-- VECTORANALYS昭 L1PropertiesofVectorsandCoordinateSystems Ordinarynumbersarccallcdscqlqrs・Theymayberealorcomplexnumbcrs・In contrasttoscalars,wehaveotherquantitiescalledDectors、Thesequantitiescom‐ binewitheachotherdiHbrcntlythanscalarsJnphysicstheyareusedtorepresent objectsthathavebothmq9"imdeanddirectio",theprototypeofwhichisadisplace‐ ment,Mathematically,vectorsaresimplyquantitiesthatbehaveandcombineaccordingtothelbllowingrules、 1.Thcsumoftwovectorsuandvisanothcrvector:u+v=w、Thesumisa commutativebinaryOperation;thatis,u+v=v+Ⅲ 2.Undersummation,theassociativelawholds,Forvectorsu,v,andw, (u+v)+w=u+(v+w)=u+v+w 3.Anyvectorcanbe“multiplied,,byascalartoyieldanothervector・ Weshallrepresentvectorsgeometricallybydirectedlinesegments(i、e、,arrows). Themagnitudeofthevectorisproportionaltotbelengthofthelinesegment,and thedirectionisgivenbytheorientationofthearrow-thatis,thcdirectioninwhich itpoints,TherulestobeMlowedinperfbrmingthis(vector)additiongeometrically arethese(seeFig.11):Onadiagramdrawntoscalelayoutthedisplacementvector u;thendrawvwithitstailattheheadofu,anddrawalinefromthetailofutothe headofvtoconstructthevectorsumw・Thisisadisplacementequivalentinlength anddirectiontc1thcsuccessivcdisplacementsuandv、Thisproccdurecanbcgen‐ cralizedtoobtainthesumofanynumberofsuccessivedisplacemcnts. 1.1.1B日seVI3CtorsamdCoordinateSystems ChoosingacoordinatesysteminspaceisessentiaIlyequivalenttochoosingasetof basevectors・Ifwechooseacartesiancoordinatesystem(Fig.1.2),ourbasevectors arechosenlLobealongthreelixedmutuaUyperpendicular(orthogonal)fixcddirec‐ tionscalledthex,y,andzdirections・Ifwerepresentavectorbyanarrow,the 1 ZVECTORANALYSIS 鋤uureLJGeometricaldefinitionofthesum oftwovectorsuandv. Z y X 胸"「B1.2Definitionofthecartesiancoordi‐ natesystem,showmgtheassociatedunit vectors. perpendicularprqjectionsofthearrowuponthethreecoordinateaxcsarecalledthe cartesjq〃compo"e"tsofthevectorinthesedirections、Intermsofthesccomponcnts, themagnitudeofavectorAisaslbllows*: MagnitudeofA=|Al=い:十A;+A:)uz AunitvectorAisthatvectorwhichwhenmultiplicdbythemagmtudelAl,yields thevcctorA;thatis,A=|AlAItprovidesameanslbrindicatingdirection・Unit vectorsalongx,y,andzcoordinateaxcs(cartesian)aredenotedbyk,’'2,respec‐ tively、Theyprovideaconvenicntandlimdamentalsetofbasevectors・Intermsof cartcsianunitvcctors,anyvectorAisrcpresentedby A=dェjt+A,,+Az2(11) whcreA罪,4,,andA葱arethecomponcntsofAalongtheA,,,and2directions, respectively、 Weshallrestrictourattentiontocaseswherethebasevectorsfbrmanorthog- onalset・Moreoverthemagnitudeofeachbasevectorwillbetakenasunity(orthonormal).ThemostcommonlyusedbasevectorsinordinarythrCe-dimensionalspacc aretheunitbasevectors(n,'’2).ThesevectorsareconsideredtobecoFIsm"WCC‐ tors・Neithertheirdircctionsnortheirmagnitudesdepcndonwhelctheyarelocated withrespecttosomerelerencepointinspaceItistheconstancyofthisortho‐ normalsetofbasevectorsthatwewishtoemphasizebythewordcq汀esjq". *ThroughOutthisbookscalarsareinitalicsandvcctorsaにinboldItlce. 1.1PROPERTIESOFVECTORSANDCOORDINATESYSTEMS3 TherepresentationofvectorsusingtheunitvectorsisveryuselUlinvector manipulations・Forexample,toaddAtoBwesimplyaddthecartesian components: A+B=仏兵+Bェ)R+い,+B,),+(Aご+Bご)2 Itisfrequentlyconvenienttouseothcrsetsofbasevectorswhosedirectionsdo happentodependontheirlocations(curvilinearbasevectors)Forexample,we shalldelineandoftenuseasphericalcoordinatesystemandacylindricalcoordinate system・ForeachofthesesystemsweshalIfindanorthonormalsetofassociated basevectorsthatdependonwhereinspacetheyarelocated RecallthatthecartesianunitvectorRmaybedefinedastheunitvectorthatis perpendiculartoanyplanex=constantSimilarly,lbrtheland2unitvectorswe respectivelyassociatetheplanesy=constantan。z=constant・Now,thereare othersurfacesthatoncca、describethatcorrcspondtosomegeometricalvariable beingconstant・Ifwecanfindthreesurlaces,definedby(three)geometricalvari‐ ables,thatintersecteachotherperpendicularlyatapoint,thenatthispointwecan definethreeassociatedmutuaUypelpendicularvectorsthatar己normaltothese surfaces・Indescribingthesphericalandcylmdricalsystems,wecitetwoinstances wherewelindituselilltodoso.(Therearemanyothcrs.) Inthecylindricalcoordinatesystem(Fig.1.3)wedefineasetofbasevectorsata pointbyconsideringsurfaces,twoofwhichareplanesandoneofwhichisa cylinder・Thesurfacesaredenotedbythelbllowingequations: (a)z=constant 〔I,)β=constant=、/〕FTア, (c)①=constant=tan-1(y/x) Inequation(a)thezcoordmatespecifiesasetofparallelplanes・Itisdefinedby relbrcncetoareferenceplanecaUedthez=OplaneTheunitvector2isthena constantvectorpointinginthe(positive)dircction(whichmaybechosenarbitrari‐ ly),perpendiculartothez=constantplanes・Thezaxisischosentobealine pointinginthezdirection(fbr-oo<z<+CO). Z ソ 死 鞠H、℃1.3DefinitionofthecylindricaIcoordinate system,showingtheassociatedmitvectors. 