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Blackbodies

Atmospheres

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Outline

• Review• Kinetic Energy/Temperature• Blackbodies• Atmospheres• Green House Effect

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Tutor (Shane)

Thursday 5-6 (or longer if needed) BH640

Register clickers… Homework…

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Review

• What was the most important thing you learned?• There are charged particles bouncing back and

forth on the (field lines in the) magnetosphere• Aurora… “excited” molecules (or atoms) being

“bumped” in the atmosphere• Temperature is a measure of the average

kinetic energy of a body’s molecules.

Ek = 1/2Mv2

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Review

• What was the most important thing you learned?• The solar wind is NOT the only reason

Mars has no atmosphere.• The magnetosphere of Jupiter is the

largest object in the solar system.

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Tornado

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Tornado

• Tornado photo courtesy of Gary Gianniny and email from Scott White.

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Which temperature will freeze water?

A) 65° F

B) 5° C

C) 263 K

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Temperature

• Temperature is just a measure of the average kinetic energy of a body’s molecules.

• Go to Solar System Collaboratory to see temperature scales.

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More Precisely 2-1The Kelvin Temperature Scale

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Temperature

• With enough kinetic energy (temperature), molecules can “escape” from a planet.

• http://www.xkcd.com/681/

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Small Group Exercise

• Normal Human body temperature is 37 ° C.

• What is this temperature in Kelvins?

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Heat Transfer

• Conduction• Transfer from one mass to another mass that is touching.

• Convection• Transfer though a gas (air) that moves across a mass

• Radiation• Transfer using electromagnetic radiation• This is the least efficient method• This is the only method available in/through space

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Albedo

• Albedo - the fraction of light that is reflected from a planet (or other solar system body).• A unitless number from 0 to 1.• 0 - all light is absorbed (very black)• 1 - all light is reflected (very white)• Earth's average albedo is 0.30, so 30% of the

sunlight is reflected. • The light absorbed would be one minus the

albedo. For the Earth, 1.00-0.30=0.70, so 70% of the light is absorbed.

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Blackbody Radiation

• “Blackbodies” are not black.• Blackbody Radiation is from an “ideal”

object with albedo = 0.• Any dense, warm, object can be

approximated as a “blackbody”.• The “peak” of the radiation “curve” is

related to the temperature of the radiator.

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Figure 2.9Ideal Blackbody Curve

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Radiation

• Higher temperature bodies radiate energy in shorter wavelength radiation.

• The Sun radiates at visible wavelengths• The Earth (and other planets) radiate at

much longer wavelengths.• Go to Solar System Collaboratory to see

black body page.

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Figure 2.10Blackbody Curves

• Note the logarithmic temperature scale.• For linear scale, go look at the “black body” section of: http://solarsystem.colorado.edu/• example - oven

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Wein’s Law

• The “peak” frequency of the radiation “curve” is directly proportional to the temperature of the radiator.

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Small Group Exercise

• Normal Human body temperature is 37 ° C.

• What is this temperature in Kelvins?

• What is the peak wavelength emitted by a person at this temperature?

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Figure 2.8Electromagnetic Spectrum

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What is the peak wavelength emitted by an object with temperature 6,000 K?

A) 4.8x10-5 cm

B) 4.8x10-4 cm

C) 2.9x10-5 cm

D) 2.9x10-4 cm

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Planet Temperature

• Go to Solar System Collaboratory on EVM “physics” page.

• A planet must balance absorbed light and radiated light to get a temperature.

• Light intensity decreases with distance. (another 1/r2 law)

• Farther from the sun, the absorbed light is less.

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Planet Temperature

• Go to Solar System Collaboratory on EVM “physics” page.

• A planet must balance absorbed light and radiated light to get a temperature.

• Light intensity decreases with distance. (another 1/r2 law)

• Farther from the sun, the absorbed light is less.• Go to Solar System Collaboratory on planet

temperature page.

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Figure 5.7About 30% of the sunlight hitting the Earth is reflected

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To Atmosphere or Not

• Once you know a planet’s temperature you can see if it will have an atmosphere, and how that atmosphere can affect a planet’s temperature.

• Compare kinetic energy of molecules with “escape velocity” from the planet.

• Light molecules (of a given temperature T) move faster than heavy molecules of the same temperature.

• A small fraction will always escape.

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To Atmosphere or Not

• Primary atmosphere• What a planet had after formation• Mostly H, He - almost all gone from the

terrestrial planets (never really was here)• Secondary atmosphere

• Heavier molecules N2, CO2 From rock outgassing

• H2O from outgassing and comet impacts.

• O2 from Life

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Earth’s Atmosphere

• 78% nitrogen• 21% oxygen - this is from living organisms• Plus Ar, CO2, H2O.• Note layers

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Figure 5.5Earth’s Atmosphere

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Meteorology

• Science dealing with phenomena in the atmosphere (Not the study of meteors)

• Warm air rises and expands• Cold air sinks and shrinks• Must conserve linear and angular momentum.

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Figure 5.6Convection

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Removing all greenhouse gasses from the Earth’s atmosphere would be good

A) True

B) False

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Three Minute Paper

• Write 1-3 sentences.• What was the most important thing

you learned today?• What questions do you still have

about today’s topics?