Mystery Clicker Number: 54161 check if belongs to you. Get LOTS of points if it does!! NEW SEAT AND GROUP ASSIGNMENTS TODAY!!. CHECK PRINTOUTS TO SEE WHERE YOU BELONG. MINOR CHANGES TO FIX PROBLEMS OF LAST THURSDAY. Reading quiz on chapter 8.1 air-cleaners, on thurs. Today: 1) finish Lightbulbs • How they work? What determines color? • Why do they “burn out”? • Why are fluorescent lights are more efficient? • Why physics makes it hard to improve efficiency? Hot filament & everything 2) infrared radiation- else gives off all the “light” we don’t see. “electromagnetic energy” 3) Greenhouse effect- The physics of global warming. Electric What’s behind CO2 emissions Current debate. Electrons (the basic physics, not the complex atmospheric modeling stuff) IR Visible UV power sun max power in visible where eye works. light bulb longer shorter wavelength matches book, (= color) but not most max power in infrared (IR) places, like where eye not sensitive simulator just make brief note to this slide if they want to look at it on website The funny shape of the black body spectrum The shape of the spectrum for the thermal radiation of an object is rather complicated, but with some effort it can be understood and a formula derived that describes it. That is too advanced for this class, but in case you are interested, the formula for the amount of power at each frequency of an electromagnetic wave is given by P = (8h/c3)3 (eh/kT -1)-1, where k is the Boltzmann constant (= 1.38 x 10-23 Joules/Kelvin), c is the speed of light (3 x 108 m/s), h is Planck’s constant (6.626 x10-34 J∙s), and the temperature T is in units of Kelvin. From this equation it is possible to show that the wavelength where the power is highest is given by peak = hc/(2.82 kT) detected power peak = hc/(2.82 kT) so shifts to shorter wavelength as T increases. frequency wavelength See spectral simulator for homework a typical spectrum Spectra of radiation given off by person and spectra of block of barely frozen ice (32 F, 0 C). IR Visible 1 3 power (different scale from sun!) 2 a. 2 is person, 1 is ice, wavelength (= b. 1 is person, 2 is ice, color) c. none, because ice gives off no thermal radiation, d. 1 is both ice and person because they are almost identical e. 3 is person, one of the others is ice. ans. b. both person and ice give off IR, but so cold no visible. Person a bit hotter than ice, so more radiation, also hotter so shifted towards visible. in notes, don’t go through in class. IR Visible power 1 (different scale person from sun!) 2 ice (32 F, 0 C). wavelength (= color) to check precise numbers: amount of power goes as T4, so 273 k = ice, 273 +37= 310 K = person. T4p/T4ice= (310/273)4 = Pperson/Pice =1.66 times more power from person. Peak wavelength goes as 1/T, so moved over about 20% (310/273 = 1.2) Spectra of radiation given off by person and spectra of block of barely frozen ice (32 F, 0 C). IR Visible power 1 (different scale from sun!) 2 wavelength (= color) Everything giving off electromagnetic radiation all the time! People, floor, rocks, … Unless very hot, in IR. look at amount of radiation (IR) to measure temperature. remote temperature sensor-- looks at IR to measure temp., demos. heat metal, look with IR sensitive camera. Measure hot plate temperature with remote temp sensor. T= 300 degrees F. Now we put a piece of glass in the way. What will temp sensor read? (decide as a group, and be prepared to explain your answer) a. same temp., b. slightly less, c. about 73 degrees F, d. much colder than 73 F. ans. c. 73 (room temp). Temp sensor gun detects the IR coming from the glass. Amount tells it glass is room temp. Visible light (red laser beam) goes through glass but IR does not. Back to light bulbs. What would the ideal temperature be for a bulb filament? a. Temp of standard bulb filament (2500 C). b. Temp of the sun (5800 C). c. Hotter than the sun. Why? The higher the temperature, the less power being wasted as IR light, so hotter means more visible light produced per Watt of electric power used. (More EFFICIENT). show simulator Temperature of sun is spectrum best matched to eye. So temperature at least as hot as sun would be best. Colder means lots of power wasted as infrared. Heats but does not illuminate. Hotter would produce bunch of UV light … bad for eyes/skin. IR Visible UV power sun 5800 C max power in visible where eye works. light bulb 2500 C wavelength (= color) max power in infrared (IR) where eye not sensitive, wasted. If hotter is better, why are filaments in standard bulb not heated to 5800 C? a. because they melt if heated up any hotter b. because they vaporize if heated up any hotter c. because they oxidize (“burn up”) if heated any hotter d. would be too expensive to build Why not c ? Demo of filament with bulb removed. It burns up very quickly! Solution- put inside bulb- inert (no oxygen) gas. demo With no oxygen in bulb, can heat a lot farther until: Melting: (Edison thought was biggest problem so used carbon, melts at highest temp. But more sublimation so bulbs N, Ar burnt out faster.) So not a, but if picked, as smart as T. E. Sublimation: going into a vapor turns out to be limit before melting. So tungsten better than carbon. ans. b. Light bulb filaments vaporize (“sublimation”) too fast at temperatures above 2500 C. Tungsten melts a bit lower than carbon used by Edison, but has lower sublimation rate, so bulbs last longer. “sublimation”, direct solid to gas- vapor Examples: • cold dry sunny day in Boulder- snow vanishes before melting • “Dry ice” carbon dioxide. .solid Rate of sublimation increases rapidly as gets hotter. Hotter T, more visible light = better electrical to visible light conversion (“efficiency”), but faster sublimation. vapor solid Light bulbs today use tungsten, least sublimation rate. T = 2500 C Wastes 88% of power!! tradeoff between efficiency and filament lifetime Eventually sublimates away. Black stuff on glass. When all the way through, filament breaks off rattles around in bulb. Not “Burned out”, actually “evaporated out”! run hotter, evaporate away much sooner skip in class, come back to if time How to get more efficient light bulbs? 