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1、.....Chapter15ElectricFieldsSupposewefixapositivelychargedparticleq1inplaceandthenputasecondpositivelychargedparticleq2nearit.FromCoulomb'slawweknowthatq1exertsarepulsiveelectrostaticforceonq2.Thenyoumayaskhowq1"know"ofthepresenceofq2?Thatis,sincethechargesdonottouch,howcanq1exertaforceonq2?Thi
2、squestionaboutactionatadistancecanbeansweredbysayingthatq1setupanelectricfieldinthespacesurroundingit.AtanygivenpointPinthatspace,thefieldhasbothmagnitudeanddirection.Thuswhenweplaceq2atP,q1interactswithq2throughtheelectricfieldatP.Themagnitudeanddirectionofthatelectricfielddeterminethemagnitud
3、eanddirectionoftheforceactingonq2.Anotheraction-at-a-distanceproblemarisesifwemoveq1,say,towardq2.Coulomb’slawtellsusthatwhenq1isclosertoq2,therepulsiveelectrostaticforceactingonq2mustbegreater,anditis.Doestheelectricfieldatq2,andthustheforceactingonq2,changeimmediately?Theanswerisno.Instead,th
4、einformationaboutthemovebyq1travelsoutwardfromq1aselectromagneticwaveatthe学习参考.....speedoflightc.Thechangeintheelectricfieldatq2,andthusthechangeintheforceactingonq2,occurswhenthewavefinallyreachq2.15.1TheElectricField1.Thetemperatureateverypointinaroomhasadefinitevalue.Wecalltheresultingdistri
5、butionoftemperatureastemperaturefield.Inmuchthesameway,youcanimagineapressurefieldintheatmosphere:itconsistsofthedistributionofairpressurevalues,oneforeachpointintheatmosphere.Thesestwoexamplesareofscalarfield,becausetemperatureandair-pressurearescalarquantities.2.Theelectricfieldisavectorfield
6、(1)Itconsistsofadistributionofvectors,oneforeachpointintheregionaroundachargedobject.(2)Inprinciple,wecandefinetheelectricfieldatsomepointnearthechargedobjectbyplacingapositivechargeq0,calledatestcharge,atthepoint.(3)Wethenmeasuretheelectrostaticforcethatactsonthetestcharge.Theelectricfieldatpo
7、intPduetothechargedobjectisdefinedas.(4)WerepresenttheelectricfieldatpointPwithavector学习参考.....whosetailisatP,asshowninthefigure.(1)TheSIunitfortheelectricfieldisthenewtonpercoulomb(N/C).15.2ElectricFieldLines1.MichaelFraday,whoin
