of the strokes of the cycle because it occurs in a very short time interval while the piston is at its highest position. The combustion represents a rapid energy transformation from potential energy stored in chemical bonds in the fuel to internal energy associated with molecular motion, which is related to temperature. During this time interval, the mixture’s pressure and temperature increase rapidly, with the temperature rising from TB to TC . The volume, however, remains approximately constant because of the short time interval. As a result, approximately no work is done on or by the gas. We can model this process in the PV diagram (Active Fig. 22.12) as that process in which the energy |Q h| enters the system. (In reality, however, this process is a conversion of energy already in the cylinder from process O S A.) In the power stroke (Active Fig. 22.11d and C S D in Active Fig. 22.12), the gas expands adiabatically from V2 to V1. This expansion causes the temperature to drop from TC to TD . Work is done by the gas in pushing the piston downward, and the value of this work is equal to the area under the curve CD. Release of the residual gases occurs when an exhaust valve is opened (Active Fig. 22.11e and D S A in Active Fig. 22.12). The pressure suddenly drops for a short time interval. During this time interval, the piston is almost stationary and the volume is approximately constant. Energy is expelled from the interior of the cylinder and continues to be expelled during the next process. In the final process, the exhaust stroke (Active Fig. 22.11e and A S O in Active Fig. 22.12), the piston moves upward while the exhaust valve remains open. Residual gases are exhausted at atmospheric pressure, and the volume decreases from V1 to V2. The cycle then repeats. 637 P TA TC C Adiabatic processes Qh B D O A V2 Qc V1 V ACTIVE FIGURE 22.12 PV diagram for the Otto cycle, which approximately represents the processes occurring in an internal combustion engine. If the air–fuel mixture is assumed to be an ideal gas, the efficiency of the Otto cycle is e512 1 1 V1 /V2 2 g21 1 Otto cycle 2 (22.7) where V1/V2 is the compression ratio and g is the ratio of the molar specific heats CP/CV for the air–fuel mixture. Equation 22.7, which is derived in Example 22.5, shows that the efficiency increases as the compression ratio increases. For a typical compression ratio of 8 and with g 5 1.4, Equation 22.7 predicts a theoretical efficiency of 56% for an engine operating in the idealized Otto cycle. This value is much greater than that achieved in real engines (15% to 20%) because of such effects as friction, energy transfer by conduction through the cylinder walls, and incomplete combustion of the air–fuel mixture. Diesel engines operate on a cycle similar to the Otto cycle, but they do not employ a spark plug. The compression ratio for a diesel engine is much greater than that for a gasoline engine. Air in the cylinder is compressed to a very small volume, and, as a consequence, the cylinder temperature at the end of the compression stroke is very high. At this point, fuel is injected into the cylinder. The temperature is high enough for the air–fuel mixture to ignite without the assistance of a spark plug. Diesel engines are more efficient than gasoline engines because of their greater compression ratios and resulting higher combustion temperatures. 27819_22_c22_p625-656.indd 637 6/30/09 12:43:15 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 638 Ex a m pl e 22.5 Efficiency of the Otto Cycle Show that the thermal efficiency of an engine operating in an idealized Otto cycle (see Active Figs. 22.11 and 22.12) is given by Equation 22.7. Treat the working substance as an ideal gas. SOLUTION Conceptualize Study Active Figures 22.11 and 22.12 to make sure you understand the working of the Otto cycle. Categorize As seen in Active Figure 22.12, we categorize the processes in the Otto cycle as isovolumetric and adiabatic. Analyze Model the energy input and output as occurring by heat in processes B S C and D S A. (In reality, most of the energy enters and leaves by matter transfer as the air–fuel mixture enters and leaves the cylinder.) Use Equation 21.8 to find the energy transfers by heat for these processes, which take place at constant volume: BSC |Q h | 5 nCV (TC 2 TB) DSA |Q c | 5 nCV (TD 2 TA) Substitute these expressions into Equation 22.2: (1) e 5 1 2 Apply Equation 21.20 to the adiabatic processes A S B and C S D: ASB TAVAg21 5 TBV Bg21 CSD TCVCg21 5 TDV D g21 Solve these equations for the temperatures TA and TD , noting that VA 5 V D 5 V1 and V B 5 VC 5 V2: 0Qc0 T D 2 TA 512 0Qh0 TC 2 TB (2) TA 5 TB a V2 g21 VB g21 5 TB a b b VA V1 (3) TD 5 TC a V2 g21 VC g21 5 TC a b b VD V1 V2 g21 TD 2 T A 5a b TC 2 TB V1 Subtract Equation (2) from Equation (3) and rearrange: (4) Substitute Equation (4) into Equation (1): e512 1 1 V1/V2 2 g21 Finalize This final expression is Equation 22.7. Pitfall Prevention 22.4 Entropy Is Abstract Entropy is one of the most abstract notions in physics, so follow the discussion in this and the subsequent sections very carefully. Do not confuse energy with entropy. Even though the names sound similar, they are very different concepts. 27819_22_c22_p625-656.indd 638 22.6 Entropy The zeroth law of thermodynamics involves the concept of temperature, and the first law involves the concept of internal energy. Temperature and internal energy are both state variables; that is, the value of each depends only on the thermodynamic state of a system, not on the process that brought it to that state. Another state variable—this one related to the second law of thermodynamics—is entropy S. In this section, we define entropy on a macroscopic scale as it was first expressed by Clausius in 1865. Entropy was originally formulated as a useful concept in thermodynamics. Its importance grew, however, as the field of statistical mechanics developed because the analytical techniques of statistical mechanics provide an alternative means of interpreting entropy and a more global significance to the concept. In statistical mechanics, the behavior of a substance is described in terms of the statistical behavior of its atoms and molecules. An important finding in these studies is that 6/30/09 12:43:15 PM a © Cengage Learning/George Semple isolated systems tend toward disorder, and entropy is a measure of this disorder. For example, consider the molecules of a gas in the air in your room. If half the gas molecules had velocity vectors of equal magnitude directed toward the left and the other half had velocity vectors of the same magnitude directed toward the right, the situation would be very ordered. Such a situation is extremely unlikely, however. If you could view the molecules, you would see that they move haphazardly in all directions, bumping into one another, changing speed upon collision, some going fast and others going slowly. This situation is highly disordered. The cause of the tendency of an isolated system toward disorder is easily explained. To do so, let’s distinguish between microstates and macrostates of a system. A microstate is a particular configuration of the individual constituents of the system. For example, the description of the ordered velocity vectors of the air molecules in your room refers to a particular microstate, and the more likely haphazard motion is another microstate. A macrostate is a description of the system’s conditions from a macroscopic point of view. For a thermodynamic system, macrostates are described by macroscopic variables such as pressure, density, and temperature. For any given macrostate of the system, a number of microstates are possible. Let’s first consider some nonthermodynamic systems for simplicity. For example, the macrostate of a 4 on a pair of dice can be formed from the possible microstates 1–3, 2–2, and 3–1. The macrostate of 2 has only one microstate, 1–1. It is assumed all microstates are equally probable. When all possible macrostates are examined, however, it is found that macrostates associated with disorder have far more possible microstates than those associated with order. Therefore, 4 is a more disordered macrostate for two dice than 2 because there are three microstates for a 4 and only one for a 2. There is only one microstate associated with the macrostate of a royal flush in a poker hand of five spades, laid out in order from ten to ace (Fig. 22.13a). Figure 22.13b shows another poker hand. The macrostate here is “worthless hand.” The particular hand (the microstate) in Figure 22.13b is as equally probable as the hand in Figure 22.13a. There are, however, many other hands similar to that in Figure 22.13b; that is, there are many microstates that also qualify as worthless hands. The more microstates that belong to a particular macrostate, the higher the probability that macrostate will occur. The macrostate of a royal flush in spades is ordered, of low probability, and of high value in poker. The macrostate of a worthless hand is disordered, of high probability, and of low poker value. 