Announcements 9/14/11 Prayer Remember, HW solutions are being posted in hallway around corner from where you turn in the homework, right next to the CSR.
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Announcements 9/14/11 Prayer Remember, HW solutions are being posted in hallway around corner from where you turn in the homework, right next to the CSR office. Calvin and Hobbes
Reading quiz (graded) Which of the following is not a way heat can be transferred? a. a.conduction b. b.convection c. c.liquification d. d.radiation
Thought question (ungraded) Which of the following does not continuously emit electromagnetic waves? a. a.A light bulb which is turned on b. b.A light bulb which is turned off c. c.A star d. d.A hot electric stove burner e. e.None of the above
Blackbody Radiation Hot objects glow! a. a.That glow carries away energy Surroundings also glow! a. a.That glow adds energy
Demo Burning ants with magnifying glass a. a.OK, not really
Color of emission You’ll learn/derive the equation in Phys 360, if you take it. Some results: Area ~ T 4 Peak ~ 1/T Chromaticity diagram
More on Emissivity “Fudge factor” between 0 and 1 Different for different surfaces a. a.0.05 for “highly polished aluminum” b. b.0.8 for “anodized aluminum” Same as “absorptivity” a. a.Why? Different for different wavelengths a. a.Greenhouse effect
A metal sphere is heated to 1200 K, and puts out 1000 W of radiation energy. If it is cooled to 600 K, it will put out ______ W of radiation energy. (Don’t worry about heat absorbed from surroundings. Assume emissivity is the same for the two temperatures.) a. a.31.25 b. b.62.5 c. c.125 d. d.250 e. e.500 Thought question
Reading quiz (graded) The rate that heat is conducted through a wall (temperature T 1 on one side, T 2 on other side) is proportional to… a. a.T 2 -T 1 b. b.(T 2 -T 1 ) 2 c. c.(T 2 -T 1 ) 1/2 d. d.log(T 2 -T 1 ) e. e.exp(T 2 -T 1 )
Thought Question You put the end of a rod in a fire and the other end in a tub of water. The rod that would heat the water fastest will be: a. a.short and fat b. b.long and fat c. c.short and thin d. d.long and thin
Thermal Conduction A hot T 1 cold L Really: Warning: what is meant by time? “Steady state” vs. “Thermal equilibrium”
Thermal Conductivity Some Thermal Conductivities (from your textbook) Materialk (J/s∙m∙ C) Copper397 Aluminum238 Iron79.5 Glass0.84 Wood0.10 Air0.0234 What “feels” colder, a metal car or a wooden box?
Thought question If I heat the left end of an iron rod such that its temperature is a constant 200 degrees C, and I put the right end in ice water, what will the temperature of the middle of the rod be when the rod approaches “steady state”? a. a.0 ºC b. b.50 ºC c. c.100 ºC d. d.150 ºC e. e.200 ºC What if left half of rod is iron, but the right half is copper?
Thought question: If I heat one end of an iron rod to 150 degrees C and I put the other end in ice water, I get a heat flow of 10 J/s through it. If I do the same with a particular copper rod, I get 25 J/s. If I stick the two of them together, side by side, how much heat will flow through the combined rod? a. a.10 Watts or less b. b.More than 10 but not greater than 25 c. c.More than 25 but less than 35 d. d.35 Watts e. e.More than 35 Watts T 2 hot T 1 cold iron Cu
Thought question: I put an iron rod and a copper rod end-to-end to form one long rod. The total heat flow through the combined rod is 100 J/s. How does the heat flow (J/s) through the iron compare to the heat flow through the copper? (k iron = 79.5 W/m C; k Cu = 397 W/m C) a. a.P iron < P copper b. b.P iron > P copper c. c.P iron = P copper T 2 hot T 1 cold iron Cu
R-values Some R-values (from your textbook) MaterialR (ft 2 F hr/Btu) Brick, 4” thick 4 Styrofoam, 1” thick 5 Fiberglass insulation, 3.5” thick10.9 Drywall, 0.5” thick 0.45 Why useful: R values of wall materials add Yuck!
Worked Problem You foolishly decide to build the walls of your new house out of solid aluminum (k = 238 W/m C), 5 cm thick. As a result, in the wintertime heat leaks out like a sieve. How much money will this cost you each day? The inside temp is 70 F (21.1 C), the average outside temperature is 25 F (-3.9 C). The surface area is 280 m 2. The gas company charges you $0.89 per “therm” (1.055 10 8 J). Only count heat loss through conduction. Answer: $27,288 Class survey: guess the answer
Quick Discussion If air is such a poor thermal conductor, why is it beneficial to use fiberglass insulation in your attic? Materialk (J/s∙m∙ C) Air0.0234 Fiberglass0.045