Intro and Basic Concepts-Therm

M.BahramiENSC388(F09)IntroandBasicConcepts 1

BasicConceptsofThermodynamicsEverysciencehasitsownuniquevocabularyassociatedwithit.Precisedefinitionofbasicconcepts formsasound foundation fordevelopmentofascienceandpreventspossiblemisunderstandings.Carefulstudyoftheseconceptsisessentialforagoodunderstandingoftopicsinthermodynamics.ThermodynamicsandEnergyThermodynamicscanbedefinedas thestudyofenergy,energytransformationsand itsrelationtomatter.Theanalysisofthermalsystemsisachievedthroughtheapplicationofthe governing conservation equations, namely Conservation of Mass, Conservation ofEnergy (1st lawof thermodynamics), the2nd lawof thermodynamicsand thepropertyrelations.Energycanbeviewedastheabilitytocausechanges.First law of thermodynamics: one of the most fundamental laws of nature is theconservationofenergyprinciple. It simplystates thatduringan interaction,energycanchangefromoneformtoanotherbutthetotalamountofenergyremainsconstant.Second law of thermodynamics: energy has quality as well as quantity, and actualprocessesoccurinthedirectionofdecreasingqualityofenergy.Whenever there is an interaction between energy and matter, thermodynamics isinvolved. Some examples include heating and airconditioning systems, refrigerators,waterheaters,etc.DimensionsandUnitsAny physical quantity can be characterized by dimensions. The arbitrary magnitudesassignedtothedimensionsarecalledunits.Therearetwotypesofdimensions,primaryorfundamentalandsecondaryorderiveddimensions.Primarydimensionsare:mass,m;length,L;time,t;temperature,TSecondarydimensionsaretheonesthatcanbederivedfromprimarydimensionssuchas:velocity(m/s2),pressure(Pa=kg/m.s2).TherearetwounitsystemscurrentlyavailableSI(InternationalSystem)andUSCS(UnitedStatesCustomarySystem)orEnglishsystem.We,however,willuseSIunitsexclusivelyinthiscourse.TheSIunitsarebasedondecimal relationshipbetweenunits.TheprefixesusedtoexpressthemultiplesofthevariousunitsarelistedinTable11.

Table1:StandardprefixesinSIunits.MULTIPLE 1012 109 106 103 102 103 106 109 1012PREFIX tetra,T giga,G mega,M kilo,k centi,c mili,m micro, nano,n pico,p


Important note: in engineering all equationsmust be dimensionally homogenous. Thismeansthateveryterminanequationmusthavethesameunits.Itcanbeusedasasanitycheckforyoursolution.Example1:UnitConversionTheheatdissipationratedensityofanelectronicdeviceisreportedas10.72mW/mm2bythemanufacturer.ConvertthistoW/m2.

2

2

2 1072010001

1100072.10

mW

mWW

mmm

mmmW

ClosedandOpenSystemsAsystem isdefinedasaquantityofmatteroraregion inspacechosen forstudy. Themassorregionoutsidethesystemiscalledthesurroundings.

Fig.1:System,surroundings,andboundaryBoundary:therealorimaginarysurfacethatseparatesthesystemfromitssurroundings.Theboundariesofasystemcanbefixedormovable.Mathematically,theboundaryhaszerothickness,nomass,andnovolume.Closedsystemorcontrolmass:consistsofafixedamountofmass,andnomasscancrossitsboundary.But,energy inthe formofheatorwork,cancrosstheboundary,andthevolumeofaclosedsystemdoesnothavetobefixed.Opensystemorcontrolvolume:isaproperlyselectedregioninspace.Itusuallyenclosesadevicethat involvesmassflowsuchasacompressor.Bothmassandenergycancrosstheboundaryofacontrolvolume.Importantnote:somethermodynamicsrelationsthatareapplicabletoclosedandopensystemsaredifferent.Thus,itisextremelyimportanttorecognizethetypeofsystemwehavebeforestartanalyzingit.Isolated system:Aclosed system thatdoesnotcommunicatewith the surroundingsbyanymeans.Rigidsystem:Aclosedsystemthatcommunicateswiththesurroundingsbyheatonly.

SYSTEMBOUNDARY

SURROUNDINGS


Adiabatic system: A closed or open system that does not exchange energy with thesurroundingsbyheat.

Fig.2:Closedsystem,masscannotcrosstheboundaries,butenergycan.

Fig.3:Controlvolume,bothmassandenergycancrosstheboundaries.

