لخّصلي

INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC.The continuum idealiza- tion is implicit in many statements we make, such as "the density of water in a glass is the same at any point." To have a sense of the distance involved at the molecular level, consider a container filled with oxygen at atmospheric conditions. The diameter of the oxygen molecule is about 3  10
10 m and its mass is 5.3  10
26 kg. Also, the mean free path of oxygen at 1 atm pressure and 20?C is 6.3  10
8 m. That is, an oxygen molecule travels, on average, a distance of 6.3  10
8 m (about 200 times of its diameter) before it collides with another molecule. Also, there are about 3  1016 molecules of oxygen in the tiny volume of 1 mm3 at 1 atm pressure and 20?C (Fig. 1-21). The continuum model is applicable as long as the characteristic length of the system (such as its O2 1 atm, 20?C 3 x 1016 molecules/mm3 VOID FIGURE 1-21 Despite the large gaps between molecules, a substance can be treated as a continuum because of the very large number of molecules even in an extremely small volume. diameter) is much larger than the mean free path of the molecules. At very high vacuums or very high elevations, the mean free path may become large (for example, it is about 0.1 m for atmospheric air at an elevation of 100 km). For such cases the rarefied gas flow theory should be used, and the impact of individual molecules should be considered. In this text we will limit our consideration to substances that can be modeled as a continuum. 1-5 ? DENSITY AND SPECIFIC GRAVITY Density is defined as mass per unit volume (Fig. 1-22). Density: r
m 1kg>m3 2 (1-4) V The reciprocal of density is the specific volume v, which is defined as vol- ume per unit mass. That is, v
V
1 (1-5) mr For a differential volume element of mass dm and volume dV, density can be expressed as r
dm/dV. The density of a substance, in general, depends on temperature and pres- sure. The density of most gases is proportional to pressure and inversely proportional to temperature. Liquids and solids, on the other hand, are essentially incompressible substances, and the variation of their density with pressure is usually negligible. At 20?C, for example, the density of water changes from 998 kg/m3 at 1 atm to 1003 kg/m3 at 100 atm, a change of just 0.5 percent. The density of liquids and solids depends more strongly on temperature than it does on pressure. At 1 atm, for example, the density of water changes from 998 kg/m3 at 20?C to 975 kg/m3 at 75?C, a change of 2.3 percent, which can still be neglected in many engi- neering analyses. Sometimes the density of a substance is given relative to the density of a well-known substance. Then it is called specific gravity, or relative den- sity, and is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4?C, for which r
1000 kg/m3). That is, Use actual data from the experiment shown here to obtain the density of water in the neighborhood of 4?C. See end-of-chapter problem 1-129. (C) Ronald Mullisen Chapter 1 | 13 INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC. 5 ON THE DVD. EXPERIMENT H2O Specific gravity: SG
r rH2O (1-6) Note that the specific gravity of a substance is a dimensionless quantity. However, in SI units, the numerical value of the specific gravity of a sub- stance is exactly equal to its density in g/cm3 or kg/L (or 0.001 times the density in kg/m3) since the density of water at 4?C is 1 g/cm3
1 kg/L
1000 kg/m3. The specific gravity of mercury at 0?C, for example, is 13.6. Therefore, its density at 0?C is 13.6 g/cm3
13.6 kg/L
13,600 kg/m3. The specific gravities of some substances at 0?C are given in Table 1-3. Note that substances with specific gravities less than 1 are lighter than water, and thus they would float on water. FIGURE 1-22 Density is mass per unit volume; specific volume is volume per unit mass. V = 12 m3 m=3kg ? = 0.25 kg/m 3 v = -1 = 4 m3/kg ? 14 | Thermodynamics TABLE 1-3 Specific gravities of some substances at 0?C Substance SG Water 1.0 Blood 1.05 Seawater 1.025 Gasoline 0.7 Ethyl alcohol 0.79 Mercury 13.6 Wood 0.3-0.9 Gold 19.2 Bones 1.7-2.0 Ice 0.92 Air (at 1 atm) 0.0013 INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC. 6 ON THE DVD. The weight of a unit volume of a substance is called specific weight and is expressed as Specificweight: g rg s 1N>m32 (1-7) where g is the gravitational acceleration. The densities of liquids are essentially constant, and thus they can often be approximated as being incompressible substances during most processes without sacrificing much in accuracy. m = 2 kg T1 = 20?C V1 = 1.5 m3 (a) State 1 FIGURE 1-23 m = 2 kg T2 = 20?C 3 V2 = 2.5 m (b) State 2 Consider a system not undergoing any change. At this point, all the proper- ties can be measured or calculated throughout the entire system, which gives us a set of properties that completely describes the condition, or the state, of the system. At a given state, all the properties of a system have fixed values. If the value of even one property changes, the state will change to a different one. In Fig. 1-23 a system is shown at two different states. Thermodynamics deals with equilibrium states. The word equilibrium implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings. There are many types of equilibrium, and a system is not in thermody- namic equilibrium unless the conditions of all the relevant types of equilib- rium are satisfied. For example, a system is in thermal equilibrium if the temperature is the same throughout the entire system, as shown in Fig. 1-24. That is, the system involves no temperature differential, which is the driving force for heat flow. Mechanical equilibrium is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time. However, the pressure may vary within the system with elevation as a result of gravitational effects. For example, the higher pressure at a bottom layer is balanced by the extra weight it must carry, and, therefore, there is no imbalance of forces. The variation of pres- sure as a result of gravity in most thermodynamic systems is relatively small and usually disregarded. If a system involves two phases, it is in phase equilibrium when the mass of each phase reaches an equilibrium level and stays there. Finally, a system is in chemical equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur. A system will not be in equilibrium unless all the relevant equilibrium crite- ria are satisfied. The State Postulate As noted earlier, the state of a system is described by its properties. But we know from experience that we do not need to specify all the properties in order to fix a state. Once a sufficient number of properties are specified, the rest of the properties assume certain values automatically. That is, specifying a certain number of properties is sufficient to fix a state. The number of prop- erties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two independent, intensive properties. A system at two different states. (a) Before FIGURE 1-24 (b) After 1-6 ? STATE AND EQUILIBRIUM ?? 20 C 30 C 35 C 40 C ? ?? ? 42 C 23 C A closed system reaching thermal equilibrium. ?? 32 C 32 C ? ? ?? 32 C 32 C 32 C 32 C

                                        A system is called a simple compressible system in the absence of elec- trical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineer- ing problems. Otherwise, an additional property needs to be specified for each effect that is significant. If the gravitational effects are to be consid- ered, for example, the elevation z needs to be specified in addition to the two properties necessary to fix the state. The state postulate requires that the two properties specified be indepen- dent to fix the state. Two properties are independent if one property can be varied while the other one is held constant. Temperature and specific vol- ume, for example, are always independent properties, and together they can fix the state of a simple compressible system (Fig. 1-25). Temperature and pressure, however, are independent properties for single-phase systems, but are dependent properties for multiphase systems. At sea level (P  1 atm), water boils at 100?C, but on a mountaintop where the pressure is lower, water boils at a lower temperature. That is, T  f(P) during a phase-change process; thus, temperature and pressure are not sufficient to fix the state of a two-phase system. Phase-change processes are discussed in detail in Chap. 3. 1-7 ? PROCESSES AND CYCLES

Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a sys- tem passes during a process is called the path of the process (Fig. 1-26). To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings. When a process proceeds in such a manner that the system remains infin- itesimally close to an equilibrium state at all times, it is called a quasi- static, or quasi-equilibrium, process. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts. This is illustrated in Fig. 1-27. When a gas in a piston-cylinder device is compressed suddenly, the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston, thus creating a high-pressure region there. Because of this pressure difference, the system can no longer be said to be in equilib- rium, and this makes the entire process nonquasi-equilibrium. However, if the piston is moved slowly, the molecules will have sufficient time to redis- tribute and there will not be a molecule pileup in front of the piston. As a result, the pressure inside the cylinder will always be nearly uniform and will rise at the same rate at all locations. Since equilibrium is maintained at all times, this is a quasi-equilibrium process. It should be pointed out that a quasi-equilibrium process is an idealized process and is not a true representation of an actual process. But many actual processes closely approximate it, and they can be modeled as quasi- equilibrium with negligible error. Engineers are interested in quasiequilib- rium processes for two reasons. First, they are easy to analyze; second, Nitrogen ? The state of nitrogen is fixed by two independent, intensive properties. INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC. 7 ON THE DVD. FIGURE 1-25 Chapter 1 | 15 T = 25 C v = 0.9 m3/kg Property A State 1 FIGURE 1-26 State 2 Process path A process between states 1 and 2 and the process path. (a) Slow compression (quasi-equilibrium) (b) Very fast compression (nonquasi-equilibrium) FIGURE 1-27 Quasi-equilibrium and nonquasi- equilibrium compression processes. Property B