4VECTORANALYSIS InCquation(b)thepcoordinateisdefincdwithreierencetothezaxisbyasetof cylindricaIlycircularsurfaccsthatintersectthez=constantplanesperpendicularly・ Thedistanccphomaparticularsurlacetothezaxisistheradiusofthecylindrical surfaceTheunitvectorPisperpendiculartothecylindricalsurface,pointingaway lromthczaxisJtsdirectiondependsuponwhatgeometricalpointofthecircle resultingfromintersectionoftheplanesz=constantandp=constantiscon‐ sidered・Thus,inFig.13,Pisaiimctionofthevariableddefinedinequation(c) (lbrp>O). Inequation(c)theonlysurfacesthatcanperpendicularlyintersectthepreviously definedsurfacesatallpointsofintersectionareplancsthatcontainthezaxis・One ▲ suchplaneiscallcdthe′=OplancandischosenarbitrarUy、The○unitvectorslie perpendiculartotheの=constantsurfblces,anddependupontheangleのofthe planewithrelerencetotheの=Oplane Theintersectionofthesuriacesdescribedbyequations(a),(b),and(C)above locatepointsinspacejustastheintcrsectionofthecartesiancoordinateplanesdo However,thecylindricalunitvectorsarewellspecifiedonlywhenapoint(noton thezaxis)isspccilicd.(Theoriginisspecifiedbysettingz=Oandp=0.)Oncethis hasbcendoncbyassigningvaluesof(,,巾,z)。r(x,y,z)tot1Jepoint,anyvcctormay beexpressedintermsofthecylindricalunitvectors(2,0,巾)atthatpoint、 Onecaneasilyshowthatthesecylindricalunitvectorsarerelatedtothecartesian unitvectorsbythefbllowingrclations. A P==fcosの+,sinの巾=-Rsinj+,cosj2=2 (1.2) jb RcmcmbcrthatPand巾dependuponthecoordinaPj.Thus,IbranyvcctorAand 鯛.Wlhi;搬紺鯛i鰯聯嚇。,i,綜繩槻紬:源 basevcctorsusedtoexpressAwillbethatdefinedbythelocationr・Notethatthe displacementvectorTtoapoint(z,p,の)isgivenbyr==叩+z2. Weshallnotdescribethesphericalcoordinatesystem(Fig.1.4)inthedetailused abovelbrtlibcylindricalsystem,excepttonotethattheconstantsurlaceschosenare asfbllows: (a) (b) に) r=constant=、/;5ZT7ZT三whichdescribcsasphcreofradiusrwith respecttotheorigin O=constant=COS~'(z/rLwhichdescribesarightcircularconewithopen‐ inganglea d=constant=tan~'(y/X),whichdescribesaplanecontainingtheaxisof theconein(b). Theunitvcct・rsprescribedbythcsesurlacesaredcnotedf,04,respectivcly,and fbrmanorthonormalsetoncethepoint(notattheorigmandnotatthezaxis) locatedbytheintcrsectionofthethleeorthogonalsurfacesisdeterminedThese unitvectorsarcgivenintermsofthecartesianunitvcctorsbythefbllowing relations. f=父sinOcos中+jsinOsinj+zcosO ▲ O=父cosOcosの+jcosOsind-2sinO 6=-ksin中+,COS巾 (1.3) LlPROPERTIESOFVECTORSANDCOORDINATESYSTEMS5 b Z 」〉 y X 胸"r21.‘Delinitionofthesphericalcoordinatesystem, showingtheassociateduniWectors. IfAisamemberofavectorlield,A(r),thenatevcrypointgivenbythedisplace‐ mcntronccan目xpressA(r)intcrmsofthebasevectorsassociatedwiththatpoint: A(r)=Arf+A00+A中中,whereA,istheprqjectionofAonAandsoon、Thc displacementvectortoapoint(「,0,.)isgivensimplyr=G7、 1.1.2TheScalarProduct(DotProduct) Animportantconceptinvectoralgebraisthatofthescalarproductoftwovectors・ ItisdenotedbyA・Bandalsocalled‘otprod"ctori""erprod皿α、Itisdefined accordingtotheIbllowingruIe:A・B=|AllBlcoso上,wherelAlandlBlarethc magnitudesofAandB,andoCistheanglebetweenAandB・Itcaneasilybeseen thatthescalarproduct,asdefined,hasthelbllowingproperties:Twovectorswhose scalarproductiszeroaresaidtobeorthogonal;thatis,ifAB=0,Aissaidtobe orthogonaltoBTheunitcartesianvectorsR,,,and2arcsaidtoconstitutean o〃ho"0mmlsetofbasevectorsbecausetheyareorthogonaltoeachotherandtheir magnitudesarenormalizedtounity. 1.1.3TY1eVectorPmduct(CrossProdUct) WehaveseenthatwecanassignascalartoanypairofvectorsTheoperationthat doesthisiscalledthescalarproduct・Wenowwishtoassignavectorquantityto anypairofvectors,AandB,andsowedefinewhatisknownasaDectorprodtJct(or crossprod脚α);itisde、otedbyAxBThedirectionofthevectorproductistaken tobeperpendiculartotheplanedeterminedbythepairofvectorsItsmagnitudeis givenbytheareaoftheparallelogramwhosesidesarefbrmedbythevectorpalr・ Therelbre,ifhisaunitvectorperpendiculartotheplanelbrmedbythevectorpair (A,B),thenthcvectorproductisde6nedaccordingtothelbUowingrule AxB=lAllBlsinaii(1.4) 6VECrORANALYSIs 霧 JYU"「F1・sDefinitionoftheright-handscrew convention,whichgivesthesenseofthecross productofthevectorsAandB. SofarthedefinitiongivenremainsambiguousbecausethenormaltotheplancofA andBmaypoint“up,,or“down."Tospccilywhichwayiipoints,weusetherjght‐ hα"dscrewco"DC"tjoPl、WesaythatifAisrotatedtothCdirectionofB,throughthe angleaに180。),thenthesamcsenceofrotationgiventoaright-handedscrew determinesii,whichpointsalongthedirectionofadvanceofthescrewasitis rotated(Fig.15l IntelTnsofcartesianumtvectorsthevectorproductisexpressedas AxB=jt[Aj,B霊-A二B,]+,[AzBx-AxBz]+2似蒸By-AyBz](L5) ,4酌 父上瓜 、AxB= 鮴…‘ (1.6) Wheneverwehavcasctofthreeorthonormalvectors,、1,62,ande3,wesaywe havca'i9ht-hα"dedsystemwhen6ix§=Gkwhcrej,j,kareintheorder(1,2,3), (2,3,1),or(3,1,2).ThesearecyclicpermutationsoftheintegCrsl,2,3.Notethat fbraright-handedsystemgivenbythctriplet{6,,62,63},onehase1・(e2xe3)=L Thecartcsiancoordinatesystemwehaveusedisright-handedifweidentilyxwith l,Jノwith2,andzwith3、Alq/t-liqndedsystemisamirrorimagCofaright-handed systCm・ TWousefUlidentitiestobcrcmcmberedareaslbllows: 1.Triplcscalarproduct A・(BxC)=(AxB)。