1 intro to magnetism - santa monica high school physicsintro to magnetism a little history… •...
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IntrotoMagnetism
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Alittlehistory…• Untiltheearly19th century,scientiststhoughtelectricityandmagnetismwereunrelated• In1820,DanishscienceprofessorHansChristianOersted wasdemonstratingelectriccurrentsinfrontofaclassofstudents• Whenelectriccurrentwaspassedinawirenearamagneticcompass,thecompassneedlemoved!
ElectricityandMagnetismarerelated!
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MagneticPoles• Allmagnetshavebothanorth andasouthpole• Likepolesrepel• Oppositepolesattract• Magneticpolesalways existinpairs
ComparingElectricity&Magnetism• Magnetsexert forcesononeanother• Similar toelectriccharges:• Canattractandrepelwithout touching• Strengthofforce dependsonthedistancebetweenthetwomagnets
• Different fromelectriccharges:• Electricchargesproduceelectricalforces• Regionscalledmagneticpolesproducemagneticforces
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MagneticFields(Bfields)
• MagneticField(B):Thespacearoundamagnetinwhichamagneticforceisexerted.• MeasuredinunitsofTesla– T• MagneticFieldisaVECTOR(hasmagnitude anddirection)
• Thedirectionofthemagneticfieldoutsideamagnetisfromthenorthpoletothesouthpole.• MagneticfieldlinespointAWAYfromnorthpolesandINTOsouthpoles.
N S
BField
N NS
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HowareElectricity&Magnetismrelated?
• ChargesinmotionhavebothE(electric)fieldsandB(magnetic)fieldsassociatedwiththem.• Inabarmagnet,electronsinsideareconstantlymoving.• Microscopiccurrentscreatepermanentmagnets.Thecurrentiscausedbytheelectronsorbitingthenuclei.Allofthenucleineedtobealignedtoproduceanetmagneticfield.• Movingcharge(alsocalledcurrent)create magneticfields!
Movingelectricchargescreatemagneticfields.
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Electromagnets
• Ifacurrentcarryingwireisbentintoaloop,themagneticfieldlinesbecomebunchedupinsidetheloop• Acurrent-carryingcoilofwirewithmanyloopsisanelectromagnet.
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• Wecancalculatethemagnitude(size)ofthemagnetic(B)fieldcreatebyasinglecurrentcarryingwireusingAmpere’sLaw.
• B=magneticfield[T]• μ0=permeabilityconstant=4πx10-7 [Henrys/m]• I=current[A]• r =distancefromcenterofwire[m]
Ampere’sLaw
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TheRightHandRule(PartI)• Ampere’slawhelpsusfindthesize(ormagnitude)ofthemagneticfield.• Sincemagneticfieldisavector,italsoneedstohavedirection• Wecanfindthedirectionofthemagneticfieldusingthe“RightHandRule.”
RightHandRuleInstructions: StraightWire• Tofindthedirectionofthemagneticfieldinastraightwire,pointthethumbofyourright handinthedirectionofcurrentflow.• Yourfingerspointinthedirectionofthemagneticfield.
Notation:Comingoutofthepage
Goingintothepage
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RightHandRuleExample:
Yourthumb followsthecurrent,soyoupointyour thumbup.Thefingersonyourrighthandcurlintothepageon therightsideofthewireandoutofthepageontheleftsideofthewire.Thisisthedirectionofthemagneticfield.
Werepresentthedirectionofthemagneticfield likethis:
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• Ifwewanttofindthedirectionofthemagneticfieldinacurledwire,weswitchtherolesofourthumbandfingers.
RightHandRuleInstructions: Curled(orcoiled)Wire• Tofindthedirectionofthemagneticfieldinacoiledwire,pointthefingers ofyourright handinthedirectionofcurrentflow.• Yourthumb pointinthedirectionofthemagneticfield.
TheRightHandRule(PartII)
Thecurrentisrepresentedbytheblackarrows.Wrapyourfingers inthedirectionofthearrows,andyoufindthatyourthumbpoints totheleft.Thatisthedirectionofthemagneticfield.
Ex:
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TheRightHandRulePractice
• Whatisthedirectionofthemagneticfieldinthiswire?
• Whatisthedirectionofthemagneticfieldinthiscoilorwire?
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• Whatisthedirectionofthemagneticfieldinthiswire?
• Whatisthedirectionofthemagneticfieldinthiscoilofwire?
ANSWERS
BField
BField
ThemagneticfieldpointsINTOthepagebelowthewireandOUTofthepageabovethewire.