《大学物理简明教程》 课件 11-2 Conductors and dielectrics

TypesofmaterialsConductorsSemiconductorsInsulatorsSuperconductors(negative)

chargesfreetomoveallchargesfixedsomechargescanmovenoresistanceatall!metals,someliquidsolutionsglass,ceramicsilicon,germaniummostmetals,someceramics,verylowtemperaturesAtomsinmetal

:onlyoneorafewelectronsintheoutershellTheseelectronscanmoverelatively

freelybetweenatomsElectronsinCu1s(2)2s(2)2p(6)3s(2)3p(6)3d(10)4s(1)diameter:1cmcopperhas1freeelectronperatomvolume:=0.52cm3density:8.96g/cm3mass:4.69gatomicmass:63.5g/molehowmanyatoms?1moleis:Numberofatomsinthesphere:6.02x1023atoms4.44x1022atomsmolesinsphere:0.074Totalnumberoffreeelectronsinthesphere,inCoulombs:~7122CHowmanyfreeelectronsarethereinasmallcoppersphere?4.44x1022chargedwith1microCoulomb7122Conlyadding10-6/7000=oneelectronin7billion!Evenabigchargeadds/removesaninsignificantamountofelectrons,anddoesnotdisturbthematerialConductorinequilibrium(electrostatic,nocurrent!)All‘excess’chargesgotothesurfaceBecausetheyrepeleachother

andcanmovefreelyintheconductorThenetforce

onchargesinsidetheconductor

mustbezero!Thepotentialisthesame

everywhereonaconductorSo:theelectricfieldinsideaconductoris0And:theelectricfieldisperpendiculartothesurfaceConductorinelectricfield+++---

-------HollowmetalshellwithchargeinsideInnersurface:inducedcharge-qOutersurface:inducedchargeq------HollowmetalshellwithchargeinsideIfthemetalshellhasanetchargeQIfthemetalshellisgrounded------Innersurface:inducedcharge-qOutersurface:inducedchargeQ+qInnersurface:inducedcharge-qPotential:0Outersurface:noinducedchargeAnelectricdipoleconsistsofapairofequalbutoppositepointchargesof

Qseparatedbyadistanceofl.WhatistheelectricfieldatthepointP,locateddabovethemid-pointbetweenthetwocharges?P+Q-Qld

dipolemoment

WhatistheelectricfieldatthepointP,whichisontheextensionbetweenthetwochargesofanelectricdipole,and

adistancedaway?P+Q-Qld

dipolemoment

Q-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+polarizedatomDielectricsNotallthechargesininsulatorsareperfectly“fixed”unpolarizedElectricfieldbecomesweakerduetothepolarizationofthedielectric!Allnon-conductingmaterialscanberegardedasdielectricsUsually,the“additional”fieldduetopolarizationisproportionaltotheoriginalexternalfield

sodielectric

isthepermittivityofthedielectric

capacitorwithdielectricmaterialBiggercapacitanceCanstoremorecharge(withcertainvoltage)

electriccharges

maybedeflectedbetween2“charged”platesCathodeRayTubeJJThomson’sCathodeRayTube1897CRTtube(CathodeRayTube)computermonitortelevisionoscilloscopeyouwantmaximumdeflectionforashortertube!anodevoltage:platevoltage:plateseparation:anodelength:platelength:AccelerationbythefirstanodeDeflectionbytheplatetransittime:changeinm