Discoveryprocess
Aristotle’svacuumprincipleoftheabsenceofnaturehasbeenusedforcenturiestoillustratethefactthatifyouwanttoemptyabarrelofwine,youmustbeatthetopatthesametime.Andthebottomopening.Becauseitisnaturallynotallowedtocreateavacuuminthebarrel,unlessairentersthebarrelfromthetop,thewinewillnotflowoutfromthebottom.Foralongtime,peoplehavebelievedthatthisprincipleholdstrueinallsituationsincludingvacuum.Butinthe16thcentury,peoplediscoveredthefactthatifthevacuumpumpusedtopumpwaterfromtheminewasplaced30feetabovethewaterlevel,thepumpwouldnotwork.Thisledpeopletothinkaboutthenaturalabsenceofvacuumforwinebarrels.Oneprinciplehascertainrestrictionsonthepump.
In1630,GiovanniBallianiofGenoadiscoveredasimilarlimitationonthesiphon.Whenhetriedtosuckwaterfromareservoirover60feethigh,hefoundthatthesiphonwasnotworking.Whenthesiphoniscompletelyfilledwithwater,removetheplugsfrombothends,itseemsthatavacuumiscreatedonthetopofthestraw.
About1641,afterlearningofthesediscoveries,GasparoBertiofRometriedtouseamorescientificmethodtodeterminewhetheravacuumcouldbecreated.Tothisend,hedesignedadevicethatconsistsofasphericalglasscontainerwithatubeabout40feetlong.Bertiattachedthedeviceverticallytothesideofthetower,andafterclosingthevalveattheendofthetubeatthelowerposition,hepouredalargeamountofwaterfromthetopopeningoftheglasscontaineratthehigherposition.Whenthetubeandglasscontainerarecompletelyfilled,thetopopeningofthecontainerissealed,andthevalveattheendisopened.Waterimmediatelygushesoutfromthebottomofthetube,thusformingavacuumintheglasscontainer.Thisexperimenthasmadeitclearformanyyears:Ifnaturedoesnothaveavacuum,howcanitbeallowedtocreateavacuuminaglasscontainer?Andwhenthisexperimentisrepeated,whydoesthewateralwaysdroptothesamepositioninthetube??
EvangelistaTorricelli,whoisworkingasGalileo’sassistant,learnedofBerti’stestresultsinFlorence.Galileobelievedthatthepowerofthevacuumsupportedthewaterinthepipe.AfterGalileo'sdeathin1642,Torricelliputforwardhishypothesisthatatmosphericpressuresupportedthewaterinthepipe.Hereasonedlikethis:Weliveatthebottomoftheatmosphericocean,andtheairpressurepropelsthewateratthebottomofthetubetoacertainheight.Hebelievesthattheequilibriumpointisreachedwhentheweightoftheremainingwaterinthepipeequalstheweightoftheupperairpushingdown.Startingfromthishypothesis,Torrizzalidrewseveralinferences:Oneisthatatmosphericpressureinatubewithasealedtopcansupportacolumnofmercuryabout29incheshigh.Thisconclusioncomesfromthefactthatatmosphericpressurecansupporta33-foot-highwatercolumn,andthedensityofmercuryis13.6timesthedensityofwater.Thus,33/13.6×12inches=29inches.Thesecondinferenceisthatsuchatubefilledwithmercurycanbeusedtomeasurechangesinatmosphericpressure.ItwasthisinferencethatfinallyearnedTorrizzalithehonorofproposingthebarometertheory.Intheend,Torizelliconcludedthatifsuchadeviceisplacedinaplacewheretheairisthin,suchasthetopofamountain,theheightofthemercurycolumnwilldrop.
In1647,theFrenchphilosopherBlaisePascalundertooktheverificationofthethirdinferenceofTorriChali.AfterlearningthenewsofTorízalí’sbarometertest,Pascalbuiltabarometerhimself,andhefirmlybelievedinthecorrectnessofTorízalí’shypothesis.Inordertoprovethecorrectnessofthetheory,thatiscontrarytothevacuumprinciple,heaskedhisbrother-in-lawParrytotransportabarometertothetopoftheDome,oneofthehighestmountainsinAuvergne(region)inFrance.Ayearlater,Parryagreedtothisrequest,andheassembledtwobarometersinthemonasteryatthefootofthemountaintostarttheexperiment.Afterobservingthatthetwomercurycolumnshadrisentothesameheight,Parrydisassembledthenextbarometerandorderedamonktomonitorthemercuryheightoftheotherbarometerthroughouttheday.Then,Parrytooktheunloadedbarometerandsetoffforthemountainwithagroupofwitnesses.Assoonashereachedthetopofthemountain,heassembledthebarometer.Everyonewassurprisedtofindthatthemercurysurfaceatthistimewasmorethan3incheslowerthantheoneatthefootofthemountain.Inthesecondtest,heremovedthebarometerandreinstalledittofivedifferentpointsonthetopofthemountain.Theresultswereexactlythesameeachtime.