                                     16 | Thermodynamics

P Final state 2 Process path Initial state 1 V2 V1 (2) (1) V System FIGURE 1-28 The P-V diagram of a compression process. work-producing devices deliver the most work when they operate on quasi- equilibrium processes. Therefore, quasi-equilibrium processes serve as stan- dards to which actual processes can be compared. Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes. Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v). Figure 1-28 shows the P-V diagram of a compression process of a gas. Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasi- equilibrium processes only. For nonquasi-equilibrium processes, we are not able to characterize the entire system by a single state, and thus we cannot speak of a process path for a system as a whole. A nonquasi-equilibrium process is denoted by a dashed line between the initial and final states instead of a solid line. The prefix iso- is often used to designate a process for which a particular property remains constant. An isothermal process, for example, is a process during which the temperature T remains constant; an isobaric process is a process during which the pressure P remains constant; and an isochoric (or isometric) process is a process during which the specific vol- ume v remains constant. A system is said to have undergone a cycle if it returns to its initial state at the end of the process. That is, for a cycle the initial and final states are identical. The Steady-Flow Process The terms steady and uniform are used frequently in engineering, and thus it is important to have a clear understanding of their meanings. The term steady implies no change with time. The opposite of steady is unsteady, or transient. The term uniform, however, implies no change with location over a specified region. These meanings are consistent with their everyday use (steady girlfriend, uniform properties, etc.). A large number of engineering devices operate for long periods of time under the same conditions, and they are classified as steady-flow devices. Processes involving such devices can be represented reasonably well by a somewhat idealized process, called the steady-flow process, which can be defined as a process during which a fluid flows through a control volume steadily (Fig. 1-29). That is, the fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process. Therefore, the volume V, the mass m, and the total energy content E of the control volume remain constant during a steady- flow process (Fig. 1-30). Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, con- densers, and heat exchangers or power plants or refrigeration systems. Some cyclic devices, such as reciprocating engines or compressors, do not satisfy any of the conditions stated above since the flow at the inlets and the exits will be pulsating and not steady. However, the fluid properties vary with Mass in Mass in FIGURE 1-29 Time: 1 PM Time: 3 PM Mass out Mass out 300?C 250?C Control volume 225?C 200?C 150?C 300?C 250?C Control volume 225?C 200?C 150?C During a steady-flow process, fluid properties within the control volume may change with position but not with time. time in a periodic manner, and the flow through these devices can still be analyzed as a steady-flow process by using time-averaged values for the properties. 1-8 ? TEMPERATURE AND THE ZEROTH LAW OF THERMODYNAMICS Although we are familiar with temperature as a measure of "hotness" or "coldness," it is not easy to give an exact definition for it. Based on our physiological sensations, we express the level of temperature qualitatively with words like freezing cold, cold, warm, hot, and red-hot.m)," which is called a joule (J). That is, 1 J  1 N # m (1-3) A more common unit for energy in SI is the kilojoule (1 kJ  103 J). In the English system, the energy unit is the Btu (British thermal unit), which is defined as the energy required to raise the temperature of 1 lbm of water at 68?F by 1?F. In the metric system, the amount of energy needed to raise the temperature of 1 g of water at 14.5?C by 1?C is defined as 1 calorie (cal), and 1 cal  4.1868 J. The magnitudes of the kilojoule and Btu are almost identical (1 Btu  1.0551 kJ). FIGURE 1-9 A body weighing 150 lbf on earth will weigh only 25 lbf on the moon. Chapter 1 | 7 kg g = 9.807 m/s2 W = 9.807 kg . m/s2 = 9.807 N = 1 kgf FIGURE 1-10 lbm g = 32.174 ft/s2 W = 32.174 lbm . ft/s2 = 1 lbf The weight of a unit mass at sea level. 8 | Thermodynamics Dimensional Homogeneity We all know from grade school that apples and oranges do not add. But we somehow manage to do it (by mistake, of course). In engineering, all equa- tions must be dimensionally homogeneous. That is, every term in an equa- tion must have the same unit (Fig. 1-11). If, at some stage of an analysis, we find ourselves in a position to add two quantities that have different units, it is a clear indication that we have made an error at an earlier stage. So checking dimensions can serve as a valuable tool to spot errors. EXAMPLE 1-1 Spotting Errors from Unit Inconsistencies While solving a problem, a person ended up with the following equation at some stage: E  25 kJ  7 kJ>kg where E is the total energy and has the unit of kilojoules. Determine how to correct the error and discuss what may have caused it. Solution During an analysis, a relation with inconsistent units is obtained. A correction is to be found, and the probable cause of the error is to be determined. Analysis The two terms on the right-hand side do not have the same units, and therefore they cannot be added to obtain the total energy. Multiplying the last term by mass will eliminate the kilograms in the denominator, and the whole equation will become dimensionally homogeneous; that is, every term in the equation will have the same unit. Discussion Obviously this error was caused by forgetting to multiply the last term by mass at an earlier stage. FIGURE 1-11 To be dimensionally homogeneous, all the terms in an equation must have the same unit. (C) Reprinted with special permission of King Features Syndicate. We all know from experience that units can give terrible headaches if they are not used carefully in solving a problem. However, with some attention and skill, units can be used to our advantage. They can be used to check for- mulas; they can even be used to derive formulas, as explained in the follow- ing example. EXAMPLE 1-2 Obtaining Formulas from Unit Considerations A tank is filled with oil whose density is r  850 kg/m3. If the volume of the tank is V  2 m3, determine the amount of mass m in the tank. Solution The volume of an oil tank is given. The mass of oil is to be deter- mined. Assumptions Oil is an incompressible substance and thus its density is con- stant. Analysis A sketch of the system just described is given in Fig. 1-12. Sup- pose we forgot the formula that relates mass to density and volume. However, we know that mass has the unit of kilograms. That is, whatever calculations we do, we should end up with the unit of kilograms. Putting the given infor- mation into perspective, we have r  850 kg>m3 and V  2 m3 OIL ? = 850 kg/m3 FIGURE 1-12 Schematic for Example 1-2. V = 2 m3 m=? s2 s2 They can also be expressed more conveniently as unity conversion ratios as N lbf #2 #2 kg m>s  1 and 32.174 lbm ft>s  1 Unity conversion ratios are identically equal to 1 and are unitless, and thus such ratios (or their inverses) can be inserted conveniently into any calcula- tion to properly convert units. Students are encouraged to always use unity conversion ratios such as those given here when converting units. Some textbooks insert the archaic gravitational constant g defined as g  32.174 cc lbm . ft/lbf . s2  kg . m/N . s2  1 into equations in order to force units to match. This practice leads to unnecessary confusion and is strongly discour- aged by the present authors. We recommend that students instead use unity conversion ratios. Chapter 1 | 9 It is obvious that we can eliminate m3 and end up with kg by multiplying these two quantities. Therefore, the formula we are looking for should be Thus, m  rV m  1850 kg>m3 2 12 m3 2  1700 kg Discussion Note that this approach may not work for more complicated formulas. You should keep in mind that a formula that is not dimensionally homo- geneous is definitely wrong, but a dimensionally homogeneous formula is not necessarily right. Unity Conversion Ratios Just as all nonprimary dimensions can be formed by suitable combinations of primary dimensions, all nonprimary units (secondary units) can be formed by combinations of primary units. Force units, for example, can be expressed as N  kg m and lbf  32.174 lbm ft EXAMPLE 1-3 The Weight of One Pound-Mass Using unity conversion ratios, show that 1.00 lbm weighs 1.00 lbf on earth (Fig. 1-13). Solution A mass of 1.00 lbm is subjected to standard earth gravity. Its weight in lbf is to be determined. Assumptions Standard sea-level conditions are assumed. Properties The gravitational constant is g  32.174 ft/s2. lbm FIGURE 1-13 A mass of 1 lbm weighs 1 lbf on earth. 10 | Thermodynamics Net weight: One pound (454 grams) Analysis We apply Newton's second law to calculate the weight (force) that corresponds to the known mass and acceleration. The weight of any object is equal to its mass times the local value of gravitational acceleration. Thus, W  mg  11.00 lbm2 132.174 ft>s22 a 1 lbf b  1.00 lbf 32.174 lbm # ft>s2 Discussion Mass is the same regardless of its location. However, on some other planet with a different value of gravitational acceleration, the weight of 1 lbm would differ from that calculated here. When you buy a box of breakfast cereal, the printing may say "Net weight: One pound (454 grams)."The term weight is often incorrectly used to express mass, particularly by the "weight watchers." Unlike mass, weight W is a force. It is the gravita- tional force applied to a body, and its magnitude is determined from New- or In SI, the force unit is the newton (N), and it is defined as the force required The relative magnitudes of the force units newton (N), kilogram-force (kgf), and pound-force (lbf). ton's second law, W  mg 1N2 (1-2) medium apples (m total

                                     where m is the mass of the body, and g is the local gravitational acceleration (g is 9.807 m/s2 or 32.174 ft/s2 at sea level and 45? latitude). An ordinary bathroom scale measures the gravitational force acting on a body. The weight of a unit volume of a substance is called the specific weight g and is determined from g  rg, where r is density. The mass of a body remains the same regardless of its location in the uni- verse. Its weight, however, changes with a change in gravitational accelera- tion. A body weighs less on top of a mountain since g decreases with altitude. On the surface of the moon, an astronaut weighs about one-sixth of what she or he normally weighs on earth (Fig. 1-9). At sea level a mass of 1 kg weighs 9.807 N, as illustrated in Fig. 1-10. A mass of 1 lbm, however, weighs 1 lbf, which misleads people to believe that pound-mass and pound-force can be used interchangeably as pound (lb), which is a major source of error in the English system. It should be noted that the gravity force acting on a mass is due to the attraction between the masses, and thus it is proportional to the magnitudes of the masses and inversely proportional to the square of the distance between them. Therefore, the gravitational acceleration g at a location depends on the local density of the earth's crust, the distance to the center of the earth, and to a lesser extent, the positions of the moon and the sun. The value of g varies with location from 9.8295 m/s2 at 4500 m below sea level to 7.3218 m/s2 at 100,000 m above sea level. However, at altitudes up to 30,000 m, the variation of g from the sea-level value of 9.807 m/s2 is less than 1 percent. Therefore, for most practical purposes, the gravitational acceleration can be assumed to be constant at 9.81 m/s2. It is interesting to note that at locations below sea level, the value of g increases with distance from the sea level, reaches a maximum at about 4500 m, and then starts decreasing. (What do you think the value of g is at the center of the earth?)

The primary cause of confusion between mass and weight is that mass is usually measured indirectly by measuring the gravity force it exerts. This approach also assumes that the forces exerted by other effects such as air buoyancy and fluid motion are negligible. This is like measuring the dis- tance to a star by measuring its red shift, or measuring the altitude of an air- plane by measuring barometric pressure. Both of these are also indirect measurements. The correct direct way of measuring mass is to compare it to a known mass. This is cumbersome, however, and it is mostly used for cali- bration and measuring precious metals. Work, which is a form of energy, can simply be defined as force times dis- tance; therefore, it has the unit "newton-meter (N .(See Fig. Using Newton's second law, the actual weight of the cereal in the metric system is W  mg  1453.6 g2 19.81 m>s22 a 1 N b a 1 kg b  4.45 N 1 kg #m>s2 1000 g 1-3 ? SYSTEMS AND CONTROL VOLUMES A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surround- ings is called the boundary. These terms are illustrated in Fig. 1-15. The boundary of a system can be fixed or movable. Note that the boundary is the contact surface shared by both the system and the surroundings. Mathemati- cally speaking, the boundary has zero thickness, and thus it can neither con- tain any mass nor occupy any volume in space. Systems may be considered to be closed or open, depending on whether a fixed mass or a fixed volume in space is chosen for study. That is, no mass can enter or leave a closed system, as shown in Fig. 1-16. But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed. If, as a special case, even energy is not allowed to cross the boundary, that system is called an isolated system. Consider the piston-cylinder device shown in Fig. 1-17. Let us say that we would like to find out what happens to the enclosed gas when it is heated. Since we are focusing our attention on the gas, it is our system. The inner surfaces of the piston and the cylinder form the boundary, and since no mass is crossing this boundary, it is a closed system. Everything outside the gas, including the piston and the cylinder, is the surroundings. An open system, or a control volume, as it is often called, is a prop- erly selected region in space. It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Flow through these FIGURE 1-14 A quirk in the metric system of units. INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC. 3 ON THE DVD. SURROUNDINGS SYSTEM BOUNDARY FIGURE 1-15 System, surroundings, and boundary. CLOSED SYSTEM m = constant FIGURE 1-16 Mass NO Energy YES Mass cannot cross the boundaries of a closed system, but energy can. devices is best studied by selecting the region within the device as the control volume. Both mass and energy can cross the boundary of a con- trol volume. A large number of engineering problems involve mass flow in and out of a system and, therefore, are modeled as control volumes. In general, any arbitrary region in space can be selected 1 m3 as a control volume. There are no concrete rules for the selection of control volumes, but the proper choice certainly makes the analysis much easier. If we were to analyze the flow of air through a nozzle, for example, a good choice for the control volume would be the region within the nozzle. The boundaries of a control volume are called a control surface, and they can be real or imaginary. In the case of a nozzle, the inner surface of the noz- zle forms the real part of the boundary, and the entrance and exit areas form the imaginary part, since there are no physical surfaces there (Fig. 1-18a).5 Dimension Length Mass Time Temperature Electric current Amount of light Amount of matter Unit meter (m) kilogram (kg) second (s) kelvin (K) ampere (A) candela (cd) mole (mol) TABLE 1-2 Standard prefixes in SI units Multiple Prefix 1012 tera, T 109 giga, G 106 mega, M 103 kilo, k 102 hecto, h 101 deka, da 10
1 deci, d 10
2 centi, c 10
3 milli, m 10
6 micro, m 10
9 nano, n 10
12 pico, p

