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CBSEClass10Science
RevisionNotes
CHAPTER–12
ELECTRICITY
Intherecenttimes,lifewouldbeimpossiblewithoutelectricity.Fromlightingourhomes,
chargingourdevices,electricityhasbecomeanintegralpartofourday-to-daylive.Soit
becomesnecessarytounderstandtheorigin,characteristicsandutilisationofelectricity.In
thecurrentchapterwewilldiscuss:
ElectricCurrentandCircuit
ElectricPotentialandPotentialDifference
CircuitDiagram
OHM'sLaw
Factorsonwhichtheresistanceofaconductordepends
Resistanceofasystemofresistors
Heatingeffectofelectriccurrent
ElectricPower
Charge (q)
Chargeisacharacteristicunitofmatterbymeansofwhichmatterexperienceselectric
forces.Itcanbeeithernegative(electron)orpositive(proton)
“Coulomb”istheSIunitofcharge,representedbyC.
Netcharge(Q)–Totalcharge
1CNetchargeisequivalenttothechargecontainedinnearlyelectrons electrons.
Q=ne
(nisno.ofelectrons)
IfQ=1C, (negativechargeonelectron)
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=
electron
Current(I)
Rateofflowofnetchargeiscalledcurrent.Denotedby(I)
Electriccurrentcanalsobedefinedastheamountofchargeflowingthroughaunitcross-
sectioninpersecond.
SIunitofcurrentis“Ampere”.DenotedbyA.
Ampere Definedasonecoulombofchargeflowingpersecond.
Inanelectriccircuittheelectriccurrentflowinanoppositedirectiontotheflowofelectron
(–vecharge).Thiscurrentiscalledconventionalcurrent.Itflowsfromthe+veterminal
batteryorcellto–veterminal.Smallquantityofcurrentareexpressedin
mA(milliAmpere)=
µA(microAmpere)=
Ammeter:Itisaninstrumentusedtomeasuretheelectriccurrentinacircuit.Itisalways
connectedinseriesinacircuit.Ithaslowresistance(practically).Ideallyammeter's
resistancemustbezero.Itisrepresentedas
ElectricCircuit:Itisaclosedpathalongwhichtheelectriccurrentflows.
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Theelectronscanonlyflowwhenthereisdifferenceofelectricpotential.Forexample
“waterflowingthroughatube”.Itisonlypossiblewhenthereishighpressureatoneside
andlowatanotherside,thenitwillmovefromhighpressuretolowpressure.
Incaseofelectriccurrent,theflowofchargeismadepossibleduetochemicalactionwithin
acellthatgeneratesthepotentialdifferenceacrosstheterminalsofthecell.
ElectricpotentialDifference:Itisdefinedastheworkdoneincarryingaunitchargefrom
onepointtoanotherinanelectriccircuit.
V–PotentialDifference
W–Work
Q–NetCharge
SIunitofpotentialdifference–Volts.Denotedby“V”.ItisnamedafterAlessandroVolta.
OneVolt When1Jouleofworkisdoneincarryingonecoulomb(1C)ofchargefromone
pointtoanotherofacurrentcarryingconductorthenthepotentialdifferenceissaidtobeIV.
Voltmeter Itisaninstrument,usedtomeasurethepotentialdifferenceandrepresented
bythesymbol inanelectriccircuit.Itisalwaysconnectedin
parallelacrossthepointsbetweenwhichthepotentialdifferenceistobemeasured.Ithas
highresistance.
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CircuitDiagram:Acircuitdiagram(electricaldiagram,elementarydiagram,electronic
schematic)isagraphicalrepresentationofanelectriccircuit.
Symbolsforsomecommonlyusedinstrumentincircuitdiagram:
OHM'sLAW
GeorgeSimonOhmfoundtherelationshipbetweenthecurrent(I)flowingthrougha
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conductorandpotentialdifference(V)acrosstheterminalsofaconductorusingthecircuit
diagram.Heobservedthatthepotentialdifferenceandcurrentvariedlinearly.
Ohm’sLaw Hestatedthattheelectriccurrentflowingthroughaconductorisdirectly
proportionaltothepotentialdifferenceacrossitsends,understandardtemperatureand
pressurconditions.
