Physics 101: Lecture 28 The Transfer of HeatConcept QuestionHeat Transfer: ConvectionHeat Transfer: ConductionSlide 5Heat Transfer: RadiationSlide 7NASA’s Thermal Imaging SystemSlide 9Physics 101: Lecture 28, Pg 1Physics 101: Physics 101: Lecture 28Lecture 28The Transfer of HeatThe Transfer of HeatToday’s lecture will cover Textbook Chapter 13The Transfer of Heat»Convection»Conduction»RadiationPhysics 101: Lecture 28, Pg 2Concept QuestionConcept QuestionWhich of the following is an example of convective, conductive and radiative heat transfer? 1. You stir some hot soup with a silver spoon and notice that the spoon warms up. 2. You stand watching a bonfire, but can’t get too close because of the heat. 3. Its hard for central air-conditioning in an old house to cool the attic.Physics 101: Lecture 28, Pg 3Heat Transfer: ConvectionHeat Transfer: ConvectionConvection: heat is transferred by the bulk movement of a gas or liquid.Example: Fluid is sitting on a burner is heated from below.The fluid right above the flame is getting hot and thusexpands: V increases => density decrease Thus, the hotter fluid experiences a net force upward (buoyant force) FB = cold V g >hot V gArchimedes Principle: low density floats on high density => warmer fluid moves upward and is replaced by cooler fluid ->fluid is warmed -> moves upward -> and so onCycle continues with net result of circulation of fluid that carries heat.Practical aspects:heater ducts on floorA/C ducts on ceilingstove heats water from bottomPhysics 101: Lecture 28, Pg 4Heat Transfer: ConductionHeat Transfer: ConductionConduction: Heat is transferred directly trough a material (bulk motion does not play a role).Atoms/molecules of hotter materials have more KE than atoms/molecules of cooler materials.Gas/fluids: high-speed atoms/molecules collide with low-speed atoms/molecules: energy transferred to lower-speed atoms/molecules heat transfers from hot to coldMetals: electrons can “freely” move and can transport energyP = rate of heat transfer = Q/t [J/s]  Q = k A (TH-TC)/L k = “thermal conductivity”»Units: J/(s m C)»good thermal conductors…high k (e.g. metals)»good thermal insulators … low k (e.g. wood)THHotTCColdL = xArea APhysics 101: Lecture 28, Pg 5Example with 2 layers: find P=Q/t in J/sKey Point: Continuity (just like fluid flow)»P1 = P2»k1A(T0-TC)/x1 = k2A(TH-T0)/x2» solve for T0 = temp. at junction»then solve for P1 or P2answers: T0=5.8 C P=265 Wattsx1 = 0.019 m A1 = 35 m2 k1 = 0.080 J/s-m-Cx2 = 0.076 m A2 = 35 m2 k2 = 0.030 J/s-m-CInside: TH = 25COutside: TC = 4CP1P2T0Physics 101: Lecture 28, Pg 6Heat Transfer: RadiationHeat Transfer: RadiationRadiation: All things radiate electromagnetic energy.Pemit = Q/t = eAT4»e = emissivity (between 0 and 1)perfect “black body” has e=1»T is Kelvin temperature = Stefan-Boltzmann constant = 5.67 x 10-8 J/(s m2K4)No “medium” requiredAll things absorb electromagnetic energy from surroundings.  Pabsorb = eAT04»good emitters (e close to 1) are also good absorbersTSurroundings at T0Hot stovePhysics 101: Lecture 28, Pg 7Heat Transfer: RadiationHeat Transfer: RadiationAll things radiate and absorb electromagnetic energy.Pemit = Q/t = eAT4Pabsorb = eAT04Pnet = Pemit - Pabsorb = eA(T4 - T04) »if T>T0, object cools down if T<T0, object heats upTSurroundings at T0Hot stovePhysics 101: Lecture 28, Pg 8NASA’s Thermal Imaging SystemNASA’s Thermal Imaging Systemhttp://mars.jpl.nasa.gov/During day time the sun heats the surface of Earth orMars. At night the materials on the surface emit thisthermal energy again in form of radiation in the infrared.Since different materials emit differently, a wholespectrum of infrared radiation is measured whichis used to identify the material.Physics 101: Lecture 28, Pg 9Concept QuestionConcept QuestionOne day during the winter, the sun has been shining all day. Toward sunset a light snow begins to fall. It collects without melting on a cement playground, but it melts immediately upon contact on a black asphalt road adjacent to the playground. How do you explain this.Black (asphalt) absorbs electromagnetic waves (radiation) more readily than white (cement) does (emissivity is larger). Hence, the black has more radiation to emit because it has absorbed more. As a result, it releases more radiation into the snow, causing the snow to heat up, and