《热力学 Thermodynamics（I）》课程教学资源：阅读书籍文献_Fundamentals of Engineering Thermodynamics, 7th Edition.pdf_P21-P25

8 Chapter 1 Getting Started TAKE NOTE... Animations reinforce many of the text presentations. You can view these anima- tions by going to the student companion site Tank for this book. Air compressor Animations are keyed to specific content by an icon in the margin. The first of these icons Fig.1.5 Air compressor and storage appears directly below.In tank. this example,the label System_Types refers to the text content while In general,the choice of system boundary is governed by two considerations: A.1-Tabs a,b&c refers to (1)what is known about a possible system,particularly at its boundaries,and(2)the the particular animation objective of the analysis. (A.1)and the tabs (Tabs a,b&c)of the animation FoR EXAMPLE Figure 1.5 shows a sketch of an air compressor connected to a recommended for viewing storage tank.The system boundary shown on the figure encloses the compressor,tank, now to enhance your and all of the piping.This boundary might be selected if the electrical power input understanding. is known,and the objective of the analysis is to determine how long the compressor must operate for the pressure in the tank to rise to a specified value.Since mass crosses the boundary,the system would be a control volume.A control volume System_Types enclosing only the compressor might be chosen if the condition of the air entering A.1-Tabs a,b,c and exiting the compressor is known,and the objective is to determine the electric power input. Describing Systems and Their Behavior Engineers are interested in studying systems and how they interact with their surround- ings.In this section,we introduce several terms and concepts used to describe systems and how they behave. 1.3.1t Macroscopic and Microscopic Views of Thermodynamics Systems can be studied from a macroscopic or a microscopic point of view.The mac- roscopic approach to thermodynamics is concerned with the gross or overall behavior. This is sometimes called classical thermodynamics.No model of the structure of matter at the molecular,atomic,and subatomic levels is directly used in classical thermody- namics.Although the behavior of systems is affected by molecular structure,classical thermodynamics allows important aspects of system behavior to be evaluated from observations of the overall system. The microscopic approach to thermodynamics,known as statistical thermodynamics, is concerned directly with the structure of matter.The objective of statistical thermo- dynamics is to characterize by statistical means the average behavior of the particles making up a system of interest and relate this information to the observed macro- scopic behavior of the system.For applications involving lasers,plasmas,high-speed gas flows,chemical kinetics,very low temperatures (cryogenics),and others,the meth- ods of statistical thermodynamics are essential.The microscopic approach is used in this text to interpret internal energy in Chap.2 and entropy in Chap 6.Moreover,as

8 Chapter 1 Getting Started In general, the choice of system boundary is governed by two considerations: (1) what is known about a possible system, particularly at its boundaries, and (2) the objective of the analysis. Figure 1.5 shows a sketch of an air compressor connected to a storage tank. The system boundary shown on the figure encloses the compressor, tank, and all of the piping. This boundary might be selected if the electrical power input is known, and the objective of the analysis is to determine how long the compressor must operate for the pressure in the tank to rise to a specified value. Since mass crosses the boundary, the system would be a control volume. A control volume enclosing only the compressor might be chosen if the condition of the air entering and exiting the compressor is known, and the objective is to determine the electric power input. b b b b b Air Air compressor Tank + – Fig. 1.5 Air compressor and storage tank. 1.3 Describing Systems and Their Behavior Engineers are interested in studying systems and how they interact with their surroundings. In this section, we introduce several terms and concepts used to describe systems and how they behave. 1.3.1 Macroscopic and Microscopic Views of Thermodynamics Systems can be studied from a macroscopic or a microscopic point of view. The macroscopic approach to thermodynamics is concerned with the gross or overall behavior. This is sometimes called classical thermodynamics. No model of the structure of matter at the molecular, atomic, and subatomic levels is directly used in classical thermodynamics. Although the behavior of systems is affected by molecular structure, classical thermodynamics allows important aspects of system behavior to be evaluated from observations of the overall system. The microscopic approach to thermodynamics, known as statistical thermodynamics, is concerned directly with the structure of matter. The objective of statistical thermodynamics is to characterize by statistical means the average behavior of the particles making up a system of interest and relate this information to the observed macroscopic