The worksheet for the simulation will be in the student’s booklets. • Ask the candidates: – Try all of the controls to get a sense of what they do. – How does x affect y? (draw graph) Simulation of “blackbody” is in Phet • Heineman has a greenhouse gas sim Simulation of microwave oven is in Phet (has to do w why N2 and O2 do not act as greenhouse gases) Simulation of greenhouse gases is in Heineman, and probably Phet too
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The worksheet for the simulation will be in the student’s booklets.
Simulation of “blackbody” is in Phet. The worksheet for the simulation will be in the student’s booklets. Heineman has a greenhouse gas sim. Simulation of microwave oven is in Phet (has to do w why N2 and O2 do not act as greenhouse gases). Ask the candidates: - PowerPoint PPT Presentation
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The worksheet for the simulation will be in the student’s booklets.
• Ask the candidates:– Try all of the controls to get a sense of what they do.– How does x affect y? (draw graph)
Simulation of “blackbody” is in Phet• Heineman has a greenhouse gas simSimulation of microwave oven is in Phet (has to do w why N2 and O2 do not act as greenhouse gases)
Simulation of greenhouse gases is in Heineman, and probably Phet too
Why do cars get so hot in the summertime?(And why doesn’t the color of the paint matter?)
Intro to the Greenhouse effect
“The ability of the atmosphere to capture and recycleenergy emitted by the Earth surface is the
defining characteristic of the greenhouse effect.”http://en.wikipedia.org/wiki/Greenhouse_effect
Greenhouse Effect in briefThe greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere. As a result, the temperature there is higher than it would be if direct heating by solar radiation were the only warming mechanism.
Details of the greenhouse effect
Understanding this effect gives us the opportunity to apply a lot of the physics that we have studied this year (and even a bit more).
Molecules, Energy, and Light• Molecules can absorb light in the same way that
atoms can absorb light, and in a different way too.• Molecules can do things that atoms cannot do:
rotate (spin), vibrate (compress and extend), bend, …
• Molecules can absorb photons that have energies that resonate with all the things that they can do. These photons all are lower energy (frequency) than electron transitions. These photons are in the infrared (“heat”) part of the E-M spectrum.
Atoms, Molecules, Gases, Liquids, Solids
• As we consider systems with more particles, and as those particles get closer together, the systems are better at absorbing photons.
• When light hits the surface of the earth, it is usually absorbed.
Approximately what is the temperature of an object that radiates most of its energy as yellow light?
Solution
Yellow light has a wavelength of about 570 nm.570 nm = 570 x 10-9 m
lmax = B / Tlmax = 2.89 X 10-3 m K / TT = 2.89 X 10-3 m K / lmax
T = 2.89 X 10-3 m K / 570 x 10-9 m T ≈ 5000 K
( This is approximately the temperature of the surface of the sun. )
“Infrared” vs. “Heat”All objects radiate electromagnetic energy. When it’s in the visible range for humans, we call it
“light”, but it is light no matter what the frequency.
Objects at room temperature emit almost no light that we can see, but they do emit plenty of light in the ‘infrared’ region.
Since hotter objects emit more infrared light than cooler objects, we sometimes call infrared light “heat”, but it is not heat, it is just light that comes from hot objects.
• The hotter the object, the greater the intensity of radiation that is emitted:
• Power per unit area = s T4
• s = 5.67 x 10-8 W m-2 K-4
ProblemLight from the sun is most intense around the color blue. What temperature is the surface of the sun?
Solution
From , lmax = 2.89 x 10-3 m K / TT = 2.89 x 10-3 m K / lmax Blue light has a wavelength of about 420 nm.So T = 2.89 x 10-3 m K / (420 x 10-9 m) T = 6900 K
The earth absorbs the sun’s light…The earth is getting a lot of energy every second.
Why doesn’t the earth get hotter and hotter?Tell your buddy.
The earth radiates energy back into space…If the earth radiated less than it does, then the
earth would cool.If the earth radiated more than it does, then the
earth would warm.The earth radiates energy at the same rate that
the earth absorbs energy.
What if you put a very cold earth in our place?At first the earth would radiate very little energy. As the earth warmed up, the earth would radiate more and more.At equilibrium, the earth would radiate as much as it received.
Our first climate calculationIf the earth had no atmosphere, then the energy coming in from the sun would balance the energy it radiated out. What would be the average temperature of the earth? Givens:Solar constant: 1380 W m-2 Radius of earth = 6.4 x 106 m(Value to check at the end: Average temperature of the earth is 59˚F = 15˚C = 288 K)
Solution (1 of 5)• Since the temperature of the earth is steady, the
energy arriving equals the energy leaving.• The energy arriving is from the sun. • The energy leaving is by radiation.
Solution (2 of 5)• The energy arriving is from the sun. The product
of the solar constant and area gives the power (energy per second).Rate of energy arriving = {1380 W m-2}×{Area}
• Which area should we use? …
The area receiving the radiation is equivalent to a disc.
Solution (2 of 5)• The energy arriving is from the sun. The product
of the solar constant and area gives the power (energy per second).Rate of energy arriving = {1380 W m-2}×{Area}
• Which area should we use?• Area of a disc = pR2
Power arriving = {1380 W m-2}×{p(6.4 x 106 m)2}Power arriving = 1.75 x 1017 W
Solution (3 of 5)• Energy arrives from the sun at 1.75 x 1017 W. • But a portion of it goes right back out into space,
due to reflection. The reflected portion does not contribute to the energy balance on earth.