639 © Cengage Learning/George Semple 22.6 | Entropy b Figure 22.13 (a) A royal flush has low probability of occurring. (b) A worthless poker hand, one of many. Quick Quiz 22.4 (a) Suppose you select four cards at random from a standard deck of playing cards and end up with a macrostate of four deuces. How many microstates are associated with this macrostate? (b) Suppose you pick up two cards and end up with a macrostate of two aces. How many microstates are associated with this macrostate? We can also imagine ordered macrostates and disordered macrostates in physical processes, not just in games of dice and poker. The result of a dice throw or a poker hand stays fixed once the dice are thrown or the cards are dealt. Physical systems, on the other hand, are in a constant state of flux, changing from moment to moment from one microstate to another. Based on the relationship between the probability of a macrostate and the number of associated microstates, we therefore see that the probability of a system moving in time from an ordered macrostate to a disordered macrostate is far greater than the probability of the reverse because there are more microstates in a disordered macrostate. The original formulation of entropy in thermodynamics involves the transfer of energy by heat during a reversible process. Consider any infinitesimal process in which a system changes from one equilibrium state to another. If dQ r is the amount of energy transferred by heat when the system follows a reversible path between the 27819_22_c22_p625-656.indd 639 6/30/09 12:43:17 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 640 states, the change in entropy dS is equal to this amount of energy for the reversible process divided by the absolute temperature of the system: Change in entropy for an X infinitesimal process dS 5 dQ r (22.8) T We have assumed the temperature is constant because the process is infinitesimal. Because entropy is a state variable, the change in entropy during a process depends only on the endpoints and therefore is independent of the actual path followed. Consequently, the entropy change for an irreversible process can be determined by calculating the entropy change for a reversible process that connects the same initial and final states. The subscript r on the quantity dQ r is a reminder that the transferred energy is to be measured along a reversible path even though the system may actually have followed some irreversible path. When energy is absorbed by the system, dQ r is positive and the entropy of the system increases. When energy is expelled by the system, dQ r is negative and the entropy of the system decreases. Notice that Equation 22.8 does not define entropy but rather the change in entropy. Hence, the meaningful quantity in describing a process is the change in entropy. To calculate the change in entropy for a finite process, first recognize that T is generally not constant during the process. Therefore, we must integrate Equation 22.8: f Change in entropy X for a finite process f DS 5 3 dS 5 3 i i dQ r T (22.9) As with an infinitesimal process, the change in entropy DS of a system going from one state to another has the same value for all paths connecting the two states. That is, the finite change in entropy DS of a system depends only on the properties of the initial and final equilibrium states. Therefore, we are free to choose a particular reversible path over which to evaluate the entropy in place of the actual path as long as the initial and final states are the same for both paths. This point is explored further in Section 22.7. Quick Quiz 22.5 An ideal gas is taken from an initial temperature Ti to a higher final temperature Tf along two different reversible paths. Path A is at constant pressure, and path B is at constant volume. What is the relation between the entropy changes of the gas for these paths? (a) DS A . DS B (b) DS A 5 DS B (c) DS A , DS B Ex a m pl e 22.6 Change in Entropy: Melting A solid that has a latent heat of fusion Lf melts at a temperature Tm . Calculate the change in entropy of this substance when a mass m of the substance melts. SOLUTION Conceptualize Imagine placing the substance in a warm environment so that energy enters the substance by heat. The process can be reversed by placing the substance in a cool environment so that energy leaves the substance by heat. The mass m of the substance that melts is equal to Dm, the change in mass of the higher-phase (liquid) substance. Categorize Because the melting takes place at a fixed temperature, we categorize the process as isothermal. Analyze Use Equation 20.7 in Equation 22.9, noting that the temperature remains fixed: DS 5 3 dQ r T 5 L f Dm Lfm Qr 1 5 5 3 dQ r 5 Tm Tm Tm Tm Finalize Notice that Dm is positive so that DS is positive, representing that energy is added to the ice cube. 27819_22_c22_p625-656.indd 640 6/30/09 12:43:18 PM 22.7 | Entropy and the Second Law 641 22.6 cont. WHAT IF? Suppose you did not have Equation 22.9 available to calculate an entropy change. How could you argue from the statistical description of entropy that the changes in entropy should be positive? Answer When a solid melts, its entropy increases because the molecules are much more disordered in the liquid state than they are in the solid state. The positive value for DS also means that the substance in its liquid state does not spontaneously transfer energy from itself to the warm surroundings and freeze because to do so would involve a spontaneous increase in order and a decrease in entropy. Let’s consider the changes in entropy that occur in a Carnot heat engine that operates between the temperatures Tc and Th . In one cycle, the engine takes in energy |Q h| from the hot reservoir and expels energy |Q c | to the cold reservoir. These energy transfers occur only during the isothermal portions of the Carnot cycle; therefore, the constant temperature can be brought out in front of the integral sign in Equation 22.9. The integral then simply has the value of the total amount of energy transferred by heat. Therefore, the total change in entropy for one cycle is DS 5 0Qh0 Th 2 0Qc0 Tc where the minus sign represents that energy is leaving the engine. In Example 22.3, we showed that for a Carnot engine, 0Qc0 0Qh0 5 Tc Th Using this result in the previous expression for DS, we find that the total change in entropy for a Carnot engine operating in a cycle is zero: DS 5 0 Now consider a system taken through an arbitrary (non-Carnot) reversible cycle. Because entropy is a state variable—and hence depends only on the properties of a given equilibrium state—we conclude that DS 5 0 for any reversible cycle. In general, we can write this condition as dQ r C T 50 1 reversible cycle 2 (22.10) where the symbol r indicates that the integration is over a closed path. 22.7 Entropy and the Second Law By definition, a calculation of the change in entropy for a system requires information about a reversible path connecting the initial and final equilibrium states. To calculate changes in entropy for real (irreversible) processes, remember that entropy (like internal energy) depends only on the state of the system. That is, entropy is a state variable, and the change in entropy depends only on the initial and final states. You can calculate the entropy change in some irreversible process between two equilibrium states by devising a reversible process (or series of reversible processes) between the same two states and computing DS 5 edQ r /T for the reversible process. In irreversible processes, it is important to distinguish between Q, the actual energy transfer in the process, and Q r , the energy that would have been transferred by heat along a reversible path. Only Q r is the correct value to be used in calculating the entropy change. 27819_22_c22_p625-656.indd 641 6/30/09 12:43:19 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 642 If we consider a system and its surroundings to include the entire Universe, the Universe is always moving toward a higher-probability macrostate, corresponding to greater disorder. Because entropy is a measure of disorder, an alternative way of stating this behavior is as follows: Entropy statement X of the second law of thermodynamics The entropy of the Universe increases in all real processes. This statement is yet another wording of the second law of thermodynamics that can be shown to be equivalent to the Kelvin-Planck and Clausius statements. When dealing with a system that is not isolated from its surroundings, remember that the increase in entropy described in the second law is that of the system and its surroundings. When a system and its surroundings interact in an irreversible process, the increase in entropy of one is greater than the decrease in entropy of the other. Hence, the change in entropy of the Universe must be greater than zero for an irreversible process and equal to zero for a reversible process. Ultimately, because real processes are irreversible, the entropy of the Universe should increase steadily and eventually reach a maximum value. At this value, the Universe will be in a state of uniform temperature and density. All physical, chemical, and biological processes will have ceased at this time because a state of perfect disorder implies that no energy is available for doing work. This gloomy state of affairs is sometimes referred to as the heat death of the Universe. Quick Quiz 22.6 True or False: The entropy change in an adiabatic process must be zero because Q 5 0. Entropy Change in Thermal Conduction Let’s now consider a system consisting of a hot reservoir and a cold reservoir that are in thermal contact with each other and isolated from the rest of the Universe. A process occurs during which energy Q is transferred by heat from the hot reservoir at temperature Th to the cold reservoir at temperature Tc . The process as described is irreversible (energy would not spontaneously flow from cold to hot), so we must find an equivalent reversible process. Because the temperature of a reservoir does not change during the process, we can replace the real process for each reservoir with a reversible, isothermal process in which the same amount of energy is transferred by heat. Consequently, for a reservoir, the entropy change does not depend on whether the process is reversible or irreversible. Because the cold reservoir absorbs energy Q, its entropy increases by Q/Tc . At the same time, the hot reservoir loses energy Q, so its entropy change is 2Q/Th . Because Th . Tc , the