1.6 Pressure 17 force,is F=A(P2-P1）=A(Patm+PgL2）-A(Pam+PgLi) P8A(L2-L1) pgV Liquid with densityp where V is the volume of the block and p is the density of the PA surrounding liquid.Thus,the magnitude of the buoyant force acting on the block is equal to the weight of the displaced liquid. Block 1.6.3t Pressure Units Area=A D.A The SI unit of pressure and stress is the pascal. 1 pascal 1 N/m2 Fig.1.11 Evaluation of buoyant force for a submerged body. However,multiples of the pascal:the kPa,the bar,and the MPa are frequently used. 1 kPa 103 N/m2 1 bar 105 N/m2 1 MPa 105 N/m2 Commonly used English units for pressure and stress are pounds force per square foot,Ibf/ft2,and pounds force per square inch,Ibf/in.2 Although atmospheric pressure varies with location on the earth,a standard refer- ence value can be defined and used to express other pressures. 1.01325×105N/m2 1 standard atmosphere (atm)= 14.696 Ibf/in.2 (1.13) 、760mmHg=29.92inHg Since 1 bar (10 N/m2)closely equals one standard atmosphere,it is a convenient pressure unit despite not being a standard SI unit.When working in SI,the bar,MPa, and kPa are all used in this text. Although absolute pressures must be used in thermodynamic relations,pressure- measuring devices often indicate the difference between the absolute pressure of a system and the absolute pressure of the atmosphere existing outside the measuring device.The magnitude of the difference is called a gage pressure or a vacuum pressure. gage pressure The term gage pressure is applied when the pressure of the system is greater than vacuum pressure the local atmospheric pressure,Patm. p(gage)=p(absolute)-patm(absolute) (1.14 When the local atmospheric pressure is greater than the pressure of the system,the TAKE NOTE... term vacuum pressure is used. In this book.the term pres sure refers to absolute p(vacuum)=patm(absolute)-p(absolute) (1.15) pressure unless indicated otherwise. Engineers in the United States frequently use the letters a and g to distinguish between absolute and gage pressures.For example,the absolute and gage pressures in pounds force per square inch are written as psia and psig,respectively.The relationship among the various ways of expressing pressure measurements is shown in Fig.1.12
force, is F 5 A1p2 2 p12 5 A1patm 1 rgL22 2 A1patm 1 rgL12 5 rgA1L2 2 L12 5 rgV where V is the volume of the block and r is the density of the surrounding liquid. Thus, the magnitude of the buoyant force acting on the block is equal to the weight of the displaced liquid. b b b b b 1.6.3 Pressure Units The SI unit of pressure and stress is the pascal. 1 pascal 5 1 N/m2 However, multiples of the pascal: the kPa, the bar, and the MPa are frequently used. 1 kPa 5 103 N/m2 1 bar 5 105 N/m2 1 MPa 5 106 N/m2 Commonly used English units for pressure and stress are pounds force per square foot, lbf/ft 2 , and pounds force per square inch, lbf/in. 2 Although atmospheric pressure varies with location on the earth, a standard reference value can be defined and used to express other pressures. 1 standard atmosphere 1atm2 5 • 1.01325 3 105 N/m2 14.696 lbf/in.2 760 mmHg 5 29.92 inHg (1.13) Since 1 bar (10 5 N/m 2 ) closely equals one standard atmosphere, it is a convenient pressure unit despite not being a standard SI unit. When working in SI, the bar, MPa, and kPa are all used in this text. Although absolute pressures must be used in thermodynamic relations, pressuremeasuring devices often indicate the difference between the absolute pressure of a system and the absolute pressure of the atmosphere existing outside the measuring device. The magnitude of the difference is called a gage pressure or a vacuum pressure. The term gage pressure is applied when the pressure of the system is greater than the local atmospheric pressure, patm . p1gage2 5 p1absolute2 2 patm1absolute2 (1.14) When the local atmospheric pressure is greater than the pressure of the system, the term vacuum pressure is used. p1vacuum2 5 patm1absolute2 2 p1absolute2 (1.15) Engineers in the United States frequently use the letters a and g to distinguish between absolute and gage pressures. For example, the absolute and gage pressures in pounds force per square inch are written as psia and psig, respectively. The relationship among the various ways of expressing pressure measurements is shown in Fig. 1.12 . L2 L1 patm p2A p1A Area = A Liquid with density ρ Block Fig. 1.11 Evaluation of buoyant force for a submerged body. gage pressure vacuum pressure 1.6 Pressure 17 TAKE NOTE... In this book, the term pressure refers to absolute pressure unless indicated otherwise. c01GettingStarted.indd Page 17 5/13/10 5:41:55 PM user-s146 /Users/user-s146/Desktop/Merry_X-Mas/New
