1.3 Describing Systems and Their Behavior as noted in Chap.3,the microscopic approach is instrumental in developing certain data,for example ideal gas specific heats. For a wide range of engineering applications,classical thermodynamics not only provides a considerably more direct approach for analysis and design but also requires far fewer mathematical complications.For these reasons the macroscopic viewpoint is the one adopted in this book.Finally,relativity effects are not significant for the systems under consideration in this book. 1.3.2 Property,State,and Process To describe a system and predict its behavior requires knowledge of its properties and how those properties are related.A property is a macroscopic characteristic of a property system such as mass,volume,energy,pressure,and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior (history)of the system. The word state refers to the condition of a system as described by its properties. state Since there are normally relations among the properties of a system,the state often can be specified by providing the values of a subset of the properties.All other prop- erties can be determined in terms of these few. When any of the properties of a system changes,the state changes and the system is said to undergo a process.A process is a transformation from one state to another. process If a system exhibits the same values of its properties at two different times,it is in the same state at these times.A system is said to be at steady state if none of its steady state properties changes with time. Many properties are considered during the course of our study of engineering thermodynamics.Thermodynamics also deals with quantities that are not properties, such as mass flow rates and energy transfers by work and heat.Additional examples of quantities that are not properties are provided in subsequent chapters.For a way Prop_State_Process to distinguish properties from nonproperties,see the box on p.10. A.2-Tab a 1.3.3 Extensive and Intensive Properties Thermodynamic properties can be placed in two general classes:extensive and inten- sive.A property is called extensive if its value for an overall system is the sum of its extensive property values for the parts into which the system is divided.Mass,volume,energy,and sev- eral other properties introduced later are extensive.Extensive properties depend on the size or extent of a system.The extensive properties of a system can change with time,and many thermodynamic analyses consist mainly of carefully accounting for changes in extensive properties such as mass and energy as a system interacts with its surroundings. Intensive properties are not additive in the sense previously considered.Their val- intensive property ues are independent of the size or extent of a system and may vary from place to place within the system at any moment.Intensive properties may be functions of both position and time,whereas extensive properties can vary only with time.Specific volume(Sec.1.5),pressure,and temperature are important intensive properties;sev- eral other intensive properties are introduced in subsequent chapters. FREXAMPLE to illustrate the difference between extensive and intensive prop- erties,consider an amount of matter that is uniform in temperature,and imagine that it is composed of several parts,as illustrated in Fig.1.6.The mass of the whole is the sum of the masses of the parts,and the overall volume is the sum of the volumes of the parts.However,the temperature of the whole is not the sum of the temperatures of the parts;it is the same for each part.Mass and volume are extensive,but tem- Ext Int Properties perature is intensive.bbbb A.3-Tab a
1.3 Describing Systems and Their Behavior 9 as noted in Chap. 3, the microscopic approach is instrumental in developing certain data, for example ideal gas specific heats. For a wide range of engineering applications, classical thermodynamics not only provides a considerably more direct approach for analysis and design but also requires far fewer mathematical complications. For these reasons the macroscopic viewpoint is the one adopted in this book. Finally, relativity effects are not significant for the systems under consideration in this book. 1.3.2 Property, State, and Process To describe a system and predict its behavior requires knowledge of its properties and how those properties are related. A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure, and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior (history) of the system. The word state refers to the condition of a system as described by its properties. Since there are normally relations among the properties of a system, the state often can be specified by providing the values of a subset of the properties. All other properties can be determined in terms of these few. When any of the properties of a system changes, the state changes and the system is said to undergo a process. A process is a transformation from one state to another. If a system exhibits the same values of its properties at two different times, it is in the same state at these times. A system is said to be at steady state if none of its properties changes with time. Many properties are considered during the course of our study of engineering thermodynamics. Thermodynamics also deals with quantities that are not properties, such as mass flow rates and energy transfers by work and heat. Additional examples of quantities that are not properties are provided in subsequent chapters. For a way to distinguish properties from nonproperties, see the box on p. 10. 