behavior of the system. For applications involving lasers, plasmas, high-speed gas flows, chemical kinetics, very low temperatures (cryogenics), and others, the methods of statistical thermodynamics are essential. The microscopic approach is used in this text to interpret internal energy in Chap. 2 and entropy in Chap 6. Moreover, as TAKE NOTE... Animations reinforce many of the text presentations. You can view these animations by going to the student companion site for this book. Animations are keyed to specific content by an icon in the margin. The first of these icons appears directly below. In this example, the label System_Types refers to the text content while A.1–Tabs a,b&c refers to the particular animation (A.1) and the tabs (Tabs a,b&c) of the animation recommended for viewing now to enhance your understanding. A System_Types A.1 – Tabs a, b, & c c01GettingStarted.indd Page 8 8/2/10 10:48:10 AM users-133 /Users/users-133/Desktop/Ramakant_04.05.09/WB00113_R1:JWCL170/New

1.3 Describing Systems and Their Behavior 9 noted in Chap.3,the microscopic approach is instrumental in developing certain data, for example ideal gas specific heats. For a wide range of engineering applications,classical thermodynamics not only provides a considerably more direct approach for analysis and design but also requires far fewer mathematical complications.For these reasons the macroscopic viewpoint is the one adopted in this book.Finally,relativity effects are not significant for the systems under consideration in this book. 1.3.2 t Property,State,and Process To describe a system and predict its behavior requires knowledge of its properties and how those properties are related.A property is a macroscopic characteristic of a property system such as mass,volume,energy,pressure,and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior (history)of the system. The word state refers to the condition of a system as described by its properties. state Since there are normally relations among the properties of a system,the state often can be specified by providing the values of a subset of the properties.All other prop- erties can be determined in terms of these few. When any of the properties of a system change,the state changes and the system is said to have undergone a process.A process is a transformation from one state to process another.However,if a system exhibits the same values of its properties at two dif- ferent times,it is in the same state at these times.A system is said to be at steady steady state state if none of its properties change with time. Many properties are considered during the course of our study of engineering thermodynamics.Thermodynamics also deals with quantities that are not properties, such as mass flow rates and energy transfers by work and heat.Additional examples Prop State Process of quantities that are not properties are provided in subsequent chapters.For a way A.2-Tab a to distinguish properties from nonproperties,see the box on p.10. 1.3.3t Extensive and Intensive Properties Thermodynamic properties can be placed in two general classes:extensive and inten- sive.A property is called extensive if its value for an overall system is the sum of its extensive property values for the parts into which the system is divided.Mass,volume,energy,and sev- eral other properties introduced later are extensive.Extensive properties depend on the size or extent of a system.The extensive properties of a system can change with time,and many thermodynamic analyses consist mainly of carefully accounting for changes in extensive properties such as mass and energy as a system interacts with its surroundings. Intensive properties are not additive in the sense previously considered.Their val- intensive property ues are independent of the size or extent of a system and may vary from place to place within the system at any moment.Thus,intensive properties may be functions of both position and time,whereas extensive properties can vary only with time.Specific volume (Sec.1.5),pressure,and temperature are important intensive properties; several other intensive properties are introduced in subsequent chapters. FREXAMPLE to illustrate the difference between extensive and intensive prop- erties,consider an amount of matter that is uniform in temperature,and imagine that it is composed of several parts,as illustrated in Fig.1.6.The mass of the whole is the sum of the masses of the parts,and the overall volume is the sum of the volumes of the parts.However,the temperature of the whole is not the sum of the temperatures Ext Int Properties of the parts;it is the same for each part.Mass and volume are extensive,but tem- A.3-Tab a perature is intensive