                                     6 |Some examples include the electric or gas range, the heating and air-conditioning systems, the refrigerator, the humidifier, the pressure cooker, the water heater, the shower, the iron, and even the computer and the TV. On a larger scale, thermodynamics plays a major part in the design and analysis of automotive engines, rockets, jet engines, and conventional or nuclear power plants, solar collectors, and the design of vehicles from ordi- nary cars to airplanes (Fig.In 1960, the CGPM produced the SI, which was based on six fundamental quantities, and their units were adopted in 1954 at the Tenth General Conference of Weights and Measures: meter (m) for length, kilo- gram (kg) for mass, second (s) for time, ampere (A) for electric current, degree Kelvin (?K) for temperature, and candela (cd) for luminous inten- sity (amount of light).Some familiar proper- ties are pressure P, temperature T, volume V, and mass m. The list can be extended to include less familiar ones such as viscosity, thermal conductiv- ity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation.To avoid this nuisance, we consider force to be a secondary dimension whose unit is derived from Newton's second law, that is,

Force  1Mass2 1Acceleration2 F  ma m = 1 kg a = 1 m/s2 F =1N a = 1 ft/s2 F = 1 lbf 200 mL 1 kg m = 32.174 lbm FIGURE 1-7 The definition of the force units.The first and second laws of thermodynamics emerged simultaneously in the 1850s, primarily out of the works of William Rankine, Rudolph Clau- sius, and Lord Kelvin (formerly William Thomson).Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary or fun- damental dimensions, while others such as velocity V, energy E, and vol- ume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions.The industries that are heavily involved in international trade (such as the automotive, soft drink, and liquor industries) have been quick in converting to the metric system for economic reasons (having a single worldwide design, fewer sizes, smaller inventories, etc.).The temperature scales used in the SI and in the English system today are the Celsius scale (formerly called the centigrade scale; in 1948 it was renamed after the Swedish astronomer A. Celsius, 1702-1744, who devised it) and the Fahrenheit scale (named after the German instrument maker G. Fahrenheit, 1686-1736), respectively.In this international treaty, meter and gram were established as the metric units for length and mass, respectively, and a General Conference of Weights and Measures (CGPM) was established that was to meet every six years.Once a and b are known, the temperature of a medium can be calculated from this relation by immersing the rigid vessel of the gas thermometer into the medium and measuring the gas pressure when thermal equilibrium is established between the medium and the gas in the vessel whose volume is held constant.The English system, however, has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily (12 in  1 ft, 1 mile  5280 ft, 4 qt  gal, etc.), which makes it confusing and difficult to learn.Generally, uppercase letters are used to denote extensive properties (with mass m being a major exception), and lowercase letters are used for intensive properties (with pressure P and temperature T being the obvious exceptions).The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinu- ities.Fortunately, several properties of materials change with temperature in a repeatable and predictable way, and this forms the basis for accurate tem- perature measurement.The second law of thermodynamics asserts that energy has qual- ity as well as quantity, and actual processes occur in the direction of decreasing quality of energy.Although the principles of thermodynamics have been in existence since the creation of the universe, thermodynamics did not emerge as a science until the construction of the first successful atmospheric steam engines in England by Thomas Savery in 1697 and Thomas Newcomen in 1712.Despite strong efforts in the scientific and engineering community to unify the world with a single unit system, two sets of units are still in common use today: the English system, which is also known as the United States Cus- tomary System (USCS), and the metric SI (from Le Systeme International d' Unites), which is also known as the International System.The systematic efforts to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up with such a unit system.The mass and length units in the two systems are related to each other by 1 lbm  0.45359 kg 1 ft  0.3048 m In the English system, force is usually considered to be one of the pri- mary dimensions and is assigned a nonderived unit.That is, 1 N  1 kg # m>s2 1 lbf  32.174 lbm # ft>s2 A force of 1 N is roughly equivalent to the weight of a small apple (m  102 g), whereas a force of 1 lbf is roughly equivalent to the weight of four  454 g), as shown in Fig.A water heater, a car radiator, a turbine, and a compressor all involve mass flow and should GAS be analyzed as control volumes (open systems) instead of as control masses 2 kg (closed systems).GAS 2 kg 3 m3 Fixed boundary WATER HEATER (control volume) Control surface Hot water out Imaginary boundary Real boundary CV (a nozzle) Cold water in (a) A control volume with real and imaginary boundaries FIGURE 1-18 Moving boundary CV Fixed boundary (b) A control volume with fixed and moving boundaries A control volume can involve fixed, moving, real, and imaginary boundaries.The lowest temperature on the Kelvin scale is absolute zero, or 0 K. Then it follows that only one nonzero reference point needs to be assigned to establish the slope of this linear scale.An ideal-gas temperature scale can be developed by measuring the pres- sures of the gas in the vessel at two reproducible points (such as the ice and the steam points) and assigning suitable values to temperatures at those two points.The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which is most descriptive of the early efforts to convert heat into power.PE = 10 units KE = 0 PE = 7 units KE = 3 units Potential energy Kinetic energy FIGURE 1-1 vation of energy principle, and it asserts that energy is a thermodynamic property.The heart is constantly pumping blood to all parts of the human body, various energy conversions occur in trillions of body cells, and the body heat generated is constantly rejected to the environment.An ordi- nary house is, in some respects, an exhibition hall filled with wonders of thermodynamics (Fig.Many ordinary household utensils and appli- ances are designed, in whole or in part, by using the principles of thermody- namics.Thermodynamics The human body Air conditioning systems Airplanes Car radiators Power plants Refrigeration systems FIGURE 1-5 Some application areas of thermodynamics.This puts an extra burden on today's engineering students, since they are expected to retain their understanding of the English system while learning, thinking, and working in terms of the SI. Given the position of the engineers in the transi- tion period, both unit systems are used in this text, with particular emphasis on SI units.In the English system, the force unit is the pound-force (lbf) and is defined as the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2 (Fig.An easy way to determine whether a property is intensive or extensive is to divide the system into two equal parts with an imaginary partition, as shown in Fig.As the name suggests, its value as a fundamental physical principle was rec- ognized more than half a century after the formulation of the first and the second laws of thermodynamics.Using nonconventional refrigeration techniques, scientists have approached absolute zero kelvin (they achieved 0.000000002 K in 1989).The temperatures on this scale are measured using a constant-volume gas thermometer, which is basically a rigid vessel filled with a gas, usually hydrogen or helium, at low pressure.This thermometer is based on the principle that at low pressures, the tem- perature of a gas is proportional to its pressure at constant volume.Then the relationship between the temperature and the pressure of the gas in the vessel can be expressed as T  a  bP (1-8) where the values of the constants a and b for a gas thermometer are deter- mined experimentally.This is the lowest temperature that can be obtained by a gas thermometer, and thus we can obtain an absolute gas temperature scale by assigning a value of zero to the constant a in Eq. 1-8.It should be noted that the absolute gas temperature scale is not a thermo- dynamic temperature scale, since it cannot be used at very low temperatures (due to condensation) and at very high temperatures (due to dissociation and ionization).Today the same name is broadly interpreted to include all aspects of energy and energy transformations, including power generation, refrigeration, and relationships among the properties of matter.The first thermodynamic textbook was written in 1859 by William Rankine, a profes- sor at the University of Glasgow.A more elabo- rate approach, based on the average behavior of large groups of individual particles, is called statistical thermodynamics.Application Areas of Thermodynamics All activities in nature involve some interaction between energy and matter; thus, it is hard to imagine an area that does not relate to thermodynamics in some manner.Thermodynamics is commonly encountered in many engineering systems and other aspects of life, and one does not need to go very far to see some application areas of it. In fact, one does not need to go anywhere.The SI is a simple and logical system based on a decimal relationship between the various units, and it is being used for scientific and engineering work in most of the industrialized nations, including England.An early version of the metric system was soon developed in France, but it

                                    did not find universal acceptance until 1875 when The Metric Convention Treaty was prepared and signed by 17 nations, including the United States.10 apples m = 1 kg

