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CU-Boulder PHYS 1010 - Greenhouse Effect

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10/19/20101HW9 available onlineNote: you actually have to hit the “submit” button.Remember reading: 10.1 static electricityQuiz ThursdayToday: greenhouse effectintroduction to electricity (static cling)The greenhouse effect. Why does the temperature of the earth’s surface depend on CO2and H2O (water)and other “greenhouse gases” in atmosphere?Simple picture- why these gases cause heating. (Without them, life would not survive.)Exactly how does the temperature change with more CO2? This is complicated.Depends on how atmosphere and world changes with heating. We will not get into this in class.Your car is sitting in the bright sun. The inside of the car gets much hotter thanthe air next to it. Why?a. The 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 incar causes it to heat up.d. The windows let in energy but do not let it escape.e. None of the above make sense, must be different explanation.answer d: this is the greenhouse effect. Sunlight passes through glass to inside of car, heating us the seats, the dashboard, etc. These hot things emit IR radiation, which is not able to leave through the glassRemember this? The glass blocked the IR from the hot plate, so we could not detect the IR from the hot plate. How is this possible? Electromagnetic energy changes color! powerwavelengthIRUVVisiblesun 40 C inside carEnergy from sun is mostly visible light, which goes through the window.The interior of the car absorbs the light and come to some temperature. It gives off radiation in the IR, which cannot go through the glass. The glass itself radiates IR, some of which goes back inside the car, and some of which escapes. Amount of energy given off by glass depends on its temperature.Temperature of earth- conservation of energy1) Visible light hits the earth and is absorbed – how much power?2) The warm earth (at temperature T) radiates power as IR radiation – how fast?As Earth heats, IR radiation power out increases until it balances visible radiation power in.Power in from sun = Power out from radiation into space. Knowing this you can calculate Temperature of earth!Preliminary: if Reis the radius of the earth, what is the area of the earth absorbing sunlight?a) 2pReb) pRe2c) 4pRe2d) 4/3 p Re3Answer: b the area as seem from the sun10/19/20102Temperature of earth- conservation of energy1) Visible light hits the earth and is absorbed – how much power?2) The warm earth (at temperature T) radiates power as IR radiation – how fast?As Earth heats, IR radiation power out increases until it balances visible radiation power in.Power in from sun = Power out from radiation into space. Knowing this you can calculate Temperature of earth!Preliminary: if Reis the radius of the earth, what is the area of the earth that emits IR?a) 2pReb) pRe2c) 4pRe2d) 4/3 p Re3Answer: c the total surface area of the earthTemperature of earth- conservation of energy1) Visible light hits the earth and is absorbed – how much power?2) The warm earth (at temperature T) radiates power as IR radiation – how fast?As Earth heats, IR radiation power out increases until it balances visible radiation power in.Power in from sun = Power out from radiation into space. Knowing this you can calculate Temperature of earth!Powerfrom sun= 1380 W/m2x πR2earthx 0.7 = 1.22 x 1017Wsolar power/m2at earth area of earth absorbing sunlightfraction of sunlight absorbed by earthTemperature of earth- conservation of energy1) Visible light hits the earth and is absorbed – how much power?2) The warm earth (at temperature T) radiates power as IR radiation – how fast?As Earth heats, IR radiation power out increases until it balances visible radiation power in.Power in from sun = Power out from radiation into space. Knowing this you can calculate Temperature of earth!If there are no greenhouse gases in the atmosphere, f = 1. What temperature does this imply?Stefan-Boltzmann Law says Powerout= T4 x surface area of Earth X ffraction of radiation that actually gets to space.Greenhouse effect and temperature of earth.Powerinfrom Sun = Poweroutfrom earth1380 W/m2x πR2earthx 0.7 =  X T4X 4πR2earth X f Solve to get T4= (1380 W/m2X 0.7)/(4 X  X f)T = sqrt (sqrt 4.26 x 109 K4) = 255 K = -18 C ~ 0 F How hot would earth be if all the IR from the surface left directly (f = 1)?Water would freeze! Life would be impossible!The surface of the earth is not this cold, because the atmosphere contains greenhouse gases.Because of these gases, the amount of IR radiated from the surface that actually gets to space is f = 0.61 (this is an average value).Why doesn’t all of the IR get out of the atmosphere?Certain types of molecules “greenhouse gases” absorb IR and send it back towards earth. Carbon dioxide, H2O, but not oxygen or nitrogen. Only part of IR getting out,Earth’s surface temperature rises untilPower in from sun = Power out to space .T up from 255 K (0 F) to 285 K (54 F)Greenhouse effect and temperature of earth.Powerinfrom Sun = Poweroutfrom earth1380 W/m2x πR2earthx 0.7 =  X T4X 4πR2earth X f Solve to get T4= (1380 W/m2X 0.7)/(4 X  X f)T = sqrt (sqrt 4.26 x 109 /0.61 K4) = 289 K = 16 C ~ 61 F Now some IR from surface is absorbed, so in reality f = 0.61Naturally occurring greenhouse gases keep the planet habitable.10/19/20103When the concentration of greenhouse gases goes up, the total power emitted by earth to space, a. goes upb. goes downc. stays the samec. stays the same. It always has to just balance theamount coming in from sun or earth will rapidly heatup. 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 outIf we look at Earth from space with temperature probe and measure IR coming off of earth to measure temperature of earth, what temperature do we measure?a. 255K b. 273 K c. 295 K d. 2000 K When the concentration of greenhouse gases goesup, most of the IR radiation emitted to space comes from...a. a lower altitude , b. a higher altitude.ans. a. Seen from space, the amount of EM radiation matches that sent in by the sun, which is what a 255 K earth with no atmosphere would give. ans. b. higher. IR from lower just gets absorbed by moleculesabove it. Only IR from top layer escapes into space. Now


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CU-Boulder PHYS 1010 - Greenhouse Effect

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