noted in Chap. 3, the microscopic approach is instrumental in developing certain data, for example ideal gas specific heats . For a wide range of engineering applications, classical thermodynamics not only provides a considerably more direct approach for analysis and design but also requires far fewer mathematical complications. For these reasons the macroscopic viewpoint is the one adopted in this book. Finally, relativity effects are not significant for the systems under consideration in this book. property process state steady state 1.3.2 Property, State, and Process To describe a system and predict its behavior requires knowledge of its properties and how those properties are related. A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure, and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior ( history ) of the system. The word state refers to the condition of a system as described by its properties. Since there are normally relations among the properties of a system, the state often can be specified by providing the values of a subset of the properties. All other properties can be determined in terms of these few. When any of the properties of a system change, the state changes and the system is said to have undergone a process. A process is a transformation from one state to another. However, if a system exhibits the same values of its properties at two different times, it is in the same state at these times. A system is said to be at steady state if none of its properties change with time. Many properties are considered during the course of our study of engineering thermodynamics. Thermodynamics also deals with quantities that are not properties, such as mass flow rates and energy transfers by work and heat. Additional examples of quantities that are not properties are provided in subsequent chapters. For a way to distinguish properties from non properties, see the box on p. 10. extensive property intensive property 1.3 Describing Systems and Their Behavior 9 1.3.3 Extensive and Intensive Properties Thermodynamic properties can be placed in two general classes: extensive and intensive. A property is called extensive if its value for an overall system is the sum of its values for the parts into which the system is divided. Mass, volume, energy, and several other properties introduced later are extensive. Extensive properties depend on the size or extent of a system. The extensive properties of a system can change with time, and many thermodynamic analyses consist mainly of carefully accounting for changes in extensive properties such as mass and energy as a system interacts with its surroundings. Intensive properties are not additive in the sense previously considered. Their values are independent of the size or extent of a system and may vary from place to place within the system at any moment. Thus, intensive properties may be functions of both position and time, whereas extensive properties can vary only with time. Specific volume (Sec. 1.5 ), pressure, and temperature are important intensive properties; several other intensive properties are introduced in subsequent chapters. to illustrate the difference between extensive and intensive properties, consider an amount of matter that is uniform in temperature, and imagine that it is composed of several parts, as illustrated in Fig. 1.6 . The mass of the whole is the sum of the masses of the parts, and the overall volume is the sum of the volumes of the parts. However, the temperature of the whole is not the sum of the temperatures of the parts; it is the same for each part. Mass and volume are extensive, but temperature is intensive. b b b b b A Prop_State_Process A.2 – Tab a A Ext_Int_Properties A.3 – Tab a c01GettingStarted.indd Page 9 6/30/10 11:38:57 AM user-s146 /Users/user-s146/Desktop/Merry_X-Mas/New

10 Chapter 1 Getting Started Fig.1.6 Figure used to discuss the extensive and intensive property concepts. (回 Distinguishing Properties from Nonproperties At a given state each property has a definite value that can be assigned without knowl- edge of how the system arrived at that state.Therefore,the change in value of a prop- erty as the system is altered from one state to another is determined solely by the two end states and is independent of the particular way the change of state occurred.That is,the change is independent of the details of the process.Conversely,if the value of a quantity is independent of the process between two states,then that quantity is the change in a property.This provides a test for determining whether a quantity is a prop- erty:A quantity is a property if,and only if,its change in value between two states is independent of the process.It follows that if the value of a particular quantity depends on the details of the process,and not solely on the end states,that quantity cannot be a property. 1.3.4 t Equilibrium Classical thermodynamics places primary emphasis on equilibrium states and changes equilibrium from one equilibrium state to another.Thus,the concept of equilibrium is fundamen- tal.In mechanics,equilibrium means a condition of balance maintained by an equality of opposing forces.In thermodynamics,the concept is more far-reaching,including not only a balance of forces but also a balance of other influences.Each kind of influence refers to a particular aspect of thermodynamic,or complete,equilibrium Accordingly,several types of equilibrium must exist individually to fulfill the condi- tion of complete equilibrium;among these are mechanical,thermal,phase,and chem- ical equilibrium. Criteria for these four types of equilibrium are considered in subsequent discus- sions.For