1.3.3 Extensive and Intensive Properties Thermodynamic properties can be placed in two general classes: extensive and intensive. A property is called extensive if its value for an overall system is the sum of its values for the parts into which the system is divided. Mass, volume, energy, and several other properties introduced later are extensive. Extensive properties depend on the size or extent of a system. The extensive properties of a system can change with time, and many thermodynamic analyses consist mainly of carefully accounting for changes in extensive properties such as mass and energy as a system interacts with its surroundings. Intensive properties are not additive in the sense previously considered. Their values are independent of the size or extent of a system and may vary from place to place within the system at any moment. Intensive properties may be functions of both position and time, whereas extensive properties can vary only with time. Specific volume (Sec. 1.5), pressure, and temperature are important intensive properties; several other intensive properties are introduced in subsequent chapters. property state process steady state extensive property intensive property Prop_State_Process A.2 – Tab a Ext_Int_Properties A.3 – Tab a to illustrate the difference between extensive and intensive properties, consider an amount of matter that is uniform in temperature, and imagine that it is composed of several parts, as illustrated in Fig. 1.6. The mass of the whole is the sum of the masses of the parts, and the overall volume is the sum of the volumes of the parts. However, the temperature of the whole is not the sum of the temperatures of the parts; it is the same for each part. Mass and volume are extensive, but temperature is intensive. b b b b b
Chapter 1 Getting Started Fig..Figure used to discuss the extensive and intensive property concepts. (a) (b) Distinguishing Properties from Nonproperties At a given state,each property has a definite value that can be assigned without knowledge of how the system arrived at that state.The change in value of a property as the system is altered from one state to another is determined,therefore,solely by the two end states and is independent of the particular way the change of state occurred.The change is independent of the details of the process.Conversely,if the value of a quantity is independent of the process between two states,then that quan- tity is the change in a property.This provides a test for determining whether a quan- tity is a property:A quantity is a property if,and only if,its change in value between two states is independent of the process.It follows that if the value of a particular quantity depends on the details of the process,and not solely on the end states,that quantity cannot be a property. 1.3.4 Equilibrium Classical thermodynamics places primary emphasis on equilibrium states and changes equilibrium from one equilibrium state to another.Thus,the concept of equilibrium is fundamen- tal.In mechanics,equilibrium means a condition of balance maintained by an equal- ity of opposing forces.In thermodynamics,the concept is more far-reaching,including not only a balance of forces but also a balance of other influences.Each kind of influence refers to a particular aspect of thermodynamic,or complete,equilibrium. Accordingly,several types of equilibrium must exist individually to fulfill the condi- tion of complete equilibrium;among these are mechanical,thermal,phase,and chem- ical equilibrium. Criteria for these four types of equilibrium are considered in subsequent discus- sions.For the present,we may think of testing to see if a system is in thermodynamic equilibrium by the following procedure:Isolate the system from its surroundings and watch for changes in its observable properties.If there are no changes,we conclude that the system was in equilibrium at the moment it was isolated.The system can be equilibrium state said to be at an equilibrium state. When a system is isolated,it does not interact with its surroundings;however,its state can change as a consequence of spontaneous events occurring internally as its intensive properties,such as temperature and pressure,tend toward uniform values. When all such changes cease,the system is in equilibrium.At equilibrium,tempera- ture is uniform throughout the system.Also,pressure can be regarded as uniform throughout as long as the effect of gravity is not significant;otherwise,a pressure variation can exist,as in a vertical column of liquid. It is not necessary that a system undergoing a process be in equilibrium during the process.Some or all of the intervening states may be nonequilibrium states.For many such processes,we are limited to knowing the state before the process occurs and the state after the process is completed
