Heat TransferPAL #13 First LawSlide 3ConductionConduction Through a SlabConductive Heat TransferThermal ConductivitiesComposite SlabsConduction Through Multiple LayersHeat Loss Through a WallThe Surface of the SunConvectionConvection Rate FactorsRadiationAbsorption of RadiationBlackbody RadiationStructure of the SunHeat Transfer in The SunHeat TransferPhysics 202Professor Lee CarknerLecture 14PAL #13 First LawFinal temperature of 20 g, 0 C ice cube dropped into 300 g of hot tea at 90 C.Add up all heats (Q = cm T and Q = Lm)Heat 1: melt ice Heat 2: warm up now melted ice cube Heat 3: cool down tea Step 4: add up heat 6660 + 83.8Tf + 1257Tf –113130 = 01340.6Tf = 106470Heat Transfer What is moving? In heat transfer the analogous methods are convection and conduction both a particle and a wave (but not really)Conduction The end in the fire experiences a large vibration of the molecules of the metal  The movement of heat from a high temperature region to a low temperature region through another materialConduction Through a SlabConductive Heat TransferThe rate at which heat is transferred by conduction is given byH = Q/t = kA (TH - TC)/LWhere: Q is heat and t is time A is the cross sectional area of the material (in the direction of heat transfer) T is the temperature (hot or cold)Thermal ConductivitiesMetals generally have high k Al and Cu make good pots and pans For air, k=0.026 for polyurethane foam, k=0.024 Down filled winter coats trap air for insulationComposite Slabs H = Q/t = A (TH - TC)/ (L/k)Where  (L/k) is the sum of the ratios of the thickness and thermal conductivity of each layer of the slabConduction Through Multiple LayersHeat Loss Through a WallThe Surface of the SunConvectionHot air (or any fluid) expands and becomes less dense than the cooler air around it If the hot air cools as it rises it will eventually fall back down to be re-heated and rise again Examples: baseboard heating, boiling water, Earth’s atmosphereConvection Rate FactorsFluidity Energy exchange with environment How rapidly will the material lose heat? A small temperature difference may result in not enough density difference to moveRadiationEnergy can be directly transported by photons The power (in Watts) that is emitted by an object depends on its temperature (T), its area (A) and it emissivity ()Pr = AT4 Emissivity has a value between 0 and 1Absorption of RadiationEvery object also absorbs radiation at a rate determined by its properties and the temperature of its environment Where Tenv is the temperature of the environment Pn = Pa -Pr = A(Tenv4 - T4)Blackbody Radiation They absorb all of the radiation incident on them Every object whose temperature is above 0 K emits thermal radiationPeople emit thermal radiation at infrared wavelengths and thus can be detected at night with IR gogglesStructure of the SunCoreRadiative ZoneConvective ZonePhotosphereChromosphereCoronaHeat Transfer in The Sun Near the core (where the energy is produced via hydrogen fusion) energy is transported by radiation About 75% of the way out, the opacity increases to a level where convection becomes dominant Convection transports the energy to the surface where it radiates away into