C=(CxA)・B(1.7) (ItisthepollJmeofaparallelepipedwhoseedgesareA,B,andC) 2.Triplevectorproduct Ax(BxC)=B(AC)-C(A・B)(1.8) Thelatterlisficqucntlyknownasthe“backcab”rule、ItwillbenotedinEq.(1.7) thatthcdot(・)andthecross(×)maybefrcclyimerchangedsolongas{A,B,C) rcmainincyclicordcr. L2ElementsofDisplacement,Area,andVolume;SolidAngle 1.2.1ElementofDiSplacement Considertwopointsinspace(x,y,x)and(x+△x,y+△y'2+A2).Thefirstpointis displacedrelativetothesecondbythcdisplacemcntAr;thatis, Ar=jtAx+,△y+2A2(19) L2ELEMENTsoFDIsPLAcEMENT,AREA,ANDvoLuME;soLIDANGLE7 Z y X 胸ElreJ.‘Thecoordinatesoftwonearbypointsin cylindricalcoordinatesthatmaybeusedtodefinethedilL lerCntialdisplacementsinthissystem expressedinacartesiansystemAdiflbrcntialelementofdisplacementisconscquentlywritten ‘r=dxR+dyl+dz2 (1.10) WenowwishtoexprcssAr(ordr)intermsofcylindricalandsphcricalcoordinatcs andtheirassociatedunitvectors、WeagainassumethatArmaybemadearbitrarily small,inthelimitcallingitdr・ considerFig.1.6,wheretwopointsarしdisplacedbyAr・InthecylindricalcoordinatcsystemwchavebasevectorsthatarcdilYbrcntatthetwopointsland2Thus Ar=p2p2+2222-(ハハ+z12,)(1.11) where Pz=p,+APP2=ハ+APZ2=z,+Az2z=2,=2(1.12) SubstitutingEq.(1.12)inEq.(1.11)anddroppingproductsofdifYbrcntials,weobtain Ar=Apハ+p1Ap十A22 (113) Ifpoints(1)and(2)areclosecnoughtogetherthen,togoodapproximation, ムト鶉帥 ForApsufYicientlysmall(6,-62=⑤wccansecthatAp=|ハlAji・AsaEcsult, Eq.(1.12)becomes Ar=Apハ+p,△。,+Az2 (1.14) Aspoint2approachespointl,wecanwritethedifYerentialdisplacementas ‘r=dlbp+dノ`巾十.【’ (1.15) dl,=dpd4=pdddL=。z (1.16) whelc aretheelcmentsofdisplacemcntinthep,の,andzdircctions,respcctively・Thus ‘lr=dpp+,。の‘+dz2 (1.17) 8vEcroRANALYsIs zHl 皿an 吻醒1.7DiHerentialdisplacementsinspher‐ O icalcoordinates. whereいゆ,z}arcdelinedatthepointwhcrethedisplacementdrismade・GCC‐ metricallythisisanaturalresult,sinceclosecnoughtoanypointofspace(2,,,①) wecandefinea“cartesianlike,,systemintermsofwhichanyelementoflengthmay beexpresseddirectly Insteadofderivingtheelcmentofdisplaccmentatapointintermsofspherical unitvectorsatthatpomt,weshaUsimplymaketheremarkthatatanypointthe unitvcctorsM,6}servetoestablishacartcsiansystemlocaUy(nearthepoint)It wiUbesecnfromFig・L7thatthcclementsoflengthalongthethreedircctionsncar thispointaregivenby ‘し=`「。l0=r`0.10=rsinOdの(L18) andthat へ 。『=drf+,dOO+,sinOd仰=。!『f+d40+`!`巾(119) へ 1.2.2EIementofSurfaceArea Havingdeterminedexpressionsfbrelementsofdisplacemcntinvanouscoordinate systems,wecannowdetermineclementsofsurfacearea・Thcrcarethrceclementsof surfaceareafbrcverycoordinatesystem;theseareofthefbrm‘I,〃2,dl2‘13,and dl3dl1・Forcartesiancoordinates,wchave dxdydydzdzdx (1.20) correspondingtothesurfacesz=constant,x=constant,andy=constant,respec- tively・Similarly,fbrcylindricalcoordinates,clementsofsurfaceareaonthesurfaces thatdefinethecoordinatesare dzdppdp⑭pdの。2 0.21) Forsphericalcoordinateswehave wlrdOr2sinOdO`のrsinOdj力(1.22) Adirectionmaybeassociatedwithanclementofarea・Thisdirectionisnormalto thearea・IfdJidぃstheclemcntofarea,thenormaldirectionisgivenbythccross productejxq,andwemaydenotetheareaasaVCctor(6,.4)×(`川)=Mlldlj. l2ELEMENTSOFDISPLACEMENT,AREA・ANDVOLUME;SOLIDANGLE9 0 D゛ 〃、  ̄ 伽 Z =、 。 3㎡Bad y X (6) 胸luyごI、8DehnitionofplaneandsoIidangles.(α)P1arueangIedO (6)SolidangledQ. Olienweshallmoresimplydenotetheelemcntofareaasda=h‘α,wherethesensc ofnmustbespecifiedbutisalwaysnormaltothesurface. 1.2.3SOlidAungle Whenanarcelementdsofacircleinaplaneisreierredtoitscenter,weusethc conceptofanangledO=CMwherCristheradiusofthecircle(Fig.18α).Onthc otherhandwhenanelementofsurfaceareaisrefbrredtoanorigin,itjsolten convenicnttousetheconccptofsoljdan此(seeFig.1.86).ThediHbrmtialelement dQofsolidanglcwithrespecttotheoriginisdefinedaslbllows: …等=竿=型 r2 (1.23) Here,thesurfaceelementdtJislocatedatapointdisplacedfromtheoriginbythe vectorr=fr,andhenceγistheanglebetweenOandfSinccda・fisjustthe elementofareaofasphereofradiusr,thensubstitutingdq=r2sinOdOd中,wesec thatdQisalsogivenby dQ-血誓。藤。-'2sin8`6`の=sinodOdのM) rz whichisanelementofareaofaunitsphere Physically,thesolidangleisthe“openingangle”ofaconewhosesidesintercept theareaelemcntinqucstionThus,justasfbrordmaryangularelementsdO,wherc l0VECTORANALYSIS 〈、 〈、 ' 、 〈、 】I 3 0 胸”e1.9111ustratingwhythesolidangle subtendedbyaclosedsurlacewithrespectto anorigininsidethesurklceis4兀andtoan originoutsidethesurlaceiszero. wehaver`O=dlc…=小Csし,fbraninfinitesimalelementofarcaofasphere (whichapproximatesarectangularplanararea)wehavedasphcrc=r2sinOdOd①,。