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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.