8.1: Transfer of Heat 8.2: Conduction - eLearn

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8. Transmission of Heat eLearn.Punjab

Wehave learnt in classVI thatkineticenergy is theenergyofanymatter inmotion.Thesmallparticlesthatmakeupmatterareconstantlymoving.Theyhavekineticenergy.Thekineticenergyofparticlesinmatteriscalledthermal energy.Whenthermelenergyistransferred,itisknownasheat.Thewordthermalmeans‘heat’.Heatisthethermalenergythatflowsfromanobject.Heatflowsfromanobjectathighertemperaturetoanobjectatlowertemperature.Inthischapter,weshall learnaboutdifferentmodesofheattransfer.

8.1: Transfer of Heat

Imagineyour teacherhasbroughtpacketsofdates todistribute inyourclass.The teacherhasthreeoptions.Option 1:Theteachercangiveapacketofdatestofirststudentandaskhim/hertopassittothenextstudentandthennextstudentpassesitfurther.Inthisway,eachoneofyouwillreceiveapacketwithoutmovingfromyourplace.Option 2:Theteachercanaskstudentstolineupandcometohistableonebyone.Afterreceivingthepacketeverystudentmovesbacktohis/herseat.Option 3:Theteachercanjustthrowapackettowardseachoneofyouwithoutanymovementonyourpart.Muchthesamewayheatenergytransfersfromahotbodytoacoldbody.The transferofheatenergy fromoneobject to theother is calledtransmission of heat.Heatenergytransfersinthreeways,i.e.conduction,convectionandradiation.

8.2: Conduction

Ifoneendofametalspoonisheatedwithaflame,theotherendwillalsogetheatedupafterawhile(Fig.8.1).Theheatenergyistransferredfromoneendofthespoontotheotherwithouttheactualmovement of particles (atoms or molecules) ofthespoon.Suchamodeof transmissionofheatis called conduction. It resembles the examplegivenaboveinoption1. Fig. 8.1: Heat from the candle flame also warms the part

of the spoon in the hand due to conduction.

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The transfer of heat throughmatterwithout the actualmovement of particles from theirpositioniscalledconduction.Conductionoccursinsolids,liquidsandgases,butsolidsusuallyconductheatbetterthanliquidsorgases.

Insolids,theparticlesareheldveryclosetoeachother.Theyvibrateconstantly.Whenweheatonepartofasolid,theparticlesgainheatenergyandstartvibratingfaster.Duringtheirvibrationtheybumpintonearbyparticlesandalsocausethemtovibratefast.Inthisway,theparticlesofhotpartofasolidtransferheattothoseinthecolderparts.

Activity 8.1 Observing Conduction

You will need: • metalrodorknittingneedle• cork• candleProcedure1.Pushoneendofthemetalrodorknittingneedleintothecork.Usethecorkasahandle.2.Lightthecandle.Burningcandlewillmeltandchangeintoliquidwax.Dropthismeltingwaxontotherodatthreedifferentpoints.Letthewaxcool.3.Heatthefreeendoftherodonthecandle.Notethetimewhenthewaxatthreedifferentpointsmelts.Onwhichpointwillthewaxmeltfirst?

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8.3: Good and Bad Conductors

Differentmaterialsconductheatatdifferentrates.Materialswhichallowheattoflowthroughthemeasilyarecalledgood conductorsofheat.Solidssuchasmetalsaregood conductors ofheat.Materialswhichdonotallowheattoflowthroughthemeasilyarecalledbad conductorsofheator heat insulators.Solidssuchaswood,glass,plastic,styrofoam,etc.arebadconductorsofheat.Allliquids(exceptmercurywhichisaliquidmetal)andgasesarealsobadconductorsofheat.

Table 8.1: Heat Conductors and InsulatorsGood Conductors Insulators

silver airoranygascopper cork

aluminium glassiron plastic

mercury wood Fig. 8.2: The metal part of the frying pan allows heat to pass through but its plastic handle does not.

Activity 8.2 Some materials are conductors of heat and some are insulators

Takesomehotwaterinabeaker.Takeasteelspoon,aglassrod,aplasticscale,awoodenscaleandapieceofthickcopperwire.Diponeendofeachoftheseinthehotwater.Waitfor2-3minutes.Thentouchtheotherendofeacharticlewithyourfingers.Whichonesbecomehot(conductorsofheat)andwhichonesdonot(insulators).