Halfofthewaydownthemountain,Parryassembledthebarometeragain.Hefoundthatthemercurysurfacewasapproximatelybetweenthemercurysurfacewhenitwasatthetopandbottomofthemountain.Finally,assoonashereturnedtothemonastery,heaskedthemonktoobservethebarometer.Themonk'sreportwasthatthemercurysurfacehadnotchangedsincetheyleftthatmorning.Thus,Pascalannouncedtheresultsofthisexperimenttotheworld'sintellectuals,thussuccessfullyoverturningtheprincipleoftheabsenceofavacuuminnature.
Introduction
Duetotheactionofgravity,nearthesurfaceoftheearth,theearth’sattractiveforceislarge,theairmoleculesaredenselypacked,andthefrequencyofhittingthesurfaceoftheobjectishigh.Theresultingatmosphericpressureislarge.Farawayfromtheearth’ssurface,theearth’sattractionislow,thedensityofairmoleculesislow,andthefrequencyofhittingthesurfaceofanobjectisalsolow,resultinginlowatmosphericpressure.Therefore,theatmosphericpressureatdifferentaltitudesontheearthisdifferent,thehighertheposition,thelowertheatmosphericpressure.Inaddition,thetemperatureandhumidityoftheairalsoaffecttheatmosphericpressure.
Inphysics,theannualaverageatmosphericpressureatalatitudeof45degreessealevel(ie,zeroaltitude)isspecifiedas1standardatmosphere(atm).Thisstandardatmosphericpressureisacertainvalue.Itsvalueis1standardatmosphericpressure=760mmHg=1.0133engineeringatmosphericpressure=1.0133×105Pa=0.10133MPa
Cause
AtmosphericpressureThegenerationistheresultoftheearth'sgravitationalforce.Duetotheearth'sgravitationalforce,theatmosphereis"sucked"towardtheearth,whichgeneratespressure,andtheatmosphericpressureisthehighestneartheground.Theatmosphericpressureinmeteorologicalsciencereferstotheweightoftheatmosphericcolumnperunitarea(atmosphericpressure),thatis,thepressureexertedbytheatmosphericcolumnperunitarea.
Therearetwotypesofairpressureunits:millimetersandmillibars:theheightofthemercurycolumnisusedtoexpresstheunitofairpressure,usingmillimeters(mm).Forexample,apressureof760mmmeansthattheatmosphericpressureatthattimeisequaltothepressureproducedbythemercurycolumnataheightof760mm.Theotheristhemillibar(mb)thatisoftenheardinweatherforecastbroadcasts.Itisaunitthatexpressesthelevelofatmosphericpressurebythesizeoftheatmosphericpressureperunitarea.1mbar=1000dynes/cm²(1bar=1000mbar).Therefore,1millibarmeansaforceof1000dynesonanareaof1squarecentimeter.Whentheairpressureis760mmHg,itisequivalentto1013.25mbar.Thisairpressurevalueiscalledastandardatmosphericpressure.
Variationlaw
Atmosphericpressureiscloselyrelatedtoaltitude,thatis,atmosphericpressuredecreasesasaltitudeincreases.Inthevicinityof1000hPanearsealevel,theairpressuredropsby1hPaforevery10mriseinaltitude;at500hPa(5500m),theairpressuredropsby1hPaforevery20mriseinaltitude;near200hPa(12000m),theairpressuredropsby1hPaforevery30mriseinaltitude;Itisusedinaviationtodeterminethealtitudeoftheaircraft.Thealtimeterontheairplaneistoconvertthealtitudefromtheairpressureheightoftheemptyboxbarometerastherulerofthealtimeter.TheInternationalCivilAviationOrganization(ICAO)assumesthatunderstandardatmosphericconditionssuchasdryair,averagesealevelpressureandtemperatureof1013.25hPaand15°C,andatemperaturedeclinerateofabout11kmbelowthetropopausewithaltitudedecreasingby6.5°Cperkilometer,asThereferencedatumofthealtimeter,theatmosphereinthisstateiscalledtheICAOStandardAtmosphere(ICAOStandardAtmosphere).
Theairpressurechangeswiththealtitudeoftheatmosphere.Thehigherthealtitude,thelowertheatmosphericpressure;thegreaterthedifferenceinaltitudebetweenthetwoplaces,thegreaterthedifferenceinairpressure.
Theweightoftheatmosphericcolumnisalsoaffectedbychangesindensity.Thegreaterthedensityofair,thatis,thegreaterthemassofairperunitvolume,thegreatertheatmosphericpressureitgenerates.
Becausethequalityoftheatmosphereisdenserasitgetsclosertotheground,andthinnerasitgoestotheupperair,thechangeinairpressurewithaltitudeisalsogreaterasitgetsclosertotheground.Forexample,atalowlevel,theairpressuredecreasesbyabout10mbarforevery100meters;atanaltitudeof5to6kilometers,theairpressuredecreasesbyabout7mbarforevery100meters;andatanaltitudeof9to10kilometers,thepressureisreducedby100meters.,Theairpressureisonlyreducedbyabout5mbar.
Theairpressurechangesallthetime.Undernormalcircumstances,theairpressurerisesinthemorningandfallsintheafternoon;theairpressureishighestinwinterandthelowestinsummer.Butsometimes,suchasundertheinfluenceofacoldwave,theairpressurewillrisequickly,butwhenthecoldairpasses,theairpressurewillslowlydecrease.