18 Chapter 1 Getting Started p(gage) Absolute pressure that is Atmospheric greater than the local pressure atmospheric pressure p(absolute) p(vacuum) Absolute pressure that is (absolute) less than the local atmospheric pressure p(absolute) Zero pressure Zero pressure Fig.1.12 Relationships among the absolute,atmospheric,gage,and vacuum pressures. BIOCONNECTIONS One in three Americans is said to have high blood pres- sure.Since this can lead to heart disease,strokes,and other serious medical compli- cations,medical practitioners recommend regular blood pressure checks for everyone. Blood pressure measurement aims to determine the maximum pressure(systolic pressure) in an artery when the heart is pumping blood and the minimum pressure(diastolic pressure) when the heart is resting,each pressure expressed in millimeters of mercury,mmHg.The systolic and diastolic pressures of healthy persons should be less than about 120 mmHg and 80 mmHg,respectively. The standard blood pressure measurement apparatus in use for decades involving an inflatable cuff,mercury manometer,and stethoscope is gradually being replaced because of concerns over mercury toxicity and in response to special requirements,including mon- itoring during clinical exercise and during anesthesia.Also,for home use and self-monitoring, many patients prefer easy-to-use automated devices that provide digital displays of blood pressure data.This has prompted biomedical engineers to rethink blood pressure measure- ment and develop new mercury-free and stethoscope-free approaches.One of these uses a highly-sensitive pressure transducer to detect pressure oscillations within an inflated cuff placed around the patient's arm.The monitor's software uses these data to calculate the systolic and diastolic pressures,which are displayed digitally. 1.7 Temperature In this section the intensive property temperature is considered along with means for measuring it.A concept of temperature,like our concept of force,originates with our sense perceptions.Temperature is rooted in the notion of the"hotness"or"coldness" of objects.We use our sense of touch to distinguish hot objects from cold objects and to arrange objects in their order of"hotness,"deciding that 1 is hotter than 2,2 hotter
18 Chapter 1 Getting Started Atmospheric pressure p (gage) p (absolute) patm (absolute) p (absolute) p (vacuum) Zero pressure Zero pressure Absolute pressure that is less than the local atmospheric pressure Absolute pressure that is greater than the local atmospheric pressure Fig. 1.12 Relationships among the absolute, atmospheric, gage, and vacuum pressures. 1.7 Temperature In this section the intensive property temperature is considered along with means for measuring it. A concept of temperature, like our concept of force, originates with our sense perceptions. Temperature is rooted in the notion of the “hotness” or “coldness” of objects. We use our sense of touch to distinguish hot objects from cold objects and to arrange objects in their order of “hotness,” deciding that 1 is hotter than 2, 2 hotter BIOCONNECTIONS One in three Americans is said to have high blood pressure. Since this can lead to heart disease, strokes, and other serious medical complications, medical practitioners recommend regular blood pressure checks for everyone. Blood pressure measurement aims to determine the maximum pressure (systolic pressure) in an artery when the heart is pumping blood and the minimum pressure (diastolic pressure) when the heart is resting, each pressure expressed in millimeters of mercury, mmHg. The systolic and diastolic pressures of healthy persons should be less than about 120 mmHg and 80 mmHg, respectively. The standard blood pressure measurement apparatus in use for decades involving an inflatable cuff, mercury manometer, and stethoscope is gradually being replaced because of concerns over mercury toxicity and in response to special requirements, including monitoring during clinical exercise and during anesthesia. Also, for home use and self-monitoring, many patients prefer easy-to-use automated devices that provide digital displays of blood pressure data. This has prompted biomedical engineers to rethink blood pressure measurement and develop new mercury-free and stethoscope-free approaches. One of these uses a highly-sensitive pressure transducer to detect pressure oscillations within an inflated cuff placed around the patient’s arm. The monitor’s software uses these data to calculate the systolic and diastolic pressures, which are displayed digitally. c01GettingStarted.indd Page 18 6/30/10 1:31:06 PM user-s146 /Users/user-s146/Desktop/Merry_X-Mas/New