increase in entropy of the cold reservoir is greater than the decrease in entropy of the hot reservoir. Therefore, the change in entropy of the system (and of the Universe) is greater than zero: DSU 5 Q Tc 1 2Q Th .0 Suppose energy were to transfer spontaneously from a cold object to a hot object, in violation of the second law. This impossible energy transfer can be described in terms of disorder. Before the transfer, a certain degree of order is associated with the different temperatures of the objects. The hot object’s molecules have a higher average energy than the cold object’s molecules. If energy spontaneously transfers from the cold object to the hot object, the cold object becomes colder over a time interval and the hot object becomes hotter. The difference in average molecular energy becomes even greater, which would represent an increase in order for the system and a violation of the second law. In comparison, the process that does occur naturally is the transfer of energy from the hot object to the cold object. In this process, the difference in average 27819_22_c22_p625-656.indd 642 6/30/09 12:43:20 PM 22.8 | Entropy on a Microscopic Scale molecular energy decreases, which represents a more random distribution of energy and an increase in disorder. Entropy Change in a Free Expansion Let’s again consider the adiabatic free expansion of a gas occupying an initial volume Vi (Fig. 22.14). In this situation, a membrane separating the gas from an evacuated region is broken and the gas expands to a volume Vf . This process is irreversible; the gas would not spontaneously crowd into half the volume after filling the entire volume. What are the changes in entropy of the gas and of the Universe during this process? The process is neither reversible nor quasi-static. As shown in Section 20.6, the initial and final temperatures of the gas are the same. To apply Equation 22.9, we cannot take Q 5 0, the value for the irreversible process, but must instead find Q r ; that is, we must find an equivalent reversible path that shares the same initial and final states. A simple choice is an isothermal, reversible expansion in which the gas pushes slowly against a piston while energy enters the gas by heat from a reservoir to hold the temperature constant. Because T is constant in this process, Equation 22.9 gives f DS 5 3 i dQ r T f 5 1 dQ r T 3i 643 When the membrane is ruptured, the gas will expand freely and irreversibly into the full volume. Insulating wall Vacuum Membrane Gas at Ti in volume Vi Figure 22.14 Adiabatic free expansion of a gas. The container is thermally insulated from its surroundings; therefore, Q 5 0. f For an isothermal process, the first law of thermodynamics specifies that ei dQ r is equal to the negative of the work done on the gas during the expansion from Vi to Vf , which is given by Equation 20.14. Using this result, we find that the entropy change for the gas is Vf DS 5 nR ln a b Vi (22.11) Because Vf . Vi , we conclude that DS is positive. This positive result indicates that both the entropy and the disorder of the gas increase as a result of the irreversible, adiabatic expansion. It is easy to see that the gas is more disordered after the expansion. Instead of being concentrated in a relatively small space, the molecules are scattered over a larger region. Because the free expansion takes place in an insulated container, no energy is transferred by heat from the surroundings. (Remember that the isothermal, reversible expansion is only a replacement process used to calculate the entropy change for the gas; it is not the actual process.) Therefore, the free expansion has no effect on the surroundings, and the entropy change of the surroundings is zero. 22.8 Entropy on a Microscopic Scale As we have seen, entropy can be approached by relying on macroscopic concepts. Entropy can also be treated from a microscopic viewpoint through statistical analysis of molecular motions. Let’s use a microscopic model to investigate once again the free expansion of an ideal gas, which was discussed from a macroscopic point of view in Section 22.7. In the kinetic theory of gases, gas molecules are represented as particles moving randomly. Suppose the gas is initially confined to the volume Vi shown in Figure 22.14. When the membrane is removed, the molecules eventually are distributed throughout the greater volume Vf of the entire container. For a given uniform distribution of gas in the volume, there are a large number of equivalent microstates, and the entropy of the gas can be related to the number of microstates corresponding to a given macrostate. Let’s count the number of microstates by considering the variety of molecular locations available to the molecules. Let’s assume each molecule occupies some 27819_22_c22_p625-656.indd 643 6/30/09 12:43:21 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 644 microscopic volume Vm . The total number of possible locations of a single molecule in a macroscopic initial volume Vi is the ratio wi 5 Vi /Vm , which is a huge number. We use wi here to represent either the number of ways the molecule can be placed in the initial volume or the number of microstates, which is equivalent to the number of available locations. We assume the probabilities of a molecule occupying any of these locations are equal. As more molecules are added to the system, the number of possible ways the molecules can be positioned in the volume multiplies. For example, if you consider two molecules, for every possible placement of the first, all possible placements of the second are available. Therefore, there are wi ways of locating the first molecule, and for each way, there are wi ways of locating the second molecule. The total number of ways of locating the two molecules is wiwi 5 wi2. Neglecting the very small probability of having two molecules occupy the same location, each molecule may go into any of the Vi /Vm locations, and so the number of ways of locating N molecules in the volume becomes Wi 5 wi N 5 1 Vi /Vm 2 N. (Wi is not to be confused with work.) Similarly, when the volume is increased to Vf , the number of ways of locating N molecules increases to Wf 5 wf N 5 1 Vf /Vm 2 N. The ratio of the number of ways of placing the molecules in the volume for the initial and final configurations is Wf Wi 5 1 Vf /Vm 2 N Vf N 5a b 1 Vi /Vm 2 Vi N Taking the natural logarithm of this equation and multiplying by Boltzmann’s constant gives k B ln a Wf Vf N Vf b 5 k B ln a b 5 nNA k B ln a b Wi Vi Vi where we have used the equality N 5 nNA . We know from Equation 19.11 that NAk B is the universal gas constant R; therefore, we can write this equation as Vf k B lnWf 2 k B lnWi 5 nR ln a b Vi (22.12) From Equation 22.11, we know that when a gas undergoes a free expansion from Vi to Vf , the change in entropy is Vf Sf 2 Si 5 nR ln a b Vi (22.13) Notice that the right sides of Equations 22.12 and 22.13 are identical. Therefore, from the left sides, we make the following important connection between entropy and the number of microstates for a given macrostate: Entropy (microscopic X definition) S ; k B ln W (22.14) The more microstates there are that correspond to a given macrostate, the greater the entropy of that macrostate. As discussed previously, there are many more microstates associated with disordered macrostates than with ordered macrostates. Therefore, Equation 22.14 indicates mathematically our earlier statement that entropy is a measure of disorder. Although our discussion used the specific example of the free expansion of an ideal gas, a more rigorous development of the statistical interpretation of entropy would lead us to the same conclusion. We have stated that individual microstates are equally probable. Because there are far more microstates associated with a disordered macrostate than with an ordered macrostate, however, a disordered macrostate is much more probable than an ordered one. 27819_22_c22_p625-656.indd 644 6/30/09 12:43:21 PM 22.8 | Entropy on a Microscopic Scale 645 ACTIVE FIGURE 22.15 a (a) One molecule in a container has a 1-in-2 chance of being on the left side. (b) Two molecules have a 1-in-4 chance of being on the left side at the same time. (c) Three molecules have a 1-in-8 chance of being on the left side at the same time. b c Let’s explore this concept by considering 100 molecules in a container. At any given moment, the probability of one molecule being in the left part of the container shown in Active Figure 22.15a as a result of random motion is 12. If there are two molecules as shown in Active Figure 22.15b, the probability of both being in the left part is 1 12 2 2, or 1 in 4. If there are three molecules (Active Fig. 22.15c), the probability of them all being in the left portion at the same moment is 1 12 2 3, or 1 in 8. For 100 independently moving molecules, the probability that the 50 fastest ones will be found in the left part at any moment is 1 12 2 50. Likewise, the probability that the remaining 50 slower molecules will be found in the right part at any moment is 1 12 2 50. Therefore, the probability of finding this fast–slow separation as a result of random motion is the product 1 12 2 50 1 12 2 50 5 1 12 2 100, which corresponds to about 1 in 1030. When this calculation is extrapolated from 100 molecules to the number in 1 mol of gas (6.02 3 1023), the ordered arrangement is found to be extremely improbable! Conceptual Example 22.7 Let’s Play Marbles! Suppose you have a bag of 100 marbles of which 50 are red and 50 are green. You are allowed to draw four marbles from the bag according to the following rules. Draw one marble, record its color, and return it to the bag. Shake the bag and then draw another marble. Continue this process until you have drawn and returned four marbles. What are the possible macrostates for this set of events? What is the most likely macrostate? What is the least likely macrostate? SOLUTION Because each marble is returned to the bag before the TABLE 22.1 Possible Results of Drawing Four Marbles next one is drawn and the bag is then shaken, the probfrom a Bag ability of drawing a red marble is always the same as Total the probability of drawing a green one. All the possible Number of microstates and macrostates are shown in Table 22.1. Macrostate Possible Microstates Microstates As this table indicates, there is only one way to draw a All R RRRR 1 macrostate of four red marbles, so there is only one 1G, 3R RRRG, RRGR, RGRR, GRRR 4 microstate for that macrostate. There are, however, four 2G, 2R RRGG, RGRG, GRRG, 6 RGGR, GRGR, GGRR possible microstates that correspond to the macrostate 3G, 1R GGGR, GGRG, GRGG, RGGG 4 of one green marble and three red marbles, six microAll G GGGG 1 states that correspond to two green marbles and two red marbles, four microstates that correspond to three green marbles and one red marble, and one microstate that corresponds to four green marbles. The most likely, and most disordered, macrostate—two red marbles and two green marbles—corresponds to the largest number of microstates. The least likely, most ordered macrostates—four red marbles or four green marbles—correspond to the smallest number of microstates. 