1. Halogen lamps- run hotter, undue sublimation. Incredibly clever!! hot filament glass envelope (cooler) halogen molecule picks up tungsten off cool glass carries it back to center, releases it to hot filament 2. Florescent lights even better. Will learn about at end of term. III. The greenhouse effect. Why the temperature of the earth depends on CO2 and H2O and other “greenhouse gases” in atmosphere. Simple picture- why these cause heating. (life depends on) Exactly how temp changes with more CO2? Complicated. Depends on how atmosphere and world changes with heating. Car sitting in bright sun. Inside gets much hotter than the air next to it. Why? a. car absorbs light energy from sun better than the pavement. b. sunlight causes chemical reactions in car materials that give off heat. c. electrical appliances such as clock that run all the time in car causes it to heat up. d. windows let in energy but do not let it escape. e. none of the above make sense, must be different explanation. ans. d- greenhouse effect. Sunlight passes through glass to inside of car, IR absorbed by glass and part reemitted back into car … not able to escape directly. How possible? Electromagnetic energy changes color! Visible light from sun, IR light off hot car materials. IR also off hot ground, but it goes through air, but not glass. Remember temperature sensor energy from sun mostly visible light. Goes through window IR Visible UV sun Some energy leaves car when power IR radiation emitted to outside 100 F car inside by glass and car frame. Amount wavelength of energy given off by glass depends on its temperature. Why not “hot car seat effect”? same physics used to keep green houses warm before cars- so “green-house effect”. Glass keeps IR from escaping. energy from sun mostly visible light. Glass warms up some, Goes through window gives off some IR, ½ sent back into house. IR emitted by inside plants, etc hits glass and is absorbed Glass warms up emits IR … ½ radiation to outside, ½ radiation to inside Energy absorbed by glass = Energy emitted by glass But some energy goes back into greenhouse! Temperature of earth- conservation of energy Sunlight putting energy into earth - light energy into thermal Keep adding energy, keeps getting hotter. Cons. of energy! As Earth heats, IR radiation out increases until balance… Power in from sun = Power out from radiation into space. Knowing this can calculate Temperature of earth! Powerin = solar power/m2 at earth x area of sunlight intersected by earth x fraction of sunlight absorbed by earth = 1380 W/m2 x πR2earth x 0.7 = = 1.22 x 1017 Watts(!) Stefan-Boltzmann Law says Powerout = Power radiated to space by Earth with surface temperature T = T4 x surface area of Earth x fraction of IR emitted that makes it to space = 5.67 x 10-8 J/(s m2 K4) x T4 x 4πR2earth m2 x fraction of IR escaping = 2.89 x 107 W/K4 x T4 x fraction of IR escaping How hot would earth be if all IR got out like shown? Greenhouse effect and temperature of earth. How hot would earth be if all IR got out like shown? Powerin from Sun = Powerout from earth 1.22 x 1017 Watts = 2.89 x 107 W/K4 x Tavg4 Solve to get Tavg = sqrt (sqrt 4.22 x 109 K4) = 255 K = -18 C ~ 0 F Brrr! Earth not that cold because of greenhouse effect. Atmosphere acts like the glass. Not all IR gets out. Why doesn’t all of the IR get out of atmosphere? Certain types of molecules “greenhouse gases” absorb IR and send it back towards earth. Carbon dioxide, H2O, Not oxygen or nitrogen. Only part of IR getting out, Earth’s surface temperature rises until Power in from sun = Power out to space . T up from 255 K (0 F) to 285 K (54 F) When the concentration of greenhouse gases goes up, the total power emitted by earth to space, a: goes up, b. goes down, c. stays the same c. stays the same. It always has to just balance the amount coming in from sun or earth will rapidly heat up. Effect of greenhouse gases is to block some going from ground to space, so need more leaving ground to have same amount get into space. power in = power out go to simulation. show if no greenhouse. what temp has to be add greenhouse gas, see temp rise. complicated stuff, clouds, etc. How to calculate temperature when greenhouse effect? Powerin = solar power/m2 at earth x area of sunlight intersected by earth x fraction of sunlight absorbed by earth = 1380 W/m2 x πR2earth x 0.7 = = 1.22 x 1017 Watts(!) Temperature of surface of earth has to be value so that Power out to space = Power in from sun Stefan-Boltzmann Law says Power out to space = 2.89 x 107 W/K4 x T4 x fraction of IR escaping if greenhouse gases absorb and emit IR such that only 61% of power radiated by earth’s surface gets into space. So Pout = 0.61 x 2.89 x 107 W/K4 x T4. To conserve energy Pin = Pout, which means T is higher than before. 1.22 x 1017 W = 0.61 x 2.89 x 107 W/K4 x T4, solving for T, get T = (6.92 x 109)1/4 = sqrt (sqrt(6.92 x 109)) = 288 K. What is the effect of clouds? a. Decrease the temperature of the ground b. Increase the temperature of the ground c. No effect on the temperature of the ground Issues: 1. Clouds reflect part of sunlight back to space. 2. Clouds absorb IR radiation from earth and send it back to ground. Net global effect is cooling by a few decrees C.
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