《大学物理简明教程》 课件 14-1 Magnetic field

MagnetismMagnetsNSSNNSNSSNSNMagnetshavetwopoles:NorthandSouthSNNSCutamagnetintwoEachparthasthesametwopoles!NSNORTHSOUTHThe“Northpole”oftheEarthisactuallyaSouthmagneticpole!Asof2017,

theNorthMagneticPolewaslocatedatapproximately

86.5°N172.6°WTheCompassMagneticfieldFieldlineMagneticfieldgeneratedbyelectriccurrentTheeffectofmagneticfieldonmovingchargesand

currentHalleffectElectricmotorsMagneticmaterialsMagneticfieldlinesMagneticfieldlinesDirection:wheretheNORTHendofacompassispointingOutsidethemagnet:FromnorthtosouthInsidethemagnet:fromsouthtonorthThemagneticfieldlinesarealwayscontinuousmagneticfieldgeneratedbymagnetsThedensityoffieldlinesatanylocationisproportionaltothemagnitudeofthefieldatthatpointThedirectionoffieldistangentialtothefieldlineFieldlinesnevercrosseachother!-Magneticfieldlinesarealwayscontinuous

(loop)SameforbothelectricandmagneticfieldlinesDifferencebetweenelectricandmagneticfieldlines-Electrostaticfieldlinesarealways

fromonechargetoanotherorinfinitelyfarawayFieldstrength–fluxdensity(B)

“B-Field”:avector.

ItsSIunitisTeslaOthernamesofB-field:magneticinduction“Magneticfieldstrength/intensity”~oftenreferstoH-field!Flux:thenumberoffieldlinesgoingthroughareaAMagneticforceonamovingchargeThemagneticfield

(B)strengthcanbedefinedfromtheforceexertedonamovingcharge:RighthandruleSIUnitofB-field:TeslaThecrossproduct

oftwovectorsandtheright-handrule

Showingthedirectionofavectorperpendiculartothe(paper)plane········××××××××Earth’smagneticfield-25to65μT~0.25to0.65GRefrigeratormagnet100GPullforce:90kg18mm*75mmN52magnet7300GnearitspolesSIUnitofB-field:TeslaGaussianunit:GaussElectromagnet–usuallyupto10Tsuperconductingmagnet–upto45TQ=20nC10m/sWestNorthtowardsthegroundB=5x10-5TAchargedrodmovingintheEarth’smagneticfieldA

uniformmagneticfield(perpendiculartotheplaneofthepicture)circulartrajectoryofachargedparticlemagneticforcessuppliesthecentripetalforceInitialvelocityperpendiculartothemagneticfieldConstantspeed,circularmotionintheplaneperpendiculartothemagneticfieldMagneticforceoncurrent-carryingconductorsMagneticforceonacurrent-carryingconductorThechargesmovewiththe“drift”velocity:vdN=numberofchargesmovinginthesectionofwireA=crosssectionofthewirel=lengthofthewire(diameterofthemagnet)n=densityofchargecarriersMagneticforcesonmovingchargeschargespersecondthroughtheareaAθ:anglebetweenBandIrighthandruleElectricmotorThemagneticforceontheconductorcreatestorqueforcesonthese

twoparts

arevertical(notorque)torque::anglebetween

F

and

r:distancefromthepivotThetorqueoneachverticalsegmentisthesametotaltorque:torqueisnegativemotorwillstop?Thedirectionofthecurrentisreversedwhenevertheloopgoesthroughθ=0keepthemotorgoingbyhavingthetorquealwaysinthesamedirectionHowtomakea

chargemovingstraightlyinaperpendicularmagneticfield？

××××××××Bq

Applyanelectricfield!E

EfieldshouldperpendiculartoBfield,andalsoperpendiculartothemotionTheHalleffectnegativechargesmovingpositivechargesmovingthechargeswouldbedeflected

themagneticforceanelectricfieldbuildsupacrossthematerialdeflectionofthechargesstopswhentheelectricforceduetothefieldbalances

themagneticforcemagneticforceelectricforce-------------VIBnegativechargecarriers:-+++++++++++-+positivechargecarriers:ThesignoftheHallvoltagegivesthesignofthechargecarriersTotalforceonthechargecarriers

(inthecaseofsteadycurrent)electricmagnetic=Hallvoltage=widthThecurrentisrelatedtothe“driftvelocity”=carrierdensity=chargeofelectron=crosssectionareaCurrentcarriercharge:negativeforelectronspositivefor“holes”carrierdensityMeasuringtheHallvoltagegivesusefulinformationonthee