1 apple m = 102 g 1 N FIGURE 1-8 (1-1) 1 kgf 4 apples m = 1 lbm 1 lbf to accelerate a mass of 1 kg at a rate of 1 m/s2.Total mass, total vol- ume, and total momentum are some examples of extensive properties.The zeroth law of thermodynamics states that if two bodies are in ther- mal equilibrium with a third body, they are also in thermal equilibrium with each other.By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.A mixture of ice and water that is in equilib- rium with air saturated with vapor at 1 atm pressure is said to be at the ice point, and a mixture of liquid water and water vapor (with no air) in equilib- rium at 1 atm pressure is said to be at the steam point.In that case Eq. 1-8 reduces to T  bP, and thus we need to specify the temperature at only one point to define an absolute gas temperature scale.The conservation of energy principle also forms the back- bone of the diet industry: A person who has a greater energy input (food) than energy output (exercise) will gain weight (store energy in the form of fat), and a person who has a smaller energy input than output will lose weight (Fig.The high-temperature energy of the coffee is degraded (transformed into a less useful form at a lower temperature) once it is trans- ferred to the surrounding air.This macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles is called classical thermodynamics.Therefore, developing a good understanding of basic principles of thermodynamics has long been an essential part of engineering education.The size, location, and the power input of the fan of your computer is also selected after an analysis that involves thermodynamics.Solar collectors Shower Hot water Cold water FIGURE 1-4 The design of many engineering systems, such as this solar hot water system, involves thermodynamics.A/C unit, fridge, radiator: (C) The McGraw-Hill Companies, Inc./Jill Braaten, photographer; Plane: (C) Vol.In 1971, the CGPM added a seventh fundamental quantity and unit: mole (mol) for the amount of matter.Congress continued to promote a voluntary switch to the metric system by passing the Metric Conversion Act in 1975.A trade bill passed by Congress in 1988 set a September 1992 deadline for all federal agencies to convert to the metric system.Most car owners probably do not realize this until they try an English socket wrench on a metric bolt.Most industries, however, resisted the change, thus slowing down the conversion process.Presently the United States is a dual-system society, and it will stay that way until the transition to the metric system is completed.1 M (0.2 L) (103 g) (106 ) Some SI and English Units In SI, the units of mass, length, and time are the kilogram (kg), meter (m), and second (s), respectively.This is a source of con- fusion and error that necessitates the use of a dimensional constant (gc) in many formulas.Another force unit in common use in many European countries is the kilogram-force (kgf), which is the weight of 1 kg mass at sea level (1 kgf  9.807 N).A control volume can be fixed in size and shape, as in the case of a noz- zle, or it may involve a moving boundary, as shown in Fig.Instead, we can concentrate our attention on the volume formed by the interior surfaces of the tank and consider the hot and cold water streams as mass leaving and entering the control volume.Extensive properties Intensive properties FIGURE 1-20 Criterion to differentiate intensive and extensive properties.In most cases, the system investigated is quite simple and obvious, and defining the system may seem like a tedious and unnecessary task.Yet it is very convenient to disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum.It is a common experience that a cup of hot coffee left on the table even- tually cools off and a cold drink eventually warms up. That is, when a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (Fig.However, it cannot be concluded from the other laws of thermodynamics, and it serves as a basis for the validity of temperature measurement.Temperature Scales Temperature scales enable us to use a common basis for temperature mea- surements, and several have been introduced throughout history.On the Celsius scale, the ice and steam points were originally assigned the values of 0 and 100?C, respec- tively.The temperature unit on this scale is the rankine, which is designated by R. A temperature scale that turns out to be nearly identical to the Kelvin scale is the ideal-gas temperature scale.That is, the temperature of a gas of fixed volume varies linearly with pressure at sufficiently low pressures.Considering that only one straight line passes through two fixed

                                      points on a plane, these two measurements are sufficient to determine the constants a and b in Eq. 1-8.The values of the constants will be different for each thermometer, depending on the type and the amount of the gas in the vessel, and the temperature values assigned at the two reference points.If the ice and steam points are assigned the values 0?C and 100?C, respec- tively, then the gas temperature scale will be identical to the Celsius scale.That is, on a P-T diagram, all the straight lines passing through the data points in this case will intersect the temperature axis at 
273.15?C when extrapolated, as shown in Fig.Although every- body has a feeling of what energy is, it is difficult to give a precise defini- tion for it. Energy can be viewed as the ability to cause changes.A rock falling off a cliff, for example, picks up speed as a result of its potential energy being converted to kinetic energy (Fig.The change in the energy content of a body or any other system is equal to the difference between the energy input and the energy output, and the energy balance is expressed as E 
 E  E.The term thermody- namics was first used in a publication by Lord Kelvin in 1849.Energy storage (1 unit)

FIGURE 1-2 Energy in (5 units) in out The first law of thermodynamics is simply an expression of the conser- Energy out (4 units) Conservation of energy principle for the human body.It would be sufficient to attach a pressure gage to the container.IMPORTANCE OF DIMENSIONS AND UNITS Any physical quantity can be characterized by dimensions.For example, the SI unit of force, which is named after Sir Isaac Newton (1647-1723), is newton (not Newton), and it is abbreviated as N. Also, the full name of a unit may be pluralized, but its abbreviation cannot.The prefixes used to express the multiples of the various units are listed in Table 1-2.They are standard for all units, and the student is encouraged to memorize them because of their widespread use (Fig.Thermodynamics FIGURE 1-6 The SI unit prefixes are used in all branches of engineering.1-14.) Technically, this means that the cereal inside the box weighs 1.00 lbf on earth and has a mass of 453.6 g (0.4536 kg).The interior sur- faces of the tank form the control surface for this case, and mass is cross- ing the control surface at two locations.Thermodynamics INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC.Continuum Matter is made up of atoms that are widely spaced in the gas phase.This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules.However, we cannot assign numerical values to temperatures based on our sensations alone.Tem- perature is also measured by using several other temperature-dependent properties.The equality of temperature is the only requirement for thermal equilibrium.The zeroth law was first formulated and labeled by R. H. Fowler in 1931.IRON 150?C COPPER 20?C IRON 60?C COPPER 60?C FIGURE 1-31 Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure.In thermodynamics, it is very desirable to have a temperature scale that is independent of the properties of any substance or substances.Then the unknown temperature T of a medium corresponding to a pressure reading P can be determined from that equation by a simple calculation.One of the most fundamental laws of nature is the conservation of energy principle.It is well-known that a substance consists of a large number of particles called molecules.For example, the pressure of a gas in a container is the result of momentum transfer between the molecules and the walls of the container.3 particles to determine the pressure in the container.This microscopic approach is rather involved and is used in this text only in the supporting role.The human comfort is closely tied to the rate of this metabolic heat rejection.The magnitudes assigned to the dimensions are called units.INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC.121/PhotoDisc; Power plant: (C) Corbis Royalty Free A number of unit systems have been developed over the years.The United States is the only industrialized country that has not yet fully converted to the metric system.Based on the notational scheme introduced in 1967, the degree symbol was officially dropped from the absolute temperature unit, and all unit names were to be written without capitalization even if they were derived from proper names (Table 1-1).However, the abbreviation of a unit was to be capitalized if the unit was derived from a proper name.For example, the length of an object can be 5 m or 5 meters, not 5 ms or 5 meter.Finally, no period is to be used in unit abbreviations unless they appear at the end of a sen- tence.TABLE 1-1 The seven fundamental (or primary) dimensions and their units in SI Chapter 1 |The respective units in the English system are the pound-mass (lbm), foot (ft), and second (s).The pound symbol lb is actually the abbreviation of libra, which was the ancient Roman unit of weight.The English retained this symbol even after the end of the Roman occupation of Britain in 410.Most control volumes, however, have fixed boundaries and thus do not involve any moving boundaries.A control volume can also involve heat and work interactions just as a closed system, in addition to mass interaction.In an engineering analysis, the system under study must be defined care- fully.In other cases, however, the system under study may be rather involved, and a proper choice of the system may greatly simplify the analysis.Properties are considered to be either intensive or extensive.Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density.Extensive properties are those whose values depend on the size--or extent--of the system.Each part will have the same value of intensive properties as the original system, but half the value of the extensive properties.Some examples of specific properties are specific volume (v  V/m) and specific total energy (e  E/m).This is the case in prac- tically all problems, except some specialized ones.A metal chair, for exam- ple, will feel much colder than a wooden one even when both are at the same temperature.The commonly used mercury-in-glass thermometer, for example, is based on the expansion of mercury with temperature.It was named the zeroth law since it should have preceded the first and the second laws of thermodynamics.17 Control volume mcV = const.INTERACTIVE TUTORIAL SEE TUTORIAL CH. 1, SEC.These are often referred to as two-point scales since temperature values are assigned at two different points.EcV = const.1-1 ?18 |

INTERACTIVE TUTORIAL
SEE TUTORIAL CH. 1, SEC. 1 ON THE DVD.
1–1 ■ THERMODYNAMICS AND ENERGY
Thermodynamics can be defined as the science of energy. Although every- body has a feeling of what energy is, it is difficult to give a precise defini- tion for it. Energy can be viewed as the ability to cause changes.
The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which is most descriptive of the early efforts to convert heat into power. Today the same name is broadly interpreted to include all aspects of energy and energy transformations, including power generation, refrigeration, and relationships among the properties of matter.
One of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. That is, energy cannot be created or destroyed. A rock falling off a cliff, for example, picks up speed as a result of its potential energy being converted to kinetic energy (Fig. 1–1). The conservation of energy principle also forms the back- bone of the diet industry: A person who has a greater energy input (food) than energy output (exercise) will gain weight (store energy in the form of fat), and a person who has a smaller energy input than output will lose weight (Fig. 1–2). The change in the energy content of a body or any other system is equal to the difference between the energy input and the energy output, and the energy balance is expressed as E
E  E.
PE = 10 units KE = 0
PE = 7 units KE = 3 units
Potential energy
Kinetic energy
FIGURE 1–1
vation of energy principle, and it asserts that energy is a thermodynamic property. The second law of thermodynamics asserts that energy has qual- ity as well as quantity, and actual processes occur in the direction of decreasing quality of energy. For example, a cup of hot coffee left on a table eventually cools, but a cup of cool coffee in the same room never gets hot by itself (Fig. 1–3). The high-temperature energy of the coffee is degraded (transformed into a less useful form at a lower temperature) once it is trans- ferred to the surrounding air.
Although the principles of thermodynamics have been in existence since the creation of the universe, thermodynamics did not emerge as a science until the construction of the first successful atmospheric steam engines in England by Thomas Savery in 1697 and Thomas Newcomen in 1712. These engines were very slow and inefficient, but they opened the way for the development of a new science.
The first and second laws of thermodynamics emerged simultaneously in the 1850s, primarily out of the works of William Rankine, Rudolph Clau- sius, and Lord Kelvin (formerly William Thomson). The term thermody- namics was first used in a publication by Lord Kelvin in 1849. The first thermodynamic textbook was written in 1859 by William Rankine, a profes- sor at the University of Glasgow.
It is well-known that a substance consists of a large number of particles called molecules. The properties of the substance naturally depend on the behavior of these particles. For example, the pressure of a gas in a container is the result of momentum transfer between the molecules and the walls of the container. However, one does not need to know the behavior of the gas
Energy cannot be created or destroyed; it can only change forms (the first law).
Energy storage (1 unit)
FIGURE 1–2
Energy in (5 units)
in out
The first law of thermodynamics is simply an expression of the conser-
Energy out (4 units)
Conservation of energy principle for the human body.

