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NATS 101 Lecture 4 TR Radiation Selective Absorption

NATS 101 Lecture 4 TR Radiation Selective Absorption. Modes of Heat Transfer. Energy is only converted from one form to another or transferred from one place to another. Energy is transferred from hot to cold. Conduction - Molecules colliding; most efficient at interface.

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NATS 101 Lecture 4 TR Radiation Selective Absorption

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  1. NATS 101Lecture 4 TRRadiationSelective Absorption

  2. Modes of Heat Transfer • Energy is only converted from one form to another or transferred from one place to another. • Energy is transferred from hot to cold. Conduction - Molecules colliding; most efficient at interface. Convection - Requires movement of a fluid or gas.

  3. Modes of Heat Transfer • Heat-Energy transfer due to temperature differences Three modes of heat transfer Conduction – molecule to molecule Convection – transport of fluid Radiation – electromagnetic waves (This lecture) Latent Heat – energy of phase changes

  4. Latent Heat • Energy associated with phase of matter. • Must be either added to or taken from a substance when it changes its phase. • To turn liquid water into solid ice, must remove energy from the liquid water. • To turn liquid water into vapor, must add a lot of energy to the liquid water.

  5. Latent Heat Take 2 Polarity Williams, p 63 Ice =>Liquid =>Vapor Takes energy from environment 80 cal/gm 540 cal/gm Vapor =>Liquid =>Ice Emits energy to environment

  6. Modes of Heat Transfer Latent Heat Williams, p. 19

  7. New Business • Radiation • Selective Absorption and Emission

  8. Radiation • Any object that has a temperature greater than 0 K, emits radiation. • This radiation is in the form of electromagnetic waves, produced by the acceleration of electric charges. • These waves do not need matter in order to propagate; they move at the “speed of light” (3x105 km/sec) in a vacuum.

  9. Electromagnetic Waves • Two important aspects of waves are: • What kind: Wavelength or distance between adjacent peaks. • How much: Amplitude or distance between crests and valleys. Wavelength Distance btw peaks Amplitude Height of crests/troughs Frequency # crests per unit time 9

  10. Why Electromagnetic Waves? • Radiation has an Electric Field Component and a Magnetic Field Component • Electric Field is Perpendicular to Magnetic Field

  11. Photons • Can also think of radiation as individual packets of energy or PHOTONS. Electron volts • In simplistic terms, radiation with shorter wavelengths corresponds to photons with more energy (i.e. more BB’s per second) and with higher wave amplitude (i.e. bigger BB’s)

  12. White Light from Flash Light Red Purple Green Emitted Spectrum • Emitted radiation has many wavelengths. Prism (Danielson, Fig. 3.14)

  13. Electromagnetic Spectrum Wavelengths of Meteorological Significance Danielson, Fig. 3.18 WAVELENGTH

  14. Plank’s Law: Emitted Spectrum Energy from Sun is spread unevenly over all wavelengths. Emission spectrum of Sun Planck’s Law Energy Emitted Ahrens, Fig. 2.7 Wavelength

  15. Planck’s Law and Wien’s Law The hotter the object, the shorter the brightest wavelength. Danielson, Fig. 3.19

  16. Wien’s Law Relates the wavelength of maximum emission to the temperature of mass λMAX= (0.29×104 μm K) × T-1 Warmer Objects => Shorter Wavelengths • Sun-visible light λMAX= (0.29×104 μm K)×(5800 K)-1 ≈ 0.5 μm • Earth-infrared radiation λMAX= (0.29×104 μm K)×(290 K)-1 ≈ 10 μm

  17. Wien’s Law What is the radiative temperature of an incandescent bulb whose wavelength of maximum emission is near 1.0 μm ? • Apply Wien’s Law: λMAX= (0.29 ×104 μm K)×T-1 • Temperature of glowing tungsten filament T= (0.29 × 104 μm K) × (λMAX)-1 T= (0.29 × 104 μm K) × (1.0 μm)-1 ≈ 2900K

  18. What is Radiative Temperature of Sun if Max Emission Occurs at 0.5 μm? • Apply Wien’s Displacement Law

  19. Stefan-Boltzmann’s (SB) Law • The hotter the object, the more radiation emitted. • Double the temperature: Total emitted radiation increases by a factor of16! • Stefan-Boltzmann’s Law E= (5.67×10-8 Wm-2K-4 )×T4 E=2×2×2×2=16 24 Sun Temp: 6000K Earth Temp: 300K Aguado, Fig. 2-7

  20. How Much More Energy is Emitted by the Sun per m2 than the Earth? • ApplyStefan-Boltzman Law • The Sun is 160,000 Times More Energetic per m2 Than the Earth, Plus Its Area is Much Bigger!

  21. How Much More Energy is Emitted by the Sun than the Earth? • ApplyStefan-Boltzman Law

  22. Radiative Equilibrium • Radiation absorbed by an object increases the energy of the object. • Increased energy causes temperature to increase(warming). • Radiation emitted by an object decreases the energy of the object. • Decreased energy causes temperature to decrease(cooling).