1.7 Temperature 19 than 3,and so on.But however sensitive human touch may be,we are unable to gauge Ext_Int Properties this quality precisely. A.3-Tab e A definition of temperature in terms of concepts that are independently defined or accepted as primitive is difficult to give.However,it is possible to arrive at an objective understanding of equality of temperature by using the fact that when the temperature of an object changes,other properties also change. To illustrate this,consider two copper blocks,and suppose that our senses tell us that one is warmer than the other.If the blocks were brought into contact and isolated from their surroundings,they would interact in a way that can be described as a thermal (heat)interaction.During this interaction,it would be observed that thermal (heat)interaction the volume of the warmer block decreases somewhat with time,while the volume of the colder block increases with time.Eventually,no further changes in volume would be observed,and the blocks would feel equally warm.Similarly,we would be able to observe that the electrical resistance of the warmer block decreases with time. and that of the colder block increases with time;eventually the electrical resis- tances would become constant also.When all changes in such observable properties cease,the interaction is at an end.The two blocks are then in thermal equilibrium. thermal equilibrium Considerations such as these lead us to infer that the blocks have a physical prop- erty that determines whether they will be in thermal equilibrium.This property is called temperature,and we postulate that when the two blocks are in thermal equi- temperature librium,their temperatures are equal. It is a matter of experience that when two objects are in thermal equilibrium with a third object,they are in thermal equilibrium with one another.This statement,which is sometimes called the zeroth law of thermodynamics,is tacitly assumed in every mea- zeroth law of surement of temperature.Thus,if we want to know if two objects are at the same thermodynamics temperature,it is not necessary to bring them into contact and see whether their observable properties change with time,as described previously.It is necessary only to see if they are individually in thermal equilibrium with a third object.The third object is usually a thermometer. 1.7.1 t Thermometers Any object with at least one measurable property that changes as its temperature changes can be used as a thermometer.Such a property is called a thermometric thermometric property property.The particular substance that exhibits changes in the thermometric property is known as a thermometric substance. A familiar device for temperature measurement is the liquid-in-glass thermometer pictured in Fig.1.13a,which consists of a glass capillary tube connected to a bulb filled with a liquid such as alcohol and sealed at the other end.The space above the liquid is occupied by the vapor of the liquid or an inert gas.As temperature increases, the liquid expands in volume and rises in the capillary.The length L of the liquid in the capillary depends on the temperature.Accordingly,the liquid is the thermometric substance and L is the thermometric property.Although this type of thermometer is commonly used for ordinary temperature measurements,it is not well suited for appli- cations where extreme accuracy is required. More accurate sensors known as thermocouples are based on the principle that when two dissimilar metals are joined,an electromotive force (emf)that is primarily a function of temperature will exist in a circuit.In certain thermocouples,one ther- mocouple wire is platinum of a specified purity and the other is an alloy of platinum and rhodium.Thermocouples also utilize copper and constantan(an alloy of copper and nickel),iron and constantan,as well as several other pairs of materials.Electrical- resistance sensors are another important class of temperature measurement devices. These sensors are based on the fact that the electrical resistance of various materials changes in a predictable manner with temperature.The materials used for this purpose are normally conductors (such as platinum,nickel,or copper)or semiconductors
