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CBSEClass10Science

RevisionNotes

CHAPTER–13

MAGNETICEFFECTSOFELECTRICCURRENT

Inthischapter,wewillstudytheeffectsofelectriccurrent:Movingchargesorelectric

currentgeneratesamagneticfield.

1. HansChristianOersted(1777-1851)

Oerstedshowedthatelectricityandmagnetismarerelatedtoeachother.Hisresearchlater

usedinradio,televisionetc.

TheunitofmagneticfieldstrengthisnamedOerstedinhishonour.

2. OerstedExperiment

OnpassingthecurrentthroughthecopperwireXYinthecircuit,thecompassneedlewhich

isplacedneartheconductorgetsdeflected.Ifwereversethedirectionofcurrent,the

compassneedledeflectinreversedirection.Ifwestoptheflowofcurrent,theneedlecomes

atrest.

Hence,itcanbeconcludedthatelectricityandmagnetismarelinkedtoeachother.Itshows

thatwheneverthecurrentwillflowthroughtheconductor,thenamagneticfieldwill

develop.

3.MagneticField:Itistheregionsurroundingamagnet,inwhichforceofmagnetcanbe

detected.Itisavectorquantity,havingbothdirection&magnitude.

4. CompassNeedle:Itisasmallbarmagnet,whosenorthendispointingtowardsnorth

poleandsouthendispointingtowardssouthpoleofearth.

5.MagneticFieldLines:Thetangenttothemagneticfieldlineatapointgivesthedirection

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ofmagneticfieldatthatpoint.

Hence,magneticfieldlineisapathalongwhichahypotheticalfreenorthpoletendtomove

towardssouthpole.

6. CharacteristicsofMagneticfieldlines:

(a) Thedirectionofmagneticfieldlinesoutsidethemagnetisalwaysfromnorthpoleto

southpoleofbarmagnetandareindicatedbyanarrow.Insidethemagnet,thedirectionof

fieldlinesisfromitssouthpoletonorthpole.Thusmagneticfieldlinesareclosedcurves.

(b) Thestrengthofmagneticfieldisexpressedbytheclosenessofmagneticfieldlines.Closer

thelines,morewillbethestrengthandfartherthelines,lesswillbethemagneticfield

strength.

(c) Notwofieldlineswillintersecteachother.Iftheyintersects,thenatpointofintersection

thecompassneedlewillshowtwodirectionsofmagneticfieldwhichisnotpossible.

7.MagneticfieldduetoCurrentCarryingConductor

Theaboveelectriccircuitinwhichacopperwireisplacedparalleltoacompassneedle,

showsthedeflectioninneedlegetsreversed,whenthedirectionofcurrentreversed.Hence

electricityandmagnetismarerelatedtoeachother.

8. RightHandThumbRule

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Itisaconvenientwayoffindingthedirectionofmagneticfieldassociatedwithcurrent

carryingconductor.Holdthestraightwirecarryingcurrentinyourrighthandsuchthat

thumbpointstowardsthedirectionofcurrent,thenyourfoldedfingersaroundthe

conductorwillshowthedirectionofmagneticfield.

ThisrulealsocalledMaxwell’scorkscrewrule.

9.MagneticFieldduetoCurrentthroughaStraightConductor

Letacurrentcarryingconductorbesuspendedverticallyandtheelectriccurrentisflowing

fromsouthtonorth.Inthiscase,thedirectionofmagneticfieldwillbeanticlockwise.Ifthe

currentisflowingfromnorthtosouth,thedirectionofmagneticfieldwillbeclockwise.

Acurrentcarryingstraightconductorhasmagneticfieldintheformofconcentriccircles;

aroundit.Magneticfieldofcurrentcarryingstraightconductorcanbeshownbymagnetic

fieldlines.

10.MagneticFieldduetoCurrentthroughacircularLoop

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Everypointonthewirecarryingcurrentgiverisetothemagneticfield,appearingasa

straightlineatthecentreofloop.ByapplyingRighthandThumbrule,wecanfindthe

directionofmagneticfieldateverysectionofthewire.

11. Solenoid:ACoilofmanycircularturnsofinsulatedcopperwirewrappedcloselyinthe

shapeofacylinderiscalledsolenoid.

12. Magneticfieldduetoacurrentinasolenoid:

UsingR.H.ThumbRule,wecandrawthepatternofmagneticfieldlinesarounda

currentcarrying‘Solenoid’.