10 Chapter 1 Getting Started 1.3.4 Equilibrium Classical thermodynamics places primary emphasis on equilibrium states and changes from one equilibrium state to another. Thus, the concept of equilibrium is fundamental. In mechanics, equilibrium means a condition of balance maintained by an equality of opposing forces. In thermodynamics, the concept is more far-reaching, including not only a balance of forces but also a balance of other influences. Each kind of influence refers to a particular aspect of thermodynamic, or complete, equilibrium. Accordingly, several types of equilibrium must exist individually to fulfill the condition of complete equilibrium; among these are mechanical, thermal, phase, and chemical equilibrium. Criteria for these four types of equilibrium are considered in subsequent discussions. For the present, we may think of testing to see if a system is in thermodynamic equilibrium by the following procedure: Isolate the system from its surroundings and watch for changes in its observable properties. If there are no changes, we conclude that the system was in equilibrium at the moment it was isolated. The system can be said to be at an equilibrium state. When a system is isolated, it does not interact with its surroundings; however, its state can change as a consequence of spontaneous events occurring internally as its intensive properties, such as temperature and pressure, tend toward uniform values. When all such changes cease, the system is in equilibrium. At equilibrium, temperature is uniform throughout the system. Also, pressure can be regarded as uniform throughout as long as the effect of gravity is not significant; otherwise, a pressure variation can exist, as in a vertical column of liquid. It is not necessary that a system undergoing a process be in equilibrium during the process. Some or all of the intervening states may be nonequilibrium states. For many such processes, we are limited to knowing the state before the process occurs and the state after the process is completed. equilibrium equilibrium state (a) (b) Fig. 1.6 Figure used to discuss the extensive and intensive property concepts. Distinguishing Properties from Nonproperties At a given state, each property has a definite value that can be assigned without knowledge of how the system arrived at that state. The change in value of a property as the system is altered from one state to another is determined, therefore, solely by the two end states and is independent of the particular way the change of state occurred. The change is independent of the details of the process. Conversely, if the value of a quantity is independent of the process between two states, then that quantity is the change in a property. This provides a test for determining whether a quantity is a property: A quantity is a property if, and only if, its change in value between two states is independent of the process. It follows that if the value of a particular quantity depends on the details of the process, and not solely on the end states, that quantity cannot be a property
1.4 Measuring Mass,Length,Time,and Force Measuring Mass,Length, Time,and Force When engineering calculations are performed,it is necessary to be concerned with the units of the physical quantities involved.A unit is any specified amount of a quantity by comparison with which any other quantity of the same kind is measured. For example,meters,centimeters,kilometers,feet,inches,and miles are all units of length.Seconds,minutes,and hours are alternative time units. Because physical quantities are related by definitions and laws,a relatively small number of physical quantities suffice to conceive of and measure all others.These are called primary dimensions.The others are measured in terms of the primary dimen- sions and are called secondary.For example,if length and time were regarded as primary,velocity and area would be secondary. A set of primary dimensions that suffice for applications in mechanics is mass, length,and time.Additional primary dimensions are required when additional phys- ical phenomena come under consideration.Temperature is included for thermody- namics,and electric current is introduced for applications involving electricity. Once a set of primary dimensions is adopted,a base unit for each primary dimen- base unit sion is specified.Units for all other quantities are then derived in terms of the base units.Let us illustrate these ideas by considering briefly two systems of units:SI units and English Engineering units. 1.4.1 SI Units In the present discussion we consider the SI system of units that takes mass,length, and time as primary dimensions and regards force as secondary.SI is the abbreviation for Systeme International d'Unites (International System of Units),which is the legally accepted system in most countries.The conventions of the SI are published and controlled by an international treaty organization.The Sl base units for mass, SI base units length,and time are listed in Table 1.3 and discussed in the following paragraphs.The SI base unit for temperature is the kelvin,K. The SI base unit of mass is the kilogram,kg.It is equal to the mass of a particular cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures near Paris.The mass standard for the United States is maintained by the National Institute of Standards and Technology(NIST).The kilogram is the only base unit still defined relative to a fabricated object. The SI base unit of length is the meter (metre),m,defined as the length of the path traveled by light in a vacuum during a specified time interval.The base unit of time is the second,s.The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition of the cesium atom. The SI unit of force,called the newton,is a secondary unit,defined in terms of the base units for mass,length,and time.Newton's second law of motion states that the net force acting on a body is proportional to the product of the mass and the TABLE 1.3 Units for Mass,Length,Time,and Force English Quantity Unit Symbol Unit Symbol mass kilogram kg pound mass Ib length meter 0 foot ft time second second force newton v pound force lbf (=1kg·m/s (=32.1740b·t/s)