than 3, and so on. But however sensitive human touch may be, we are unable to gauge this quality precisely. A definition of temperature in terms of concepts that are independently defined or accepted as primitive is difficult to give. However, it is possible to arrive at an objective understanding of equality of temperature by using the fact that when the temperature of an object changes, other properties also change. To illustrate this, consider two copper blocks, and suppose that our senses tell us that one is warmer than the other. If the blocks were brought into contact and isolated from their surroundings, they would interact in a way that can be described as a thermal (heat) interaction. During this interaction, it would be observed that the volume of the warmer block decreases somewhat with time, while the volume of the colder block increases with time. Eventually, no further changes in volume would be observed, and the blocks would feel equally warm. Similarly, we would be able to observe that the electrical resistance of the warmer block decreases with time, and that of the colder block increases with time; eventually the electrical resistances would become constant also. When all changes in such observable properties cease, the interaction is at an end. The two blocks are then in thermal equilibrium. Considerations such as these lead us to infer that the blocks have a physical property that determines whether they will be in thermal equilibrium. This property is called temperature, and we postulate that when the two blocks are in thermal equilibrium, their temperatures are equal. It is a matter of experience that when two objects are in thermal equilibrium with a third object, they are in thermal equilibrium with one another. This statement, which is sometimes called the zeroth law of thermodynamics, is tacitly assumed in every measurement of temperature. Thus, if we want to know if two objects are at the same temperature, it is not necessary to bring them into contact and see whether their observable properties change with time, as described previously. It is necessary only to see if they are individually in thermal equilibrium with a third object. The third object is usually a thermometer . 1.7.1 Thermometers Any object with at least one measurable property that changes as its temperature changes can be used as a thermometer. Such a property is called a thermometric property. The particular substance that exhibits changes in the thermometric property is known as a thermometric substance. A familiar device for temperature measurement is the liquid-in-glass thermometer pictured in Fig. 1.13a , which consists of a glass capillary tube connected to a bulb filled with a liquid such as alcohol and sealed at the other end. The space above the liquid is occupied by the vapor of the liquid or an inert gas. As temperature increases, the liquid expands in volume and rises in the capillary. The length L of the liquid in the capillary depends on the temperature. Accordingly, the liquid is the thermometric substance and L is the thermometric property. Although this type of thermometer is commonly used for ordinary temperature measurements, it is not well suited for applications where extreme accuracy is required. More accurate sensors known as thermocouples are based on the principle that when two dissimilar metals are joined, an electromotive force (emf) that is primarily a function of temperature will exist in a circuit. In certain thermocouples, one thermocouple wire is platinum of a specified purity and the other is an alloy of platinum and rhodium. Thermocouples also utilize copper and constantan (an alloy of copper and nickel), iron and constantan, as well as several other pairs of materials. Electricalresistance sensors are another important class of temperature measurement devices. These sensors are based on the fact that the electrical resistance of various materials changes in a predictable manner with temperature. The materials used for this purpose are normally conductors (such as platinum, nickel, or copper) or semiconductors. thermal (heat) interaction thermal equilibrium temperature zeroth law of thermodynamics thermometric property 1.7 Temperature 19 A Ext_Int_Properties A.3 – Tab e c01GettingStarted.indd Page 19 6/30/10 11:40:18 AM user-s146 /Users/user-s146/Desktop/Merry_X-Mas/New