Oneendofthesolenoidbehavesasamagneticnorthpole,whiletheotherendbehave

astheSouthPole.

Thefiledlinesinsidethesolenoidareinformofparallelstraightlines,thatimplies

thatmagneticfieldinsidethesolenoidissameatallpointsi.e.Fieldisuniform.

Thestrengthofthemagneticfieldproduceddependsupon

(a) thenumberofturns

(b) Strengthofcurrentinthesolenoidusedinmakingsolenoid.

13. Electromagnet:Strongmagneticfieldinsidethesolenoidcanbeusedtomagnetisea

magneticmaterialforexamplesoftiron,whenitisplacedinsidethecoil.Themagnetso

formediscalledelectromagnet.Itisatemporarymagnet.

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PropertiesofMagneticField:

Themagnitude;ofmagneticfieldincreaseswithincreaseinelectriccurrentand

decreaseswithdecreaseinelectriccurrent.

Themagnitudeofmagneticfield;producedbyelectriccurrent;decreaseswith

increaseindistanceandvice-versa.Thesizeofconcentriccirclesofmagneticfield

linesincreaseswithdistancefromtheconductor,whichshowsthatmagneticfield

decreaseswithdistance.

Magneticfieldlinesarealwaysparalleltoeachother.

Notwofieldlinescrosseachother.

14. Forceonacurrentcarryingconductorinamagneticfield.

AndreMarieAmpere(1775–1836)suggestedthatthemagnetalsoexertanequalandopposite

forceonthecurrentcarryingconductor.

Wewillobservethattherodwilldisplacei.e.therodwillexperienceaforce,whenitis

placedinmagneticfield,inaperpendiculardirectiontoitslength.

Thedirectionoftheexertedforcewillbereversedifthedirectionofcurrentthrough

theconductorisreversed.

Ifwechangethedirectionoffieldbyinterchangingthetwopolesofthemagnet,

againthedirectionofexertforcewillchange.

Thereforethedirectionofexertedforcedependson

(a) directionofcurrent

(b) directionofmagneticfieldlines.

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15. LeftHandflemingRule

OR

Accordingtothisrule,stretchthumb,forefinger,andmiddlefingerofyourleft

handsuchthattheyaremutuallyperpendiculartoeachother.

Ifforefingerrepresentdirectionofmagneticfield&middlefingerrepresentdirectionof

current,thenthumbwillpointinthedirectionofmotionorforceactingontheconductor.

– ELECTRICMOTOR:

Electricalenergyisconvertedintomechanicalenergybyusinganelectricmotor.Electric

motorworksonthebasisofrulesuggestedbyMarieAmpereandFleming’sLeftHandRule.

Inanelectricmotor,arectangularcoilissuspendedbetweenthetwopolesofamagnetic

field.Theelectricsupplytothecoilisconnectedwithacommutator.Commutatorisadevice

whichreversesthedirectionofflowofelectriccurrentthroughacircuit.

Whenelectriccurrentissuppliedtothecoilofelectricmotor,itgetsdeflectedbecauseof

magneticfield.Asitreachesthehalfway,thesplitringwhichactsascommutatorreverses

thedirectionofflowofelectriccurrent.Reversalofdirectionofcurrentreversesthe

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directionofforcesactingonthecoil.Thechangeindirectionofforcepushesthecoil;andit

movesanotherhalfturn.Thus,thecoilcompletesonerotationaroundtheaxle.Continuation

ofthisprocesskeepsthemotorinrotation.

Incommercialmotor,electromagnet;insteadofpermanentmagnet;andarmatureisused.

Armatureisasoftironcorewithlargenumberofconductingwireturnsoverit.Large

numberofturnsofconductingwireenhancesthemagneticfieldproducedbyarmature.

16.MichaelFaraday:GavethelawofElectromagneticInduction:Whenaconductoris

settomoveinsideamagneticfieldoramagneticfieldissettobechangingarounda

conductor,electriccurrentisinducedintheconductor.

17. Galvanometer:Itisaninstrumentthatcandetectthepresenceofacurrentinacircuit.

Ifpointerisatzero(thecentreofscale)thentherewillbenoflowofcurrent.

Ifthepointerdeflectoneithersiderightorleft,thiswillshowthedirectionofcurrent.

