浙江大学大学物理甲下 chapter 30

《浙江大学大学物理甲下 chapter 30》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、Chapter30Capacitance130-1CapacitorsAdevicethatstorescharge(andthenenergyinelectrostaticfield)iscalledacapacitor.21.CapacitanceofaisolatedconductorqVcapacitanceThecapacitanceofanisolatedsphericalconductorreads:aTheSIunitofcapacitanceisfarad(法拉):1farad=1F=1coulomb/volt

2、32.CapacitorqADB-qqThesetwoconductorsarecalledplates(nomatterwhattheirshapes).4Symbolically,acapacitorisrepresentedas:CCstandsforthecapacitanceofacapacitor.Remarks:1.Acapacitorissaidtobechargedifitsplatescarryequalandoppositecharges+qand-q.qisnotthenetchargeonthecapa

3、citor,whichiszero.52.Thecapacitorscanbeusedtoproduceelectricfield.⊕⊕⊕⊕⊕⊕+q⊖⊖⊖⊖⊖⊖-qForthetimebeing,weassumethattheplatesofacapacitorexistinavacuum.3.Incircuits,capacitorsareoftenusedtosmoothoutthesuddenvariationsinlinevoltagethatmaydestroycomputermemories.630-2Capacit

4、anceThecapacitanceisageometricalfactorthatdependsonthesize,shapeandseparationofthecapacitorplates,aswellasthematerialthatoccupiesthespacebetweentheplates.730-3CalculatingtheCapacitanceProcedure:Supposethatthecapacitorischarged,with±qonthetwoplatesrespectively.2.Findt

5、heelectricfieldEintheregionbetweentheplates.3.Evaluatethepotentialdifferencebetweenthepositiveandnegativeplates,byusingtheformula:84.Theexpectedcapacitanceisthen:AParallel-plateCapacitor9ASphericalCapacitor10Example:WhatisthecapacitanceoftheEarth,viewedasanisolatedco

6、nductingsphereofradiusR=6370km?1mF=10-6F1pF=10-12F11ACylindricalCapacitorThecapacitorhaslengthL,andL>>b.12Quickquiz-1AcapacitorstoreschargeQatapotentialdifferenceΔV.Ifthevoltageappliedbyabatterytothecapacitorisdoubledto2ΔV,thecapacitancefallstohalfitsinitialvalueandt

7、hechargeremainsthesame.(2)thecapacitanceandthechargebothfalltohalftheirinitialvalues.(3)thecapacitanceandthechargebothdouble.(4)thecapacitanceremainsthesameandthechargedoubles.13Quickquiz-2Manycomputerkeyboardbuttonsareconstructedofcapacitors,asshowninFigure.Whenakey

8、ispusheddown,thesoftinsulatorbetweenthemovableplateandthefixedplateiscompressed.Whenthekeyispressed,thecapacitance(1)increases,(2)d