                                     Chapter 1

| 3
particles to determine the pressure in the container. It would be sufficient to attach a pressure gage to the container. This macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles is called classical thermodynamics. It provides a direct and easy way to the solution of engineering problems. A more elabo- rate approach, based on the average behavior of large groups of individual particles, is called statistical thermodynamics. This microscopic approach is rather involved and is used in this text only in the supporting role.
Application Areas of Thermodynamics
All activities in nature involve some interaction between energy and matter; thus, it is hard to imagine an area that does not relate to thermodynamics in some manner. Therefore, developing a good understanding of basic principles of thermodynamics has long been an essential part of engineering education.
Thermodynamics is commonly encountered in many engineering systems and other aspects of life, and one does not need to go very far to see some application areas of it. In fact, one does not need to go anywhere. The heart is constantly pumping blood to all parts of the human body, various energy conversions occur in trillions of body cells, and the body heat generated is constantly rejected to the environment. The human comfort is closely tied to the rate of this metabolic heat rejection. We try to control this heat transfer rate by adjusting our clothing to the environmental conditions.
Other applications of thermodynamics are right where one lives. An ordi- nary house is, in some respects, an exhibition hall filled with wonders of thermodynamics (Fig. 1–4). Many ordinary household utensils and appli- ances are designed, in whole or in part, by using the principles of thermody- namics. Some examples include the electric or gas range, the heating and air-conditioning systems, the refrigerator, the humidifier, the pressure cooker, the water heater, the shower, the iron, and even the computer and the TV. On a larger scale, thermodynamics plays a major part in the design and analysis of automotive engines, rockets, jet engines, and conventional or nuclear power plants, solar collectors, and the design of vehicles from ordi- nary cars to airplanes (Fig. 1–5). The energy-efficient home that you may be living in, for example, is designed on the basis of minimizing heat loss in winter and heat gain in summer. The size, location, and the power input of the fan of your computer is also selected after an analysis that involves thermodynamics.
1–2 ■ IMPORTANCE OF DIMENSIONS AND UNITS
Any physical quantity can be characterized by dimensions. The magnitudes assigned to the dimensions are called units. Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary or fun- damental dimensions, while others such as velocity V, energy E, and vol- ume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions.
Hot coffee 70°C
Cool environment 20°C
Heat
FIGURE 1–3
Heat flows in the direction of decreasing temperature.
Solar collectors
Shower
Hot water
Cold water
FIGURE 1–4
The design of many engineering systems, such as this solar hot water system, involves thermodynamics.
INTERACTIVE TUTORIAL
SEE TUTORIAL CH. 1, SEC. 2 ON THE DVD.
Hot water tank Heat Pump
exchanger

                                    4 |

Thermodynamics
The human body Air conditioning systems Airplanes
Car radiators Power plants Refrigeration systems
FIGURE 1–5
Some application areas of thermodynamics.
A/C unit, fridge, radiator: © The McGraw-Hill Companies, Inc./Jill Braaten, photographer; Plane: © Vol. 14/PhotoDisc; Humans: © Vol. 121/PhotoDisc; Power plant: © Corbis Royalty Free
A number of unit systems have been developed over the years. Despite strong efforts in the scientific and engineering community to unify the world with a single unit system, two sets of units are still in common use today: the English system, which is also known as the United States Cus- tomary System (USCS), and the metric SI (from Le Système International d’ Unités), which is also known as the International System. The SI is a simple and logical system based on a decimal relationship between the various units, and it is being used for scientific and engineering work in most of the industrialized nations, including England. The English system, however, has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily (12 in  1 ft, 1 mile  5280 ft, 4 qt  gal, etc.), which makes it confusing and difficult to learn. The United States is the only industrialized country that has not yet fully converted to the metric system.
The systematic efforts to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up with such a unit system. An early version of the metric system was soon developed in France, but it

                                    did not find universal acceptance until 1875 when The Metric Convention Treaty was prepared and signed by 17 nations, including the United States. In this international treaty, meter and gram were established as the metric units for length and mass, respectively, and a General Conference of Weights and Measures (CGPM) was established that was to meet every six years. In 1960, the CGPM produced the SI, which was based on six fundamental quantities, and their units were adopted in 1954 at the Tenth General Conference of Weights and Measures: meter (m) for length, kilo- gram (kg) for mass, second (s) for time, ampere (A) for electric current, degree Kelvin (°K) for temperature, and candela (cd) for luminous inten- sity (amount of light). In 1971, the CGPM added a seventh fundamental quantity and unit: mole (mol) for the amount of matter.

Based on the notational scheme introduced in 1967, the degree symbol was officially dropped from the absolute temperature unit, and all unit names were to be written without capitalization even if they were derived from proper names (Table 1–1). However, the abbreviation of a unit was to be capitalized if the unit was derived from a proper name. For example, the SI unit of force, which is named after Sir Isaac Newton (1647–1723), is newton (not Newton), and it is abbreviated as N. Also, the full name of a unit may be pluralized, but its abbreviation cannot. For example, the length of an object can be 5 m or 5 meters, not 5 ms or 5 meter. Finally, no period is to be used in unit abbreviations unless they appear at the end of a sen- tence. For example, the proper abbreviation of meter is m (not m.).
The recent move toward the metric system in the United States seems to have started in 1968 when Congress, in response to what was happening in the rest of the world, passed a Metric Study Act. Congress continued to promote a voluntary switch to the metric system by passing the Metric Conversion Act in 1975. A trade bill passed by Congress in 1988 set a September 1992 deadline for all federal agencies to convert to the metric system. However, the deadlines were relaxed later with no clear plans for the future.
The industries that are heavily involved in international trade (such as the automotive, soft drink, and liquor industries) have been quick in converting to the metric system for economic reasons (having a single worldwide design, fewer sizes, smaller inventories, etc.). Today, nearly all the cars manufactured in the United States are metric. Most car owners probably do not realize this until they try an English socket wrench on a metric bolt. Most industries, however, resisted the change, thus slowing down the conversion process.
Presently the United States is a dual-system society, and it will stay that way until the transition to the metric system is completed. This puts an extra burden on today’s engineering students, since they are expected to retain their understanding of the English system while learning, thinking, and working in terms of the SI. Given the position of the engineers in the transi- tion period, both unit systems are used in this text, with particular emphasis on SI units.
As pointed out, the SI is based on a decimal relationship between units. The prefixes used to express the multiples of the various units are listed in Table 1–2. They are standard for all units, and the student is encouraged to memorize them because of their widespread use (Fig. 1–6).
TABLE 1–1
The seven fundamental (or primary)
dimensions and their units in SI
Chapter 1
| 5
Dimension
Length
Mass
Time Temperature Electric current Amount of light Amount of matter
Unit
meter (m) kilogram (kg) second (s) kelvin (K) ampere (A) candela (cd) mole (mol)
TABLE 1–2
Standard prefixes in SI units
Multiple Prefix
1012 tera, T 109 giga, G 106 mega, M 103 kilo, k 102 hecto, h 101 deka, da 10
1 deci, d 10
2 centi, c 10
3 milli, m 10
6 micro, m 10
9 nano, n 10
12 pico, p

                                     6 | Thermodynamics

      FIGURE 1–6

The SI unit prefixes are used in all branches of engineering.
1 M (0.2 L) (103 g) (106 )
Some SI and English Units
In SI, the units of mass, length, and time are the kilogram (kg), meter (m), and second (s), respectively. The respective units in the English system are the pound-mass (lbm), foot (ft), and second (s). The pound symbol lb is actually the abbreviation of libra, which was the ancient Roman unit of weight. The English retained this symbol even after the end of the Roman occupation of Britain in 410. The mass and length units in the two systems are related to each other by
1 lbm  0.45359 kg 1 ft  0.3048 m
In the English system, force is usually considered to be one of the pri- mary dimensions and is assigned a nonderived unit. This is a source of con- fusion and error that necessitates the use of a dimensional constant (gc) in many formulas. To avoid this nuisance, we consider force to be a secondary dimension whose unit is derived from Newton’s second law, that is,
Force  1Mass2 1Acceleration2 F  ma
m = 1 kg
a = 1 m/s2
F =1N
a = 1 ft/s2
F = 1 lbf
200 mL 1 kg
m = 32.174 lbm
FIGURE 1–7
The definition of the force units.
10 apples m = 1 kg
1 apple m = 102 g
1 N
FIGURE 1–8
(1–1)
1 kgf
4 apples m = 1 lbm
1 lbf
to accelerate a mass of 1 kg at a rate of 1 m/s2. In the English system, the force unit is the pound-force (lbf) and is defined as the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2 (Fig. 1–7). That is,
1 N  1 kg # m>s2
1 lbf  32.174 lbm # ft>s2
A force of 1 N is roughly equivalent to the weight of a small apple (m  102 g), whereas a force of 1 lbf is roughly equivalent to the weight of four
 454 g), as shown in Fig. 1–8. Another force unit in common use in many European countries is the kilogram-force (kgf), which
is the weight of 1 kg mass at sea level (1 kgf  9.807 N).
The term weight is often incorrectly used to express mass, particularly by the “weight watchers.” Unlike mass, weight W is a force. It is the gravita- tional force applied to a body, and its magnitude is determined from New-
or
In SI, the force unit is the newton (N), and it is defined as the force required
The relative magnitudes of the force units newton (N), kilogram-force (kgf), and pound-force (lbf).
ton’s second law,
W  mg 1N2 (1–2)
medium apples (m
total

                                     where m is the mass of the body, and g is the local gravitational acceleration (g is 9.807 m/s2 or 32.174 ft/s2 at sea level and 45° latitude). An ordinary bathroom scale measures the gravitational force acting on a body. The weight of a unit volume of a substance is called the specific weight g and is determined from g  rg, where r is density.