α =r2dQ、Theunitofsolidangleisknownasthesterqdmnltisclearlyanalogous totheunitofangle,theradianAnylinitesolidangleisexpresscdas Q=IdQ=IsinOdOdj,whereOand①arethesphericalcoordinatesofthcspher‐ icalsurfaceelementinterceptedIfwehaveasurfacethatcompletelyenclosesthe origin,thenQ=4兀.Ifwehaveaclosedsurlacethatdoesnotenclosetheoriginand ifwechoosethedirectionofdaalwaystopointoutlromthecloscdsurlace(orinto thcsuriacc),thenQ=OThisis(seeFig.1.9)essentiallyduetothelactthatibreach positivecontributionofsolidanglethereisanequalcontributionofnegativesolid angle,asseenlromtheorigin 1.2.4EIementofVoIume RememberingthatA・(BxC)isthevolumeofaparallelepiped,wehavethatthe volumeelcmentlbrasystcmofbasevectors{6,,62,,3}issimplygivenbythevcctor tripleproduct: ‘、=dノ,、,.(。!262×,ノノ363) or ‘、='6,.(e2xe3)|dl,。ノュdl3 wherethed↓arcthemagnitudcsoftheelcmentsofdisplaccmcntsalongthedirec‐ tionofthercspcctivcbasevcctors§・Thus,inthecartcsiansystcm, 。U='2.(,×2)Mxdydz=dxdlydz (1.25) Inthecylindricalsystemwehave へ 。、='2・(px0)ldzdpp⑭=pd2dpdj(126) andinthesphcricalcoordinatesystcmwehave ▲へ 。u='6.(O×。)|`rrdOrsinOd①=r2sinOdrdO⑭(1.27) L2ELEMENTSOFDISPLACEMENT,AREA,ANDVOLUME;SOLIDANGLE11 1.3Gradient lfwewisbtoexprcssthechangeinascalarfUnctionofposition/(r)atthelocation spCcifiedbyr,thenwriting/andrincartesiancomponents,wenndthediHerential changetobc …襄十`,静雄墓 WenowdefinealineardilYerentiaI“vectoroperator,,caUcddel,andsymboIizedV asfbllows: ,臺徽急十,急十m是 (1.28) ‘=(WMr (1.29) Sincedr=Rdx+,。〕+2.z,thenlromthedefinitionofthedotproduct, whercWisavectorpointfimctionjandiscalledthegradie"tqノノ: 狐にw-捌癸+,筌十2壬 (1.30) Someapplicationsinvolvetheoperationoftheabovegradientoperatoronvector ficldslfwewishtoexpressthechangeinavectoriieldAatthelocationspecifiedby r,thenwritingAandrincartesiancomponents,wefindthediHbrentialchangetobe ‘…姜十`,警十`帯[`蕊云十`,鼻+`釘△ UsingEq.(1.30LonecanshowthatthcscalarproductofdrandVis Thus ‘w-["呈川,急+`塾呈] (131) 。A=(drV)A(r) (1.32) Inwords,thescalaroperator(dr.V)actingonavectorpointhmctionA(r)generates thespatialdifYerentialofA,。A,atthepointinquestion. 0 c+〃 胸耀1.IOTheuseofthesmface(『)=c=constanttoshow thatthegradientofthelimction/(r)isnormaltothissurface. 12vEcroRANALYSIs ThegradientgiveninEq.(1.28)hasanmterestinginterpretation(seeFig.1.10). SupposcwehavethescalarfUnction/(r).Asurlnceisgeneratedifweset/(r)equaltoa co"stqFDt,c・Thisbeingthccase,ifwefindthedifYerentialof/(r)whendrconnectstwo pointsoftheSurface/(r)=c,then(callingdいhissurfacedisplacement)/(r+`⑲= /(r)=C,sothat〃=OThisimpliesthat‘=(W)・`r図=0,whichinturnim‐ pliesthat(W)isperpendiculartodr,、Sincedrswasdelinedtolieinthetangentplane tothesurfaceatr,wesCethatWisperpeJMic卿lqrtothesurfacc、Nowthevalueofthc changCin/(r)whenwcmovetoancighboringpoint(r+伽)notinthesurlacc,isgiven byEq.(1.29):ガーW・`r・Therefbre,ifldrl=。s,〃/as=(WMVlis=W・i,wherei isaunitvectorinthedirectionofdr・Thederivatived//【lsisknownasthedirectio"ql dlerjDatipeofメThemaximumvalueoMWlswillbeobtainedwheniliesalongWIn thiscaselW・il=|W|=〃/ds…;thatis,themaximumratcofincrcascinthe lilnctionノ(r)withrcspecttodisplacementsdTisgivenbyW;andWpointsintbe dircctionofmaximumincreasaThesepropcrtiesofWaresummarizedbytheIbrmula w-m藍 (1.33) whcreiid"isthcdisplaccmentdrperpendiculartothesurlaceノーconstant(mthe directionofthemaximumincreaseinDItisfbrthisrcasonthatitiscalledthegradient ofメAsanexample,consider/=x=c,whichdefinesasetofplancs,onefbreach valueofc,perpendiculartothexaxis・clearly,thelactthatW=jtshowsthat(a)the perpendiculartotheplanesliesinthexdirection,and(b)therateofincreaseof/in thisdirectionisunity,Asanotherexample,letノーx2+y2+z2=72.Thenonefinds W=276,wherer=xi+y,+z2Theperpendiculartothesurlacesノーcareinthe directionsoftheradiitothepointsonthespheresgivenbyxz+y2+22=72.Therate ofchangeof/inthesedircctionsisjustd/Wlr=27.Finally,notethatsinceWis perpendiculartothesurfaceノーconstant,itcanbcusedtogeneratetheunitvectors ofcoordinatesystcmsThus -- i vlM (1.34) isaunitvectorpcrpendiculartothesurlace/(x,y,z)=constant、 WenowiindanexpressionfbrWwhere/isanyscalarpointhmctionintermsof cylindricalcoordinates・Todothisweturntoourdefiningequationfbrthegradient: ▲ `rW=`lfWritmgdr=Pdp+ipdの+2`lzandW=P(W),+巾(W〃+2(W)塁, aswecanalwaysdoatanypoint{,,の,z},gives "=(W),。'+(Wbpdj+(W)zdz Notingthattherulesofcalculusrequire 炸莞`,+莞"+筌傘 andequatingthetwoexpressionsfbr〃gives 腓嘉(wルー潟(wルー美 or w-,差十$鵠+鶚 L4THEDIvERGENcEoFAvEcroRANDGAuss,THEoREMI3 Hence,thcdel(V)operatormcylindricalcoordmatesis ’一,完+`誌十塾呈川 Similarly,onecanobtaincxpressionslbrWandVinsphericalpolarcoordinates: 可一助0|〃 l|》1|“ 〈抑へⅦ 可一”6|卵 十+ llrllr A0へ0 〃一介6房 |’|| ▽▽ (1.36) 1.4TheDivergenceofaVectorandGauss,Theorem Foravectorfield,A(r),wcshalldelinethedivergence,writtenvariouslyasdivAor V.A,bytheCxpression ,A一隅[半]m whercAレisadiHbrentialvolumeandSisitsSurface、MoreoverV・Amaybeshown tobeequivalentlydefinableasthcscalarproductofVwiththevectorA・WewiU firstusetheintegraldelinitiontoderivethedivergenceofavectorcxplicitly・ConsiderasurflceSwhosesurlaceelementisdenotedbWla,thentheintegral ’一L…川 iscalledthe`yh4xofAthroughS.”clearly,asinourdiscussionofsolidangle,Sis assumedtohaveanonentation(withrespecttotheoriginofthecoordinatesystem used),andingcneralmayormaynotbeclosedTheelementofUuxthroughdais correspondinglygivenbydF=A・dawhereAisevaluatedatthecenteroMa・Now considerthcinfinitesimalvolumeelementdp,showninFig、1.11.ThesidesofdDare givenbythesixspherical-coordinatesurlaces:「=c,「=c+d「;0=c'’0=c'+do, (6) 〉 FiD…1.11Determinationofthedivergenceofavec‐ torinsphericalcoordinates. 