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Why metals are better conductors than non-metals?Allsolids (metalsandnon-metals)aremadeof tinyparticlescalledmolecules.But,somesolidsconductheatbetterthanothers.Letusputametalandaplasticspooninhotwater.Whichoneisagoodconductorofheat,metalorplastic?Whenbothspoonsreceiveheatenergy,theparticles(atomsormolecules)atthehotendofeachspoonvibratefastandbumpintoneighbouringparticles.Theseparticlestransferheatenergytonextneighboringparticles(Fig.8.3,8.4).

Particlesinmetalsarepackedmorecloselytogetherthannon-metals.Metalscantransmitheatenergymorereadilythannon-metals(wood,plastic,etc.).Thepresenceoffreeelectronsalsospeedsupthetransferofheatinmetals.Whenthemetalspoongetsheated,thefreeelectronsgainkineticenergyandmovefarthertowardsthecolderpartsofthemetalspoon.Theybumpintotheatomsinthecolderpartsandtransferheatenergytothem.Inmetals,heatenergyistransferredfromoneplacetoanotherbothbythevibrationsofparticlesaswellasbythemovementoffreeelectrons.Thatiswhymetalsaregoodconductorsofheatthannon-metals(insulators).

Conduction in Liquids and Gases

Theprocessofconductioninliquidsandgasesisveryslowascomparedtosolids(metals).Theparticlesinliquidsandgasesarenotheldcloselytogether.Theparticleshavelesschancestobumpintootherparticlesinliquidsandevenlesseringases.Thatiswhy,thetransferofheatenergyfromfast-movingparticlestoneighbouringparticlesisslow.Waterandairarebadconductorsofheat.

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Activity 8.3 Water is a bad conductor of heat.

Waterandmostotherliquidsexceptmercuryarebadconductors.Toshowthis:1. Wrapanicecubeinwiregauzeanddropitinatesttubealmost

filledwithwater.Itwillsettledown.2. HoldthetesttubeandheatitneartheneckwithaBunsenflame/

spiritlamp.

ObserveSoonthewaterintheupperpartofthetesttubewillstarttoboil,buttheiceatthebottommeltsveryslowly.Itshowsthatwaterisabadconductorofheat.

8.4: Everyday Applications of Conduction of Heat

Conductionplaysanimportantroleinourlives.1. Cookingutensils,electrickettle,iron,solderingiron,etc.aremadeof

metalstoconductheatquickly.Theirhandlesaremadeofplasticorwoodwhicharebadconductors.

2. Birdshavefeatherswhichkeeptheirbodieswarmbecausefeathersarebadconductorsofheat(Fig.8.5).

3. Woolen clothes andblankets slowdown the transfer of heat. It sohappensbecausethewooltrapsairinit.Theairisabadconductorofheat.

4. Iceiscoveredwithjuterugstoreduceitsmeltingspeed.Juteisabadconductorofheat.

5. An insulatingmaterial (e.g. styrofoam) is filledbetween thedoublewallsofarefrigerator.Itreducesthetransferofheatacrossthewallsoftherefrigerator.

6. Double-panwindows are used in buildings to slow the transfer ofheat.Airbetweenthetwolayersofglassactsasaninsulator.

7. Thermosbottlesuseairoravacuumtoslowthetransferofheatby

Fig. 8.5: Birds have feathers which are bad conductor of heat.

Fig. 8.6: Polyester is a poor con-ductor of heat. It can keep our

body warm during winter

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• Theplasticwatercoolerandhotpotshavedoublewalls.Thestyrofoamandairbetweenthewallsreducetransferofheatbyconduction.

8.5: Convection

Unlikeparticlesofsolids,particlesin liquidsandgasesmovefromoneplacetoanother.Takeabeakerandputsmallpiecesofpaperinit.Fillhalfofthebeakerwithwater.Heatthebeakerbyaspiritlamp.Weshallseethatpiecesofpaperrisetothetopofwater,movesidewaysandsinktothebottom.Thewaterinthebeakeralsogetswarm.Themoleculesofwaterabsorbheatenergyfromthebottomofthebeakerandrisetothetop.Othersurroundingmoleculesofwatercometothebottomtoabsorbheatenergy.

