Physics 101 Lecture 28 The Transfer of Heat Today s lecture will cover Textbook Chapter 13 The Transfer of Heat Convection Conduction Radiation Physics 101 Lecture 28 Pg 1 Concept Question Which 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 2 Heat 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 thus expands V increases density decrease Thus the hotter fluid experiences a net force upward buoyant force FB cold V g hot V g Archimedes 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 bottom Physics 101 Lecture 28 Pg 3 Heat 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 cold Metals 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 TH Units J s m C Hot good thermal conductors high k e g metals good thermal insulators low k e g wood L x TC Cold Area A Physics 101 Lecture 28 Pg 4 Example with 2 layers find P Q t in J s Key Point Continuity just like fluid flow P1 P 2 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 P1 P2 Inside TH 25C Outside TC 4C T0 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 C Physics 101 Lecture 28 Pg 5 Heat Transfer Radiation Radiation All things radiate electromagnetic energy Pemit Q t eA T4 Surroundings at T0 e emissivity between 0 and 1 perfect black body has e 1 Hot stove T 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 absorbers Physics 101 Lecture 28 Pg 6 Heat Transfer Radiation All things radiate and absorb electromagnetic energy Pemit Q t eA T4 Pabsorb eA T04 Surroundings at T0 Pnet Pemit Pabsorb eA T T0 4 4 T Hot stove if T T0 object cools down if T T0 object heats up Physics 101 Lecture 28 Pg 7 NASA s Thermal Imaging System http mars jpl nasa gov During day time the sun heats the surface of Earth or Mars At night the materials on the surface emit this thermal energy again in form of radiation in the infrared Since different materials emit differently a whole spectrum of infrared radiation is measured which is used to identify the material Physics 101 Lecture 28 Pg 8 Concept Question One 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 melt Physics 101 Lecture 28 Pg 9