Hewitt/Suchocki/HewittConceptual Physical ScienceConceptual Physical ScienceFourth EditionChapter 7:Chapter 7:HEAT TRANSFER ANDCHANGE OF PHASECopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyThis lecture will help you understand:• Conduction• Convection• Radiation• Newton’s Law of Cooling Gl b l W i d th G h Eff t•Global Warming and the Greenhouse Effect• Heat Transfer and Change of Phase•BoilingBoiling• Melting and Freezing• Energy and Change of PhaseCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyHeat TransferHeat TransferProcesses of thermal energy transfer:• ConductionCti•Convection• RadiationCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConductionConductionConduction• Transfer of internal energy by electron and gy ymolecular collisions within a substanceCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyHeat Transfer: ConductionHeat Transfer: ConductionConduction occursConduction occurs predominately in solids where the molecules remainwhere the molecules remain in relatively restricted locations.When you stick a nail into ice, does cold flow from the ice to your hand, or heat fhdtthi?from your hand to the ice?Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConductionConductionInsulation• Doesn’t prevent the flow of internal energypgy• Slows the rate at which internal energy flowsElExample: Rock wool or fiberglass between walls slows the transfer of internal energy from a warm house to a cool exterior in winter and thehouse to a cool exterior in winter, and the reverse in summerCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConduction ApplicationConduction Application• Snow patterns on the roof of a house show areas of conduction and insulation.• Bare parts show where heat from inside has conducted through the roof and melted the snow.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyHeat Transfer: ConductionHeat Transfer: ConductionGd dtGood conductors:• Composed of atoms with “loose” outer electronsKilt•Known as poor insulators• Examples—all metals to varying degreesPoor conductors:• Delay the transfer of heat• Known as good insulatorsEldlt Stf•Examples—wood, wool, straw, paper, Styrofoam, cork, liquid, gases, air, or materials with trapped airCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConductionCo duct oDramatic example:Author John Suchocki walksDramatic example:Author John Suchocki walks barefoot without burning his feet on red-hot coals,due to pooron redhot coals,due to poor conduction between the coals and his feetCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConvectionConvection• Transfer of heat involving only bulk motion of fluidsExamples:•Visible shimmer of air above a•Visible shimmer of air above a hot stove or above asphalt on a hot dayy• Visible shimmers in water due to temperature differenceCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConvectionCooling by expansion• Opposite to the warming that occurs when air is compressedExample: The “cloudy” region above hot steam issuing from the nozzle of a pressure cooker isnozzle of a pressure cooker is cool to the touch (a combination of air expansion and mixing withexpansion and mixing with cooler surrounding air).Careful, the part at the nozzle th t ’tithat you can’t see issteam—ouch!Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConvection CurrentsConvection Currents• Convection currents produced by unequal heating of land and water. • During the day, warm air above the land rises, and cooler air over the water moves in to replace it.•At night, the direction of air flow is reversed.At night, the direction of air flow is reversed.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyConvectionConvectionReason warm air rises• Warm air expands, becomes less dense, and is p, ,buoyed upward•Air rises until its density equals that of theAir rises until its density equals that of the surrounding airElSf fExample:Smoke from a campfire rises and blends with the surrounding cool air.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyRadiationRadiationRadiation• Transfer of energy via electromagnetic waves gy gthat can travel through empty spaceCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyHeat Transfer: RadiationHeat Transfer: RadiationWavelength of radiation is related to the frequency of vibration.qyLow-frequency vibrations ⇒ long wavesHi hfib tiHigh-frequency vibrations ⇒ short wavesCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyRadiationRadiationEmission of radiant energy•Every object above absolute yjzero radiates•From the Sun’s surface comesFrom the Sun s surface comes light, or solar radiation•From the Earth’ssurfaceis•From the Earth s surface is terrestrial radiation in the form of infrared waves below ourinfrared waves below our threshold of sightCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyWave FrequencyTemperatureWave Frequency -Temperature(a) A low-temperature (cool) source emits primarily low-frequency, long wavelength waves.(b) A medium-temperature source emits primarily ymedium-frequency.(c) A high-temperature(c) A hightemperature source emits primarily high-frequency, shortCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesleyhighfrequency, short wavelength waves.RadiationRadiationEi i f di tEmission of radiant energy• Peak frequency of radiation is proportional to the absolute temperature of the source ( ) f~TCopyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyEmission and AbsorptionEmission and AbsorptionThe surface of any material both absorbs and emits radiant energy.gyWhen a surface absorbs more energy than it emits it is anet absorberandemits, it is a net absorber, and temperature tends to rise.When a surface emits more energy than it absorbs, it is anet emitter, andabsorbs, it is a net emitter, and temperature tends to fall.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison WesleyEmission and AbsorptionEmission and
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