14VECrORANALYSIS の=c",の=c"+dの,anditsvolumeis「ZsinOdrdOd①.WecomputethefluxesdF1, .F2,…,ulF6throughthesidcsl,2,…,60fthevolumeelementdcscribedabove (andUlustratedintheligure). 。F,+dF2=-(「2A,sinOdOdj),+(r2ArsinOdOd‘)2=。[r2sinOd8dのA』2,, .F3+dF4=-(rsinOdのdrAo}3+(rsinOd①drAルー。(rsinOdjdrd`}4,3 .F5+`IF`=-{rdOdrA小+(rdOdrA中}`=。(rdOdrAや)6,5 Thequantiticsinthebracescanbeevaluatcdatthecenterofthcfaces、Thus,lbr surlacesland2onlylimctionsofrarediflbrent: ‘(んin",`“M_`(r2win,`,`トー(『塾4}…`M Similarly, ‘(『Sim`…)…`(圏inMM岬晶(sinO』`M`『`‘ Therdbre ‘(MM…-念い,M`M 研A薑ト鵲鶚`,] thatis, 。A÷烏(柵赤[烏(…)十割(L,,) WenowusethedirectscalarproductV・Atoarriveatthedivergenceinthe variouscoordinatesystems、TheVoperatorinsphericalcoordinatesisgivenbyEq. (1.36),andthus …[,多半:烏+論念]M(。+"] Expandingtheindicatedscalarproduct,wegctninetcrms: A ▽乱→呈[……咄鵠[M…十帖] A A aL A +鈴急M…Aj Jh Now,perfbrmtheilnpliedderivatives,notmg,fbrcxample,that 2ドル筌峨'等 67 andsofbrth・ThcrewiUbel8individualterms,butifwerememberthat 6.0=f‘=06=0,andsoon,weobtainjustl2nonzeroterms: A A v…二A,+等+臘豐…祭4. 十二諾4+鴻砧鶚十鵠糺, +鈴覺4+論景朏鈴慕い元而而 ▲A 1M① 1.4THEDIVERGENCEOFAVECTORANDGAUSS,THEOREM15 ZiJ6化1.J 6 A 0 O U房6|”6|姉 0 0 ▲ 0 ▲ -6 中sinO 0 `cosO-[OsinO+Ocosm 濡搬i§,,樵離篭嚇鵬臘嚇瞬蝋纈柵鯏霊 glvenprcviouslymEq.(1.3).Forexample,sinceO=kcosOcosd+jcosOsin① -2sinO、andthecartesianunitvectorsareconstanMO/60=R(-sinO)com +,(-Sim)sin①-2cosO=-G.Substitutionwillyicldthesameresultwegotin L39・ TheoperatorVincylindricalcoordinateswasshowntobc[seeEq.(1.35)] v-,急十$鵠+2基 ProceediPl9bjノthedjrectpJ・odHctmethodtocql“lateV・Aincylindricalcoordinates andusingthefactthat2isaconstantunitvectorandthatOand中dependonthe ▲ coordinateのonly[seeEq.(1.2)],wefindthattheonlynonzeroderivativcofthe unitvectorsare6P/6①=6and6巾/6の=-P・Thcrefbre wA-急い:』け:急峨筈 or …蒜(",)+誌峨筈 (1.40) Finany,inthecaseofcartesiancoordinatestheoperatorVwasshowninEq.(1.28) tobe v-m呈十'二十2基 ExpandingV・Adircctlygives wA-等+等等 (1.41) Z1beDilwge"CCZ】b巴”emrQzuuss,Z1bBorEum).Finallyweprovearelationthatis veryusefUlinelectrostatics:thedivergcncetheorcm,whichinvoIvesthedivergence operationFromourdelinitionsofV・Awehave a 口α A r‐、T血 (V・A)△、= 16VECTORANALYSIS '1 si,△zIi S,し ノ 鋤"℃1.J2Provingthedivergencetheorem(Gauss, law)bysubdividingthevolumeyofsurlaccSinto manyinlinitesimalvolumcsADlofsurlacessiandthe applicationofthedefinitionofthedivergenceinterms oftheinlinitesimalvolumes. ● lbrsmallAp、Consideravolumel′withasurfaceSsubdividedintoNsmallvolume elementsVDi,withsurlacessiasshowninFi9.1.12.Then zI,Aw弓菫f… IfwetakethcIimitasN→ooandADi→0,ontheleItwehaveavolumeintegral andontherightasurlaceintegral Thelactthat 〃A`,-仏。川 JimLP妙。 becomesanimegraloverthesurfaceextemaltothewholevolumccanbesecnlrom Fig.1.12.NotethatthenetHuxthroughanyintemalsurfaceiszero,bccausesucha surlaceiscommontotwocontiguousvolumeelementswhoseoutwardnormals pointoppositelyonthecommonsurface・AllthatremainsareHuxesthroughsur‐ ibIccsnotcommontotwovolumeelements-namcly,surlhlcesonthesurfaccofy itself Equation(1.42LwhichisknownasthediDeP・ge"cetheorem(orGa卿ss,theorem), provesveryusehllwhenonewishestorelatevaluesofavectoriieldonthesurface ofaregiontovaluesintheinterior・ItoltenwiUhappenthatwemaywantto convertasurlaceintegraltoavoIumeintegral,orviceversa,aswillbeshownin Chapters3and4. L5TheCurlandStokes,Theorem WenowintroduceanotherusclUloperationmvolvingV・Foravectorfield,A(r),we shalldelinethccurlwrittenasVxAorcurlAbytheexpression VxA= 隅戯`…] (1.43) 1.5THEcuRLANDsToKEsTHEoREM17 Z y X Fi〃…1.J3Determinationoftheexplicit lbrmofthecurlofavectorincartesianco- ordinatesusingadillbrentiaIrectangularbox. whereAuisasmaUvolumeofsurfaceareas,Thisdefinitioncannowbeusedto determineVxAinacartesianrepresentation、Considerthevolumcclcment dp=dxdydzshowninFig、1.13.Thecontributionslromtheoppositesidesland2 totheintegralldaxAare l"…(-M,…-…,-,幼 L`…-…xAI-`MM,軌 Thesumofthesetwointegralsisthe(partial)difYbrentialoftheexpressionshownin bracestakenbetwcenfacesland2: L`…-…ル拠籔)止加`急(2,WMェ -`假一,等] Similarly,ibrtheoppositesides3and4andoppositesides5and6,wecanreadily showthat. 〃…-`僻-`制し`…-`催-, 割 SummingaUthecontributionsgives 州一興[等一等ル'謄一筈胖際一等] (1.44) SimUarprocedurescanbeusedtodctermineVxAinsphericalandcylindrical coordinates・WemayalsolbrmthccrossproductoftheoperatorVwiththcvector pointlimctionAbyemployingthecrossproductoftwovcctors・OnecaneasUy showthatthisoperationgivesexactlyEq.