Unit 6: Transfer Of Thermal Energy - MT Physics Portal

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Unit 6: Transfer of Thermal EnergySec 2 Science - Physics

Learning Outcomes1. Thermal Equilibrium show understanding that thermal energy is transferred from region ofhigher temperature to a region of lower temperature show understanding that regions of equal temperature will be inthermal equilibrium2. Heat Transfer Conduction: Describe in molecular terms, how energy transfer occursin solids Convection: Describe, in terms of density changes, convection in fluids Radiation: Show understanding that energy transfer by radiation doesnot require a material medium and its rate is affected by: (i) colour andtexture of the surface, (ii) surface temperature and (iii) surface area3. Applications of Thermal Energy Transfer Concept Describe how the concept of heat transfer can be applied to everydaypractical applications and problem solving.Sec 2 Science - Physics

Sec 2 Science - Physics

Sec 2 Science - Physics

What causes transfer of thermal energy? Thermal energy is transferred only when there is adifference in temperature. Thermal energy always flows from a region of highertemperature to a region of lower temperature. When thermal equilibrium is reached between twobodies (i.e. both bodies are at the sametemperature), there is no net flow of thermal energybetween them.What are the processes that enables heat to travel?Sec 2 Science - Physics

Heat Transfer ProcessThere are three processes by which heat maybe transmitted: Conduction Convection RadiationSec 2 Science - Physics

Suppose We have a cool glass which ispoured with boiling hot water.Will it be easy for us to lift it upwith our bare hands after awhile?How come?Why did the glass become hottoo?Sec 2 Science - Physics

So what has happened?Heat has flowed from water through the glass.The process of transferring heat in solids isconductionIt does not involve movement or flow in the matteritselfSo how is the heat transferred? What happened?Sec 2 Science - Physics

ConductionConduction is the process by which heat is transferredfrom one particle to another through a medium from aregion of higher temperature to a region of 9A02.swfSec 2 Science - Physics

Conductiona) When the medium is heated at one end, the particles at theheated end, gain energy and vibrate faster about their fixedpositions.b) These vibrating particles collide with their less energeticneighboursc) Some energy is transferred to these neighbouring particleswhich in turn gain kinetic energyd) In this way, heat is passed along the medium by thevibrating particlese) There is no net movement of particles during the process ofconduction.Note: This is a rather slow process of heat transfer!Occurs in all matters.Sec 2 Science - Physics

ConductionSolidLiquidGasWhich matter is a better conductor of heat? Solids are better conductors than liquids and gasesbecause the particles are closer together Liquids and gases are poor conductors of heatbecause their particles are very far apart and thecollisions between the particles are less frequent ascompared to solids. Heat conduction through a fluid is ll.htmlSec 2 Science - Physics

Conduction Different solids conduct heat differently – some fast,some slow. Good conductors, like metals, conduct heat quickly fromone end to another. Poor conductors (or good INSULATORS), like wool orcloth, conduct heat SLOWLY from one end to /genscience/scienceflashfiles/frame hotpotato flash3.htm

ConductionGood Conductor In metals, another much faster mechanism of thermalenergy transfer takes place at the same time -freeelectron diffusion. The free electrons gain kinetic energy and move faster. The fast-moving electrons then diffuse into cooler parts ofthe metal. They also transfer theirkinetic energy to themolecules by collidingwith 8CB-AE71-11D4-BD55-0050DA64A4E9%7d/PH09A03.swfSec 2 Science - Physics

ConductionInsulators In insulators, thetransfer of thermalenergy is solely theresults of vibratingatoms and molecules. There is no freeelectrons.Sec 2 Science - Physics

Summary1. Conduction is the transfer of thermal energywithout any flow of the material medium.2. The two mechanisms for conduction areatomic or molecular vibrations (for bothmetals and non-metals) and free electrondiffusion (for metals only).3. Liquids and gases are poor conductors ofheat compared to solids.

Demonstration on Conduction in fluid:Which setup’s icecube melts faster?Setup BSetup ASlowerprocessFasterprocessSetup A shows water is a poor conductor of heat

Sec 2 Science - Physics

ConvectionConvection – Heat transfer process that involves thebulk movement of fluid molecules that carry heat withthem.http://www.youtube.com/watch?v p wr kSDPPw&feature related

How Convection Works2. As water becomes lessdense, will start to rise tothe surface of the water.This water that rise upwill take the place of thecooler water above.1. Water directly aboveheat source heats up,with molecular structureexpanding. Hencebecoming less dense thansurrounding fluid3. The denser,cooler water willmove down totake the place ofthe void createdwhen warm waterrises. A circularmotion created4. This movement of liquid dueto difference in density sets upthe convection current

Thinking QuestionWill there be any change ifthe Bunsen burner is shiftedto the right side if the beaker?What will the convectioncurrent look like?Sec 2 Science - Physics

Observations of ConvectionKMn04 serves as a markershowing the movement ofwater molecules3. Trail of KMn04moves across.2. Trail of KMn04rises – indicatingmovement of fluid.1. Potassium permanganate,KMn04 melts.Heat4. Trail of KMn04 completescircular motion. Indicatesthat fluid moving in circularmanner

Application of ConductionUses of insulators: Air is poor conductor of heat. Animals like bears, sheep have fur that trap air toprotect them from cold. It minimises heat loss in thebody Man uses woolen clothes that trap air Double-glazing used to insulate homes AirGlassOutsideIn

Application of ConductionUses of insulators: Fibreglass, felt, expandedpolystyrene trap air as well. Fibreglass: insulate roofs intemperate countries Expanded polystyrene: insulatefood Handles of pots and pans madeof plastic or wood which arepoor conductors of heat

Application of ConductionUses of conductors: Used in situations where heat has to betransmitted quickly. Cooking utensils Heating elements in ovens, kettles

Convection in Nature: Sea breezesLand mass heats up fasterthan water. Air above landheats up, rising up as it is lessdenseWater body takes longer timeto heat up. Air above water iscool. Will flow towards land asair above land rises.

