Physics 101: Chapter 13 The Transfer of Heat Textbook Chapter 13 The Transfer of Heat convection conduction Radiation 1
Chapter 13, Preflight Which of the following is an example of convective 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.
Chapter 13, Preflight Which of the following is an example of conductive 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 cant get too close because of the heat. 3. Its hard for central air-conditioning in an old house to cool the attic.
Chapter 13, Preflight Which of the following is an example of 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 cant get too close because of the heat. 3. Its hard for central air-conditioning in an old house to cool the attic.
Heat Transfer: Convection Air heats at bottom Thermal expansion…density gets smaller Lower density air rises Archimedes: low density floats on high density Cooler air pushed down Cycle continues with net result of circulation of air Practical aspects heater ducts on floor A/C ducts on ceiling stove heats water from bottom heater
Heat Transfer: Conduction Hot molecules have more KE than cold molecules High-speed molecules on left collide with low-speed molecules on right energy transferred to lower-speed molecules heat transfers from hot to cold H = rate of heat transfer = Q/t [J/s] H = k A (TH-TC)/L Q/t = k A T/x k = “thermal conductivity” Units: J/s-m-C good thermal conductors…high k good thermal insulators … low k L = Dx TH Hot TC Cold Area A
Chapter 13, Preflight On a cool night you make your bed with a thin cotton sheet covered by a thick wool blanket. As you lay there all covered up, heat is leaving your body, flowing though the sheet and the blanket and into the air of the room. (The trick, of course, is to have enough blankets to make the flow of heat just right...if not enough flows you will be too hot and if too much flows you will be cold). Compare the amount of heat that flows though the sheet to the amount of heat that flows through the blanket. 1. More heat flows through sheet than through the blanket. 2. More heat flows through blanket than through the sheet. 3. The same amount of heat flows through sheet and through the blanket. correct Sort of like the plywood and insulation thing in the book...it's not like the area between the sheet and blanket is getting hotter and hotter or colder and colder, so the same amount must flow through each. Also, kind of like the whole "hose doesn't leak" thing, I guess.
H1 H2 Example with 2 layers: find H=Q/t in J/s Key Point: Continuity (just like fluid flow) H1 = H2 k1A(T0-TC)/Dx1 = k2A(TH-T0)/Dx2 solve for T0 = temp. at junction then solve for H1 or H2 answers: T0=2.27 C H=318 Watts H1 H2 Outside: TC = 0C Inside: TH = 25C T0 Dx1 = 0.02 m A1 = 35 m2 k1 = 0.080 J/s-m-C Dx2 = 0.075 m A1 = 35 m2 k1 = 0.030 J/s-m-C
Heat Transfer: Radiation All things radiate electromagnetic energy Hemit = Q/t = eAT4 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-m2-K4 No “medium” required All things absorb energy from surroundings Habsorb = eAT04 good emitters (e close to 1) are also good absorbers T Surroundings at T0 Hot stove
Heat Transfer: Radiation All things radiate and absorb electromagnetic energy Hemit = Q/t = eAT4 Habsorb = eAT04 Hnet = Hemit - Habsorb = eA(T4 - T04) if T>T0, object cools down if T<T0, object heats up T Surroundings at T0 Hot stove
Chapter 13, Preflight 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. 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. the asphalt is closer to being a perfect blackbody. It absorbes more heat from the sun as well as emits this heat. That is why the snow melts.