(1.44). l8vEcroRANALYsIs …-賎l Thcdeterminantmustbeexpandedmtermsofthetoprowtohavemcanin9. SinceVxAislbrmedlikeacrossproductoftwovectors,VandAitiscxpected tohaveavalueindependentofthcparticularcoordinatesysteminwhichitis representedWeshallsee,lbrexample,thatVxAhasaphysicalmeaningand retainsthatmeaningindependentofthesystemofcoordinatcsmwhichAiscx‐ pressed・Thus,weshallassumethatVxAisavector,justasweassumcdWwasa OnecanlbrmaUyobtaindiHerentcoordinatereprcsentationsfbrVxAbywrit‐ ingtheoperatorVandthevectorAinaconsistentsctofcoordinates・Wcneedonly partialderivativescqualtozcro(unlikcthccartesian{父,9,2}unitvectors,whosc derivativesarealwayszero).Thismethodgivesthelbllowingexpressionlbrthecurl ”A-僻一等]+`[讐一劉化院1M′鶏](!") Equation(1.46)indicatesthatthercisnosimpledctcrminantnotationthatcanbe appUedtothecurlofavectorincylindricalcoordinates・However,onecanstm …|; ;Ⅱ Similarly,onemayderivethefbllowihg expressionfbrVxAinspherical coordinates. ”A一命隈(い、昨念仏・)ル`[志詩-号(M・)] +`隠仙汁淵 …満際l (1.47) SYoAes,Z1beorEm・FinaUyweproveaveryusefi」lrclationinmagnetostaticsthat 1.5THECURLANDSTOKES,THEOREM19 =、 e み、 n 町”e1.IjDeterminationOfthecomponent ofacurlofavectoralongaunitvcctoraby appIyingthedefinitionofthecurltoadilL lelcntialpillboxofnegligibleheight6,andaxis alonge. elementofsurfaccS,showninFig、114,intheibrmofacylinderwhosetopand bottomeachofareaSbareperpendiculartoGandwhosesidesofwidth6andarea Sbisparallelto6UsingEq.(1.43)lbrthisvolumeelement,wehave ▽…去曵… WenowcalculatethecomponentofVxAalong6、Takingthedotproductof6 withVxAgives: …A-鈍…△仏鯛) 。……Wl戦岼~…T If6issuflicientlysmall,thenonewritesda=Jdlii,where‘lisanelementoflength alongtheband,andiiisaunitvectornormaltotheband、Notingthatnx6=i, 舵i縦:i蝿9搬雛雛競臘冊:蜜'…副…u…`nA …A一三{", ASD→0,scistheareaborderedbythepathofi、tcgrationandcontainsthevector i・Thesenseofcirculationisrelatedtoeasthetumingofaright-handscrewistoits advancaWrittcnlbrmally. …-概臥A欧]川 whereSerepresentsanareawhosenormalisparaUeltoeandwhoseperimeterof lengthCisthepathofintegrationNotethatthemtegraljcA・driscaUedthe circulationofAaroundCWithEq.(1.49)onecanreadilyobtainthccomponentsof VxAinorthogonalcurviUnCarcoordinates. Z0vEcToRANALYsIs WenowuseEq.(1.49)toprovcStokes,theorem,whichrelatcstheHuxofthecurl ofAthroughasurfacetothecirculationofAaroundtheedgeofthesurface・ ConsideranopensurlaceSwhoscperipheryisaclosedcurveC,WetakeCtobe sjmp/yco""ccに。;thatis,itcanbccontinuouslyshrunkdowntoapointwithoutthe curvesleavmgthespacaThesurlaceShastwosides,oncofwhichwedeclarethe positiveside,Wenowsubdividethissurlacemtovectorelcmentsofarea(ejAqj) whichareessentiallyplanarifAajaresmallenoughForeachoftheseareaelements wemayapplyEq.(149)ibrthecomponentofthecurlofAinthedirection6j;as fO11ows: ⅢA岼釧/W,。,mAM・’一妙U`O) WenowlbrmthesumofthecxpressionsmEq.(1.50)fbrqllthesurlaceelements ofthesurfaceSThus zWW4・Fzf心`‘ IngoingtothelimitasAaj→Oandthenumberofelementstcndstoinfinity,the left-handsidebecomesanintegraloverthe(open)surfaceS、Theright-handside bec・mesthelineintegralaroundcurveC,SincecontributionsoflA・`ribrallline elcmentsinternaltoScancel,andonlythecontributionsoftherimofS(thatis,on qremain・Thcrelbre LいAM-li A・ar (1.51) whichisknownasStokes,theorcmThepositivesideofSandthesenseinwhichC istraversedarerelatedviatheright-handconvention. AnimmediateutilizationofStokes,thcoremisthederivationofthecriterionfbr determiningwhetherafieldisc…ruQ`iueornotJfiA.。r=0,fbrallpossible closedpaths,inaregionofspace,thenitlbllowsthatVxA=Ocvcrywhereinthis region・Theconverseisalsotrue、Suchavectoriscalledaconservativcvector・We maytherefbresummarizethecriteriathatdeterminewhetherornotavectorfieldA isconservativeinaregionofspacelfinsomesimplyconnectedregiononeofthe lbUowmgrelationsholds, い-゜…… VxA=O (1.52) A=Wlbrsomescalarlimction/(seeExampleL2) thenAisaconsewativefieldOneofthesccriteriabemgtruethroughoutasimply connectedregionofspaceimpliesthattheothertwoalsoaretrue. L6VectorManipulationsofV 1.6.1SingIeDelOperations Wehavediscussedthemeaningsofthegradient,divergcnce,andcurloperations、In sodoingwehavecometoregardthedeloperator,V,asaquantitythatacquires meamngonlybyoperatingonwhatis“totherightwofit,butotherwisebehavesin 1.6VECTORMANIPULATIONSOFV21 manylcspectslikeanordinaryvector・Wewishhcretosummanzesomeofthese operations・If/and9arescalarIimctions,andAandBarcvectorfimctionsof positioninspace,then V(/+9)=W+V,(1.53) v・(A+B)=7A+v・B (1.54) V×(A+B)=VxA+VxB (155) V(〃)=(W)9+/(V,)(1.56) V・(/A)=(W).A+/(7A)(157) V×(/A)=(W)xA+八V×A)(1.58) V(A・B)=(B・V)A+Bx(VxA)+(A・V)B+A×(VxB)(1.59) v・(AxB)=(vxA)・B-(vxB).A (1.60) V×(AxB)=(B・V)A+A(VB)-(A・V)B-B(ワA)(1.61) ThesefbrmulasmayallbeprovedbyexpressingVincartesiancomponentsand comparingbothsidesofthcaboveequations. 