Where“R”istheproportionalityconstantforthegivenmetalatgiventemperatureandis
calledresistance.ThegraphbetweenVandIisalwaysstraightlinewithslopeequaltoR.
Resistance:Itisthepropertyofaconductorthatopposestheflowofcurrent.Itis
representedby‘R’andsymbolis
SIunitofresistanceis“Ohm”.
1Ohm:TheresistanceofaconductorissaidtobeoneOhm,whenthepotentialdifference
acrosstheconductoris1Vandthecurrentflowingthroughitis1A.
V=IR
IR=V
Rheostat
Asweknowthat
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Sotoincreaseordecreasethecurrentaccordinglyinthecircuitacomponentusediscalled
“Rheostat”,thatregulatesthecurrentwithoutchangingpotentialdifference.Representedby
“Rh”.
Itssymbolis
IfaconductorhaslessResistance,thenmorecurrentwillflowthroughit.
FactorsonWhichResistanceofaConductorDepends
1. Onitslength(L)
2. Onitscrosssectionalarea(A)
3. Onthenatureofmaterial.
(Resistance)R L(Directectlyprop.tolength)
(inverselyproptocross-sectionalarea)
Where“ ”(rho)isaproportionalityconstantknownasresistivityofthematerialof
conductor.
Resistivity( ):Theresistanceofferedbyawireofunitlengthandunitcross-sectionalarea
iscalledresistivity.
ItsSIunitis
Foramaterialirrespectiveoflengthandarea,theresistivityisaconstant.
ResistivityofaMaterialvarieswithTemperature
Resistivityisthemeasureofresistanceofamaterial.Inotherwords,uptowhatextent
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willthematerialresistcurrentflow.
Resistivityofanalloy(homogeneousmixtureofmetals)isgenerallyhigherthanofits
constituentmetals.Example:Constantan(alloyofCu&Ni)
Alloyshavehighresistivityanddonotoxidise(burn)readlyathightemperature,forthis
reasontheyarecommonlyusedinelectricalheatingdevices,likeelectriciron,heater,
toastersetc.Forexample“Tungsten”asfilamentofelectricbulb.
Resistanceofasystemofresistors
ResistanceinSeries(MaximumEffectiveResistance)
Letustakethreeconductors/resistorsofresistanceR1,R2andR3thatareconnectedinseries
inacircuit.
Ohm’slawstated
V=IR
Thecurrent(I)flowingthroughtheresistanceinserieswillremainsame,whereasthe
potentialdifference(V)acrosseachresistorwillbedifferent.
V=IR
Totalpotentialdifference
Thus,weconcludethateffectiveResistanceoftheseveralresistorsjoinedinseriesisequalto
thealgebraicsumoftheirindividualresistances.Also,thenetresistanceincreaseswhen
resistorsareconnectedinseries.
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ResistanceinParallel(MinimumEffectiveResistance)
Letustakethreeresistorsofresistancethatareconnectedinparallelintheelectriccircuit.
Now,
Totalcurrent substitutethevalueof
Thus,weconcludethatthereciprocaloftotaleffectiveresistanceoftheseveralresistors
connectedinparallelisequaltothesumofthereciprocalsoftheindividualresistances.Also
,thenetresistancereduceswhenresistorsareconnectedinparallel.
DisadvantageofSeriesConnectioninanElectricCircuit
1. Inseriesconnectionifanyofthecomponentfailtowork,thecircuitwillbreakandthen
noneofthecomponent(ex.TV,bulb,fan..)willwork.
2. Itisnotpossibletoconnectabulbandaheaterinseries,becausetheyneeddifferentvalue
ofcurrenttooperateproperly.
Hence,toovercomethisproblemwegenerallyuseparallelcircuit.
HeatingEffectofElectricCurrent
Explanation Batteryoracellisasourceofelectricalenergy.
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1. Batteryorcell(Chemicalreactioninitwillproducepotentialdifferenceatitstwo
terminals)
2. Electronwillcomeinmotiontoflowcurrentthroughresistor
3. Tomaintainthiscurrent,thesourceshastokeepexpendingitsenergy.
4. Partofthisenergyisconsumedinusefulwork(Likerotatingoffan)
5. Restofenergyofsourceisconvertedintoheat,thatraisesthetemperatureofgadget.
6. Thisisknownasheatingeffectofelectriccurrent.
7. Thiseffectisutilizedindevicessuchaselectricheater,ironetc.
MathematicalExpression
Letussupposethatcurrent(I)isflowingthrougharesistorofresistance(R)forthetime(t).
