IntrotoMagnetism

Alittlehistory…• Untiltheearly19th century,scientiststhoughtelectricityandmagnetismwereunrelated• In1820,DanishscienceprofessorHansChristianOersted wasdemonstratingelectriccurrentsinfrontofaclassofstudents• Whenelectriccurrentwaspassedinawirenearamagneticcompass,thecompassneedlemoved!

ElectricityandMagnetismarerelated!

MagneticPoles• Allmagnetshavebothanorth andasouthpole• Likepolesrepel• Oppositepolesattract• Magneticpolesalways existinpairs

ComparingElectricity&Magnetism• Magnetsexert forcesononeanother• Similar toelectriccharges:• Canattractandrepelwithout touching• Strengthofforce dependsonthedistancebetweenthetwomagnets

• Different fromelectriccharges:• Electricchargesproduceelectricalforces• Regionscalledmagneticpolesproducemagneticforces

MagneticFields(Bfields)

• MagneticField(B):Thespacearoundamagnetinwhichamagneticforceisexerted.• MeasuredinunitsofTesla– T• MagneticFieldisaVECTOR(hasmagnitude anddirection)

• Thedirectionofthemagneticfieldoutsideamagnetisfromthenorthpoletothesouthpole.• MagneticfieldlinespointAWAYfromnorthpolesandINTOsouthpoles.

N S

BField

N NS

HowareElectricity&Magnetismrelated?

• ChargesinmotionhavebothE(electric)fieldsandB(magnetic)fieldsassociatedwiththem.• Inabarmagnet,electronsinsideareconstantlymoving.• Microscopiccurrentscreatepermanentmagnets.Thecurrentiscausedbytheelectronsorbitingthenuclei.Allofthenucleineedtobealignedtoproduceanetmagneticfield.• Movingcharge(alsocalledcurrent)create magneticfields!

Movingelectricchargescreatemagneticfields.

Electromagnets

• Ifacurrentcarryingwireisbentintoaloop,themagneticfieldlinesbecomebunchedupinsidetheloop• Acurrent-carryingcoilofwirewithmanyloopsisanelectromagnet.

• Wecancalculatethemagnitude(size)ofthemagnetic(B)fieldcreatebyasinglecurrentcarryingwireusingAmpere’sLaw.

• B=magneticfield[T]• μ0=permeabilityconstant=4πx10-7 [Henrys/m]• I=current[A]• r =distancefromcenterofwire[m]

Ampere’sLaw

TheRightHandRule(PartI)• Ampere’slawhelpsusfindthesize(ormagnitude)ofthemagneticfield.• Sincemagneticfieldisavector,italsoneedstohavedirection• Wecanfindthedirectionofthemagneticfieldusingthe“RightHandRule.”

RightHandRuleInstructions: StraightWire• Tofindthedirectionofthemagneticfieldinastraightwire,pointthethumbofyourright handinthedirectionofcurrentflow.• Yourfingerspointinthedirectionofthemagneticfield.

Notation:Comingoutofthepage

Goingintothepage

RightHandRuleExample:

Yourthumb followsthecurrent,soyoupointyour thumbup.Thefingersonyourrighthandcurlintothepageon therightsideofthewireandoutofthepageontheleftsideofthewire.Thisisthedirectionofthemagneticfield.

Werepresentthedirectionofthemagneticfield likethis:

• Ifwewanttofindthedirectionofthemagneticfieldinacurledwire,weswitchtherolesofourthumbandfingers.

RightHandRuleInstructions: Curled(orcoiled)Wire• Tofindthedirectionofthemagneticfieldinacoiledwire,pointthefingers ofyourright handinthedirectionofcurrentflow.• Yourthumb pointinthedirectionofthemagneticfield.

TheRightHandRule(PartII)

Thecurrentisrepresentedbytheblackarrows.Wrapyourfingers inthedirectionofthearrows,andyoufindthatyourthumbpoints totheleft.Thatisthedirectionofthemagneticfield.

Ex:

TheRightHandRulePractice

• Whatisthedirectionofthemagneticfieldinthiswire?

• Whatisthedirectionofthemagneticfieldinthiscoilorwire?

• Whatisthedirectionofthemagneticfieldinthiswire?

• Whatisthedirectionofthemagneticfieldinthiscoilofwire?

ANSWERS

BField

BField

ThemagneticfieldpointsINTOthepagebelowthewireandOUTofthepageabovethewire.