Representedby

ELECTRICGENERATOR:

Thestructureofelectricgeneratorissimilartothatofanelectricmotor.Incaseofanelectric

generatorarectangulararmatureisplacedwithinthemagneticfieldofapermanent

magnet.Thearmatureisattachedtowireandispositionedinwaythatitcanmovearound

anaxle.Whenthearmaturemoveswithinthemagneticfieldanelectriccurrentisinduced.

Thedirectionofinducedcurrentchanges,whenthearmaturecrossesthehalfwaymarkofits

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rotation.Thus,thedirectionofcurrentchangesonceineveryrotation.Duetothis,the

electricgeneratorusuallyproducesalternatecurrent,i.e.AC.ToconvertanACgenerator

intoaDCgenerator,asplitringcommutatorisused.Thishelpsinproducingdirectcurrent.

18. ElectroMagneticInduction:Canbeexplainedbytwoexperiments

(a) FirstExperiment “SelfInduction”

Inthisexperiment,whenthenorthpoleofbarmagnetisbroughtclosertothecoiloraway

fromthecoil,weseemomentarydeflectionintheneedleofgalvanometeroneithersideof

nullpoint.Firstrightandthenleft.

Similarly,ifwekeepthemagnetstationaryandcoilismadetomovetowardsorawayfrom

thenorthpoleofmagnet.Againwewillobservedeflectionintheneedleofgalvanometer.

Ifbothbarmagnetandcoilarekeptstationary,therewillbenodeflectioningalvanometer.

Thisexperimentcanalsobedonewiththesouthpoleofmagnet,wewillobservethe

deflectioningalvanometer,butitwouldbeinoppositedirectiontothepreviouscase.

Itconcludesthatmotionofmagnetwithrespecttocoilorvice-versa,changesthe

magneticfield.Duetothischangeinmagneticfieldlines,potentialdifferenceisinducedin

thesamecoil,whichsetupaninducedcurrentinthecircuit.

(b) SecondExperiment:MutualInduction

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Inthisexperimentpluginthekeythatconnectscoilwithbatteryandobservethedeflection

ingalvanometer.Nowplugoutthekeythatdisconnectthecoil-1frombatteryandobserve

thedeflectioningalvanometer,whichwillbeinreversedirection.

Hence,weconcludethatpotentialdifferenceisinducedinsecondarycoil(coil-2),whenever

thereisachangeincurrent,inprimarycoil(coil-1)(byonandoffofkey).

Thisisbecause,wheneverthereischangeincurrentinprimarycoil

Magneticfieldassociatedwithitalsochanges

Now,magneticfieldlinesaroundthesecondarycoil(coil-2)willchangeandinducesthe

electriccurrentinit(observedbythedeflectionofneedleofGalvanometerinsecondary

circuit)

Thisprocess,bywhichchangingofstrengthofcurrentinprimarycoil,inducesacurrentin

secondarycoiliscalledElectromagneticInduction”

Theinducedcurrentisfoundtobehighestwhenthedirectionofmotionofcoilisatright

anglestothemagneticfield.

19. Fleming’sRightHandRule

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OR

Rulecanbedefinedas:

Stretch,thumb,forefinger,andmiddlefingerofrighthand,sothattheyareperpendicularto

eachother.Theforefingerindicatesdirectionofmagneticfield,thumbshowsthedirectionof

motionofconductor,thenthemiddlefingerwillshowsthedirectionofinducedcurrent.

Electricalgeneratorisbasedontheprincipleofelectromagneticinduction.Itconvert

mechanicalenergyintoelectricalenergy.

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21. AdvantagesofAlternateCurrent(AC)overDirectCurrent(DC)

Electricpowercanbetransmittedtolongerdistanceswithoutmuchlossofenergy.Therefore

costoftransmissionislow.

InIndiathefrequencyofACis50Hz.Itmeansafterevery1/100seconditchangesits

direction.

22. DomesticElectricCircuits:

Inourhomes,theelectricpowersuppliedisofpotentialdifferenceV=220Vandfrequency

50Hz.

Itconsistofthreewires:–

(1) Wirewithredinsulationcover–LIVEWIRE(POSITIVE)Livewireisathighpotentialof

220V

(2) Wirewithblackinsulationcover–NEUTRALWIRE(NEGATIVE)Neutralwireisatzero

potentialTherefore,thepotentialdifferencebetweenthetwois220V.

(3) WirewithGreeninsulationcover–EARTHWIRE

Itisconnectedtoacopperplatedeepintheearthnearhouse.

Themetallicbodyoftheappliancesisconnectedwiththeearthwireasasafetymeasure.