Thetransferofheat inwhichmoleculesofamediumactuallymove to the source of heat energy to absorb heat and thenmoveawayfromit,iscalledconvection.

Convection occurs in liquids and gases only because theirmolecules canmove freely. Themoleculesofa solidareheldcloselytogether.Theycannotmovefreely,therefore,convectionisnotpossibleinsolids.

Theupwardanddownwardmovementofmoleculesofwaterorairiscalledaconvection current.

Fig.8.7: Motion of the paper pieces in water is due to convection of heat.

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SImulation 8.3 : Conduction & ConvectionSource & Credit : prezi

Animation 8.2 : Metals ConvectionSource & Credit : carleton

Activity 8.4 Convection in LiquidsYou will need• abeaker• water• tripodstand• spiritlamp• potassiumpermanganatecrystals(pinky)Procedure:1. Fillthebeakertwo-thirdswithwater.2. Placethebeakeronatripodstand.3. Placeacrystalofpotassiumpermanganate(pinky)atthebottom

ofthebeakergentlyusingastraw.4. Now,heat thewaterbyplacingthespirit lamp justbelowthe

crystal.Whatdidyouobserve?ExplanationWhenwaterisheated,thewaterneartheflamegetshot.Hotwaterrisesup.Thecoldwaterfromthesidesmovesdowntowardsthesourceofheat.Thiswateralsogetshotandrises.Waterfromthesidesmovesdowntotakeitsplace.Thisprocesscontinuestillthewholewatergetsheated.Thismodeofheattransferisknownasconvection.

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Activity 8.5 Convection in Gases

You will need• aboxwithtwoholesatthetop(convectionbox)• paper/cloth• matchbox• candle• plasticsheetProcedure1. Lightacandleandplaceitunderoneoftheholesin

theconvectionbox.2. Bringaburningpieceofpaper/clothnearthehole

otherthanthecandle.Observethepathofthesmokeintheconvectionbox.What happens?Asthewarmairweighslessthanthesurroundingair,itrisesoutoftheholeabove.Thecoolerandheaviersurroundingairenterstheboxthroughtheotherholeto take the place of the air which left the box. Thismovement of air in andout of theholes due to thedifferenceinweights,setsupaconvectioncurrent.

8.6: Winds and Ocean Currents

Weknowthatconvection is thetransferof heat by the actual movement oftheparticles in materials. Winds andoceancurrentsareexamplesofeffectsofconvection.TheheatoftheSunheatsupthesurfaceoftheEarthandtheairnearitalsogetshot.Theairexpandsandgetslighter. So, it risesup and cool air fromtheneighbouringregionsmovesintofillitsspace. Fig.8.8: Convection causes winds.

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Convection and Gliding Flight of BirdsConvection currents also take place in atmosphere. The heat fromthe Sunwarms the air near the ground. Thewarm air expands andbecomes lighter inweight. Aswarmair rises, colder air rushes in tofillitsplaceneartheground.Thisprocesscontinues.Birdslikeeagles,hawks, vultures and gulls take advantage of this phenomenon. Theyenjoygliding.Duringglidingflightabirddoesnotmoveitswings,butglidesonaircurrents.Alotofenergyofbirdsissavedduringgliding

8.7: Everyday Applications of Convection CurrentsWecanobservetheuseofconvectioncurrentsinoursurroundings.1. Household ventilation canmakeourhousecool.Theairwhichwebreatheoutiswarmerand

lighter.Itmovesupintheroomtogooutoftheventilatorsnearthetopsideofthewalls.Freshandcoolairenterstheroomthroughwindowsanddoors.

2. Inadomestic water Heater,waterisheatedintheboilerbygasburnerorheatingcoil.Thehotwaterexpandsandbecomeslighterinweight.Thiswaterrisesandflowsintotheupperpartofthewaterheater.Totaketheplaceofhotwater,coldwaterfromstoragetank(cistern)fallstothelowerpartofthewaterheatertobecomehot.Wetakethehotwaterfromthetapattachedtothewaterheater,convectioncurrentshelpinthecontinuoussupplyofhotwater.

3. Anair conditioneralsousesconvectioncurrentscoola room.Airconditionersare installedneartotheceiling.Thefanofanairconditionerblowscooldryair.Thecoolair isheavier inweight,soitsinks.Thewarmairoftheroomrisesbecauseitbecomeslighterinweight.Theairconditionerdrawsthiswarmairtomakeitcool.Inthisway,theaircirculatesagainandagaintilldesiredtemperatureisreached.