20 Chapter 1 Getting Started Liquid (a ( (d Fig.1.13 Thermometers.(a)Liquid-in-glass.(b)Electrical-resistance(c)Infrared-sensing ear thermometer. Devices using conductors are known as resistance temperature detectors.Semiconductor types are called thermistors.A battery-powered electrical-resistance thermometer commonly used today is shown in Fig.1.13b. A variety of instruments measure temperature by sensing radiation,such as the ear thermometer shown in Fig.1.13c.They are known by terms such as radiation thermometers and optical pyrometers.This type of thermometer differs from those previously considered because it is not required to come in contact with the object whose temperature is to be determined,an advantage when dealing with moving objects or objects at extremely high temperatures. ENERGY ENVIRONMENT The mercury-in-glass fever thermometers,once found in nearly every medicine cabinet,are a thing of the past.The American Academy of Pediatrics has designated mercury as too toxic to be present in the home.Families are turning to safer alternatives and disposing of mercury thermometers.Proper disposal is an issue,experts say. The safe disposal of millions of obsolete mercury-filled thermometers has emerged in its own right as an environmental issue.For proper disposal,thermometers must be taken to hazardous- waste collection stations rather than simply thrown in the trash where they can be easily broken, releasing mercury.Loose fragments of broken thermometers and anything that contacted mercury should be transported in closed containers to appropriate disposal sites. The present generation of liquid-in-glass fever thermometers for home use contains patented liquid mixtures that are nontoxic,safe alternatives to mercury.Other types of thermometers also are used in the home,including battery-powered electrical-resistance thermometers. 1.7.2 t Kelvin and Rankine Temperature Scales Empirical means of measuring temperature such as considered in Sec.1.71 have inherent limitations. FRXAMPL3 the tendency of the liquid in a liquid-in-glass thermometer to freeze at low temperatures imposes a lower limit on the range of temperatures that can be
20 Chapter 1 Getting Started Devices using conductors are known as resistance temperature detectors . Semiconductor types are called thermistors . A battery-powered electrical-resistance thermometer commonly used today is shown in Fig. 1.13b . A variety of instruments measure temperature by sensing radiation, such as the ear thermometer shown in Fig. 1.13c . They are known by terms such as radiation thermometers and optical pyrometers . This type of thermometer differs from those previously considered because it is not required to come in contact with the object whose temperature is to be determined, an advantage when dealing with moving objects or objects at extremely high temperatures. L Liquid (a) ( (b) c) Fig. 1.13 Thermometers. (a) Liquid-in-glass. (b) Electrical-resistance (c) Infrared-sensing ear thermometer. 1.7.2 Kelvin and Rankine Temperature Scales Empirical means of measuring temperature such as considered in Sec. 1.7.1 have inherent limitations. the tendency of the liquid in a liquid-in-glass thermometer to freeze at low temperatures imposes a lower limit on the range of temperatures that can be ENERGY & ENVIRONMENT The mercury-in-glass fever thermometers, once found in nearly every medicine cabinet, are a thing of the past. The American Academy of Pediatrics has designated mercury as too toxic to be present in the home. Families are turning to safer alternatives and disposing of mercury thermometers. Proper disposal is an issue, experts say. The safe disposal of millions of obsolete mercury-filled thermometers has emerged in its own right as an environmental issue. For proper disposal, thermometers must be taken to hazardouswaste collection stations rather than simply thrown in the trash where they can be easily broken, releasing mercury. Loose fragments of broken thermometers and anything that contacted mercury should be transported in closed containers to appropriate disposal sites. The present generation of liquid-in-glass fever thermometers for home use contains patented liquid mixtures that are nontoxic, safe alternatives to mercury. Other types of thermometers also are used in the home, including battery-powered electrical-resistance thermometers. c01GettingStarted.indd Page 20 6/30/10 1:31:15 PM user-s146 /Users/user-s146/Desktop/Merry_X-Mas/New