EnergyInthermodynamics,wedealwithchangeofthetotalenergyonly.Thus,thetotalenergyofasystemcanbeassignedavalueofzeroatsomereferencepoint. Totalenergyofasystemhastwogroups:macroscopicandmicroscopic.Macroscopic forms of energy: forms of energy that a system posses as awholewithrespect to some outside reference frame, such as kinetic and potential energy. Themacroscopicenergyofasystem isrelatedtomotionandthe influenceofsomeexternaleffectssuchasgravity,magnetism,electricity,andsurfacetension.

energy

mass

CLOSEDSYSTEMm =const.

mass

energy

CONTROLVOLUME


Kinetic energy: energy that a system posses as a result of its relativemotionrelativetosomereferenceframe,KE

kJmVKE2

2

whereVisthevelocityofthesystemin(m/s). Potentialenergy:istheenergythatasystempossesasaresultofitselevationinagravitationalfield,PE

kJmgzPE wheregisthegravitationalaccelerationandzistheelevationofthecenterofgravityofthesystemrelativetosomearbitraryreferenceplane.

Microscopicformsofenergy:arethoserelatedtomolecularstructureofasystem.Theyareindependentofoutsidereferenceframes.Thesumofmicroscopicenergyiscalledtheinternalenergy,U.Thetotalenergyofasystemconsistsofthekinetic,potential,andinternalenergies:

kJmgzmVUPEKEUE 2

2

where the contributions of magnetic, electric, nuclear energy are neglected. Internalenergy isrelated to themolecularstructureand thedegreeofmolecularactivityand itmaybeviewedasthesumofthekineticandpotentialenergiesofmolecules.

Thesumof translational,vibrational,androtationalenergiesofmolecules is thekinetic energy of molecules, and it is also called the sensible energy. At highertemperatures,systemwillhavehighersensibleenergy.

Internalenergyassociatedwith thephaseofa system iscalled latentheat.Theintermolecularforcesarestrongestinsolidsandweakestingases.

The internal energy associated with the atomic bonds in a molecule is calledchemicalorbondenergy. The tremendousamountofenergyassociatedwith thebondswithinthenucleolusofatomitselfiscalledatomicenergy.

Energyinteractionswithaclosedsystemcanoccurviaheattransferandwork.


Fig.14:Formsofenergy.


PropertiesofaSystemAny characteristic of a system is called a property. In classical thermodynamics, thesubstanceisassumedtobeacontinuum,homogenousmatterwithnomicroscopicholes.Thisassumptionholdsaslongasthevolumes,andlengthscalesarelargewithrespecttotheintermolecularspacing.Intensiveproperties:arethosethatareindependentofthesize(mass)ofasystem,suchastemperature,pressure,anddensity.Theyarenotadditive.Extensive properties: values that are dependant on size of the system such asmass,volume,andtotalenergyU.Theyareadditive.

Generally,uppercaselettersareusedtodenoteextensiveproperties(exceptmassm), and lower case letters are used for intensive properties (except pressure P,temperatureT).

Extensive properties per unit mass are called specific properties, e.g. specificvolume(v=V/m).

Fig.15:Intensiveandextensivepropertiesofasystem.

StateandEquilibriumAtagivenstate,allthepropertiesofasystemhavefixedvalues.Thus,ifthevalueofevenonepropertychanges,thestatewillchangetodifferentone.Inanequilibriumstate,therearenounbalancedpotentials(ordrivingforces)withinthesystem. A system in equilibrium experiences no changes when it is isolated from itssurroundings.

Thermal equilibrium:when the temperature is the same throughout the entiresystem.

mVTP

0.5m0.5VTP

0.5m0.5VTP

extensiveproperties

intensiveproperties


Mechanicalequilibrium:whenthere isnochange inpressureatanypointofthesystem. However, the pressuremay varywithin the system due to gravitationaleffects.

Phaseequilibrium: inatwophasesystem,whenthemassofeachphasereachesanequilibriumlevel.

Chemical equilibrium: when the chemical composition of a system does notchangewithtime,i.e.,nochemicalreactionsoccur.

ProcessesandCyclesAnychangeasystemundergoesfromoneequilibriumstatetoanotheriscalledaprocess,andtheseriesofstatesthroughwhichasystempassesduringaprocessiscalledapath.

Fig.6:Tospecifyaprocess,initialandfinalstatesandpathmustbespecified.

Quasiequilibriumprocess: canbeviewedasa sufficiently slowprocess thatallows thesystemtoadjustitself internallyandremainsinfinitesimallyclosetoanequilibriumstateat all times. Quasiequilibrium process is an idealized process and is not a truerepresentationoftheactualprocess.Wemodelactualprocesseswithquasiequilibriumones.Moreover,theyserveasstandardstowhichactualprocessescanbecompared.Processdiagramsareusedtovisualizeprocesses.Notethattheprocesspath indicatesaseriesofequilibrium states,andwearenotable to specify the states foranonquasiequilibriumprocess.Prefixisoisusedtodesignateaprocessforwhichaparticularpropertyisconstant.