1.4 Measuring Mass, Length, Time, and Force 11 1.4 Measuring Mass, Length, Time, and Force When engineering calculations are performed, it is necessary to be concerned with the units of the physical quantities involved. A unit is any specified amount of a quantity by comparison with which any other quantity of the same kind is measured. For example, meters, centimeters, kilometers, feet, inches, and miles are all units of length. Seconds, minutes, and hours are alternative time units. Because physical quantities are related by definitions and laws, a relatively small number of physical quantities suffice to conceive of and measure all others. These are called primary dimensions. The others are measured in terms of the primary dimensions and are called secondary. For example, if length and time were regarded as primary, velocity and area would be secondary. A set of primary dimensions that suffice for applications in mechanics is mass, length, and time. Additional primary dimensions are required when additional physical phenomena come under consideration. Temperature is included for thermodynamics, and electric current is introduced for applications involving electricity. Once a set of primary dimensions is adopted, a base unit for each primary dimension is specified. Units for all other quantities are then derived in terms of the base units. Let us illustrate these ideas by considering briefly two systems of units: SI units and English Engineering units. 1.4.1 SI Units In the present discussion we consider the SI system of units that takes mass, length, and time as primary dimensions and regards force as secondary. SI is the abbreviation for Système International d'Unités (International System of Units), which is the legally accepted system in most countries. The conventions of the SI are published and controlled by an international treaty organization. The SI base units for mass, length, and time are listed in Table 1.3 and discussed in the following paragraphs. The SI base unit for temperature is the kelvin, K. The SI base unit of mass is the kilogram, kg. It is equal to the mass of a particular cylinder of platinum–iridium alloy kept by the International Bureau of Weights and Measures near Paris. The mass standard for the United States is maintained by the National Institute of Standards and Technology (NIST). The kilogram is the only base unit still defined relative to a fabricated object. The SI base unit of length is the meter (metre), m, defined as the length of the path traveled by light in a vacuum during a specified time interval. The base unit of time is the second, s. The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition of the cesium atom. The SI unit of force, called the newton, is a secondary unit, defined in terms of the base units for mass, length, and time. Newton’s second law of motion states that the net force acting on a body is proportional to the product of the mass and the base unit SI base units Units for Mass, Length, Time, and Force SI English Quantity Unit Symbol Unit Symbol mass kilogram kg pound mass lb length meter m foot ft time second s second s force newton N pound force lbf (5 1 kg ? m/s2 ) (5 32.1740 lb ? ft/s2 ) TABLE 1.3
Chapter 1 Getting Started smallest volume for which the matter can be considered a continuum and is normally small enough that it can be considered a "point."With density defined by Eq.1.6, density can be described mathematically as a continuous function of position and time. The density,or local mass per unit volume,is an intensive property that may vary from point to point within a system.Thus,the mass associated with a particular vol- ume V is determined in principle by integration pdv (1.7) and not simply as the product of density and volume. specific volume The specific volume v is defined as the reciprocal of the density,v 1/p.It is the volume per unit mass.Like density,specific volume is an intensive property and may vary from point to point.SI units for density and specific volume are kg/m3 and m/kg,respectively.They are also often expressed,respectively,as g/cm'and cm'/g.English units used for density and specific volume in this text are Ib/ft3 and ft/lb,respectively. In certain applications it is convenient to express properties such as specific vol- ume on a molar basis rather than on a mass basis.A mole is an amount of a given substance numerically equal to its molecular weight.In this book we express the molar basis amount of substance on a molar basis in terms of the kilomole (kmol)or the pound mole (Ibmol),as appropriate.In each case we use M 1.8) The number of kilomoles of a substance,n,is obtained by dividing the mass,m,in kilograms by the molecular weight,M,in kg/kmol.Similarly,the number of pound moles,n,is obtained by dividing the mass,m,in pound mass by the molecular weight, M,in Ib/lbmol.When m is in grams,Eq.1.8 gives n in gram moles,or mol for short. Recall from chemistry that the number of molecules in a gram mole,called Avogadro's number,is 6.022 X 1023.Appendix Tables A-1 and A-1E provide molecular weights for several substances. To signal that a property is on a molar basis,a bar is used over its symbol.Thus, signifies the volume per kmol or Ibmol,as appropriate.In this text,the units used for are m'/kmol and ft'/lbmol.With Eq.1.8,the relationship between v and v is U Mv (1.9) where M is the molecular weight in kg/kmol or lb/lbmol,as appropriate. 1.6 Pressure Next,we introduce the concept of pressure from the continuum viewpoint.Let us begin by considering a small area,A,passing through a point in a fluid at rest.The fluid on one side of the area exerts a compressive force on it that is normal to the area,Fommal.An equal but oppositely directed force is exerted on the area by the fluid on the other side.For a fluid at rest,no other forces than these act on the area.The pressure pressure,p,at the specified point is defined as the limit (1.10) Ext Int Properties where A'is the area at the "point"in the same limiting sense as used in the defini- A.3-Tab d 。tion of density