• The average albedo of the earth is 30 % = 0.30• So 70 % of the energy from the sun does enter
our energy balance.• 0.70 x 1.75 x 1017 W = 1.23 x 1017 W { }• This is the relevant value of solar energy that
arrives at earth.
Solution (4 of 5)• The energy leaving is by radiation. • Use the Stefan-Boltzmann law:
Rate of energy leaving per area= s T4 Rate of energy leaving = Area×s×T4
• Which area should we use?• Over what area does the earth radiate?• Area of a sphere = 4pR2
Rate of energy leaving = (4pR2)×s×T4 Power leaving =4p(6.4x106m)2(5.67x10-8Wm-2K-4)×T4
Solution (5 of 5)Power arriving = Power leaving
1.23x1017W = 4p(6.4x106m)2(5.67x10-8Wm-2K-4)×T4 Solve for TemperatureT = 255 K = -18˚C = 1˚F
This is colder than the average temperature of the earth (288 K), but we left out the earth’s atmosphere. We left out the (natural) greenhouse effect.
Emissivity (1 of 4) A big help in getting the energy equation to work out better comes from realizing that the earth does not act like a perfect “blackbody”. Real objects do not act like perfect black bodies; real objects emit less energy than a perfect black body would at the same temperature.Because the atmosphere acts like a blanket, the earth radiates energy at a rate less than sT4.
Emissivity (2 of 4) Emissivity is a unitless number that describes how it resembles a perfect black body.Emissivity is calculated by dividing the real output of the object by the perfect blackbody output.A dull, black lump of rock has an emissivity of about 0.9 A mirror has an emissivity of about 0.1
Emissivity (3 of 4) If the earth did radiate like a perfect blackbody, then it would radiate at this rate:
Power Leaving = (Area of sphere)×sT4 Power Leaving = [4p(6.4x106)2]×(5.67x10-8)×(288)4
Power Leaving = 2.0 x 1017 WThe real power leaving the earth is the same as the real power arriving at the earth:
1.23 x 1017 W { }How would you calculate the emissivity of the earth?
Emissivity (4 of 4) The emissivity of the earth is:
e = { 1.23 x 1017 W } ÷ { 2.0 x 1017 W } e = 0.6
The role of the atmosphere in climate• Every molecule can absorb radiation, and when it
does, it often emits that same frequency, but in a random direction.
• Our atmosphere has an upper layer with ozone (O3) and a lower layer with water vapor and carbon dioxide (CO2)
Greenhouse gases are transparent to incoming solar radiation…
Greenhouse gases are transparent to incoming solar radiation, but absorb the earth’s radiation.
Conceptual QuestionsImagine light that hits the ground and is reflected right back.a)What is the most likely color of the earth in that region?b)What is the most likely thing for that light to do when it hits the atmosphere?
Conceptual AnswersImagine light that hits the ground and is reflected right back.a)What is the most likely color of the earth in that region?
The earth there is likely white, due to snow or ice.b)What is the most likely thing for that light to do when it hits the atmosphere?Since the light just made it through the atmosphere, it is visible light, and it will go back through it again.
More Conceptual QuestionsImagine light that hits the ground and is absorbed, and as a result, a moment later the earth re-radiates. What is the most likely thing for that light to do when it hits the atmosphere?
More Conceptual AnswersImagine light that hits the ground and is absorbed, and as a result, a moment later the earth re-radiates. What is the most likely thing for that light to do when it hits the atmosphere?The earth is not terribly hot, so when it radiates, it sends out infrared radiation. Atmospheric CO2 and
Rise in Sea Level• As the temperature of a liquid increases, it
usually gets larger: DV = bV0DT.• We do observe an increase in sea level.• The rise could be due to warming oceans or
melting icecaps.
Global Warming• “Global Warming” refers to any long-term
increase in the temperature of the earth.• For example, if the sun got hotter, it would cause
global warming.• If global warming is observed, it could be due to a
dozens of different causes.
Computers and Global Warming• The calculations involved in global warming are
famous for being complicated, and for being important.
• In order to make responsible predictions for the long-term future of the earth, scientists have been pressing hard to accurately describe how climates work.
The forecast…• Weather is not Climate.• Day-to-day weather, and even data over just a
few years, are not the concerns of climatologists.• Most models forecast more rain in the North and
more heat around the equator.
How is global warming different from the greenhouse effect?
What is the cause and what is the effect?
IPCC• The United Nations and the World Meteorological
Organization established the Intergovernmental Panel on Climate Change.
• The IPCC reviewed the data and wrote:THE EXPERTS CONCLUDED THAT THEY ARE CERTAIN THAT EMISSIONS FROM HUMAN
ACTIVITIES ARE SUBSTANTIALLYINCREASING THE ATMOSPHERIC
CONCENTRATIONS OF GREENHOUSE GASES AND THAT THIS WILL ENHANCE THE GREENHOUSE EFFECT AND RESULT IN AN ADDITIONAL WARMING OF THE EARTH’S SURFACE.
Global Warming is an international problem.
SolutionsAll of the solutions deal with minimizing the emission of CO2.
• Cleaner cars.• Power plants that do not use coal, or at least that