27819_22_c22_p625-656.indd 645 7/13/09 12:10:13 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 646 Ex a m pl e 22.8 Adiabatic Free Expansion: One Last Time Let’s verify that the macroscopic and microscopic approaches to the calculation of entropy lead to the same conclusion for the adiabatic free expansion of an ideal gas. Suppose an ideal gas expands to four times its initial volume. As we have seen for this process, the initial and final temperatures are the same. (A) Using a macroscopic approach, calculate the entropy change for the gas. SOLUTION Conceptualize Look back at Figure 22.14, which is a diagram of the system before the adiabatic free expansion. Imagine breaking the membrane so that the gas moves into the evacuated area. The expansion is irreversible. Categorize We can replace the irreversible process with a reversible isothermal process between the same initial and final states. This approach is macroscopic, so we use a thermodynamic variable, in particular, the volume V. Analyze Use Equation 22.11 to evaluate the entropy change: Vf 4Vi DS 5 nR ln a b 5 nR ln a b 5 nR ln 4 Vi Vi (B) Using statistical considerations, calculate the change in entropy for the gas and show that it agrees with the answer you obtained in part (A). SOLUTION Categorize This approach is microscopic, so we use variables related to the individual molecules. Vi N b Vm Analyze The number of microstates available to a single molecule in the initial volume Vi is wi 5 Vi /Vm . Use this number to find the number of available microstates for N molecules: Wi 5 wi N 5 a Find the number of available microstates for N molecules in the final volume Vf 5 4Vi : Wf 5 a Use Equation 22.14 to find the entropy change: DS 5 k B ln Wf 2 k B ln Wi 5 k B ln a Vf Vm N b 5a 5 k B ln a 4Vi N b Vm P WHAT IF? In part (A), we used Equation 22.11, which was based on a reversible isothermal process connecting the initial and final states. Would you arrive at the same result if you chose a different reversible process? 27819_22_c22_p625-656.indd 646 Wi b 4Vi N b 5 k B ln 1 4N 2 5 Nk B ln 4 5 nR ln 4 Vi Finalize The answer is the same as that for part (A), which dealt with macroscopic parameters. Answer You must arrive at the same result because entropy is a state variable. For example, consider the two-step process in Figure 22.16: a reversible adiabatic expansion from Vi to 4Vi (A S B) during which the temperature drops from T1 to T2 and a reversible isovolumetric process (B S C) that takes the gas back to the initial temperature T1. During the reversible adiabatic process, DS 5 0 because Q r 5 0. Wf T1 T2 A Figure 22.16 (Example 22.8) A gas expands to four times its initial volume and back to the initial temperature by means of a two-step process. C B Vi 4Vi V 6/30/09 12:43:24 PM | Summary 647 22.8 cont. f For the reversible isovolumetric process (B S C), use Equation 22.9: DS 5 3 i dQ r T T1 53 T2 nCV dT T1 5 nCV ln a b T T2 Find the ratio of temperature T1 to T2 from Equation 21.20 for the adiabatic process: 4Vi g21 T1 5a b 5 1 4 2 g21 T2 Vi Substitute to find DS: DS 5 nCV ln 1 4 2 g21 5 nCV 1 g 2 1 2 ln 4 5 nCV a CP 2 1b ln 4 5 n 1 CP 2 CV 2 ln 4 5 nR ln 4 CV and you do indeed obtain the exact same result for the entropy change. Summary Definitions The thermal efficiency e of a heat engine is e; Weng 0Qh0 5 0Qh0 2 0Qc0 0Qc0 512 0Qh0 0Qh0 From a microscopic viewpoint, the entropy of a given macrostate is defined as (22.2) S ; k B ln W (22.14) where k B is Boltzmann’s constant and W is the number of microstates of the system corresponding to the macrostate. In a reversible process, the system can be returned to its initial conditions along the same path on a PV diagram, and every point along this path is an equilibrium state. A process that does not satisfy these requirements is irreversible. Concepts and Principles A heat engine is a device that takes in energy by heat and, operating in a cyclic process, expels a fraction of that energy by means of work. The net work done by a heat engine in carrying a working substance through a cyclic process (DE int 5 0) is Weng 5 |Q h | 2 |Q c | (22.1) where |Q h | is the energy taken in from a hot reservoir and |Q c | is the energy expelled to a cold reservoir. Two ways the second law of thermodynamics can be stated are as follows: • It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the input of energy by heat from a reservoir and the performance of an equal amount of work (the Kelvin–Planck statement). • It is impossible to construct a cyclical machine whose sole effect is to transfer energy continuously by heat from one object to another object at a higher temperature without the input of energy by work (the Clausius statement). continued 27819_22_c22_p625-656.indd 647 6/30/09 12:43:26 PM 648 CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics Carnot’s theorem states that no real heat engine operating (irreversibly) between the temperatures Tc and Th can be more efficient than an engine operating reversibly in a Carnot cycle between the same two temperatures. The thermal efficiency of a heat engine operating in the Carnot cycle is eC 5 1 2 Tc Th (22.6) The second law of thermodynamics states that when real (irreversible) processes occur, the degree of disorder in the system plus the surroundings increases. When a process occurs in an isolated system, the state of the system becomes more disordered. The measure of disorder in a system is called entropy S. Therefore, yet another way the second law can be stated is as follows: • The entropy of the Universe increases in all real processes. The change in entropy dS of a system during a process between two infinitesimally separated equilibrium states is dS 5 dQ r T (22.8) where dQ r is the energy transfer by heat for the system for a reversible process that connects the initial and final states. Objective Questions 1. A steam turbine operates at a boiler temperature of 450 K and an exhaust temperature of 300 K. What is the maximum theoretical efficiency of this system? (a) 0.240 (b) 0.500 (c) 0.333 (d) 0.667 (e) 0.150 2. An engine does 15.0 kJ of work while exhausting 37.0 kJ to a cold reservoir. What is the efficiency of the engine? (a) 0.150 (b) 0.288 (c) 0.333 (d) 0.450 (e) 1.20 3. A refrigerator has 18.0 kJ of work done on it while 115 kJ of energy is transferred from inside its interior. What is its coefficient of performance? (a) 3.40 (b) 2.80 (c) 8.90 (d) 6.40 (e) 5.20 4. Of the following, which is not a statement of the second law of thermodynamics? (a) No heat engine operating in a cycle can absorb energy from a reservoir and use it entirely to do work. (b) No real engine operating between two energy reservoirs can be more efficient than a Carnot engine operating between the same two reservoirs. (c) When a system undergoes a change in state, the change in the internal energy of the system is the sum of the energy transferred to the system by heat and the work done on the system. (d) The entropy of the Universe increases in all natural processes. (e) Energy will not spontaneously transfer by heat from a cold object to a hot object. 27819_22_c22_p625-656.indd 648 The change in entropy of a system during an arbitrary process between an initial state and a final state is f DS 5 3 i dQ r T (22.9) The value of DS for the system is the same for all paths connecting the initial and final states. The change in entropy for a system undergoing any reversible, cyclic process is zero, and when such a process occurs, the entropy of the Universe remains constant. denotes answer available in Student Solutions Manual/Study Guide 5. Consider cyclic processes completely characterized by each of the following net energy inputs and outputs. In each case, the energy transfers listed are the only ones occurring. Classify each process as (a) possible, (b) impossible according to the first law of thermodynamics, (c) impossible according to the second law of thermodynamics, or (d) impossible according to both the first and second laws. (i) Input is 5 J of work, and output is 4 J of work. (ii) Input is 5 J of work, and output is 5 J of energy transferred by heat. (iii) Input is 5 J of energy transferred by electrical transmission, and output is 6 J of work. (iv) Input is 5 J of energy transferred by heat, and output is 5 J of energy transferred by heat. (v) Input is 5 J of energy transferred by heat, and output is 5 J of work. (vi) Input is 5 J of energy transferred by heat, and output is 3 J of work plus 2 J of energy transferred by heat. 6. A compact air-conditioning unit is placed on a table inside a well-insulated apartment and is plugged in and turned on. What happens to the average temperature of the apartment? (a) It increases. (b) It decreases. (c) It remains constant. (d) It increases until the unit warms up and then decreases. (e) The answer depends on the initial temperature of the apartment. 