The mass of a body remains the same regardless of its location in the uni- verse. Its weight, however, changes with a change in gravitational accelera- tion. A body weighs less on top of a mountain since g decreases with altitude. On the surface of the moon, an astronaut weighs about one-sixth of what she or he normally weighs on earth (Fig. 1–9).
At sea level a mass of 1 kg weighs 9.807 N, as illustrated in Fig. 1–10. A mass of 1 lbm, however, weighs 1 lbf, which misleads people to believe that pound-mass and pound-force can be used interchangeably as pound (lb), which is a major source of error in the English system.
It should be noted that the gravity force acting on a mass is due to the attraction between the masses, and thus it is proportional to the magnitudes of the masses and inversely proportional to the square of the distance between them. Therefore, the gravitational acceleration g at a location depends on the local density of the earth’s crust, the distance to the center of the earth, and to a lesser extent, the positions of the moon and the sun. The value of g varies with location from 9.8295 m/s2 at 4500 m below sea level to 7.3218 m/s2 at 100,000 m above sea level. However, at altitudes up to 30,000 m, the variation of g from the sea-level value of 9.807 m/s2 is less than 1 percent. Therefore, for most practical purposes, the gravitational acceleration can be assumed to be constant at 9.81 m/s2. It is interesting to note that at locations below sea level, the value of g increases with distance from the sea level, reaches a maximum at about 4500 m, and then starts decreasing. (What do you think the value of g is at the center of the earth?)
The primary cause of confusion between mass and weight is that mass is usually measured indirectly by measuring the gravity force it exerts. This approach also assumes that the forces exerted by other effects such as air buoyancy and fluid motion are negligible. This is like measuring the dis- tance to a star by measuring its red shift, or measuring the altitude of an air- plane by measuring barometric pressure. Both of these are also indirect measurements. The correct direct way of measuring mass is to compare it to a known mass. This is cumbersome, however, and it is mostly used for cali- bration and measuring precious metals.
Work, which is a form of energy, can simply be defined as force times dis- tance; therefore, it has the unit “newton-meter (N · m),” which is called a joule (J). That is,
1 J  1 N # m (1–3)
A more common unit for energy in SI is the kilojoule (1 kJ  103 J). In the English system, the energy unit is the Btu (British thermal unit), which is defined as the energy required to raise the temperature of 1 lbm of water at 68°F by 1°F. In the metric system, the amount of energy needed to raise the temperature of 1 g of water at 14.5°C by 1°C is defined as 1 calorie (cal), and 1 cal  4.1868 J. The magnitudes of the kilojoule and Btu are almost identical (1 Btu  1.0551 kJ).
FIGURE 1–9
A body weighing 150 lbf on earth will weigh only 25 lbf on the moon.
Chapter 1
| 7
kg
g = 9.807 m/s2
W = 9.807 kg · m/s2 = 9.807 N
= 1 kgf
FIGURE 1–10
lbm
g = 32.174 ft/s2 W = 32.174 lbm · ft/s2
= 1 lbf
The weight of a unit mass at sea level.

                                    8 |

Thermodynamics
Dimensional Homogeneity
We all know from grade school that apples and oranges do not add. But we somehow manage to do it (by mistake, of course). In engineering, all equa- tions must be dimensionally homogeneous. That is, every term in an equa- tion must have the same unit (Fig. 1–11). If, at some stage of an analysis, we find ourselves in a position to add two quantities that have different units, it is a clear indication that we have made an error at an earlier stage. So checking dimensions can serve as a valuable tool to spot errors.
EXAMPLE 1–1 Spotting Errors from Unit Inconsistencies
While solving a problem, a person ended up with the following equation at
some stage:
E  25 kJ  7 kJ>kg
where E is the total energy and has the unit of kilojoules. Determine how to
correct the error and discuss what may have caused it.
Solution During an analysis, a relation with inconsistent units is obtained. A correction is to be found, and the probable cause of the error is to be determined.
Analysis The two terms on the right-hand side do not have the same units, and therefore they cannot be added to obtain the total energy. Multiplying the last term by mass will eliminate the kilograms in the denominator, and the whole equation will become dimensionally homogeneous; that is, every term in the equation will have the same unit.
Discussion Obviously this error was caused by forgetting to multiply the last term by mass at an earlier stage.
FIGURE 1–11
To be dimensionally homogeneous, all the terms in an equation must have the same unit.
© Reprinted with special permission of King Features Syndicate.
We all know from experience that units can give terrible headaches if they are not used carefully in solving a problem. However, with some attention and skill, units can be used to our advantage. They can be used to check for- mulas; they can even be used to derive formulas, as explained in the follow- ing example.
EXAMPLE 1–2 Obtaining Formulas from Unit Considerations
A tank is filled with oil whose density is r  850 kg/m3. If the volume of the
tank is V  2 m3, determine the amount of mass m in the tank.
Solution The volume of an oil tank is given. The mass of oil is to be deter- mined.
Assumptions Oil is an incompressible substance and thus its density is con- stant.
Analysis A sketch of the system just described is given in Fig. 1–12. Sup- pose we forgot the formula that relates mass to density and volume. However, we know that mass has the unit of kilograms. That is, whatever calculations we do, we should end up with the unit of kilograms. Putting the given infor- mation into perspective, we have
r  850 kg>m3 and V  2 m3
OIL
ρ = 850 kg/m3
FIGURE 1–12
Schematic for Example 1–2.
V = 2 m3 m=?

                                    s2

s2
They can also be expressed more conveniently as unity conversion ratios as N lbf
#2 #2
kg m>s  1 and 32.174 lbm ft>s  1
Unity conversion ratios are identically equal to 1 and are unitless, and thus
such ratios (or their inverses) can be inserted conveniently into any calcula-
tion to properly convert units. Students are encouraged to always use unity
conversion ratios such as those given here when converting units. Some
textbooks insert the archaic gravitational constant g defined as g  32.174 cc
lbm · ft/lbf · s2  kg · m/N · s2  1 into equations in order to force units to match. This practice leads to unnecessary confusion and is strongly discour- aged by the present authors. We recommend that students instead use unity conversion ratios.
Chapter 1
| 9
It is obvious that we can eliminate m3 and end up with kg by multiplying these two quantities. Therefore, the formula we are looking for should be
Thus,
m  rV
m  1850 kg>m3 2 12 m3 2  1700 kg
Discussion Note that this approach may not work for more complicated formulas.
You should keep in mind that a formula that is not dimensionally homo- geneous is definitely wrong, but a dimensionally homogeneous formula is not necessarily right.
Unity Conversion Ratios
Just as all nonprimary dimensions can be formed by suitable combinations of primary dimensions, all nonprimary units (secondary units) can be formed by combinations of primary units. Force units, for example, can be expressed as
N  kg m and lbf  32.174 lbm ft
EXAMPLE 1–3 The Weight of One Pound-Mass
Using unity conversion ratios, show that 1.00 lbm weighs 1.00 lbf on earth
(Fig. 1–13).
Solution A mass of 1.00 lbm is subjected to standard earth gravity. Its weight in lbf is to be determined.
Assumptions Standard sea-level conditions are assumed.
Properties The gravitational constant is g  32.174 ft/s2.
lbm
FIGURE 1–13
A mass of 1 lbm weighs 1 lbf on earth.

                                      10

| Thermodynamics
Net weight:
One pound
(454 grams)
Analysis We apply Newton’s second law to calculate the weight (force) that corresponds to the known mass and acceleration. The weight of any object is equal to its mass times the local value of gravitational acceleration. Thus,
W  mg  11.00 lbm2 132.174 ft>s22 a 1 lbf b  1.00 lbf 32.174 lbm # ft>s2
Discussion Mass is the same regardless of its location. However, on some other planet with a different value of gravitational acceleration, the weight of 1 lbm would differ from that calculated here.
When you buy a box of breakfast cereal, the printing may say “Net weight: One pound (454 grams).” (See Fig. 1–14.) Technically, this means that the cereal inside the box weighs 1.00 lbf on earth and has a mass of 453.6 g (0.4536 kg). Using Newton’s second law, the actual weight of the cereal in the metric system is
W  mg  1453.6 g2 19.81 m>s22 a 1 N b a 1 kg b  4.45 N 1 kg #m>s2 1000 g
1–3 ■ SYSTEMS AND CONTROL VOLUMES
A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surround- ings is called the boundary. These terms are illustrated in Fig. 1–15. The boundary of a system can be fixed or movable. Note that the boundary is the contact surface shared by both the system and the surroundings. Mathemati- cally speaking, the boundary has zero thickness, and thus it can neither con- tain any mass nor occupy any volume in space.
Systems may be considered to be closed or open, depending on whether a fixed mass or a fixed volume in space is chosen for study. A closed system (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass can enter or leave a closed system, as shown in Fig. 1–16. But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed. If, as a special case, even energy is not allowed to cross the boundary, that system is called an isolated system.
Consider the piston-cylinder device shown in Fig. 1–17. Let us say that we would like to find out what happens to the enclosed gas when it is heated. Since we are focusing our attention on the gas, it is our system. The inner surfaces of the piston and the cylinder form the boundary, and since no mass is crossing this boundary, it is a closed system. Notice that energy may cross the boundary, and part of the boundary (the inner surface of the piston, in this case) may move. Everything outside the gas, including the piston and the cylinder, is the surroundings.
An open system, or a control volume, as it is often called, is a prop- erly selected region in space. It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Flow through these
FIGURE 1–14
A quirk in the metric system of units.
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SURROUNDINGS
SYSTEM
BOUNDARY
FIGURE 1–15
System, surroundings, and boundary.
CLOSED SYSTEM
m = constant
FIGURE 1–16
Mass NO
Energy YES
Mass cannot cross the boundaries of a closed system, but energy can.

                                       devices is best studied by selecting the region within the device as the control volume. Both mass and energy can cross the boundary of a con- trol volume.

A large number of engineering problems involve mass flow in and out of
a system and, therefore, are modeled as control volumes. A water heater, a
car radiator, a turbine, and a compressor all involve mass flow and should GAS be analyzed as control volumes (open systems) instead of as control masses 2 kg (closed systems). In general, any arbitrary region in space can be selected 1 m3 as a control volume. There are no concrete rules for the selection of control
volumes, but the proper choice certainly makes the analysis much easier. If
we were to analyze the flow of air through a nozzle, for example, a good
choice for the control volume would be the region within the nozzle.
The boundaries of a control volume are called a control surface, and they can be real or imaginary. In the case of a nozzle, the inner surface of the noz- zle forms the real part of the boundary, and the entrance and exit areas form the imaginary part, since there are no physical surfaces there (Fig. 1–18a).
A control volume can be fixed in size and shape, as in the case of a noz- zle, or it may involve a moving boundary, as shown in Fig. 1–18b. Most control volumes, however, have fixed boundaries and thus do not involve any moving boundaries. A control volume can also involve heat and work interactions just as a closed system, in addition to mass interaction.
As an example of an open system, consider the water heater shown in Fig. 1–19. Let us say that we would like to determine how much heat we must transfer to the water in the tank in order to supply a steady stream of hot water. Since hot water will leave the tank and be replaced by cold water, it is not convenient to choose a fixed mass as our system for the analysis. Instead, we can concentrate our attention on the volume formed by the interior surfaces of the tank and consider the hot and cold water streams as mass leaving and entering the control volume. The interior sur- faces of the tank form the control surface for this case, and mass is cross- ing the control surface at two locations.
FIGURE 1–17
Chapter 1
| 11
Moving boundary
A closed system with a moving boundary.
GAS 2 kg 3 m3
Fixed boundary
WATER HEATER
(control volume)
Control surface
Hot water out
Imaginary boundary
Real boundary
CV
(a nozzle)
Cold water in
(a) A control volume with real and imaginary boundaries
FIGURE 1–18
Moving boundary
CV
Fixed boundary
(b) A control volume with fixed and moving boundaries
A control volume can involve fixed, moving, real, and imaginary boundaries.
An open system (a control volume) with one inlet and one exit.
FIGURE 1–19