the present,we may think of testing to see if a system is in thermodynamic equilibrium by the following procedure:Isolate the system from its surroundings and watch for changes in its observable properties.If there are no changes,we conclude that the system was in equilibrium at the moment it was isolated.The system can be equilibrium state said to be at an equilibrium state. When a system is isolated,it does not interact with its surroundings;however,its state can change as a consequence of spontaneous events occurring internally as its intensive properties,such as temperature and pressure,tend toward uniform values. When all such changes cease,the system is in equilibrium.At equilibrium,temperature is uniform throughout the system.Also,pressure can be regarded as uniform through- out as long as the effect of gravity is not significant;otherwise a pressure variation can exist,as in a vertical column of liquid. There is no requirement that a system undergoing a process be in equilibrium during the process.Some or all of the intervening states may be nonequilibrium states. For many such processes we are limited to knowing the state before the process occurs and the state after the process is completed

10 Chapter 1 Getting Started (a) (b) Fig. 1.6 Figure used to discuss the extensive and intensive property concepts. equilibrium equilibrium state 1.3.4 Equilibrium Classical thermodynamics places primary emphasis on equilibrium states and changes from one equilibrium state to another. Thus, the concept of equilibrium is fundamental. In mechanics, equilibrium means a condition of balance maintained by an equality of opposing forces. In thermodynamics, the concept is more far-reaching, including not only a balance of forces but also a balance of other influences. Each kind of influence refers to a particular aspect of thermodynamic, or complete, equilibrium. Accordingly, several types of equilibrium must exist individually to fulfill the condition of complete equilibrium; among these are mechanical, thermal, phase, and chemical equilibrium. Criteria for these four types of equilibrium are considered in subsequent discussions. For the present, we may think of testing to see if a system is in thermodynamic equilibrium by the following procedure: Isolate the system from its surroundings and watch for changes in its observable properties. If there are no changes, we conclude that the system was in equilibrium at the moment it was isolated. The system can be said to be at an equilibrium state. When a system is isolated, it does not interact with its surroundings; however, its state can change as a consequence of spontaneous events occurring internally as its intensive properties, such as temperature and pressure, tend toward uniform values. When all such changes cease, the system is in equilibrium. At equilibrium, temperature is uniform throughout the system. Also, pressure can be regarded as uniform throughout as long as the effect of gravity is not significant; otherwise a pressure variation can exist, as in a vertical column of liquid. There is no requirement that a system undergoing a process be in equilibrium during the process. Some or all of the intervening states may be nonequilibrium states. For many such processes we are limited to knowing the state before the process occurs and the state after the process is completed. Distinguishing Properties from Nonproperties At a given state each property has a definite value that can be assigned without knowledge of how the system arrived at that state. Therefore, the change in value of a property as the system is altered from one state to another is determined solely by the two end states and is independent of the particular way the change of state occurred. That is, the change is independent of the details of the process. Conversely, if the value of a quantity is independent of the process between two states, then that quantity is the change in a property. This provides a test for determining whether a quantity is a property: A quantity is a property if, and only if, its change in value between two states is independent of the process. It follows that if the value of a particular quantity depends on the details of the process, and not solely on the end states, that quantity cannot be a property. c01GettingStarted.indd Page 10 7/1/10 10:35:41 AM user-s146 /Users/user-s146/Desktop/Merry_X-Mas/New

1.4 Measuring Mass,Length,Time,and Force 11 4 Measuring Mass,Length, Time,and Force When engineering calculations are performed,it is necessary to be concerned with the units of the physical quantities involved.A unit is any specified amount of a quantity by comparison with which any other quantity of the same kind is measured. For example,meters,centimeters,kilometers,feet,inches,and miles are all units of length.Seconds,minutes,and hours are alternative time units. Because physical quantities are related by definitions and laws,a relatively small number of physical quantities suffice to conceive of and measure all others.These are called primary dimensions.The others are measured in terms of the primary dimen- sions and are called secondary.For example,if length and time were regarded as primary,velocity and area would be secondary. A set of primary dimensions that suffice for applications in mechanics are mass, length,and time.Additional primary dimensions are required when additional phys- ical phenomena come under consideration.Temperature is included for thermody- namics,and electric current is introduced for applications involving electricity. Once a set of primary dimensions is adopted,a base unit for each primary dimen- base unit sion is specified.Units for all other quantities are then derived in terms of the base units.Let us illustrate these ideas by considering briefly two systems of units:SI units and English Engineering units. 