Inanoven,theheaterisplacedatthebottom.Theconvectioncurrenttransfersheattoallthepartsoftheoven.

Oceancurrentsarealsosetupduetoconvectionofheat.Waterofthehotregionsofanoceangetshot,itexpandsandgetslighter,butwaterinthecolderregionsremainscoldandheavy.Hotwatermovesalongthesurfaceoftheoceantowardsthecolderregions.Thecoldwaterflowsbelowthesurfaceoftheoceantowardsthehotregions.Inthisway,oceancurrentsaresetup.

Therisingwarmairreachesuppercolderlayersoftheairandcoolsdown.CoolheavyairsinkstotheEarthincoldregionstoblowagaintotaketheplaceoftherisingair.Thus,convectioncurrentsaresetupandthewind-systemgoeson.

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8.8: Radiation

Thetransferofheatenergyfromahotbodytoacoldbodydirectly,withoutheatingthespaceinbetweenthetwobodiesiscalledradiation.Whenwesitinthesunorinfrontofaheater,wefeelwarmth.Heatenergyreachesusbyradiation.Thisheatcannotreachusbyconductionbecauseairisabadconductorofheat.Similarly,thisheatcannotreachusbyconvection,asthehotairrisesupward,ratherthansideways.Ifweputacardboardoraplasticsheetbetweenusandthesourceofheat,wenolongerfeelwarmth.So,wecansaythatheatfromtheSunoraheaterreachesusbyradiationwhichrequiresnomedium(Fig.8.10).

Fig.8.9: Convection cur-rents heat up water in a

water heater.

8.9: Experiments on Radiation and Absorption

Objectsabsorbandradiateheatatthesametime.Whetherallobjectsabsorbandradiateheatequally?Tostudyitletusperformsomeactivities.

8.10:

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Activity 8.6 A good absorber of heat is a good radiator of heat.You will need: • black-colouredcan• silver-colouredcan• laboratorythermometer• coldwater• hotwaterProcedure1. Filltwothirdspartofeachcanwithcoldwater.2. Putathermometerineachcanandrecordtemperatures.3. Placebothcansinbrightsunlight.Observeandrecordthetemperatureofeachcanafterabout10minutes.ThetemperatureofwaterinblackcanishigherbecauseithasabsorbedmoreheatfromtheSun.4. Againfillthecanswithhotwaterandplacethermometersinthem.

Recordtemperatureofwaterineachcan.5. Placebothcansinashadyplace.Observeandrecordthetemperatureof

eachcanaftersometime.Inwhichcandidthewatercoolfaster?

Activity 8.7 A good absorber of heat is a good radiator of heat.You will need: • anelectricheater• twomarbles• wax• ametalplatewithrough,darkcolouredsurface• ametalplatewithshiny,smooth,lightcolouredsurfaceProcedure1. Stickamarbleoneachplatewiththehelpofwax.2. Placetheheaterbetweenthetwometalplatessothat

eachplatereceivesthesameamountofheatfromtheheater.

3. Switchontheheater.Observethemarblesstuckontheplates.Things to thinkWhydid themarble stuck on the platewith rough, darkcolouredsurfacedropfirst?

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Inhotcountries,housesarepaintedwithlight-colouredpaints.ThelightcolourofthepaintabsorbslittleheatandreflectsmostoftheradiationfromtheSun.Whatcolourofpaintwouldyousuggest for thehouses inverycoldcountries?

8.10: Good and Bad Radiators and Absorbers of Heat

Experimentshaveprovedthatgoodabsorbersofheatarealsogoodradiatorsofheat.Blacksurfacesaregoodabsorbersandgoodradiatorsofheat,whileshinysurfacesarebadabsorbersandbadradiatorsofheat(Fig.8.11,8.12).

Fig. 8.11: A black, dull surface Fig. 8.12: A white, shiny surface

Sinceshinysurfacesarebademittersofradiation,shiny teapots and utensils can keep food or teawarmforalongertimethanblackones.Inaddition,shiny containers can keep cold liquids cool for alongertimethanblackcontainers.