                                    12 |

Thermodynamics
INTERACTIVE TUTORIAL
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m
V
T P ρ
1
–2 m
1
–2 V
T P ρ
1
–2 m
1
–2 V
T P ρ
Extensive properties
Intensive properties
FIGURE 1–20
Criterion to differentiate intensive and extensive properties.
In an engineering analysis, the system under study must be defined care- fully. In most cases, the system investigated is quite simple and obvious, and defining the system may seem like a tedious and unnecessary task. In other cases, however, the system under study may be rather involved, and a proper choice of the system may greatly simplify the analysis.
1–4 ■ PROPERTIES OF A SYSTEM
Any characteristic of a system is called a property. Some familiar proper- ties are pressure P, temperature T, volume V, and mass m. The list can be extended to include less familiar ones such as viscosity, thermal conductiv- ity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation.
Properties are considered to be either intensive or extensive. Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density. Extensive properties are those whose values depend on the size—or extent—of the system. Total mass, total vol- ume, and total momentum are some examples of extensive properties. An easy way to determine whether a property is intensive or extensive is to divide the system into two equal parts with an imaginary partition, as shown in Fig. 1–20. Each part will have the same value of intensive properties as the original system, but half the value of the extensive properties.
Generally, uppercase letters are used to denote extensive properties (with mass m being a major exception), and lowercase letters are used for intensive properties (with pressure P and temperature T being the obvious exceptions).
Extensive properties per unit mass are called specific properties. Some examples of specific properties are specific volume (v  V/m) and specific total energy (e  E/m).
Continuum
Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum. The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinu- ities. This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules. This is the case in prac- tically all problems, except some specialized ones. The continuum idealiza- tion is implicit in many statements we make, such as “the density of water in a glass is the same at any point.”
To have a sense of the distance involved at the molecular level, consider a container filled with oxygen at atmospheric conditions. The diameter of the oxygen molecule is about 3  10
10 m and its mass is 5.3  10
26 kg. Also, the mean free path of oxygen at 1 atm pressure and 20°C is 6.3  10
8 m. That is, an oxygen molecule travels, on average, a distance of 6.3  10
8 m (about 200 times of its diameter) before it collides with another molecule.
Also, there are about 3  1016 molecules of oxygen in the tiny volume of 1 mm3 at 1 atm pressure and 20°C (Fig. 1–21). The continuum model is applicable as long as the characteristic length of the system (such as its
O2
1 atm, 20°C
3 × 1016 molecules/mm3
VOID
FIGURE 1–21
Despite the large gaps between molecules, a substance can be treated as a continuum because of the very large number of molecules even in an extremely small volume.

                                    diameter) is much larger than the mean free path of the molecules. At very high vacuums or very high elevations, the mean free path may become large (for example, it is about 0.1 m for atmospheric air at an elevation of 100 km). For such cases the rarefied gas flow theory should be used, and the impact of individual molecules should be considered. In this text we will limit our consideration to substances that can be modeled as a continuum.

1–5 ■ DENSITY AND SPECIFIC GRAVITY Density is defined as mass per unit volume (Fig. 1–22).
Density: r
m 1kg>m3 2 (1–4) V
The reciprocal of density is the specific volume v, which is defined as vol- ume per unit mass. That is,
v
V
1 (1–5) mr
For a differential volume element of mass dm and volume dV, density can be expressed as r
dm/dV.
The density of a substance, in general, depends on temperature and pres- sure. The density of most gases is proportional to pressure and inversely proportional to temperature. Liquids and solids, on the other hand, are essentially incompressible substances, and the variation of their density with pressure is usually negligible. At 20°C, for example, the density of water changes from 998 kg/m3 at 1 atm to 1003 kg/m3 at 100 atm, a change of just 0.5 percent. The density of liquids and solids depends more strongly on temperature than it does on pressure. At 1 atm, for example, the density of water changes from 998 kg/m3 at 20°C to 975 kg/m3 at 75°C, a change of 2.3 percent, which can still be neglected in many engi- neering analyses.
Sometimes the density of a substance is given relative to the density of a well-known substance. Then it is called specific gravity, or relative den- sity, and is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C, for which r
1000 kg/m3). That is,
Use actual data from the experiment shown here to obtain the density of water in the neighborhood of 4°C. See end-of-chapter problem 1–129.
© Ronald Mullisen
Chapter 1
| 13
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EXPERIMENT
H2O
Specific gravity: SG

r
rH2O
(1–6)
Note that the specific gravity of a substance is a dimensionless quantity. However, in SI units, the numerical value of the specific gravity of a sub- stance is exactly equal to its density in g/cm3 or kg/L (or 0.001 times the density in kg/m3) since the density of water at 4°C is 1 g/cm3
1 kg/L
1000 kg/m3. The specific gravity of mercury at 0°C, for example, is 13.6. Therefore, its density at 0°C is 13.6 g/cm3
13.6 kg/L
13,600 kg/m3. The specific gravities of some substances at 0°C are given in Table 1–3. Note that substances with specific gravities less than 1 are lighter than water, and thus they would float on water.
FIGURE 1–22
Density is mass per unit volume; specific volume is volume per unit mass.
V = 12 m3 m=3kg
ρ = 0.25 kg/m 3
v = –1 = 4 m3/kg ρ

                                      14 | Thermodynamics TABLE 1–3

Specific gravities of some
substances at 0°C
Substance SG
Water 1.0 Blood 1.05 Seawater 1.025 Gasoline 0.7 Ethyl alcohol 0.79 Mercury 13.6 Wood 0.3–0.9 Gold 19.2 Bones 1.7–2.0 Ice 0.92 Air (at 1 atm) 0.0013
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The weight of a unit volume of a substance is called specific weight and is expressed as
Specificweight:
g rg s
1N>m32
(1–7)
where g is the gravitational acceleration.
The densities of liquids are essentially constant, and thus they can often
be approximated as being incompressible substances during most processes without sacrificing much in accuracy.
m = 2 kg T1 = 20°C V1 = 1.5 m3
(a) State 1
FIGURE 1–23
m = 2 kg T2 = 20°C
3 V2 = 2.5 m
(b) State 2
Consider a system not undergoing any change. At this point, all the proper- ties can be measured or calculated throughout the entire system, which gives us a set of properties that completely describes the condition, or the state, of the system. At a given state, all the properties of a system have fixed values. If the value of even one property changes, the state will change to a different one. In Fig. 1–23 a system is shown at two different states.
Thermodynamics deals with equilibrium states. The word equilibrium implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings.
There are many types of equilibrium, and a system is not in thermody- namic equilibrium unless the conditions of all the relevant types of equilib- rium are satisfied. For example, a system is in thermal equilibrium if the temperature is the same throughout the entire system, as shown in Fig. 1–24. That is, the system involves no temperature differential, which is the driving force for heat flow. Mechanical equilibrium is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time. However, the pressure may vary within the system with elevation as a result of gravitational effects. For example, the higher pressure at a bottom layer is balanced by the extra weight it must carry, and, therefore, there is no imbalance of forces. The variation of pres- sure as a result of gravity in most thermodynamic systems is relatively small and usually disregarded. If a system involves two phases, it is in phase equilibrium when the mass of each phase reaches an equilibrium level and stays there. Finally, a system is in chemical equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur. A system will not be in equilibrium unless all the relevant equilibrium crite- ria are satisfied.
The State Postulate
As noted earlier, the state of a system is described by its properties. But we know from experience that we do not need to specify all the properties in order to fix a state. Once a sufficient number of properties are specified, the rest of the properties assume certain values automatically. That is, specifying a certain number of properties is sufficient to fix a state. The number of prop- erties required to fix the state of a system is given by the state postulate:
The state of a simple compressible system is completely specified by two independent, intensive properties.
A system at two different states.
(a) Before FIGURE 1–24
(b) After
1–6 ■
STATE AND EQUILIBRIUM
°°
20 C
30 C
35 C 40 C
°
°°
°
42 C
23 C
A closed system reaching thermal equilibrium.
°°
32 C
32 C
°
°
°°
32 C
32 C
32 C
32 C

                                        A system is called a simple compressible system in the absence of elec- trical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineer- ing problems. Otherwise, an additional property needs to be specified for each effect that is significant. If the gravitational effects are to be consid- ered, for example, the elevation z needs to be specified in addition to the two properties necessary to fix the state.

The state postulate requires that the two properties specified be indepen- dent to fix the state. Two properties are independent if one property can be varied while the other one is held constant. Temperature and specific vol- ume, for example, are always independent properties, and together they can fix the state of a simple compressible system (Fig. 1–25). Temperature and pressure, however, are independent properties for single-phase systems, but are dependent properties for multiphase systems. At sea level (P  1 atm), water boils at 100°C, but on a mountaintop where the pressure is lower, water boils at a lower temperature. That is, T  f(P) during a phase-change process; thus, temperature and pressure are not sufficient to fix the state of a two-phase system. Phase-change processes are discussed in detail in Chap. 3.
1–7 ■ PROCESSES AND CYCLES
Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a sys- tem passes during a process is called the path of the process (Fig. 1–26). To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings.
When a process proceeds in such a manner that the system remains infin- itesimally close to an equilibrium state at all times, it is called a quasi- static, or quasi-equilibrium, process. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts.
This is illustrated in Fig. 1–27. When a gas in a piston-cylinder device is compressed suddenly, the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston, thus creating a high-pressure region there. Because of this pressure difference, the system can no longer be said to be in equilib- rium, and this makes the entire process nonquasi-equilibrium. However, if the piston is moved slowly, the molecules will have sufficient time to redis- tribute and there will not be a molecule pileup in front of the piston. As a result, the pressure inside the cylinder will always be nearly uniform and will rise at the same rate at all locations. Since equilibrium is maintained at all times, this is a quasi-equilibrium process.
It should be pointed out that a quasi-equilibrium process is an idealized process and is not a true representation of an actual process. But many actual processes closely approximate it, and they can be modeled as quasi- equilibrium with negligible error. Engineers are interested in quasiequilib- rium processes for two reasons. First, they are easy to analyze; second,
Nitrogen
°
The state of nitrogen is fixed by two independent, intensive properties.
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FIGURE 1–25
Chapter 1
| 15
T = 25 C
v = 0.9 m3/kg
Property A
State 1
FIGURE 1–26
State 2
Process path
A process between states 1 and 2 and the process path.
(a) Slow compression (quasi-equilibrium)
(b) Very fast compression (nonquasi-equilibrium)
FIGURE 1–27
Quasi-equilibrium and nonquasi- equilibrium compression processes.
Property B

