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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?
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