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Announcements 9/12/12

Announcements 9/12/12. Prayer Register your clickers by today at 5 pm! Unregistered numbers from Monday: 3368A2F9, 7A8E14E. The Far Side. From warmup. Extra time on? emissivity (several people) Other comments?

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Announcements 9/12/12

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  1. Announcements 9/12/12 • Prayer • Register your clickers by today at 5 pm! Unregistered numbers from Monday: 3368A2F9, 7A8E14E The Far Side

  2. From warmup Extra time on? emissivity (several people) Other comments? You know you're a physics major when spell check underlines more words that it doesn't know, than words you don't know.

  3. Heat Transfer • conduction • convection • radiation

  4. Clicker question • Which of the following does not continuously emit electromagnetic waves? • A light bulb which is turned on • A light bulb which is turned off • A star • A hot electric stove burner • None of the above

  5. Blackbody Radiation • Hot objects glow! • That glow carries away energy • Surroundings also glow! • That glow adds energy

  6. From warmup • If the temperature of a "black body" doubles, how much does its rate of energy emission change? • It would be multiplied by a factor of 16 because P is proportional to T^4.

  7. Demo • Burning ants with magnifying glass • OK, not really

  8. “white point” Color of emission You’ll learn/derive the equation in Phys 360, if you take it. Some results: Area ~ T4 Peak l ~ 1/T Chromaticity diagram

  9. More on Emissivity • “Fudge factor” between 0 and 1 • Different for different surfaces • 0.05 for “highly polished aluminum” • 0.8 for “anodized aluminum” • Same as “absorptivity” • Why? • Different for different wavelengths • Greenhouse effect

  10. Clicker question • 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.) • 31.25 • 62.5 • 125 • 250 • 500

  11. From warmup • To decrease the energy lost by conduction from a hot object to a cold one, what can you change? • You can increase the thickness or decrease the cross-sectional area of whatever connects the two objects. Or, perhaps the simpler option, add insulation in between, particularly one such as fiberglass.

  12. T2 hot T1 cold Really: A L Thermal Conduction Warning: what is meant by time? “Steady state” vs. “Thermal equilibrium”

  13. Thermal Conductivity Some Thermal Conductivities (from your textbook) Materialk (J/s∙m∙C) Copper 397 Aluminum 238 Iron 79.5 Glass 0.84 Wood 0.10 Air 0.0234 What “feels” colder, a metal car or a wooden box?

  14. Demo • Boiling water in a paper cup • Clicker: To prevent the cup from catching on fire, would you like it to have a large or small thermal conductivity? • large • small

  15. Clicker 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”? • 0 ºC • 50 ºC • 100 ºC • 150 ºC • 200 ºC What if left half of rod is iron, but the right half is copper?

  16. T2 hot T1 cold iron Cu Clicker 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? • 10 Watts or less • More than 10 but not greater than 25 • More than 25 but less than 35 • 35 Watts • More than 35 Watts

  17. Clicker 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? (kiron = 79.5 W/mC; kCu = 397 W/mC) • Piron < Pcopper • Piron > Pcopper • Piron = Pcopper T2 hot T1 cold iron Cu

  18. R-values Some R-values (from your textbook) MaterialR (ft2 F hr/Btu) Brick, 4” thick 4 Styrofoam, 1” thick 5 Fiberglass insulation, 3.5” thick 10.9 Drywall, 0.5” thick 0.45 Yuck! Why useful: R values of wall materials add

  19. 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 m2. The gas company charges you $0.89 per “therm” (1.055  108 J). Only count heat loss through conduction. Class survey: guess the answer Answer: $27,288

  20. Quick Discussion Materialk (J/s∙m∙C) Air 0.0234 Fiberglass 0.045 • If air is such a poor thermal conductor, why is it beneficial to use fiberglass insulation in your attic?

  21. From warmup • Why do you think there are no equations in the "convection" section of the book? • It is too complicated for a simple equation to explain it. • There are too many unknown variables • Because they look like this http://en.wikipedia.org/wiki/Convection-diffusion_equation • (not quite correct) Convection is a theory that shows how conduction works in fluids.

  22. Convection • Youtube: http://www.youtube.com/watch?v=7xWWowXtuvA

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