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Earthwireprovidealowresistancetothecurrenthenceanyleakageofcurrenttothe

metallicbodyoftheappliances,keepitspotentialequaltothatofearth.Thatmeanszero

potentialandtheuserissavedfromsevereelectricshock.

PointtobenotedinDomesticCircuit

(a) EachappliancehasaseparateswitchofON/OFF

(b) Inordertoprovideequalpotentialdifferencetoeachappliance,theyshouldbe

connectedparalleltoeachother.Sothattheycanbeoperatedatanytime.

24. ShortCircuiting:Duetofaultintheappliancesordamageintheinsulationoftwowires,

thecircuitwillofferzeroornegligibleresistancetotheflowofcurrent.Duetolow

resistance,largeamountofcurrentwillflow.

AccordingtoJoule’slawofheatingeffect,heatisproducedinlivewireandproducesspark,

damagingthedeviceandwiring.

25. Overloading:Overloadingcanbecausedby(1)Connectingtoomanyappliancestoa

singlesocketor(2)accidentalriseinsupplyvoltageifthetotalcurrentdrawnbythe

appliancesataparticulartimeexceedsthebearingcapacityofthatwire,itwillgetheated

up.Thisisknownasoverloading.Fuseasafetydevicecanpreventthecircuitfrom

overloadingandshortcircuiting.

Whatyouhavelearnt

Acompassneedleisasmallmagnet.Itsoneend,whichpointstowardsnorth,iscalled

anorthpole,andtheotherend,whichpointstowardssouth,iscalledasouthpole.

Amagneticfieldexistsintheregionsurroundingamagnet,inwhichtheforceofthe

magnetcanbedetected.

Fieldlinesareusedtorepresentamagneticfield.Afieldlineisthepathalongwhicha

hypotheticalfreenorthpolewouldtendtomove.Thedirectionofthemagneticfield

atapointisgivenbythedirectionthatanorthpoleplacedatthatpointwouldtake.

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Fieldlinesareshownclosertogetherwherethemagneticfieldisgreater.

Ametallicwirecarryinganelectriccurrenthasassociatedwithitamagneticfield.The

fieldlinesaboutthewireconsistofaseriesofconcentriccircleswhosedirectionis

givenbytheright-handrule.

Thepatternofthemagneticfieldaroundaconductorduetoanelectriccurrent

flowingthroughitdependsontheshapeoftheconductor.Themagneticfieldofa

solenoidcarryingacurrentissimilartothatofabarmagnet.

Anelectromagnetconsistsofacoreofsoftironwrappedaroundwithacoilof

insulatedcopperwire.

Acurrent-carryingconductorwhenplacedinamagneticfieldexperiencesaforce.If

thedirectionofthefieldandthatofthecurrentaremutuallyperpendiculartoeach

other,thentheforceactingontheconductorwillbeperpendiculartobothandwillbe

givenbyFleming’sleft-handrule.Thisisthebasisofanelectricmotor.Anelectric

motorisadevicethatconvertselectricenergyintomechanicalenergy.

Thephenomenonofelectromagneticinductionistheproductionofinducedcurrent

inacoilplacedinaregionwherethemagneticfieldchangeswithtime.Themagnetic

fieldmaychangeduetoarelativemotionbetweenthecoilandamagnetplacednear

tothecoil.Ifthecoilisplacedneartoacurrent-carryingconductor,themagneticfield

maychangeeitherduetoachangeinthecurrentthroughtheconductororduetothe

relativemotionbetweenthecoilandconductor.Thedirectionoftheinducedcurrent

isgivenbytheFleming’sright-handrule.

Ageneratorconvertsmechanicalenergyintoelectricalenergy.Itworksonthebasis

ofelectromagneticinduction.

InourhouseswereceiveACelectricpowerof220Vwithafrequencyof50Hz.Oneof

thewiresinthissupplyiswithredinsulation,calledlivewire.Theotheroneisof

blackinsulation,whichisaneutralwire.Thepotentialdifferencebetweenthetwois

220V.

Thethirdistheearthwirethathasgreeninsulationandthisisconnectedtoametallicbody

deepinsideearth.Itisusedasasafetymeasuretoensurethatanyleakageofcurrenttoa

metallicbodydoesnotgiveanysevereshocktoauser.

Fuseisthemostimportantsafetydevice,usedforprotectingthecircuitsduetoshort-

circuitingoroverloadingofthecircuits.