1.7 Temperature 21 measured.At high temperatures liquids vaporize,and therefore these temperatures also cannot be determined by a liquid-in-glass thermometer.Accordingly,several different thermometers might be required to cover a wide temperature interval. In view of the limitations of empirical means for measuring temperature,it is desir- able to have a procedure for assigning temperature values that does not depend on the properties of any particular substance or class of substances.Such a scale is called a thermodynamic temperature scale.The Kelvin scale is an absolute thermodynamic Kelvin scale temperature scale that provides a continuous definition of temperature,valid over all ranges of temperature.The unit of temperature on the Kelvin scale is the kelvin(K) The kelvin is the SI base unit for temperature. To develop the Kelvin scale,it is necessary to use the conservation of energy principle and the second law of thermodynamics;therefore,further discussion is deferred to Sec.5.8 after these principles have been introduced.However,we note here that the Kelvin scale has a zero of 0 K,and lower temperatures than this are not defined. By definition,the Rankine scale,the unit of which is the degree rankine (R),is Rankine scale proportional to the Kelvin temperature according to TR)=1.8T(K) (1.16) As evidenced by Eg.1.16,the Rankine scale is also an absolute thermodynamic scale with an absolute zero that coincides with the absolute zero of the Kelvin scale.In thermodynamic relationships,temperature is always in terms of the Kelvin or Rankine scale unless specifically stated otherwise.Still,the Celsius and Fahrenheit scales considered next are commonly encountered. 1.7.3 t Celsius and Fahrenheit Scales The relationship of the Kelvin,Rankine,Celsius,and Fahrenheit scales is shown in Fig.1.14 together with values for temperature at three fixed points:the triple point, ice point,and steam point. By international agreement,temperature scales are defined by the numerical value assigned to the easily reproducible triple point of water:the state of equilibrium between triple point K C R Steam point Triple point of water- S 160 Ice point- .65 Absolute zero Fig.1.14 Comparison of temperature scales
measured. At high temperatures liquids vaporize, and therefore these temperatures also cannot be determined by a liquid-in-glass thermometer. Accordingly, several different thermometers might be required to cover a wide temperature interval. b b b b b In view of the limitations of empirical means for measuring temperature, it is desirable to have a procedure for assigning temperature values that does not depend on the properties of any particular substance or class of substances. Such a scale is called a thermodynamic temperature scale. The Kelvin scale is an absolute thermodynamic temperature scale that provides a continuous definition of temperature, valid over all ranges of temperature. The unit of temperature on the Kelvin scale is the kelvin (K). The kelvin is the SI base unit for temperature. To develop the Kelvin scale, it is necessary to use the conservation of energy principle and the second law of thermodynamics; therefore, further discussion is deferred to Sec. 5.8 after these principles have been introduced. However, we note here that the Kelvin scale has a zero of 0 K, and lower temperatures than this are not defined. By definition, the Rankine scale , the unit of which is the degree rankine (°R), is proportional to the Kelvin temperature according to T18R2 5 1.8T1K2 (1.16) As evidenced by Eq. 1.16, the Rankine scale is also an absolute thermodynamic scale with an absolute zero that coincides with the absolute zero of the Kelvin scale. In thermodynamic relationships, temperature is always in terms of the Kelvin or Rankine scale unless specifically stated otherwise. Still, the Celsius and Fahrenheit scales considered next are commonly encountered. 1.7.3 Celsius and Fahrenheit Scales The relationship of the Kelvin, Rankine, Celsius, and Fahrenheit scales is shown in Fig. 1.14 together with values for temperature at three fixed points: the triple point, ice point, and steam point. By international agreement, temperature scales are defined by the numerical value assigned to the easily reproducible triple point of water: the state of equilibrium between Kelvin scale Absolute zero Steam point 0.00 Kelvin 273.15 273.16 373.15 Triple point of water Ice point K –273.15 Celsius 0.00 0.01 100.0 °C 0.00 Rankine 491.67 491.69 671.67 °R –459.67 Fahrenheit 32.0 32.02 212 °F Fig. 1.14 Comparison of temperature scales. triple point 1.7 Temperature 21 Rankine scale c01GettingStarted.indd Page 21 7/1/10 10:35:54 AM user-s146 /Users/user-s146/Desktop/Merry_X-Mas/New