1.4.1 t SI Units In the present discussion we consider the system of units called SI that takes mass, length,and time as primary dimensions and regards force as secondary.SI is the abbreviation for Systeme International d'Unites (International System of Units), which is the legally accepted system in most countries.The conventions of the SI are published and controlled by an international treaty organization.The Sl base units for SI base units mass,length,and time are listed in Table 1.3 and discussed in the following paragraphs. The SI base unit for temperature is the kelvin,K. The SI base unit of mass is the kilogram,kg.It is equal to the mass of a particular cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures near Paris.The mass standard for the United States is maintained by the National Institute of Standards and Technology.The kilogram is the only base unit still defined relative to a fabricated object. The SI base unit of length is the meter (metre),m,defined as the length of the path traveled by light in a vacuum during a specified time interval.The base unit of time is the second,s.The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition of the cesium atom. TABLE 1.3 Units for Mass,Length,Time,and Force SI English Quantity Unit Symbol Unit Symbol mass kilogram kg pound mass b length meter foot 公 time second second force newton v pound force lbf (=1kg·m/s (=32.1740lb·ft/s

1.4 Measuring Mass, Length, Time, and Force When engineering calculations are performed, it is necessary to be concerned with the units of the physical quantities involved. A unit is any specified amount of a quantity by comparison with which any other quantity of the same kind is measured. For example, meters, centimeters, kilometers, feet, inches, and miles are all units of length. Seconds, minutes, and hours are alternative time units. Because physical quantities are related by definitions and laws, a relatively small number of physical quantities suffice to conceive of and measure all others. These are called primary dimensions. The others are measured in terms of the primary dimensions and are called secondary . For example, if length and time were regarded as primary, velocity and area would be secondary. A set of primary dimensions that suffice for applications in mechanics are mass, length, and time. Additional primary dimensions are required when additional physical phenomena come under consideration. Temperature is included for thermodynamics, and electric current is introduced for applications involving electricity. Once a set of primary dimensions is adopted, a base unit for each primary dimension is specified. Units for all other quantities are then derived in terms of the base units. Let us illustrate these ideas by considering briefly two systems of units: SI units and English Engineering units. base unit SI base units TABLE 1.3 Units for Mass, Length, Time, and Force SI English Quantity Unit Symbol Unit Symbol mass kilogram kg pound mass lb length meter m foot ft time second s second s force newton N pound force lbf (5 1 kg · m/s2 ) (5 32.1740 lb · ft/s2 ) 1.4 Measuring Mass, Length, Time, and Force 11 1.4.1 SI Units In the present discussion we consider the system of units called SI that takes mass, length, and time as primary dimensions and regards force as secondary. SI is the abbreviation for Système International d’Unités (International System of Units), which is the legally accepted system in most countries. The conventions of the SI are published and controlled by an international treaty organization. The SI base units for mass, length, and time are listed in Table 1.3 and discussed in the following paragraphs. The SI base unit for temperature is the kelvin, K. The SI base unit of mass is the kilogram, kg. It is equal to the mass of a particular cylinder of platinum–iridium alloy kept by the International Bureau of Weights and Measures near Paris. The mass standard for the United States is maintained by the National Institute of Standards and Technology. The kilogram is the only base unit still defined relative to a fabricated object. The SI base unit of length is the meter (metre), m, defined as the length of the path traveled by light in a vacuum during a specified time interval. The base unit of time is the second, s. The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition of the cesium atom. c01GettingStarted.indd Page 11 4/26/10 11:55:05 AM users-133 /Users/users-133/Desktop/Ramakant_04.05.09/WB00113_R1:JWCL170/New