物理学专业英语1

BasicConceptofPhysicsUnitsanddimensionsPhysicalquantitySayaplankis2meterslong.Thismeasurementiscalledaphysicalquantity.Inthiscase,itisalength.Itismadeupoftwoparts:2mMagnitude(number)unitNote:‘2m’reallymeans‘2×meter’,jusas,inalgebra,2ymeans‘2×y’SIbaseunitsScientificmeasurementsaremadeusingSIunits(standingforSystèmeInternationald'unités).Thesystemstartswithaseriesofbaseunits,themainonesbeingshowninthetablebelow.Otherunitsarederivedfromthese.PhysicalQuantityUnitNameSymbollengthMetermmasskilogramkgtimesecondscurrentampereAtemperaturekelvinKamount*molemol*:Inscience,‘amount’isameasurementbasedonthenumberofparticles(atoms,ionsormolecules)present.Onemoleis6.02×1023particles,anumberwhichgivesasimplelinkwiththetotalmass.Forexample,1mole(6.02×1023)ofcarbon-12hasamassof12grams.6.02×1023iscalledtheAvogadroconstant.SIderivedunitsThereisnoSIbaseunitfrospeed.However,speedisdefinedbyanequation.Ifanobjecttravels12min3s:m/s4s3m12takentimetravelleddistancespeedTheunitsmandshavebeenincludedintheworkingaboveandtreatedlikeanyothernumbersoralgebraicquantities.Theunitms-1isanexampleofderivedSIunit.Itcomesfromadefiningequation.Thereareotherexamplesbelow.Somederivedunitsarebasedonotherderivedunits.Andsomederivedunitshavespecialnames.Forexample,1joulepersecond(Js-1)iscalled1watt(W).PhysicalquantityDefiningequation(simplified)derivedunitSpecialsymbol(andname)speeddistance/timems-1-accelerationspeed/timems-2-forcemass×accelerationkgms-2N(newton)workforce×distanceNmJ(joule)powerwork/timeJs-1W(watt)pressureforce/areaNm-2Pa(pascal)densitymass/volumekgm-3-chargecurrent×timeAsC(coulomb)voltageenergy/chargeJC-1V(volt)resistancevoltage/currentVA-1Ω(ohm)Prefixes:PrefixescanbeaddedtoSIbaseandderivedunitstomakelargerorsmallerunits.PrefixSymbolValuePrefixSymbolValuepicop10-12kilok103nanon10-9megaM106microμ10-6gigaG109millim10-3teraT1012Forexample:1mm=10-3m1km=103mNote:1gram(10-3kg)iswritten‘1g’andnot‘1mkg’.Dimensions:Herearethreemeasurement:Length=10marea=6m2volume=4m3Thesethreequantitieshavedimensionsoflength,lengthsquared,andlengthcubed.UsingthesymbolLforlength,thesedimensionscanbewritten:[L][L2][L3]Startingwiththreebasicdimensions–length[L],mass[M],andtime[T]–itispossibletoworkoutthedimensionsofmanyotherphysicalquantitiesformtheirdefiningequations.Thereareexamplesonthebelow.Example1:][][][takentimetravelleddistancespeed1LTTLExample2:][][][volumemassdensity33MLLMPhysicalquantityDefiningequation(simplified)Dimensionsfromequationreducedformlength--[L]mass--[M]time--[T]speeddistance/time[L]/[T][LT-1]accelerationspeed/time[LT-1]/[T][LT-2]forcemass×acceleration[M]×[LT-2][MLT-2]workmass×distance[MLT-2]×[L][ML2T-2]powerwork/time[ML2T-2]/[T][ML2T-3]pressureforce/area[MLT-2]/[L-2][ML-1T-2]Usingdimensionstocheckequations:Thetwosidesofanequationmustalwayshavethesamedimensions.Forexample:Work=force×distancemoved[ML2T-2]=[MLT-2]×[L]=[ML2T-2]Anequationcannotbeaccurateifthedimensionsonbothsidesdonotmatch.Itwouldbelikeclaimingthat‘6applesequals6oranges’.Dimensionsareausefulwayofcheckingthatanequationisreasonable.Example:CheckwhethertheequationPE=mghisdimensionallycorrect.Todothis,startbyworkingoutthedimensionsoftheright-handside:mgh=[M]×[LT-2]×[L]=[ML2T-2]Thesearethedimensionsofwork,andthereforeofenergy.Sotheequationisdimensionallycorrect.Note:Thedimensionscheckcannottellyouwhetheranequationisaccurate.Forexample,bothofthefollowingaredimensionallycorrect,butonlyoneisright:PE=mghandPE=2mghDimensionlessnumbersApurenumber,suchas6,hasnodimensions.Herearetwoconsequencesofthisfact.DimensionsandunitsoffrequencyThefrequencyofavibratingsourceisdefinedasfollows:takentimesvibrationofnumberfrequencyAsnumberisdimensionless,thedimensionsoffrequencyare[T-1].TheSIunitoffrequencyinthehertz(Hz):1Hz=1s-1DimensionsandunitofanglersθOntheright,theangleθinradiansisdefinedlikethis:rss/rhasnodimensionsbecause[L]×[L-1]=1.However,whenmeasuringanangleinradians,aunitisoftenincludedforclarity:2rad,forexample.Measurement,uncertaintiesandgraphsScientificnotationTheaveragedistancefromtheEarthtothesunis150000000km.Therearetwoproblemswithquotingameasurementintheaboveform:Theinconvenienceofwritingsomaynoughts,Uncertaintyaboutwhichfiguresareimportant,(i.e.Howapproximateisthevalue?Howmanyofthefiguresaresignificant?)Theseproblemsareovercomeifthedistanceiswrittenintheform1.50×108km.‘1.50×108’tellsyouthattherearethreesignificantfigures-1,5and0.Thelastsignificantandtherefore,themostuncertain.Theonlyfunctionoftheotherzerosin150000000istoshowhowbigthenumberis.Ifthedistancewereknownlessaccurately,totwosignificantfigures,thenitwouldbewrittenas1.5×108km.Numberswrittenusingpowersof10areinscientificnotationorstandardform.Thisisalsousedforsmallnumbers.Forexample,0.002canbewrittenas2×10-3.UncertaintyWhenmakinganymeasurement,thereisalwayssomeuncertaintyinthereading.Asaresult,themeasuredvaluemaydifferfromthetruevalue.Inscience,anuncertaintyissometimescalledanerror.However,itisimportanttorememberthatitisnotthesamethingasamistake.Inexperiments,therearetwotypesofuncertainty:Systematicuncertainties:th