14 Chapter 1 Getting Started smallest volume for which the matter can be considered a continuum and is normally small enough that it can be considered a “point.” With density defined by Eq. 1.6, density can be described mathematically as a continuous function of position and time. The density, or local mass per unit volume, is an intensive property that may vary from point to point within a system. Thus, the mass associated with a particular volume V is determined in principle by integration m V rdV (1.7) and not simply as the product of density and volume. The specific volume is defined as the reciprocal of the density, 5 1/. It is the volume per unit mass. Like density, specific volume is an intensive property and may vary from point to point. SI units for density and specific volume are kg/m3 and m3 /kg, respectively. They are also often expressed, respectively, as g/cm3 and cm3 /g. English units used for density and specific volume in this text are lb/ft3 and ft3 /lb, respectively. In certain applications it is convenient to express properties such as specific volume on a molar basis rather than on a mass basis. A mole is an amount of a given substance numerically equal to its molecular weight. In this book we express the amount of substance on a molar basis in terms of the kilomole (kmol) or the pound mole (lbmol), as appropriate. In each case we use n m M (1.8) The number of kilomoles of a substance, n, is obtained by dividing the mass, m, in kilograms by the molecular weight, M, in kg/kmol. Similarly, the number of pound moles, n, is obtained by dividing the mass, m, in pound mass by the molecular weight, M, in lb/lbmol. When m is in grams, Eq. 1.8 gives n in gram moles, or mol for short. Recall from chemistry that the number of molecules in a gram mole, called Avogadro’s number, is 6.022 3 1023. Appendix Tables A-1 and A-1E provide molecular weights for several substances. To signal that a property is on a molar basis, a bar is used over its symbol. Thus, y signifies the volume per kmol or lbmol, as appropriate. In this text, the units used for y are m3 /kmol and ft3 /lbmol. With Eq. 1.8, the relationship between y and y is y My (1.9) where M is the molecular weight in kg/kmol or lb/lbmol, as appropriate. specific volume molar basis 1.6 Pressure Next, we introduce the concept of pressure from the continuum viewpoint. Let us begin by considering a small area, A, passing through a point in a fluid at rest. The fluid on one side of the area exerts a compressive force on it that is normal to the area, Fnormal. An equal but oppositely directed force is exerted on the area by the fluid on the other side. For a fluid at rest, no other forces than these act on the area. The pressure, p, at the specified point is defined as the limit p lim ASA¿ a Fnormal A b (1.10) where A9 is the area at the “point” in the same limiting sense as used in the definition of density. pressure Ext_Int_Properties A.3 – Tab d
1.6 Pressure HORIZONS Big Hopes for Nanotechnology Nanoscience is the study of molecules and tinuum model may no longer apply owing to the interactions molecular structures,called nanostructures,hav- among the atoms under consideration.Also at these scales, ing one or more dimensions less than about 100 the nature of physical phenomena such as current flow may nanometers.One nanometer is one-billionth of a meter: depend explicitly on the physical size of devices.After many 1 nm =10-9m.To grasp this level of smallness,a stack of years of fruitful research,nanotechnology is now poised to 10 hydrogen atoms would have a height of 1 nm,while a provide new products with a broad range of uses,including human hair has a diameter of about 50,000 nm.Nanotech- implantable chemotherapy devices,biosensors for glucose nology is the engineering of nanostructures into useful detection in diabetics,novel electronic devices,new energy products.At the nanotechnology scale,behavior may differ conversion technologies,and smart materials as,for example, from our macroscopic expectations.For example,the aver- fabrics that allow water vapor to escape while keeping aging used to assign property values at a point in the con- liquid water out. If the area a'was given new orientations by rotating it around the given point, and the pressure determined for each new orientation,it would be found that the pressure at the point is the same in all directions as long as the fluid is at rest.This is a consequence of the equilibrium of forces acting on an element of volume sur- rounding the point.However,the pressure can vary from point to point within a fluid at rest;examples are the variation of atmospheric pressure with elevation and the pressure variation with depth in oceans,lakes,and other bodies of water. Consider next a fluid in motion.In this case the force exerted on an area passing through a point in the fluid may be resolved into three mutually perpendicular com- ponents:one normal to the area and two in the plane of the area.When expressed on absolute pressure a unit area basis,the component normal to the area is called the normal stress,and the two components in the plane of the area are termed shear stresses.The magnitudes Gas at of the stresses generally vary with the orientation of the area.The state of stress in a pressure p