6/30/09 12:43:29 PM | Conceptual Questions 7. The second law of thermodynamics implies that the coefficient of performance of a refrigerator must be what? (a) less than 1 (b) less than or equal to 1 (c) greater than or equal to 1 (d) finite (e) greater than 0 8. A thermodynamic process occurs in which the entropy of a system changes by 28 J/K. According to the second law of thermodynamics, what can you conclude about the entropy change of the environment? (a) It must be 18 J/K or less. (b) It must be between 18 J/K and 0. (c) It must be equal to 18 J/K. (d) It must be 18 J/K or more. (e) It must be zero. 9. A sample of a monatomic ideal gas is contained in a cylinder with a piston. Its state is represented by the dot in the PV diagram shown in Figure OQ22.9. Arrows A through E represent isobaric, isothermal, adiabatic, and isovolumetric processes that the sample can undergo. In each process C P 649 except D, the volume changes by a factor of 2. All five processes are reversible. Rank the processes according to the change in entropy of the gas from the largest positive value to the largest-magnitude negative value. In your rankings, display any cases of equality. 10. Assume a sample of an ideal gas is at room temperature. What action will necessarily make the entropy of the sample increase? (a) Transfer energy into it by heat. (b) Transfer energy into it irreversibly by heat. (c) Do work on it. (d) Increase either its temperature or its volume, without letting the other variable decrease. (e) None of those choices is correct. 11. The arrow OA in the PV diagram shown in Figure OQ22.11 represents a reversible adiabatic expansion of an ideal gas. The same sample of gas, starting from the same state O, now undergoes an adiabatic free expansion to the same final volume. What point on the diagram could represent the final state of the gas? (a) the same point A as for the reversible expansion (b) point B (c) point C (d) any of those choices (e) none of those choices D B P O A E V Figure OQ22.9 Conceptual Questions 1. What are some factors that affect the efficiency of automobile engines? 2. A steam-driven turbine is one major component of an electric power plant. Why is it advantageous to have the temperature of the steam as high as possible? 3. Does the second law of thermodynamics contradict or correct the first law? Argue for your answer. 4. “The first law of thermodynamics says you can’t really win, and the second law says you can’t even break even.” Explain how this statement applies to a particular device or process; alternatively, argue against the statement. B A C Figure OQ22.11 denotes answer available in Student Solutions Manual/Study Guide electric potential energy is produced. When one leg is at a higher temperature than the other as shown in the photograph on the right, however, electric potential energy is produced as the device extracts energy from the hot reservoir and drives a small electric motor. (a) Why is the difference in temperature necessary to produce electric potential energy in this demonstration? (b) In what sense does this intriguing experiment demonstrate the second law of thermodynamics? Courtesy of PASCO Scientific Company 5. Is it possible to construct a heat engine that creates no thermal pollution? Explain. 6. (a) Give an example of an irreversible process that occurs in nature. (b) Give an example of a process in nature that is nearly reversible. 7. The device shown in Figure CQ22.7, called a thermoelectric converter, uses a series of semiconductor cells to transform internal energy to electric potential energy, which we will study in Chapter 25. In the photograph on the left, both legs of the device are at the same temperature and no 27819_22_c22_p625-656.indd 649 V Figure CQ22.7 6/30/09 12:43:31 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 650 8. Discuss three different common examples of natural processes that involve an increase in entropy. Be sure to account for all parts of each system under consideration. 9. Discuss the change in entropy of a gas that expands (a) at constant temperature and (b) adiabatically. 10. Suppose your roommate cleans and tidies up your messy room after a big party. Because she is creating more order, does this process represent a violation of the second law of thermodynamics? 11. “Energy is the mistress of the Universe, and entropy is her shadow.” Writing for an audience of general readers, argue for this statement with at least two examples. Alternatively, argue for the view that entropy is like an executive who instantly determines what will happen, whereas energy is like a bookkeeper telling us how little we can afford. (Arnold Sommerfeld suggested the idea for this question.) 12. (a) If you shake a jar full of jelly beans of different sizes, the larger beans tend to appear near the top and the smaller ones tend to fall to the bottom. Why? (b) Does this process violate the second law of thermodynamics? 13. The energy exhaust from a certain coal-fired electric generating station is carried by “cooling water” into Lake Ontario. The water is warm from the viewpoint of living things in the lake. Some of them congregate around the outlet port and can impede the water flow. (a) Use the theory of heat engines to explain why this action can reduce the electric power output of the station. (b) An engineer says that the electric output is reduced because of “higher back pressure on the turbine blades.” Comment on the accuracy of this statement. Problems The problems found in this chapter may be assigned online in Enhanced WebAssign 1. denotes straightforward problem; 2. denotes intermediate problem; 3. denotes challenging problem 1. full solution available in the Student Solutions Manual/Study Guide 1. denotes problems most often assigned in Enhanced WebAssign; these provide students with targeted feedback and either a Master It tutorial or a Watch It solution video. Section 22.1 Heat Engines and the Second Law of Thermodynamics 1. An engine absorbs 1.70 kJ from a hot reservoir at 277°C and expels 1.20 kJ to a cold reservoir at 27°C in each cycle. (a) What is the engine’s efficiency? (b) How much work is done by the engine in each cycle? (c) What is the power output of the engine if each cycle lasts 0.300 s? 2. The work done by an engine equals one-fourth the energy it absorbs from a reservoir. (a) What is its thermal efficiency? (b) What fraction of the energy absorbed is expelled to the cold reservoir? 3. A heat engine takes in 360 J of energy from a hot reservoir and performs 25.0 J of work in each cycle. Find (a) the efficiency of the engine and (b) the energy expelled to the cold reservoir in each cycle. 4. A gun is a heat engine. In particular, it is an internal combustion piston engine that does not operate in a cycle, but comes apart during its adiabatic expansion process. A certain gun consists of 1.80 kg of iron. It fires one 2.40-g bullet at 320 m/s with an energy efficiency of 1.10%. Assume the body of the gun absorbs all the energy exhaust—the 27819_22_c22_p625-656.indd 650 denotes asking for quantitative and conceptual reasoning denotes symbolic reasoning problem denotes Master It tutorial available in Enhanced WebAssign denotes guided problem shaded denotes “paired problems” that develop reasoning with symbols and numerical values other 98.9%—and increases uniformly in temperature for a short time interval before it loses any energy by heat into the environment. Find its temperature increase. 5. A particular heat engine has a mechanical power output of 5.00 kW and an efficiency of 25.0%. The engine expels 8.00 3 103 J of exhaust energy in each cycle. Find (a) the energy taken in during each cycle and (b) the time interval for each cycle. 6. A multicylinder gasoline engine in an airplane, operating at 2.50 3 103 rev/min, takes in energy 7.89 3 103 J and exhausts 4.58 3 103 J for each revolution of the crankshaft. (a) How many liters of fuel does it consume in 1.00 h of operation if the heat of combustion of the fuel is equal to 4.03 3 107 J/L? (b) What is the mechanical power output of the engine? Ignore friction and express the answer in horsepower. (c) What is the torque exerted by the crankshaft on the load? (d) What power must the exhaust and cooling system transfer out of the engine? 7. Suppose a heat engine is connected to two energy reservoirs, one a pool of molten aluminum (660°C) and the other a block of solid mercury (238.9°C). The engine runs 6/30/09 12:43:34 PM | Problems by freezing 1.00 g of aluminum and melting 15.0 g of mercury during each cycle. The heat of fusion of aluminum is 3.97 3 105 J/kg; the heat of fusion of mercury is 1.18 3 104 J/kg. What is the efficiency of this engine? Section 22.2 Heat Pumps and Refrigerators 8. A refrigerator has a coefficient of performance equal to 5.00. The refrigerator takes in 120 J of energy from a cold reservoir in each cycle. Find (a) the work required in each cycle and (b) the energy expelled to the hot reservoir. 9. During each cycle, a refrigerator ejects 625 kJ of energy to a high-temperature reservoir and takes in 550 kJ of energy from a low-temperature reservoir. Determine (a) the work done on the refrigerant in each cycle and (b) the coefficient of performance of the refrigerator. 