重邮大学物理英文版 (4)ppt课件

15 04 2020 1 2 2ElectricField ElectricFieldIntensity 1 ElectricField a withoutmedium chargecharge b ether c electricfield QuestionHowdoestheinteractionforcebetweenchargestransmit Threeviewpoints 1832 Faraday 15 04 2020 2 Electricfieldisaspecialmatterdistributesroundcharge Aregionofspacecharacterizedbytheexistenceofaforcegeneratedbyelectriccharge Definition 15 04 2020 3 thefielddistributedroundastaticchargeiscalledtheelectrostaticfield andthestaticchargeisjustcalledthesourcechargeofelectrostaticfield Coulomb sforceisreallyelectrostaticfieldforce 15 04 2020 4 Whatforcewillapositive test chargefeelifplacedintotheelectricfield 15 04 2020 5 2 ElectricFieldIntensity 1 TestPointCharge Purpose Thetestparticleisusedtomeasuretheforceandthusdetecttheexistenceofthefieldandevaluateitsstrength 15 04 2020 6 Thechargeasdetectorshouldbeapointchargewhoseelectricquantityissmallerfarthanthesourcechargeinordertodecreasethedisturbanceasmuchaspossibletothesourcechargeanditselectrostaticfield Requirement 7 2 Definition UnitN C V m Atapointinthegivenelectrostaticfieldtheratiooftheelectricfieldforceonthetestpointchargewithitselectricquantity isnotrelatedwithvalueof andiscalledtheelectricfieldintensityatthepointinelectrostaticfield 15 04 2020 8 Discussion Magnitudetheelectricfieldforceonunitpositivecharge Direction UnitN C V m theuniformelectricfield non uniformelectricfield thestaticorsteadyelectricfield thenon staticelectricfield ispresentwhetherornotweintroduceatestparticleintothefield 15 04 2020 9 Vector 15 04 2020 10 3 SuperpositionPrincipleofElectrostaticField Thetotalelectricfieldintensityatapointinspaceequalsthevectorsumoftheelectricfieldintensity 15 04 2020 11 4 Calculation 1 DuetotheSystemofPointCharges Thechargesarecountable 15 04 2020 12 TheelectricfieldatpointPduetoagroupofsourcechargescanbeexpressedas whereriisthedistancefromtheithchargeqitothepointP thelocationatwhichthefieldistobeevaluate isaunitvectordirectedfromritowardP 15 04 2020 13 2 DuetoContinuousChargeDistributions P ElementAnalysisMethod isavectordirectedfromthesourcechargetowardthefieldpoint 15 04 2020 14 Theintegralisavectoroperationandmustreduceittoascalarintegral Theintegralisoverallthechargecreatingthefield 15 04 2020 15 ChargeDensity 15 04 2020 16 Reducethevectorintegraltoascalarintegral 15 04 2020 17 Example1ElectricFieldofaDipole AnelectricdipoleconsistsoftwopointchargesQand Qseparatedbyadistantofl asinfigure Manyneutralatomsandmoleculesbehaveasdipoleswhenplacedinanexternalelectricfield Manymolecules suchasHCL arepermanentdipoles H ion CL ion Theproductiscalledelectricdipolemoment 15 04 2020 18 TheelectricfieldatthepointP perpendicularbisectortothedipoleaxis Question 15 04 2020 19 r l r r r 15 04 2020 20 Example2 TheElectricFieldDuetoaChargedRod P X a l Arodoflengthlhasauniformlinearchargedensity andatotalchargeQ CalculatetheelectricfieldatapointPalongtheaxisoftherod adistanceafromoneend 15 04 2020 21 chooseanelementofthechargedistribution Expressthechargedqoftheelementintermsoftheothervariableswithintheintegral inthisexample thereisonevariable x Usedxtorepresentthelengthofonesmallsegmentoftherod Thechargeonthesmallsegmentis ThefieldintensityduetothissegmentatthepointPisinthenegativexdirection anditsmagnitudeis 15 04 2020 22 Eachelementofthechargedistributionproducesafieldinthenegativexdirection andsothevectorsumoftheircontributionsreducestoanalgebraicsum Discussion IfthepointPisfarfromtherod thenlinthedenominatorcanbeignored and Thisisjusttheformofapointcharge 15 04 2020 23 Example3 AringofradiusRhasuniformpositivechargeperunitlength withatotalchargeQ CalculatetheelectricfieldatapointPontheaxisoftheringatadistantxfromthecenterofthering TheElectricFieldDuetoaUniformRingofCharge 2 R x 1 Discussion 15 04 2020 25 Example4 BelowfigureshowsacircularplasticdiskofradiusRthathasapositivesurfacechargeofuniformdensity onitsuppersurface WhatistheelectricfieldatpointP adistancexfromthediskalongitscentralaxis TheElectricFieldofaUniformlyChargedDisk 15 04 2020 26 5 Sum up CalculatingtheElectricField Problem solvingstrategy Analysisthedistributionofcharges Applyingthesuperpositionprincipleofthefieldintensity Ifthechargesarecountable theresultantfieldisthevectorsumofthefieldsduetotheindividualcharges whenconfrontedwithproblemsthatinvolveacontinuousdistributionofcharge youmustdividethechargedistributionintoinfinitesimalpieces andthevectorsumiscarriedoutbyintegratingovertheentirechargedistribution El