fluid in motion is a topic that is normally treated thoroughly in fluid mechanics.The deviation of a normal stress from the pressure,the normal stress that would exist were the fluid at rest,is typically very small.In this book we assume that the normal stress at a point is equal to the pressure at that point.This assumption yields results of acceptable accuracy for the applications considered.Also,the term pressure,unless Tank stated otherwise,refers to absolute pressure:pressure with respect to the zero pressure of a complete vacuum.The lowest possible value of absolute pressure is zero. Manometer liquid 1.6.1 Pressure Measurement Fig. Manometer. Manometers and barometers measure pressure in terms of the length of a column of liquid such as mercury,water,or oil.The manometer shown in Fig.1.7 has one end Mercury vapor,Pvapor open to the atmosphere and the other attached to a tank containing a gas at a uniform pressure.Since pressures at equal elevations in a continuous mass of a liquid or gas at rest are equal,the pressures at points a and b of Fig.1.7 are equal.Applying an elementary force balance,the gas pressure is p Patm pgL (1.11) where patm is the local atmospheric pressure,p is the density of the manometer liquid, g is the acceleration of gravity,and L is the difference in the liquid levels. The barometer shown in Fig.1.8 is formed by a closed tube filled with liquid mer- Mercury,Pm cury and a small amount of mercury vapor inverted in an open container of liquid mercury.Since the pressures at points a and b are equal,a force balance gives the Fig. Barometer
1.6 Pressure 15 If the area A9 was given new orientations by rotating it around the given point, and the pressure determined for each new orientation, it would be found that the pressure at the point is the same in all directions as long as the fluid is at rest. This is a consequence of the equilibrium of forces acting on an element of volume surrounding the point. However, the pressure can vary from point to point within a fluid at rest; examples are the variation of atmospheric pressure with elevation and the pressure variation with depth in oceans, lakes, and other bodies of water. Consider next a fluid in motion. In this case the force exerted on an area passing through a point in the fluid may be resolved into three mutually perpendicular components: one normal to the area and two in the plane of the area. When expressed on a unit area basis, the component normal to the area is called the normal stress, and the two components in the plane of the area are termed shear stresses. The magnitudes of the stresses generally vary with the orientation of the area. The state of stress in a fluid in motion is a topic that is normally treated thoroughly in fluid mechanics. The deviation of a normal stress from the pressure, the normal stress that would exist were the fluid at rest, is typically very small. In this book we assume that the normal stress at a point is equal to the pressure at that point. This assumption yields results of acceptable accuracy for the applications considered. Also, the term pressure, unless stated otherwise, refers to absolute pressure: pressure with respect to the zero pressure of a complete vacuum. The lowest possible value of absolute pressure is zero. 1.6.1 Pressure Measurement Manometers and barometers measure pressure in terms of the length of a column of liquid such as mercury, water, or oil. The manometer shown in Fig. 1.7 has one end open to the atmosphere and the other attached to a tank containing a gas at a uniform pressure. Since pressures at equal elevations in a continuous mass of a liquid or gas at rest are equal, the pressures at points a and b of Fig. 1.7 are equal. Applying an elementary force balance, the gas pressure is p patm rgL (1.11) where patm is the local atmospheric pressure, is the density of the manometer liquid, g is the acceleration of gravity, and L is the difference in the liquid levels. The barometer shown in Fig. 1.8 is formed by a closed tube filled with liquid mercury and a small amount of mercury vapor inverted in an open container of liquid mercury. Since the pressures at points a and b are equal, a force balance gives the absolute pressure Nanoscience is the study of molecules and molecular structures, called nanostructures, having one or more dimensions less than about 100 nanometers. One nanometer is one-billionth of a meter: 1 nm 5 1029 m. To grasp this level of smallness, a stack of 10 hydrogen atoms would have a height of 1 nm, while a human hair has a diameter of about 50,000 nm. Nanotechnology is the engineering of nanostructures into useful products. At the nanotechnology scale, behavior may differ from our macroscopic expectations. For example, the averaging used to assign property values at a point in the continuum model may no longer apply owing to the interactions among the atoms under consideration. Also at these scales, the nature of physical phenomena such as current flow may depend explicitly on the physical size of devices. After many years of fruitful research, nanotechnology is now poised to provide new products with a broad range of uses, including implantable chemotherapy devices, biosensors for glucose detection in diabetics, novel electronic devices, new energy conversion technologies, and smart materials as, for example, fabrics that allow water vapor to escape while keeping liquid water out. Big Hopes for Nanotechnology HORIZONS Fig. 1.8 Barometer. Fig. 1.7 Manometer. Tank L a b patm Manometer liquid Gas at pressure p a patm L Mercury vapor, pvapor b Mercury, ρm