Convection in Nature: Land breezesLand mass loses heat fasterthan water. Air above landcools down faster, and willflow towards water as airabove water risesWater body takes longer timeto lose heat also. Air abovewater is warm. Will rise as it isless dense

Application of Convection Heating elements in kettles– Where is the heating element usually found? Air-conditioning– Where are air conditioners normally installed? Refrigerator– Where is the freezer located?

Application of ConvectionSec 2 Science - Physics

Application of ConvectionAir conditioners are installednear to the ceiling of roomsto facilitate setting upconvection currents ascooler air sinksSec 2 Science - PhysicsFreezing unit is placed at topto cool the air and facilitatethe setting up of convectioncurrents.

Convection & ConductionDifferences Movement of moleculesIn conduction, molecules vibrate with more K.E.affecting neighbors. In convection, there is movement ofmolecules where warmer, less dense matter rises,causing cooler, denser matter to move inSimilarities Both processes require matter!Think how do we receive heat from the sun? Sincespace is vacuum

How does the heatfrom Sun reach us?Sec 2 Science - Physics

RadiationRadiation is a method of heat transfer by which aheat source transmits infra-red waves It does not require a medium. It can take place in vacuum. When absorbed, the energy of infra-red waves istransformed into thermal energy of the receiving body. All objects emits radiation. The hotter the object, the greater is the radiation.Sec 2 Science - Physics

RadiationAbsorption of infrared radiation Infrared radiation is absorbed by all objects and surfaces. The absorption of radiant heat causes a temperature rise.Emission of infrared radiation Infrared radiation is emitted by all objects and surfaces. This emission causes the temperature of the objectsthemselves to fall. In general, good emitter of radiation is also a goodabsorber of radiation. Conversely, poor emitter of radiation is also a poorabsorber of radiation.Sec 2 Science - Physics

Factors affecting Radiation Color & texture of surface Surface temperature Surface area

Factors Affecting RadiationColour & Texture of Surfaceboiling watershinydullblackwhich side feelshotter?Dull black surfaces are good emitters ofradiation than shiny science/scienceflashfiles/frame hotpotato flash3.htmSec 2 Science - Physics

Factors Affecting RadiationColour & Texture of SurfaceEg. Darker clothes-feel hotterbecause absorbs more heat thanlight coloured clothesSec 2 Science - PhysicsShiny Teapot – bademitters of heat

Factors Affecting RadiationColour & Texture of SurfaceSec 2 Science - Physics

Factors Affecting RadiationColour & Texture of SurfaceRough objects are better absorber and emitter of radiationthan smooth objects. (because of its greater surface area)Sec 2 Science - Physics

Factors Affecting RadiationSurface TemperatureBoilingwaterIce watershinyWhich cup radiates more heat?The hotter an object is, the more energy itradiatesSec 2 Science - Physics

Factors Affecting RadiationSurface AreaImagine you are lost in North Pole, with nowinter clothes, how do you keep yourself warm?The greater the surface area of the object,the faster is the rate of heat transfer from itSec 2 Science - Physics

Factors Affecting RadiationSurface Areathe big ears of theAfrican elephantprovide a largesurface area for thegiant mammal tocool off quickly in hotweatherSec 2 Science - Physics

Factors affecting Radiation Color & texture of surface– Dull, black surface better heat emitter/absorber ofradiation– Shiny, white surface poor heat emitter/absorber ofradiation Surface temperature– Higher temperature, higher heat transfer Surface area– Larger surface area, larger area to emit heat – so emitradiation at higher rate.Sec 2 Science - Physics

Application of Radiationuses of good and poor emitters of radiationGood emitters are used in situations where heathas to be quickly emitted.Good EmittersPoor EmittersCooling fins at theback of a refrigeratorpainted dull blackA shiny metalteapotKeep liquid warmerfor longer timeSec 2 Science - Physicscooling fins at theback of a refrigeratorpainted dull blackrefrigerator

Application of Radiationuses of good and poor absorbers of radiationGood absorbers are used in situations where heat has to bequickly absorbed.Good AbsorbersPoor AbsorbersSolar heating panels arepainted in dull black paintHouses in hot countriesand factory roofs arepainted in white, lightcoloured paint,aluminium paintLight colours are chosenfor clothes and cars inhot weatherSec 2 Science - Physicssolar panels

Application of Radiation Greenhouse– Glass allow IR radiationfrom sun (short wavelengthbecause of higher temp.) topass through– Do not allow IR radiationfrom soil (long wavelengthbecause of low temp.) topass.– Heat trapped in greenhouse.Good for plant growth

Application of Radiation Vacuum flask– Layer of vacuum preventconvection and conduction.NOT radiation.– Silvered glass wall minimizeheat loss due to radiation– Plastic screw top minimizeevaporation and convectionfrom flask astic

Application of Radiation

High temperatureTransfer of thermalenergy to achievethermalequilibriumLow temperatureby processConductionby1. Vibration of atomsor molecules2. Free electrondiffusionConvectionbyDensity changesRadiationbyEmission ofInfra-red radiationrate affected by colour and texture ofthe surface surface temperature surface areahaveEveryday applicationsSec 2 Science - Physics

1. Thermal Equilibrium show understanding that thermal energy is transferred from region of higher temperature to a region of lower temperature show understanding that regions of equal temperature will be in thermal equilibrium 2. Heat Transfer Conduction: Describe in molecular terms, how energy transfer occurs in solids

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