Thepotentialdifferenceacrosstheresistanceis(V).
WorkdoneinmovingthechargeQwillbe
W=VQ
Thenpower, [Rateofchangeofworkdone]
=
Heatenergysuppliedbythesourcefortimetwillbe
(H=Pt)
Putequation(i)inequation(2)
H=VIt
=(IR)It
ThisisknownasJoule’sLaw
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Thelawstatedthattheheatproducedinaresistoris:
(i) directlyproportionaltosquareofthecurrent(I)
(ii) directlyproportionaltoresistance(R)forgivencurrent
(iii) directlyproportionaltotime(t)forwhichcurrentflowthroughresistor.
ApplicationofHeatingEffectofElectricCurrent
1. Usedinelectriciron,toaster,oven,heateretc.
2. Itisalsousedinbulbtoproducelight.
(Filamentofbulbismadeofstrongmetalwithhighmeltingpointsuchastungsten
( ).Thisfilamentcanretainasmuchoftheheatgeneratedaspossible,to
becomeveryhotandemitlight)
3. Itisalsousedinthe“fuseconnectedinanelectriccircuit.Fuseisasafetydevice,that
protectthecircuitsandappliancebystoppingtheflowofhighcurrent.Thewireoffuseis
madeofanalloyofmetalse.g.,AluminiumCopper,Iron,Leadetc.Thealloyshouldbeof
lowmeltingpointandhighresistivity,fuseisalwaysconnectedinseriescircuit.Whenlarge
currentflowsthroughthecircuit,thetemperatureoffusewirewillincrease.Thismeltsthe
fusewireandbreaksthecircuit.
“Fuses”usedfordomesticpurposesareratedas1A,2A,3A,5A,10Aetc.forvariousoperation
dependinguponthepowerofapplianceusing.
Example:letusconsideranappliance“electricIron”whichconsume1KWelectricpower,at
220V
P=VI
I=4.54A
Inthiscasea5Afuseisrequired.
ElectricPower:Incaseofelectricity,itisdefinedastherateofchangeofelectricalenergy
dissipatedorconsumedinanelectriccircuit.
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P=VI
Or
Or
SIunitofelectricpoweris“Watt”(W)
1Watt Definedasthepowerconsumedbyadevice,whenIAofcurrentpassesthroughit
atthepotentialdifferenceofIV.
P=VI
1Watt=1Volt 1Ampere
ElectricalEnergy
SIunitofelectricalenergy=WsorJ
Commercialunitofelectricalenergy=KWhoroneunit
=1000Wx3600s
=36x10 Ws
=3.6x10 J(SIunitWs=J)
Onehorsepower=746W
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Whatyouhavelearnt
Astreamofelectronsmovingthroughaconductorconstitutesanelectriccurrent.
Conventionally,thedirectionofcurrentistakenoppositetothedirectionofflowofelectrons.
TheSIunitofelectriccurrentisampere.
Tosettheelectronsinmotioninanelectriccircuit,weuseacellorabattery.A
cellgeneratesapotentialdifferenceacrossitsterminals.Itismeasuredinvolts(V).
Resistanceisapropertythatresiststheflowofelectronsinaconductor.Itcontrolsthe
magnitudeofthecurrent.TheSIunitofresistanceisohm(Ω).
Ohm’slaw:Thepotentialdifferenceacrosstheendsofaresistoris
directlyproportionaltothecurrentthroughit,provideditstemperatureremainsthe
same.
Theresistanceofaconductordependsdirectlyonitslength,inverselyonitsarea
ofcross-section,andalsoonthematerialoftheconductor.
Theequivalentresistanceofseveralresistorsinseriesisequaltothesumoftheir
individualresistances.
Asetofresistorsconnectedinparallelhasanequivalentresistance givenby
TheelectricalenergydissipatedinaresistorisgivenbyW=V×I×t
Theunitofpoweriswatt(W).Onewattofpowerisconsumedwhen1Aof
currentflowsatapotentialdifferenceof1V.
Thecommercialunitofelectricalenergyiskilowatthour(kWh).1kWh=3,600,000J=
3.6×10 J.