                                     16 |

Thermodynamics
P
Final state 2
Process path
Initial state
1
V2 V1
(2) (1)
V
System
FIGURE 1–28
The P-V diagram of a compression process.
work-producing devices deliver the most work when they operate on quasi- equilibrium processes. Therefore, quasi-equilibrium processes serve as stan- dards to which actual processes can be compared.
Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes. Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v). Figure 1–28 shows the P-V diagram of a compression process of a gas.
Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasi- equilibrium processes only. For nonquasi-equilibrium processes, we are not able to characterize the entire system by a single state, and thus we cannot speak of a process path for a system as a whole. A nonquasi-equilibrium process is denoted by a dashed line between the initial and final states instead of a solid line.
The prefix iso- is often used to designate a process for which a particular property remains constant. An isothermal process, for example, is a process during which the temperature T remains constant; an isobaric process is a process during which the pressure P remains constant; and an isochoric (or isometric) process is a process during which the specific vol- ume v remains constant.
A system is said to have undergone a cycle if it returns to its initial state at the end of the process. That is, for a cycle the initial and final states are identical.
The Steady-Flow Process
The terms steady and uniform are used frequently in engineering, and thus it is important to have a clear understanding of their meanings. The term steady implies no change with time. The opposite of steady is unsteady, or transient. The term uniform, however, implies no change with location over a specified region. These meanings are consistent with their everyday use (steady girlfriend, uniform properties, etc.).
A large number of engineering devices operate for long periods of time under the same conditions, and they are classified as steady-flow devices. Processes involving such devices can be represented reasonably well by a somewhat idealized process, called the steady-flow process, which can be defined as a process during which a fluid flows through a control volume steadily (Fig. 1–29). That is, the fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process. Therefore, the volume V, the mass m, and the total energy content E of the control volume remain constant during a steady- flow process (Fig. 1–30).
Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, con- densers, and heat exchangers or power plants or refrigeration systems. Some cyclic devices, such as reciprocating engines or compressors, do not satisfy any of the conditions stated above since the flow at the inlets and the exits will be pulsating and not steady. However, the fluid properties vary with
Mass in
Mass in
FIGURE 1–29
Time: 1 PM
Time: 3 PM
Mass out
Mass out
300°C 250°C
Control volume 225°C
200°C
150°C
300°C 250°C
Control volume 225°C
200°C
150°C
During a steady-flow process, fluid properties within the control volume may change with position but not with time.

                                    time in a periodic manner, and the flow through these devices can still be analyzed as a steady-flow process by using time-averaged values for the properties.

1–8 ■ TEMPERATURE AND THE ZEROTH LAW OF THERMODYNAMICS
Although we are familiar with temperature as a measure of “hotness” or “coldness,” it is not easy to give an exact definition for it. Based on our physiological sensations, we express the level of temperature qualitatively with words like freezing cold, cold, warm, hot, and red-hot. However, we cannot assign numerical values to temperatures based on our sensations alone. Furthermore, our senses may be misleading. A metal chair, for exam- ple, will feel much colder than a wooden one even when both are at the same temperature.
Fortunately, several properties of materials change with temperature in a repeatable and predictable way, and this forms the basis for accurate tem- perature measurement. The commonly used mercury-in-glass thermometer, for example, is based on the expansion of mercury with temperature. Tem- perature is also measured by using several other temperature-dependent properties.
It is a common experience that a cup of hot coffee left on the table even- tually cools off and a cold drink eventually warms up. That is, when a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (Fig. 1–31). At that point, the heat transfer stops, and the two bodies are said to have reached thermal equilibrium. The equality of temperature is the only requirement for thermal equilibrium.
The zeroth law of thermodynamics states that if two bodies are in ther- mal equilibrium with a third body, they are also in thermal equilibrium with each other. It may seem silly that such an obvious fact is called one of the basic laws of thermodynamics. However, it cannot be concluded from the other laws of thermodynamics, and it serves as a basis for the validity of temperature measurement. By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.
The zeroth law was first formulated and labeled by R. H. Fowler in 1931. As the name suggests, its value as a fundamental physical principle was rec- ognized more than half a century after the formulation of the first and the second laws of thermodynamics. It was named the zeroth law since it should have preceded the first and the second laws of thermodynamics.
Temperature Scales
Temperature scales enable us to use a common basis for temperature mea- surements, and several have been introduced throughout history. All temper- ature scales are based on some easily reproducible states such as the freezing and boiling points of water, which are also called the ice point and
Mass in
FIGURE 1–30
Mass out
Chapter 1
| 17
Control volume
mcV = const. EcV = const.
Under steady-flow conditions, the mass and energy contents of a control volume remain constant.
INTERACTIVE TUTORIAL
SEE TUTORIAL CH. 1, SEC. 8 ON THE DVD.
IRON 150°C
COPPER 20°C
IRON 60°C
COPPER 60°C
FIGURE 1–31
Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure.

                                    18 |

Thermodynamics
the steam point, respectively. A mixture of ice and water that is in equilib- rium with air saturated with vapor at 1 atm pressure is said to be at the ice point, and a mixture of liquid water and water vapor (with no air) in equilib- rium at 1 atm pressure is said to be at the steam point.
The temperature scales used in the SI and in the English system today are the Celsius scale (formerly called the centigrade scale; in 1948 it was renamed after the Swedish astronomer A. Celsius, 1702–1744, who devised it) and the Fahrenheit scale (named after the German instrument maker G. Fahrenheit, 1686–1736), respectively. On the Celsius scale, the ice and steam points were originally assigned the values of 0 and 100°C, respec- tively. The corresponding values on the Fahrenheit scale are 32 and 212°F. These are often referred to as two-point scales since temperature values are assigned at two different points.
In thermodynamics, it is very desirable to have a temperature scale that is independent of the properties of any substance or substances. Such a temperature scale is called a thermodynamic temperature scale, which is developed later in conjunction with the second law of thermodynamics. The thermodynamic temperature scale in the SI is the Kelvin scale, named after Lord Kelvin (1824–1907). The temperature unit on this scale is the kelvin, which is designated by K (not °K; the degree symbol was officially dropped from kelvin in 1967). The lowest temperature on the Kelvin scale is absolute zero, or 0 K. Then it follows that only one nonzero reference point needs to be assigned to establish the slope of this linear scale. Using nonconventional refrigeration techniques, scientists have approached absolute zero kelvin (they achieved 0.000000002 K in 1989).
The thermodynamic temperature scale in the English system is the Ran- kine scale, named after William Rankine (1820–1872). The temperature unit on this scale is the rankine, which is designated by R.
A temperature scale that turns out to be nearly identical to the Kelvin scale is the ideal-gas temperature scale. The temperatures on this scale are measured using a constant-volume gas thermometer, which is basically a rigid vessel filled with a gas, usually hydrogen or helium, at low pressure. This thermometer is based on the principle that at low pressures, the tem- perature of a gas is proportional to its pressure at constant volume. That is, the temperature of a gas of fixed volume varies linearly with pressure at sufficiently low pressures. Then the relationship between the temperature and the pressure of the gas in the vessel can be expressed as
T  a  bP (1–8)
where the values of the constants a and b for a gas thermometer are deter- mined experimentally. Once a and b are known, the temperature of a medium can be calculated from this relation by immersing the rigid vessel of the gas thermometer into the medium and measuring the gas pressure when thermal equilibrium is established between the medium and the gas in the vessel whose volume is held constant.
An ideal-gas temperature scale can be developed by measuring the pres- sures of the gas in the vessel at two reproducible points (such as the ice and the steam points) and assigning suitable values to temperatures at those two points. Considering that only one straight line passes through two fixed

                                      points on a plane, these two measurements are sufficient to determine the constants a and b in Eq. 1–8. Then the unknown temperature T of a medium corresponding to a pressure reading P can be determined from that equation by a simple calculation. The values of the constants will be different for each thermometer, depending on the type and the amount of the gas in the vessel, and the temperature values assigned at the two reference points. If the ice and steam points are assigned the values 0°C and 100°C, respec- tively, then the gas temperature scale will be identical to the Celsius scale. In this case the value of the constant a (which corresponds to an absolute pressure of zero) is determined to be 
273.15°C regardless of the type and the amount of the gas in the vessel of the gas thermometer. That is, on a P-T diagram, all the straight lines passing through the data points in this case will intersect the temperature axis at 
273.15°C when extrapolated, as shown in Fig. 1–32. This is the lowest temperature that can be obtained by a gas thermometer, and thus we can obtain an absolute gas temperature scale by assigning a value of zero to the constant a in Eq. 1–8. In that case Eq. 1–8 reduces to T  bP, and thus we need to specify the temperature at only one point to define an absolute gas temperature scale.

It should be noted that the absolute gas temperature scale is not a thermo- dynamic temperature scale, since it cannot be used at very low temperatures (due to condensation) and at very high temperatures (due to dissociation and ionization). However, absolute gas temperature is identical to the thermody- namic temperature in the temperature range in which the gas thermometer can be used, and thus we can view the thermodynamic temperature scale at this point as an absolute gas temperature scale that utilizes an “ideal” or “imaginary” gas that always acts as a low-pressure gas regardless of the temperature. If such a gas thermometer existed, it would read zero kelvin at absolute zero pressure, which corresponds to
273.15°C on the Celsius scale (Fig. 1–33).
P
Measured data points
Extrapolation
–273.15
FIGURE 1–32
0
Gas A
Gas B
Gas C Gas D
T (°C)
Chapter 1
|
19
P versus T plots of the experimental data obtained from a constant-volume gas thermometer using four different gases at different (but low) pressures.
T (°C) 200
FIGURE 1–33
T (K)
75 50
P (kPa)
120 80
225 250
40 275 0 0
The Kelvin scale is related to the Celsius scale by
T1K2  T1°C2  273.15
The Rankine scale is related to the Fahrenheit scale by
T1R2  T1°F2  459.67
(1–9)
(1–10)
25
–273.15 0 0
Absolute vacuum
V = constant
A constant-volume gas thermometer would read
273.15°C at absolute zero pressure.
It is common practice to round the constant in Eq. 1–9 to 273 and that in Eq. 1–10 to 460.
The temperature scales in the two unit systems are related by
T1R2  1.8T1K2 (1–11) T1°F2  1.8T1°C2  32 (1–12)
A comparison of various temperature scales is given in Fig. 1–34.
The reference temperature chosen in the original Kelvin scale was 273.15 K (or 0°C), which is the temperature at which water freezes (or ice melts) and water exists as a solid–liquid mixture in equilibrium under stan- dard atmospheric pressure (the ice point). At the Tenth General Conference on Weights and Measures in 1954, the reference point was changed to a much more precisely reproducible point, the triple point of water (the state at which all three phases of water coexist in equilibrium), which is