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8.11: Everyday Applications of Radiation of Heat

Everyobjectemitsorradiatessomeamountofheat.Knowledgeofradiationcanhelpusinmanyways.1. Whenwesitbesideafire,theheatoffirereachesusbyradiation.2. Thecoolingfinsatthebackofourrefrigeratorneedtoradiateitsheatquicklytothesurroundings.

Itssurfaceismaderoughandpaintedblack(Fig.8.13).3. Duringhot summerdays, it is advised towearwhiteor light-coloured clothes.White colour

absorbslessheatthandarkcolours.4. Incoldareas,agreenhouseisusedforbettergrowthofplants.RadiationfromtheSunpasses

through the glass or plastic and warms up the soil and plants. Plants and soil absorb andemitradiationandincreasethetemperatureinthegreenhouse.Plantsgrowwellinincreasedtemperatureofthegreenhouse(Fig.8.14).

Fig. 8.13: Black cooling fins at the back of this refrigera-tor radiate heat quickly.

Fig. 8.14: A greenhouse

Ablacksmithexperiencesall threewaysofheattransfer,i.e.conduction,convectionandradiation.1. The iron in the blacksmith’s forge glows red

asheatistransferredtothemetalfromthefurnace.(conduction)

2. Theheatofthefurnacewarmstheair intheblacksmith’sshop.(convection)

3. Theblacksmithfeelstheglowofheatfromthefurnace.(radiation)

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8.12: The Vacuum Flask

Thevacuumflaskisacontainerwhichcankeephotthingshotandcoldthingscold.Thevacuumflask reduces the rateof transferofheatbyall the threeways, i.e. conduction, convectionandradiation.Thevacuumflask(thermosflask)isactuallytwothinglassormetalbottles,oneinsidetheother(Fig.8.15).Airbetweentheglasswallsisremovedtocreatevacuum.Thevacuumpreventsthetransferofheatbyconductionandconvection.Thewallsofbothbottlesarecoatedwithaluminiumonthevacuumside.Thesesilvered(likeamirror)andsmoothglasswallspreventtransferofheatbyradiation.Thelidoftheflaskismadefromabadconductorsuchascorkorplasticonlyalittleamountofheatislostbyconductionthroughthelid.Thethinwalledglassbottleisprotectedbyfixingitinametalorplasticcontainer.

Fig. 8.15 : A vacuum flask slows the transfer of heat by conduction (plastic), convection (vacuum) and radiation (shiny surface).

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Key Points• Heatisaformofenergy.Heatalwaysflowsfromanobjectathighertemperaturetoanobject

atlowertemperature.• Conductionisthetransferofheatthroughmatterwithouttheactualmovementofparticles

fromtheirpositions.• Convectionisthetransferofheatinwhichmoleculesofamediumactuallymovetothesource

ofheatenergytoabsorbheatandthenmoveawayfromit.• Radiationisthetransferofheatfromahotbodytoacoldbodydirectly,withoutheatingthe

spaceinbetweenthetwobodies.• Conductionoccursinsolids,liquidsandgases.Butmetalsarebetterheatconductors.• Convectionoccursonlyinliquidsandgases.• Radiationneedsnomaterialmediumtotransferheatenergy.• Goodandbadconductorsofheatplayveryimportantroleinourlives.• Convectioncauseswindandoceancurrents.• HeatfromtheSunreachesusbyradiation.• Somebirdstakeadvantageofconvectioncurrentsandglideintheairforhours.• Agoodradiatorofheatisalsoagoodabsorberofheat.• Avacuumflaskreducesthetransferofheatbyconduction,convectionandradiationtokeep

thingshotorcold.

Questions

3. Give short answers.i.Whydoweusecookingpotsmadeofmetals?ii.Whatisaconvectioncurrent?iii.Whichsurfacesdoabsorbmaximumheat?iv.Whydoweusewoollenclothesandblanketsduringwinterdays?v.Whatistheadvantageofglidingflightforabird?

1. Complete each of the following sentences by writing the correct term.iThetransferofheatbymovementofmoleculesfromplacetoplaceii.Itcanmaintainthetemperatureofdrinksiii.Thetransferofheatbydirectcontactofmoleculesiv.Thesurfacewhichabsorbsandradiatesheatbetter

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8.1: Transfer of Heat 8.2: Conduction - eLearn