  23. Radiative Equilibrium (cont.) • When the energy absorbed equals energy emitted, this is calledRadiative Equilibrium. • The corresponding temperature is theRadiative Equilibrium Temperature. • Concept is analogous to a bathtub with the faucet running and the drain open. If water in exceeds water out, level rises. If water in is less than water out, level falls. If water in equals water out, level is constant, or it is said to be at an equilibrium.

  24. Clicker Instructions • Turn on clicker • Make sure you’ve entered in your UA NetID correctly • Join the class: MULLEN • When a question comes up, enter the answer on your key pad. • You will have a certain number of seconds to enter an answer, as indicated on the presentation • When you successfully send your answer, you will see a “received” message on the device. • Can change your answer as long as time has not run out.

  25. Participation Scoring + Attendance Always get one point for entering an answer, whether right or wrong An additional point if you get the right answer WARNING: IF YOU DON’T ENTER ANY ANSWER THEN YOU WILL BE RECORDED AS ABSENT, SO ALWAYS ENTER SOMETHING. SEE SYLLABUS POLICY FOR ADMINSTRATIVE DROP POLICY.

  26. FOUR POSSIBLE FATES OF RADIATION: • 1.Transmitted • 2. Reflected • 3. Scattered • 4. Absorbed • The atmosphere does ALL of these…

  27. Transmitted: Radiation passes through object GLASS WINDOW SUNLIGHT TRANSMITTED SUNLIGHT

  28. Reflected: Radiation turned back REFLECTED SUNLIGHT MIRROR SUNLIGHT

  29. Scattered: Path of radiation deflected SCATTERED SUNLIGHT FROSTED GLASS SUNLIGHT

  30. Absorbed: Radiation transferred to object Blackbody: a perfect absorber and emitter of radiation in equilibrium, with no reflection or scattering. BLACK BOX SUNLIGHT

  31. Radiative equilibrium: Absorption = Emission (Kirchoff’s Law) INFRARED (LONGWAVE) EMISSION BLACK BOX SUNLIGHT (SHORTWAVE)

  32. A “Grey Body” = Not all radiation absorbedHow the atmosphere behaves SOME TRANSMISSION OF SUNLIGHT THROUGH BOX GREY BOX SUNLIGHT (SHORTWAVE) INFRARED (LONGWAVE) EMISSION

  33. Why Selective, Discrete Absorption/Emission? Life as we perceive it:A continuous world! Atomic perspective:A quantum world! Gedzelman 1980, p 103

  34. Energy States for Atoms Gedzelman 1980, p 104 Hydrogen Atom Electrons can orbit inonly permitted states A state corresponds tospecific energy level Onlyquantum jumps between states can occur Intervals correspond tospecific wavelengths of radiation Hydrogen Atom Probability States

  35. Energy States for Molecules Molecules can alsorotate, vibrate, librate But only atspecific energy levels or frequencies Quantum intervals between modes correspond tospecific wavelengths Gedzelman 1980, p 105 H2O molecule H2O Bands

  36. Selective Absorption The Bottom Line Each molecule has aunique distribution of quantum states! Each molecule has a unique spectrum of absorption and emission frequencies of radiation! H2O molecule Williams, p 63

  37. Humans are Selective Absorbers Danielson, Fig. 3.18 http://hyperphysics.phy-astr.gsu.edu/hbase/mod4.html#c1

  38. Absorption UV Visible IR Visible (0.4-0.7 μm) is absorbed very little by atmosphere UV (shorter than 0.3 μm) is absorbed by O2 an O3 Infrared (5-20 μm) is selectively absorbed by atmosphere H2O & CO2 are strong absorbers of IR Little absorption of IR around 10 μm – “atmospheric window” MODTRAN 5 (D. Archer) Akin to Ahrens, Fig. 2.9

  39. Visible radiation (0.4-0.7 μm) is not absorbed Ultraviolet radiation (<0.3 μm) is blocked Infrared radiation (5-20 μm) is selectively absorbed, but there is an emission window at 10 μm Total Atmospheric Absorption Akin to Ahrens, Fig. 2.9

  40. Take Home Points • Three modes of heat transfer Conduction: molecule-to-molecule Convection: fluid motion Radiation: electromagnetic waves • Heat transfer works to equilibrate temperature differences

  41. Review Items • All objects above 0K emit radiation • Hotter the object, shorter the wavelength of maximum emission: Wien’s Law • Hotter objects radiate more energy than colder objects: Stefan-Boltzman Law • Objects that are good absorbers of radiation are also good emitters…today’s lecture!

  42. Take Home Points • Radiative equilibrium and temperature Energy In = Energy Out (Eq. Temp.) • Each molecule has a unique distribution of permitted, quantum energy states Unique spectrum of absorption and emission frequencies of radiation • Air is transparent to incoming solar opaque to outgoing infrared

  43. Next Class AssignmentSeasons • Reading -Ahrens 3rd: Pg. 42-51 4th: Pg. 42-51 5th: Pg. 42-51 • Homework02: D2L - Due Monday Feb 1st 3rd-Pg. 52: 2.15, 16, 18 4th-Pg. 52: 2.15, 16, 18 5th-Pg. 52: 2.15, 16, 18

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