10. A heat pump has a coefficient of performance of 3.80 and operates with a power consumption of 7.03 3 103 W. (a) How much energy does it deliver into a home during 8.00 h of continuous operation? (b) How much energy does it extract from the outside air? 17. What is the coefficient of performance of a refrigerator that operates with Carnot efficiency between temperatures 23.00°C and 127.0°C? 18. Why is the following situation impossible? An inventor comes to a patent office with the claim that her heat engine, which employs water as a working substance, has a thermodynamic efficiency of 0.110. Although this efficiency is low compared with typical automobile engines, she explains that her engine operates between an energy reservoir at room temperature and a water–ice mixture at atmospheric pressure and therefore requires no fuel other than that to make the ice. The patent is approved, and working prototypes of the engine prove the inventor’s efficiency claim. 19. A heat engine is being designed to have a Carnot efficiency of 65.0% when operating between two energy reservoirs. (a) If the temperature of the cold reservoir is 20.0°C, what must be the temperature of the hot reservoir? (b) Can the actual efficiency of the engine be equal to 65.0%? Explain. 20. An ideal refrigerator or ideal heat pump is equivalent to a Carnot engine running in reverse. That is, energy |Q c | is taken in from a cold reservoir and energy |Q h | is rejected to a hot reservoir. (a) Show that the work that must be supplied to run the refrigerator or heat pump is 11. A freezer has a coefficient of performance of 6.30. It is advertised as using electricity at a rate of 457 kWh/yr. (a) On average, how much energy does it use in a single day? (b) On average, how much energy does it remove from the refrigerator in a single day? (c) What maximum mass of water at 20.0°C could the freezer freeze in a single day? Note: One kilowatt-hour (kWh) is an amount of energy equal to running a 1-kW appliance for one hour. 12. A heat pump has a coefficient of performance equal to 4.20 and requires a power of 1.75 kW to operate. (a) How much energy does the heat pump add to a home in one hour? (b) If the heat pump is reversed so that it acts as an air conditioner in the summer, what would be its coefficient of performance? Section 22.3 Reversible and Irreversible Processes W5 14. A heat engine operates between a reservoir at 25.0°C and one at 375°C. What is the maximum efficiency possible for this engine? 15. A Carnot engine has a power output of 150 kW. The engine operates between two reservoirs at 20.0°C and 500°C. (a) How much energy enters the engine by heat per hour? (b) How much energy is exhausted by heat per hour? 16. A Carnot engine has a power output P. The engine operates between two reservoirs at temperature Tc and Th . (a) How much energy enters the engine by heat in a time interval Dt ? (b) How much energy is exhausted by heat in the time interval Dt ? 27819_22_c22_p625-656.indd 651 Th 2 Tc 0Qc0 Tc (b) Show that the coefficient of performance (COP) of the ideal refrigerator is COP 5 Tc Th 2 Tc 21. What is the maximum possible coefficient of performance of a heat pump that brings energy from outdoors at 23.00°C into a 22.0°C house? Note: The work done to run the heat pump is also available to warm the house. 22. Section 22.4 The Carnot Engine 13. One of the most efficient heat engines ever built is a coalfired steam turbine in the Ohio River valley, operating between 1 870°C and 430°C. (a) What is its maximum theoretical efficiency? (b) The actual efficiency of the engine is 42.0%. How much mechanical power does the engine deliver if it absorbs 1.40 3 105 J of energy each second from its hot reservoir? 651 How much work does an ideal Carnot refrigerator require to remove 1.00 J of energy from liquid helium at 4.00 K and expel this energy to a room-temperature (293-K) environment? 23. If a 35.0%-efficient Carnot heat engine (Active Fig. 22.2) is run in reverse so as to form a refrigerator (Active Fig. 22.4), what would be this refrigerator’s coefficient of performance? 24. A Carnot heat engine operates between temperatures Th and Tc . (a) If Th 5 500 K and Tc 5 350 K, what is the efficiency of the engine? (b) What is the change in its efficiency for each degree of increase in Th above 500 K? (c) What is the change in its efficiency for each degree of change in Tc ? (d) Does the answer to part (c) depend on Tc ? Explain. 25. An ideal gas is taken through a Carnot cycle. The isothermal expansion occurs at 250°C, and the isothermal compression takes place at 50.0°C. The gas takes in 1.20 3 103 J of energy from the hot reservoir during the isothermal expansion. Find (a) the energy expelled to the cold reservoir in each cycle and (b) the net work done by the gas in each cycle. 6/30/09 12:43:34 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 652 26. 27. Argon enters a turbine at a rate of 80.0 kg/min, a temperature of 800°C, and a pressure of 1.50 MPa. It expands adiabatically as it pushes on the turbine blades and exits at pressure 300 kPa. (a) Calculate its temperature at exit. (b) Calculate the (maximum) power output of the turning turbine. (c) The turbine is one component of a model closed-cycle gas turbine engine. Calculate the maximum efficiency of the engine. 28. pressures, volumes, and temperatures as you fill in the following table: An electric power plant that would make use of the temperature gradient in the ocean has been proposed. The system is to operate between 20.0°C (surface-water temperature) and 5.00°C (water temperature at a depth of about 1 km). (a) What is the maximum efficiency of such a system? (b) If the electric power output of the plant is 75.0 MW, how much energy is taken in from the warm reservoir per hour? (c) In view of your answer to part (a), explain whether you think such a system is worthwhile. Note that the “fuel” is free. Suppose you build a two-engine device with the exhaust energy output from one heat engine supplying the input energy for a second heat engine. We say that the two engines are running in series. Let e 1 and e 2 represent the efficiencies of the two engines. (a) The overall efficiency of the two-engine device is defined as the total work output divided by the energy put into the first engine by heat. Show that the overall efficiency e is given by A B C D 29. An electric generating station is designed to have an electric output power of 1.40 MW using a turbine with two-thirds the efficiency of a Carnot engine. The exhaust energy is transferred by heat into a cooling tower at 110°C. (a) Find the rate at which the station exhausts energy by heat as a function of the fuel combustion temperature Th . (b) If the firebox is modified to run hotter by using more advanced combustion technology, how does the amount of energy exhaust change? (c) Find the exhaust power for Th 5 800°C. (d) Find the value of Th for which the exhaust power would be only half as large as in part (c). (e) Find the value of Th for which the exhaust power would be onefourth as large as in part (c). 30. At point A in a Carnot cycle, 2.34 mol of a monatomic ideal gas has a pressure of 1 400 kPa, a volume of 10.0 L, and a temperature of 720 K. The gas expands isothermally to point B and then expands adiabatically to point C, where its volume is 24.0 L. An isothermal compression brings it to point D, where its volume is 15.0 L. An adiabatic process returns the gas to point A. (a) Determine all the unknown 27819_22_c22_p625-656.indd 652 V T 1 400 kPa 10.0 L 720 K 24.0 L 15.0 L (b) Find the energy added by heat, the work done by the engine, and the change in internal energy for each of the steps A S B, B S C, C S D, and D S A. (c) Calculate the efficiency Wnet /|Q h |. (d) Show that the efficiency is equal to 1 2 TC /TA , the Carnot efficiency. 31. A heat pump used for heating shown in Figure P22.31 is essentially an air conditioner installed backward. It extracts energy from colder air outside and deposits it in a warmer room. Suppose the ratio of the actual energy entering the room to the work done by the device’s motor is 10.0% of the theoretical maximum ratio. Determine the energy entering the room per joule of work done by the motor given that the inside temperature is 20.0°C and the outside temperature is 25.00°C. Heat pump Qc e 5 e 1 1 e 2 2 e 1e 2 What If? For parts (b) through (e) that follow, assume the two engines are Carnot engines. Engine 1 operates between temperatures Th and Ti . The gas in engine 2 varies in temperature between Ti and Tc . In terms of the temperatures, (b) what is the efficiency of the combination engine? (c) Does an improvement in net efficiency result from the use of two engines instead of one? (d) What value of the intermediate temperature Ti results in equal work being done by each of the two engines in series? (e) What value of Ti results in each of the two engines in series having the same efficiency? P Qh Outside Tc Inside Th Figure P22.31 32. An ideal (Carnot) freezer in a kitchen has a constant temperature of 260 K, whereas the air in the kitchen has a constant temperature of 300 K. Suppose the insulation for the freezer is not perfect but rather conducts energy into the freezer at a rate of 0.150 W. Determine the average power required for the freezer’s motor to maintain the constant temperature in the freezer. Section 22.5 Gasoline and Diesel Engines Note: For problems in this section, assume the gas in the engine is diatomic with g 5 1.40. 33. In a cylinder of an automobile engine, immediately after combustion the gas is confined to a volume of 50.0 cm3 and has an initial pressure of 3.00 3 106 Pa. The piston moves outward to a final volume of 300 cm3, and the gas expands without energy transfer by heat. (a) What is the final pressure of the gas? (b) How much work is done by the gas in expanding? 