1.6.2DoubleDelOperations ThedeloperatormaybeappliedseveraltimesinsuccessionConsideringascalar pointfUnction/wehave,fbrexample,V・WandVxWIncartesiancoordinates, ,w-(捌会刊÷@基)(A昊十'蟇柁妥) SinceR・父=,,=2.2=1and”=父12=,.z=0,wchave l w'一傷+暴劇ノー耐 (1.62) ThisiscalledtheLaplacjα〃operator、TheLaplacianinothercoordinatescanbe detcrminedusingsimilarprocedurcs・IncyUndricalandsphericalcoordinatesitis glvenby w-誌㈹+滞十裏 w一幕('2:O+論晶(…劉半論纂 (1.63) (1.64) ThecurlofthegradientofascalarVxWcanalsobedetermmedUsingsimilar procedures・Incartesiancoordinatesone駒rites 20|ゐ 可一生 ,0|〃郎一の Wwll Z2vEcroRANALYsIs Expandinggives vx(w)-,偽-紛號,儂一紛十`偽-蒜) butfbrwcll-bchaved,continuousfUnctions6ツ〕/Oy6z=0伽zOy,etc.,thecurlofthe gradientofascalarlimctionvanishes[seeEq.(1.52)definingconservativcvectors]; thatis, VxW=0(1.65) Withavectorfieldfonecanlbrmthevariousdoubledclexpressions・Onecan show,however,that(VxV)・fan。(VxV)xfvanishThequantityV・(Vxf),the divergenceofthccurlofavectorcanalsobeshowntobezcrobydirectcalculation incartesiancoordmates;thatis, V・Vxf=O (1.66) Equation(1.66)isimportantinmagnetostaticssincethedivergcnceofthemagnetic fieldBisknowntobezero(V・B=0),thenitallowscastingofthemagneticiieldB intermsofavectorpotentialA: B=VxA FinallywediscussthecurlofthecurlofavectorVx(VxD・Thisdoubledel operationhaswideapplicationinthepropagationofelectromagneticwaves,atopic tobediscussedinthelaterchaptersofthisbook・RegardingVasavectorlVx(V xf)canbeexpandedbytheusualtriplevectorproductax(bxc)=b(a.c) -(a.b)cThisgives V×(Vxf)=V(V・f)-V.(W) (1.67) whereWisasecond-ranktensorordyadic(seeExampleL3).Incartesiancoordi‐ nateswehaveV.M)=(V・V)f=V2fwhereVzisthelaplacianoperator. L7VectorlntegralRelations HerewcdiscussanumberofextensionstothcdivergencetheoremandStokes, theorcmAlthoughwewillnotneedalloftheseextensionsfbrthedevelopmentof electricityandmagnetismatthelevelofthisbook,weincludethemfbrthesakeof completenessandasamturerelbre、Ce. DiDwgBDu“Z1IeD'F、、TheintegralrelationsgivenbelowinEqs.(1.68)to(1.72)are extensionsofthedivergencetheorem LMw川wM-l1owM‘(し`8) ThisiscalledGreen,sfiriitidentityortheorem L・閥,-WM-L(川-"。Ⅳ。u`'1 L7vEcToRINTEGRALRELATIoNs23 ThisiscalledGreen,ssecondidentityorsymmetricaltheorem llMv-炎(iM`・一生`… 作…L・iMo 1mwwM薑企EM1d。 (1.70) (1.71) (1.72) Equations(1.68)and(169)canbeprovcdeasilybyapplyingthedivergcnce theoremtothevcctorF=⑪WandF=⑩W'-しVの,respcctivcly,wherCのand しarescalarhmctions・ Equations(1.70)and(1.71)canalsobeprovcdbyapplyingthedivergence theoremtothevectorF=AxCandF=のC,respectivcly,whereCisaconstant vector. 8mA“,Z1b2o肥JPu・ThelbllowingintegralrelationsarcextensionsofStokes, theorem. {…-LM1…。 企…LいⅧ-1… (1.73) (1.74) ThesetworelationscanbeprovedbyapplyingStokes,thcoremtothevector F=BxCandF=⑩C,respectively,whereCisaconstantvector. ExampIeL1VeIocityFieldinaWaterDrain A Considcravectorfieldgivenbyv(r)=poalBdshowninFig、1.15,whercpisthedistance fromthezaxisのisaconstant,and巾isassociatcdwiththeangularcoordinatejaboUtthez axislsvconservative? い‘ -回 R(ハンゥ 殉”21.JSVelocityfieldinawaterdrain. 24vEcToRANALYsIs Thatitisclcarlynotconscrvativcissccnltomconsideringitscirculationonacircularpath ofradiusRaboutthezaxis: j……に."姉1M…, Thecirculationisnonzero,andthe”lbrevisnonconservative,Thisexamplemightbringto mindthevelocityfieldofwatergoingdownthedraininasink・Thccirculationofthewateris notingeneralzerolbrsuchasystcm. Example1.2CCI聾ewativeNatureofRadiHIVector ̄PotentialFunctions ConsideraradialvectorfieldgiveninsphericalcoordinatesbyA=/(r)fwhercノ(7)isa scalarfimctionthatdependsonronly・Wewnlshowbelowthatthisvectorisconservative・ ThecriteriathatdeterminewhetherornotavectorisconservativearegiveninEq.,(152). SubstitutingA,=/(7),andAo=Aの=OinVxAinsphericalcoordinates(Eq、1.47)immedi‐ atelygivesVxA=0;thusindicatingthatAisconservative RadialvcctorIieldsarCofimportancetoelectmstaticssincetheelcctricfieldproduccdbya pointchargeisradial,andhenceitisconservativeBecauseoftheimportanceofthisprop‐ erty,wewillexaminetheconservativenatureo「thesevectorsfromthcpointo『viewofthc lastcriterionofEq.(1.52).IfAisconscrvativc,thenitmustbewrittenasthegradientofa scalarfimction⑩;thatis,A=V⑩、ThelimctionのiscalledapotentiaMimctioncorrespond‐ ingtoA,Toshowthisweconsideralimction 。-トーい`' lftheintegralexistsandisacontinuoushmctionofr,thcnfromtheexpressionfbrthe gradientinsphericalcoordinatesweseethat -- 0 一一 ”|姉 ”|” 7 1 1 一一 〈J ”一ケ The艇sultisthatthereexistsahmctionのsuchthat V⑩=ノ(r)f (1.75), Aimdeedhasapotcntialfimction. Example1.3GmdientofaVector-Dyadics Thisexampledealswiththegradientofavector,whichwillbeusefUlwhenwedealwith lbrcesonclectricdipolesplacedinextemalelectricfields・ConsideravcctoTE=ExR+E,, +E爵2.Formally,wecandefineVEasfbIlows: Expanding,wegCt ,画一(m呈令,急糀呈)'……) ,膠一儂鰄+警船筈鋤)+傍,粁等,,半等'2) +(等鐡半警鰄÷料 (1.76) Thequantities鰍,R,,…arccalledMPIitdWuエノ3.NotcthatR,,lbrexampIe,isnotthesamc aslf;thuswehavcnincdiflbrentunitdyadsinthegradient、Aquantitythatcanbeexpandedinthelbrm ⑩=a11jtR+q1zR,+α1322+α2,,2十α22”+α2392+α3,22+α3229+α33雌(1.77) iscallCdadyadic,andtheninccocHicientsava”itscomponents. 1.8SUMMARY25 ItisusclUltoexaminethescalarproductofavectorA=A』+Aj,,+dz2andadyadic のofthefbrmgivenabovc・ConsidertheproductA.⑩FormaUy,wewrite. A・⑩=八雲1.⑩+A,,.⑩+Aga.