《大學(xué)物理簡明教程》 課件 14-2 Magnetism and Electric Current

MagneticfieldFieldlineMagneticfieldgeneratedbyelectriccurrentTheeffectofmagneticfieldonmovingchargesand

currentHalleffectElectricmotorsMagneticmaterialsHansChristian?rsted(1820)AnelectriccurrentinducesamagneticfieldrighthandruleMagneticfieldproducedbyacurrent:Ampere’slawpermeabilityoffreespacelong,straight

wire

rHowmuchcurrentisneededtoproduceamagneticfieldequaltothatoftheEarth,at2cmfromalongwire?2cmEarth’smagneticfield:

B=5x10-5Tr=2x10-2m

Magneticfieldfromacircularcurrentloopallelementsofwireproduceafieldatthecenter:forNloops:Solenoid:manyloopsofwirealongoneaxisThemagneticfieldisalmostuniform

inside:n

isthenumberofturnsperunitoflengthWhatisthefieldBinside?2mcurrent=1600A2000loops1000loops/mverylargecurrent!Awirecarryingelectriccurrentwillexertaforceonanotherwirecarryingelectriccurrent!2.Amovingchargeinamagneticfieldwillbesubjecttoamagneticforce1.AnelectriccurrentproducesamagneticfieldAnelectriccurrentismadeofmovingcharges

magneticforceonWire2fromaparallelwire(Wire1)forceperunitlengthlF2currentsinthesamedirectionattractiveforcecurrentsinoppositedirectionsrepulsiveforceOfficialdefinitionoftheAmpereTheampereisthatconstantcurrentwhich,ifmaintainedintwostraightparallelconductorsofinfinitelength,ofnegligiblecircularcrosssection,andplaced1meterapartinvacuum,wouldproducebetweentheseconductorsaforceequalto2x10-7newtonpermeteroflength.Wheredoesmagnetismcomefrom?comesfromelectriccurrentFrommovingchargesMovingelectronsinatomsMagneticfieldfromacircularcurrentloopatthecenter:movingcharge=electriccurrentcircularmotion=currentloop

chargeontheparticleperiodofthecircularmotionatthecenter:

rWhatisthemagneticfieldatthenucleus

duetothemovingelectroninahydrogenatomatitsgroundstate?Themagneticfieldatahydrogennucleus

duetothemovingelectronr

electricforceonelectron

centripetalforceneededbyelectron

Theperiod

Using

Bohrradius

r0=0.53?

forhydrogengroundstate

electrons“spin”onthemselvesThisislikeacurrent,thatcreatesamagneticfieldActuallyarelativisticeffect,notreallyspinningMagnetizationmolecular“magnets”alignunderexternalmagneticfiled

(maystrengthenthefield?)FerromagneticmaterialsCobaltIronNickelGadoliniumDysprosiumSomecompoundscontainingaboveelementsMaymakepermanentmagnetsProducingstrong

magneticfieldaftermagnetizationParamagneticmaterialsWeaklyrespondstoexternalmagneticfieldInducingaweakfieldinthesamedirectionAluminumOxygenTitaniumFeODiamagneticmaterialsInduceanopposingmagneticfield(weak)mostmaterials…GraphiteGoldSilverWater

slightlygreaterthan1

slightlysmallerthan1AlignmentresistedbythermalmotionPartialmagnetizationDifferencebetweenparamagnetismandferromagnetismStronginteractionsbetweenmolecular“magnets”micrographofmetalsurfaceshowingmagneticdomains,withredandgreenstripesdenotingoppositemagnetizationdirectionsMovingdomainwallsinagrainof

siliconsteel

causedbyanincreasing“downward”externalmagneticfield.

Whiteareasaredomainswithmagnetizationdirectedup,darkareasarewithmagnetizationdown.Usually10?4-10?6mFerromagneticmaterialRelativepermeability(μr)Iron

(99.95%pureFeannealedinH)200000Cobalt-iron(highpermeabilitystripmaterial)18000Iron

(99.8%pure)5000Electricalsteel(siliconsteel)4000Martensiticstainlesssteel

(annealed)750–950Carbonsteel100Nickel100

–600Martensiticstainlesssteel

(hardened)40–95Austeniticstainlesssteel1.003–7Auniversalquantum

effect!Superconductor–perfectdiamagnetismMeissnereffectAll

magneticfieldwillbeexpelled

froma

superconductor

duringitstransitiontothesuperconductingstateRepulsiveforce

SuperconductingmagneticlevitationSolenoidwithaferromagneticcore

n

-thenumberofturnsperunitoflength

“soft”ferromagneticmaterialshaveverylargeμrvalueMagneticcircuitAlmostallthemagneticfiledlinesarecontainedinthesoftferromagneticmaterials(havingverylargeμrvalue)Samemagneticflux

ΦSamecurrentinanelectriccircuit

lMagneticElectricNameSymbolUnitsNameSymbolUnitsMagnetomotiveforce

(MMF)ampere-turnElectromotiveforce

(EMF)voltMagneticfieldHampere/meterElectricfieldEVolt/meter

MagneticfluxΦweberElectriccurrentIampereReluctanceRm1/henryElectricalresistanceRohmPermean