34. A gasoline engine has a compression ratio of 6.00. (a) What is the efficiency of the engine if it operates in an idealized Otto cycle? (b) What If? If the actual efficiency is 15.0%, what fraction of the fuel is wasted as a result of friction and energy transfers by heat that could be avoided in a revers- 7/1/09 2:16:08 PM | Problems ible engine? Assume complete combustion of the air–fuel mixture. 35. An idealized diesel engine operates in a cycle known as the air-standard diesel cycle shown in Figure P22.35. Fuel is sprayed into the cylinder at the point of maximum compression, B. Combustion occurs during the expansion B S C, which is modeled as an isobaric process. Show that the efficiency of an engine operating in this idealized diesel cycle is 653 41. A 2.00-L container has a center partition that divides it into two equal parts as shown in Figure P22.41. The left side contains 0.044 0 mol of H2 gas, and the right side contains 0.044 0 mol of O2 gas. Both gases are at room temperature and at atmospheric pressure. The partition is removed, and the gases are allowed to mix. What is the entropy increase of the system? 0.044 0 mol H2 1 TD 2 TA e512 a b g TC 2 TB 0.044 0 mol O2 Figure P22.41 P Qh C B Adiabatic processes 42. How fast are you personally making the entropy of the Universe increase right now? Compute an order-of-magnitude estimate, stating what quantities you take as data and the values you measure or estimate for them. D Qc A V2 VB VC V1 VA V Figure P22.35 43. When an aluminum bar is connected between a hot reservoir at 725 K and a cold reservoir at 310 K, 2.50 kJ of energy is transferred by heat from the hot reservoir to the cold reservoir. In this irreversible process, calculate the change in entropy of (a) the hot reservoir, (b) the cold reservoir, and (c) the Universe, neglecting any change in entropy of the aluminum rod. 44. Section 22.6 Entropy Section 22.7 Entropy and the Second Law 36. An ice tray contains 500 g of liquid water at 0°C. Calculate the change in entropy of the water as it freezes slowly and completely at 0°C. 37. A Styrofoam cup holding 125 g of hot water at 100°C cools to room temperature, 20.0°C. What is the change in entropy of the room? Neglect the specific heat of the cup and any change in temperature of the room. 38. Two 2.00 3 103 -kg cars both traveling at 20.0 m/s undergo a head-on collision and stick together. Find the change in entropy of the surrounding air resulting from the collision if the air temperature is 23.0°C. Ignore the energy carried away from the collision by sound. 39. A 70.0-kg log falls from a height of 25.0 m into a lake. If the log, the lake, and the air are all at 300 K, find the change in entropy of the air during this process. 40. A 1.00-mol sample of H2 gas is contained in the left side of the container shown in Figure P22.40, which has equal volumes on the left and right. The right side is evacuated. When the valve is opened, the gas streams into the right side. (a) What is the entropy change of the gas? (b) Does the temperature of the gas change? Assume the container is so large that the hydrogen behaves as an ideal gas. Valve H2 When a metal bar is connected between a hot reservoir at Th and a cold reservoir at Tc , the energy transferred by heat from the hot reservoir to the cold reservoir is Q. In this irreversible process, find expressions for the change in entropy of (a) the hot reservoir, (b) the cold reservoir, and (c) the Universe, neglecting any change in entropy of the metal rod. 45. The temperature at the surface of the Sun is approximately 5 800 K, and the temperature at the surface of the Earth is approximately 290 K. What entropy change of the Universe occurs when 1.00 3 103 J of energy is transferred by radiation from the Sun to the Earth? Section 22.8 Entropy on a Microscopic Scale 46. If you roll two dice, what is the total number of ways in which you can obtain (a) a 12 and (b) a 7? 47. Prepare a table like Table 22.1 by using the same procedure (a) for the case in which you draw three marbles from your bag rather than four and (b) for the case in which you draw five marbles rather than four. 48. (a) Prepare a table like Table 22.1 for the following occurrence. You toss four coins into the air simultaneously and then record the results of your tosses in terms of the numbers of heads (H) and tails (T) that result. For example, HHTH and HTHH are two possible ways in which three heads and one tail can be achieved. (b) On the basis of your table, what is the most probable result recorded for a toss? In terms of entropy, (c) what is the most ordered macrostate, and (d) what is the most disordered? Vacuum Additional Problems Figure P22.40 27819_22_c22_p625-656.indd 653 49. The energy absorbed by an engine is three times greater than the work it performs. (a) What is its thermal efficiency? 6/30/09 12:43:36 PM 654 CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics (b) What fraction of the energy absorbed is expelled to the cold reservoir? ety of applications ever since, including the solar power application discussed on the cover of this textbook. Fuel is burned externally to warm one of the engine’s two cylinders. A fixed quantity of inert gas moves cyclically between the cylinders, expanding in the hot one and contracting in the cold one. Figure P22.57 represents a model for its thermodynamic cycle. Consider n moles of an ideal monatomic gas being taken once through the cycle, consisting of two isothermal processes at temperatures 3Ti and Ti and two constant-volume processes. Let us find the efficiency of this engine. (a) Find the energy transferred by heat into the gas during the isovolumetric process AB. (b) Find the energy transferred by heat into the gas during the isothermal process BC. (c) Find the energy transferred by heat into the gas during the isovolumetric process CD. (d) Find the energy transferred by heat into the gas during the isothermal process DA. (e) Identify which of the results from parts (a) through (d) are positive and evaluate the energy input to the engine by heat. (f) From the first law of thermodynamics, find the work done by the engine. (g) From the results of parts (e) and (f), evaluate the efficiency of the engine. A Stirling engine is easier to manufacture than an internal combustion engine or a turbine. It can run on burning garbage. It can run on the energy transferred by sunlight and produce no material exhaust. Stirling engines are not currently used in automobiles due to long startup times and poor acceleration response. 50. A steam engine is operated in a cold climate where the exhaust temperature is 0°C. (a) Calculate the theoretical maximum efficiency of the engine using an intake steam temperature of 100°C. (b) If, instead, superheated steam at 200°C is used, find the maximum possible efficiency. 51. Find the maximum (Carnot) efficiency of an engine that absorbs energy from a hot reservoir at 545°C and exhausts energy to a cold reservoir at 185°C. 52. Every second at Niagara Falls, some 5.00 3 103 m3 of water falls a distance of 50.0 m. What is the increase in entropy of the Universe per second due to the falling water? Assume the mass of the surroundings is so great that its temperature and that of the water stay nearly constant at 20.0°C. Also assume a negligible amount of water evaporates. 53. 54. Energy transfers by heat through the exterior walls and roof of a house at a rate of 5.00 3 103 J/s 5 5.00 kW when the interior temperature is 22.0°C and the outside temperature is 25.00°C. (a) Calculate the electric power required to maintain the interior temperature at 22.0°C if the power is used in electric resistance heaters that convert all the energy transferred in by electrical transmission into internal energy. (b) What If? Calculate the electric power required to maintain the interior temperature at 22.0°C if the power is used to drive an electric motor that operates the compressor of a heat pump that has a coefficient of performance equal to 60.0% of the Carnot-cycle value. In 1993, the U.S. government instituted a requirement that all room air conditioners sold in the United States must have an energy efficiency ratio (EER) of 10 or higher. The EER is defined as the ratio of the cooling capacity of the air conditioner, measured in British thermal units per hour, or Btu/h, to its electrical power requirement in watts. (a) Convert the EER of 10.0 to dimensionless form, using the conversion 1 Btu 5 1 055 J. (b) What is the appropriate name for this dimensionless quantity? (c) In the 1970s, it was common to find room air conditioners with EERs of 5 or lower. State how the operating costs compare for 10 000-Btu/h air conditioners with EERs of 5.00 and 10.0. Assume each air conditioner operates for 1 500 h during the summer in a city where electricity costs 17.0¢ per kWh. 55. An airtight freezer holds n moles of air at 25.0°C and 1.00 atm. The air is then cooled to 218.0°C. (a) What is the change in entropy of the air if the volume is held constant? (b) What would the entropy change be if the pressure were maintained at 1.00 atm during the cooling? 