Isothermal:isaprocessduringwhichthetemperatureremainsconstant Isobaric:isaprocessduringwhichthepressureremainsconstant Isometric:isprocessduringwhichthespecificvolumeremainsconstant.

Asystemissaidtohaveundergoneacycleifitreturnstoitsinitialstateattheendoftheprocess.

State2

State1

Processpath

A

B


Fig.17:AfourprocesscycleinaPVdiagram.

The stateofasystem isdescribedby itsproperties.Thestateofasimplecompressiblesystemiscompletelyspecifiedbytwoindependent,intensiveproperties.A system is called simple compressible system in the absence of electrical,magnetic,gravitational,motion,andsurfacetensioneffects(externalforcefields).Independentproperties: twopropertiesare independent ifoneproperty canbevariedwhiletheotheroneisheldconstant.PressurePressureistheforceexertedbyafluidperunitarea.

PamN 2Area

ForcePressure Influids,gasesandliquids,wespeakofpressure;insolidsthisisstress.Forafluidatrest,thepressureatagivenpointisthesameinalldirections.

Fig.8:Pressureofafluidatrestincreaseswithdepth(duetoaddedweight),butconstant

inhorizontalplanes.

V

P

1

2

3

4

P(z)

z

Area=A

ghPAAhg

AmgP

P

Area

liquid ofWeight

h

A


Theactualpressureatagivenpositioniscalledtheabsolutepressure,anditismeasuredrelativetoabsolutevacuum.

gaugepressure=absolutepressureatmosphericpressure

atmabsatmvac

atmatmabsgauge

PPPPPPPPPP

Fig.9:Absolute,gauge,andvacuumpressures.

Inthermodynamicscalculations,alwaysuseabsolutepressure.Mostpressuremeasuringdevicesarecalibrated to read zero in theatmosphere (theymeasurePgauge orPvac).Beawareofwhatyouarereading!AdevicethatmeasurespressureusingacolumnofliquidiscalledaManometer.Thecrosssectionalareaofthetubeisnotimportant.Themanometermeasuresthegaugepressure.

Fig.10:Basicmanometer,P2=P1.

kPaghPP atm 1

Patm

Pgauge

Pabs

Absolute(vacuum)=0

P

Pvac


BourdonTubeisadevicethatmeasurespressureusingmechanicaldeformation.PressureTransducersaredevicesthatusepiezoelectricstomeasurepressure. veryaccurateandrobust canmeasurefrom106to105atm canmeasurePgaugeorPabs Barometerisadevicethatmeasuresatmosphericpressure.Itisamanometerwithanearvacuumononeend

Fig.11:Burdongauge.

Example2:PressureThepistonofacylinderpistondevicehasamassof60kgandacrosssectionalareaof0.04m2,asshown inFig.12.Thedepthof the liquid in thecylinder is1.8mandhasadensityof1558kg/m3.The localatmosphericpressure is0.97bar,andthegravitationalaccelerationis9.8m/s2.Determinethepressureatthebottomofthecylinder.Solution:thepressureatthebottomofthecylindercanbefoundfromthesummationoftheforcesduetoatmosphericpressure,pistonweight,andtheweightoftheliquidinthecylinder.

ghAmgPP

WWAPW

atmbottom

Pistonliquidatmbottom


bars

mNbar

smkgmN

msmmkgm

smkgbarPbottom

3918.1/10

1./1

/1

8.1/8.9/155804.0

/8.96097.0

252

2

232

2

Fig.12:Sketchforexample2.

TemperatureTemperatureisapointerforthedirectionofenergytransferasheat.

Fig.13:Heattransferoccursinthedirectionofhighertolowertemperature.

Whenthetemperaturesoftwobodiesarethesame,thermalequilibriumisreached.Theequalityoftemperatureistheonlyrequirementforthermalequilibrium.The0thlawofthermodynamics:statesthatiftwobodiesareinthermalequilibriumwithathirdbody,theyarealsointhermalequilibriumwitheachother.The0th lawmakesathermometerpossible.

A=0.04m2

h=1.8m

P=?

mPiston =60kgPatm=0.97bar


In accordancewith the 0th law, any system that possesses an equation of state thatrelates temperature T to other accurately measurable properties can be used as athermometere.g.anidealgasobeystheequationofstate:

mRPVT

ExperimentallyobtainedTemperatureScales:theCelsiusandFahrenheitscales,arebasedonthemeltingandboilingpointsofwater.Theyarealsocalledtwopointscales.Conventional thermometry depends onmaterial properties e.g.mercury expandswithtemperatureinarepeatableandpredictablewayThermodynamic Temperature Scales (independent of the material), the Kelvin andRankinescales,aredeterminedusingaconstantvolumegasthermometer.

Documents

Intro and Basic Concepts-Therm