⑩ AsanexampIeconsidertheproductR・の.Theproducthasninetcrms,mcludingfbrexample, 父・q11jtA,父・q12fl,f・口2,批,and父.α3ユ21.Theseindividualproductscanbeevaluatedusing thefbllowingrules: jt・q11RR=α,,(2.2)A=α1,k k.q12R,=α12(R・父ルーq121 jt。α2,,A=α2,(R・,)R=0 2.α32”=ロココ(f・2),=O (1.78) AnalogousrulesexistlbrtheICstoftheproductsandlbrtheproductsl・のand2.⑩See ProblemL20fbraspecificexample. ExampleL4DiracDeltaFunction InthisexamplewemtroduceaveryusefUlfimctionfbrdcalmgwithpointcharges・WewUl dcnneithcrefromamathematicalpointofview・ItsrelevencetoelectmmagnetismwiUbe introducedlatcr、TheDiracdeltafimction,giventhesymbolNr)isdefinedasfbllows: 刑=o1brr≠0 0.79) ル)`'-1(川 wherctheintegraliscarriedoutoverallspace、ThisdehnitionshowsthatthedeltalUnctionis averyhighsingularmathematicallimction;itiszeroeverywhereexceptatasinglepointand yethasanonzeromtegral("spike,,Iimction).ThisfUnctionisobviouslynotacontmuous onc;thusitshouldnotbcdiHbrentiatcdasacontinuouslUnction・Neverthelessitisavery usefUlmathematicalpropertyifhandledcautiousIy、 AnotherpropertyoftheDiracdeltafimctioncomesfromitsrelationtotheLaplacianor divergenceoperator;thatis, ,声-,2e)-伽川 Itiseasytoshowbydi塵ctdilYerentiationthat ,声-vい÷響 SinceV.r=3,V(1/r3)=-30/P.4,thcnWvr3)iszeroIbrr≠Oandbecomesindeterminateas r→OThenatureofthedivergCnceatr=Ocanbcexaminedusingthedivergencetheorem・ ApplyingthetheoremtoasmallvolumeofradiusRgives 仁畠`。一際`・一志}… SincethisrcsultistrucregardlessofhowsmallR,thenonecanrCplacetheV・け3)by4元6(r). 1.8Summary Whenanelementofsurlacea琵adaatrisrcferredtotheorigin,itisoltenconvenienttouse theconceptofsolidangledQ: 幻一等-…`,(,川24) Z6vECToRANALYsIs IfwehaveasurlaceSthatcompletelyenclosestheorigin,then。=fsdQ=4祁,whereasifwc haveaclosedsurlacethatdoesnotenclosetheorigin,thenQ=0. ThegradientoperatorVisalineardiHerential``vectoroperator”thatcanoperateOn scalaraswellasvectorfields/andAtogivethegradientofascalar,divergenceofavector, curlofavectorbandgradientofavector. w一興二十,墓+魁筌 ,A-筈十等十等 ,…除一等ル,謄一筈胖[警一等] ,A-謄鰄+等鋤十等鰯]+… (1.30) (1.41) (1.44) (1.76) lnthelastequationtheellipsis(…)representsthreeanalogoustermsfbreachofthederiva‐ tiveswithresPccttoyandz・ThegradicntOperatormaybeappliedscvcraltimesinsuc‐ cession,ormaybeappliedtoproductsoflimctions・Theoutcomeofsuchoperationscanbe derivedfromtheabovebasicdilYbrentia]operations・OneimportantoperationistheLapJ IacianoperatoractingonascalarorvectorlimctionVW=WorV2A、Allopcrationscam alsobederivedintermsofothercoordinatesystems(e9.,cylindricalandsphericalsystems). someintegralidentitiesofthedel(gradient)operationscanbederivedbymtegratingthe dilYercntialonesoveranarbitraryvolumePboundedbyaclosedsurlaccS,oroveranopen surfnceSboundedbyaclosedcurveCTheseincludethedivCrgencetheoremandStokes, theorcm. 仁…尖… [……い, (1.42) (1.51) AvectorAissaidtobeconservativeif VxA=Oor い-゜(..…… (1.52) Ifso,Acanalsobewrittenasagradientofascalar A=一V⑪(conservativeA) Problems L1DeterminetheunitvectorperpendiculartotheplanethatcontainsthevectorsA=Zjt -61-32andB=4A+31-2. 1.2DetclmineanequationlbrtheplanepassingthroughthepointsP,(2,-1,1),P2(3,2 -1),andP3(-1,3,2). l3ThcpositionvcctorsofpointsP1andP2areA=31t+,+22andB=jR-21-42 DetermineanequationfbrtheplanepassingthroughP2andperpendiculartotheline joiningthepoints、 1.4(a)ShowthatVr風="r"~zr.(b)FmdVlnlrlandV(1/"、 nSConsiderthesurIacedefinedbythecquation2xzユー3xy-4x-7=OFindaunit vectornormaltothissurfaceatthepoint(1,-1,2). 1⑩Considerthelimction⑩=xZyz3・InwhatdirectionliromthepointP(2,1,-1)isthe directionaIderivativeof⑩amaximum?Whatisthemagnimdeofthismaximum? 句再。■伊閃筥唾い】 』勤 再酌 窪・三富ョ・{号喫)Ⅱニマ洲○・ 円二重・忌少目。■閏:。易曾く昌憲・宵・く二言(少x■騎鰯・一③:苣色ニー詳言二皇宮三富駒 = 巳 一 騨N③『○ロー竜⑰『碗③己。⑪。 (輯)ロ鴛曾曰冒③弓③8罵言ロ露角一寺》自己⑤蒔馨③ご③具○再診Ⅱ(謀+脚]+疸些坤十{胃’ぜ 1卿)》+冠H+こ+鰐)》尉胃○一畳○目一一》・口器塁農ぐ⑨.e)】)里③宮口旨③号○昏曰蔓ope》  ̄  ̄ ●  ̄ 重寄寓⑰』Ⅱ『e・ ]●■】 ニミ彦罠⑫芦。昌旦号③。。■駒冨昌色ず③萬号⑰急。(。『少Ⅱ(H+凶』)浄十(』-凹卿)》十(罵十白蜘)騨雷 ⑫○行曰。』□稗一(ず画⑫N③討○昌一く③吋媚③口。③)や 駒二.聿一曽貰号③く⑰⑤ご『画Ⅱ且『牌厨8口器弓自】く①.□③富】蔑昌回③e碗巨&昏筥閂Ⅱ-ぐe自已 e(色)ⅡP望弓③昂pVC・ 】・】い 切豈○竜二皀弓③ご⑰。【。『抄Ⅱ(②韓苣+町〕)沖十(]蔑凶1N)》十(]潴恥-竜)坤一⑫。○回⑫③『置菖ご⑰・田ご二 △。、 ● ■  ̄ ̄  ̄ ̄ 号①8目⑰舌○口昌邑碩己C冨昌旨一言ロ三○国e曽昌昏皀シⅡ『e・ 口③雷『ヨーロ③『・『曲已□岳③皆『雷。⑪一罠農『巴一助『・与冒・頭「意思唾蔚湧⑥一○麗○豊ユ画。③. 6○局一二曾号③ご⑱旦○再少Ⅱ』託鈴-吋、》+闘念騨口旦忌③『③四○口け。■ご○⑰色ず嗜識』+、Ⅱ一・ UU = ●  ̄ 軸Ⅱ。騨再○凶Ⅱ】。(聾)O巴⑪『ヨヨ③『・少.(ず)ロ⑰(。p凰口⑪号⑰巨昌(『⑪○s忌口○吋目囚]【○善③ 皆ユ芭○囲い】(いⅡ。}・駁(いⅡ』}鱒ロー言⑥。胃ぐ⑰且豊熟画。。堕』{x凶十、Ⅱ」).(o)ぐ⑰ユ辱曽⑪ 昌蔚『礪③旨⑰s8忌己註『少冨百二○宮③『号③守○巨二②&忌璽。■筈○ぐ②. 6。■⑫昼③『二③く③。ご『少Ⅱ墹誇十薦》-』宅』愚図ロ○号⑪⑫■里買溺。『(ず。○臣辱邑旦③『駒。⑰罰邑。』ず臣 批吋十託Ⅱ一○(回)ロ③(○閏昌再③二③巨已声ぐ③日。『ロ○回己騨三○一意⑫口且胃⑰。害意⑥里一言○国■陽画 冒再&○二○『蔑騨且』.(ず)向く昌冨冨昌⑱旨1宵⑤冒冨関昌一“』,二畳。ご曾号⑰印三。⑥冨昌 。、  ̄ ●  ̄ 碗昌静。⑰。『号⑰昌一旨。③『ず③豆「⑪昌悶Ⅱ。酋己○町Ⅱ山. 6。■呂二曾二⑪ご③○s『しⅡ(吋潴-])声-》恥》-、璽夢”宮。、号③臣ごロ③『ずこ『駒■ユ四○③。『吾⑰ ⑫宮:魏紬十、+純いⅡ一.この『辱浬・穴⑪二言・『§・岸『x夢・写冒Ⅱ}、少・員壽:0-鰯  ̄ ̄QC-コ い」 ̄= ̄ ■●●C 一一一一 号③ず。■■○餌昌○二二⑥豊『ず(稲吻・ 勺『○ぐ①晉皀(⑬)一切②号Ⅱ。酋国己(す)}『x》』ロⅡ◎ず閂揮ロ竃⑤一○望邑旨塁質潴・ 勺『○く③号昌一『x再号Ⅱ鈩与x因冒自皀(ご一『e署Ⅱ一s』』白・ ロ⑰[③員昌弓⑰『凶一口『》『凹型閏回已司凹{」(『}(『#。.) 。○冨匡③『弓③昌舌&○sⅡ浄蝉十二+露(鷺浦田愚目ご]⑰】・】}・口③【曾曰目⑰『.(e・閂)閏口。 (『.e)・閑・房二③忌画国電四目亘晦昌巨旨軍「閂三二媚『・ep『己 □輿⑪国ご旨⑰善⑰、園島③三○『『。
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