《大学物理简明教程》 课件 英文 11-1 Electrostatic field.pptx

Physics2:ElectricityandThermodynamics

toreviewElectricChargeElectricForceElectricFieldElectricPotentialcalculateforsomespecialcases

Electricforce

andElectricfieldElectricchargeisthephysicalpropertyofmatterthatcausesittoexperienceaforcewhenplacedinanelectromagneticfield.Therearetwotypesofelectriccharges:positiveandnegative

++ADCB-FE+ADCBFE

HydrogenHelium+Comparetheelectricforcesthatthetwoelectronsexperience.(giventhesamedistancebetweenelectronsandnuclei)

ElectrostaticForce–Coulomb’slawTheelectricforcebetween2pointchargesq1andq2is:Theelectricforceisavector!unitvector(toshowthedirection)Forceonpointchargeq2duetopointchargeq1?(Permittivityofvacuum)

Thecoulomb(unitsymbol:C)istheSIunitofelectriccharge.Itisthecharge(symbol:Qorq)transportedbyaconstantcurrentofoneampereinonesecond:1C=1A.1sThus,itisalsotheamountofexcesschargeonacapacitorofonefaradchargedtoapotentialdifferenceofonevolt:1C=1F.1VCharles-AugustindeCoulomb(1736–1806)

Electricchargeis“quantized”:Anypossiblechargeisamultipleoftheelementarychargeee=1.6×10-19CElementary/subatomicparticlescarryelementarycharge:electronme=9.11×10?31kg-emp=1.6726×10?27kgeprotonneutron0mn=1.6749×10?27kg=1.6021766208(98)×10-19C

CoulombsineverydaylifeStaticelectricitybyrubbingmaterials:Alightningbolt:TotalchargefromAAbatteryfromfullychargedtodischarged:about15Cafewmicro-coulombs(μC=10-6C)~5000C

1C1C1mF=9×109Nequivalenttotheweightof920,000metrictons!FFForceontwo1Coulombchargesseparatedby1mSo:1Cisaverybigcharge!

-+-Superpositionofelectricforces

Q2Q1Q3Q4qFtotFtotFtotForceonasmall“test”chargeqWhatifthetestchargebecomes2q?Howabouttheforces?

Atanypointinspace,wecandefine:EistheElectricFieldNowwemaydefinetheelectricfield(throughtheelectr