56. Suppose an ideal (Carnot) heat pump could be constructed for use as an air conditioner. (a) Obtain an expression for the coefficient of performance (COP) for such an air conditioner in terms of Th and Tc . (b) Would such an air conditioner operate on a smaller energy input if the difference in the operating temperatures were greater or smaller? (c) Compute the COP for such an air conditioner if the indoor temperature is 20.0°C and the outdoor temperature is 40.0°C. 57. In 1816, Robert Stirling, a Scottish clergyman, patented the Stirling engine, which has found a wide vari- 27819_22_c22_p625-656.indd 654 P Isothermal processes B C 3Ti A D Ti Vi 2Vi V Figure P22.57 58. A firebox is at 750 K, and the ambient temperature is 300 K. The efficiency of a Carnot engine doing 150 J of work as it transports energy between these constant-temperature baths is 60.0%. The Carnot engine must take in energy 150 J/0.600 5 250 J from the hot reservoir and must put out 100 J of energy by heat into the environment. To follow Carnot’s reasoning, suppose some other heat engine S could have an efficiency of 70.0%. (a) Find the energy input and exhaust energy output of engine S as it does 150 J of work. (b) Let engine S operate as in part (a) and run the Carnot engine in reverse between the same reservoirs. The output work of engine S is the input work for the Carnot refrigerator. Find the total energy transferred to or from the firebox and the total energy transferred to or from the environment as both engines operate together. (c) Explain how the results of parts (a) and (b) show that the Clausius statement of the second law of thermodynamics is violated. (d) Find the energy input and work output of engine S as it puts out exhaust energy of 100 J. Let engine S operate as in part (c) and contribute 150 J of its work output to running 6/30/09 12:43:37 PM | Problems the Carnot engine in reverse. Find (e) the total energy the firebox puts out as both engines operate together, (f) the total work output, and (g) the total energy transferred to the environment. (h) Explain how the results show that the Kelvin–Planck statement of the second law is violated. Therefore, our assumption about the efficiency of engine S must be false. (i) Let the engines operate together through one cycle as in part (d). Find the change in entropy of the Universe. (j) Explain how the result of part (i) shows that the entropy statement of the second law is violated. 59. Review. This problem complements Problem 84 in Chapter 10. In the operation of a single-cylinder internal combustion piston engine, one charge of fuel explodes to drive the piston outward in the power stroke. Part of its energy output is stored in a turning flywheel. This energy is then used to push the piston inward to compress the next charge of fuel and air. In this compression process, assume an original volume of 0.120 L of a diatomic ideal gas at atmospheric pressure is compressed adiabatically to one-eighth of its original volume. (a) Find the work input required to compress the gas. (b) Assume the flywheel is a solid disk of mass 5.10 kg and radius 8.50 cm, turning freely without friction between the power stroke and the compression stroke. How fast must the flywheel turn immediately after the power stroke? This situation represents the minimum angular speed at which the engine can operate without stalling. (c) When the engine’s operation is well above the point of stalling, assume the flywheel puts 5.00% of its maximum energy into compressing the next charge of fuel and air. Find its maximum angular speed in this case. 60. A biology laboratory is maintained at a constant temperature of 7.00°C by an air conditioner, which is vented to the air outside. On a typical hot summer day, the outside temperature is 27.0°C and the air-conditioning unit emits energy to the outside at a rate of 10.0 kW. Model the unit as having a coefficient of performance (COP) equal to 40.0% of the COP of an ideal Carnot device. (a) At what rate does the air conditioner remove energy from the laboratory? (b) Calculate the power required for the work input. (c) Find the change in entropy of the Universe produced by the air conditioner in 1.00 h. (d) What If? The outside temperature increases to 32.0°C. Find the fractional change in the COP of the air conditioner. 61. A heat engine operates between two reservoirs at T2 5 600 K and T1 5 350 K. It takes in 1.00 3 103 J of energy from the higher-temperature reservoir and performs 250 J of work. Find (a) the entropy change of the Universe DSU for this process and (b) the work W that could have been done by an ideal Carnot engine operating between these two reservoirs. (c) Show that the difference between the amounts of work done in parts (a) and (b) is T1 DSU . 62. A 1.00-mol sample of a monatomic ideal gas is taken through the cycle shown in Figure P22.62. At point A, the pressure, volume, and temperature are Pi , Vi , and Ti , respectively. In terms of R and Ti , find (a) the total energy entering the system by heat per cycle, (b) the total energy leaving the system by heat per cycle, and (c) the efficiency of an engine operating in this cycle. (d) Explain how the efficiency compares with that of an engine operating in a Carnot cycle between the same temperature extremes. 27819_22_c22_p625-656.indd 655 655 Q2 P B 3Pi C Q3 Q1 2Pi Pi D A Q4 2Vi Vi V Figure P22.62 63. A power plant, having a Carnot efficiency, produces 1.00 GW of electrical power from turbines that take in steam at 500 K and reject water at 300 K into a flowing river. The water downstream is 6.00 K warmer due to the output of the power plant. Determine the flow rate of the river. 64. A power plant, having a Carnot efficiency, produces electric power P from turbines that take in energy from steam at temperature Th and discharge energy at temperature Tc through a heat exchanger into a flowing river. The water downstream is warmer by DT due to the output of the power plant. Determine the flow rate of the river. 65. A sample consisting of n moles of an ideal gas undergoes a reversible isobaric expansion from volume Vi to volume 3Vi . Find the change in entropy of the gas by calculatf ing ei dQ /T, where dQ 5 nCP dT. 66. An athlete whose mass is 70.0 kg drinks 16.0 ounces (454 g) of refrigerated water. The water is at a temperature of 35.0°F. (a) Ignoring the temperature change of the body that results from the water intake (so that the body is regarded as a reservoir always at 98.6°F), find the entropy increase of the entire system. (b) What If? Assume the entire body is cooled by the drink and the average specific heat of a person is equal to the specific heat of liquid water. Ignoring any other energy transfers by heat and any metabolic energy release, find the athlete’s temperature after she drinks the cold water given an initial body temperature of 98.6°F. (c) Under these assumptions, what is the entropy increase of the entire system? (d) State how this result compares with the one you obtained in part (a). 67. A 1.00-mol sample of an ideal monatomic gas is taken through the cycle shown in Figure P22.67. The process A S B is a reversible isothermal expansion. Calculate (a) the net work done by the gas, (b) the energy added to P (atm) A 5 1 Isothermal process C 10 B 50 V (liters) Figure P22.67 6/30/09 12:43:37 PM CHAPTER 22 | Heat Engines, Entropy, and the Second Law of Thermodynamics 656 the gas by heat, (c) the energy exhausted from the gas by heat, and (d) the efficiency of the cycle. (e) Explain how the efficiency compares with that of a Carnot engine operating between the same temperature extremes. 68. 69. A system consisting of n moles of an ideal gas with molar specific heat at constant pressure CP undergoes two reversible processes. It starts with pressure Pi and volume Vi , expands isothermally, and then contracts adiabatically to reach a final state with pressure Pi and volume 3Vi . (a) Find its change in entropy in the isothermal process. (The entropy does not change in the adiabatic process.) (b) What If? Explain why the answer to part (a) must be the same as the answer to Problem 65. (You do not need to solve Problem 65 to answer this question.) A sample of an ideal gas expands isothermally, doubling in volume. (a) Show that the work done on the gas in expanding is W 5 2nRT ln 2. (b) Because the internal energy E int of an ideal gas depends solely on its temperature, the change in internal energy is zero during the expansion. It follows from the first law that the energy input to the gas by heat during the expansion is equal to the energy output by work. Does this process have 100% efficiency in converting energy input by heat into work output? (c) Does this conversion violate the second law? Explain. 70. Why is the following situation impossible? Two samples of water are mixed at constant pressure inside an insulated container: 1.00 kg of water at 10.0°C and 1.00 kg of water at 30.0°C. Because the container is insulated, there is no exchange of energy by heat between the water and the environment. Furthermore, the amount of energy that leaves the warm water by heat is equal to the amount that enters the cool water by heat. Therefore, the entropy change of the Universe is zero for this process. 27819_22_c22_p625-656.indd 656 Challenge Problems 71. A 1.00-mol sample of an ideal gas (g 5 1.40) is carried through the Carnot cycle described in Active Figure 22.10. At point A, the pressure is 25.0 atm and the temperature is 600 K. At point C, the pressure is 1.00 atm and the temperature is 400 K. (a) Determine the pressures and volumes at points A, B, C, and D. (b) Calculate the net work done per cycle. 72. The compression ratio of an Otto cycle as shown in Active Figure 22.12 is VA/V B 5 8.00. At the beginning A of the compression process, 500 cm3 of gas is at 100 kPa and 20.0°C. At the beginning of the adiabatic expansion, the temperature is TC 5 750°C. Model the working fluid as an ideal gas with g 5 1.40. (a) Fill in this table to follow the states of the gas: T (K) A B C D 293 P (kPa) V (cm3) 100 500 1 023 (b) Fill in this table to follow the processes: Q W DE int ASB BSC CSD DSA ABCDA (c) Identify the energy input |Q h |, (d) the energy exhaust |Q c |, and (e) the net output work Weng. (f) Calculate the thermal efficiency. (g) Find the number of crankshaft revolutions per minute required for a one-cylinder engine to have an output power of 1.00 kW 5 1.34 hp. Note: The thermodynamic cycle involves four piston strokes. 6/30/09 12:43:38 PM

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