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NATS 101 Lecture 5 Radiation

NATS 101 Lecture 5 Radiation. 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.

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NATS 101 Lecture 5 Radiation

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  1. NATS 101Lecture 5Radiation

  2. 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 don’t need matter in order to propagate; they move at the “speed of light” (3x105 km/sec) in a vacuum.

  3. Electromagnetic Waves • Two important aspects of waves are: • What kind: Wavelength or distance between peaks. • How much: Amplitude or distance between peaks and valleys. Wavelength Amplitude Frequency

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

  5. Photons • NOT TO CONFUSE YOU, but… • Can also think of radiation as individual packets of energy or PHOTONS. • In simplistic terms, radiation with • shorter wavelengths corresponds to photons with more energy and • higher wave amplitude to more BB’s per second

  6. Electromagnetic Spectrum Wavelengths of Meteorology Significance Danielson, Fig. 3.18 WAVELENGTH

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

  8. Emitted Spectrum Energy from Sun is spread unevenly over all wavelengths. Emission spectrum of Sun Energy Emitted Ahrens, Fig. 2.7 Wavelength

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

  10. Wien’s Law Relates the wavelength of maximum emission to the temperature of mass MAX= (0.29104 m K)  T-1 Warmer Objects => Shorter Wavelengths • Sun-visible light MAX= (0.29104 m K)(5800 K)-1  0.5 m • Earth-infrared radiation MAX= (0.29104 m K)(290 K)-1  10 m

  11. Wien’s Law What is the radiative temperature of an incandescent bulb whose wavelength of maximum emission is near 1.0 m ? • Apply Wien’s Law: MAX= (0.29104 m K)  T-1 • Temperature of glowing tungsten filament T= (0.29104 m K)(MAX)-1 T= (0.29104 m K)(1.0 m)-1  2900K

  12. Stefan-Boltzmann’s (SB) Law • The hotter the object, the more radiation emitted. • When the temperature is doubled, the emitted energy increases by a factor of16! • Stefan-Boltzmann’s Law E= (5.6710-8 Wm-2K-4 )T4 E=2222=16 4 times Sun Temp: 6000K Earth Temp: 300K Aguado, Fig. 2-7

  13. How Much More Energy is Emitted by the Sun per m2 Than the Earth? • ApplyStefan-Boltzman Law • The Sun Emits 160,000 Times More Energy per m2 than the Earth, • Plus Its Area is Mucho Bigger (by a factor of 10,000)!

  14. 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).

  15. Radiative Equilibrium (cont.) • When the energy absorbed equals energy emitted, this is calledRadiative Equilibrium. • The corresponding temperature is theRadiative Equilibrium Temperature.

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

  17. Key Points • Radiation is emitted from all objects that have temperatures warmer than absolute zero (0 K). • Wien’s Law: wavelength of maximum emission MAX= (0.29104 m K)  T-1 • Stefan-Boltzmann Law: total energy emission E= (5.6710-8 W/m2 )T4

  18. Key Points • Radiative equilibrium and temperature Energy In = Energy Out (Eq. Temp.) • Three modes of heat transfer due to temperature differences. Conduction: molecule-to-molecule Convection: fluid motion Radiation: electromagnetic waves

  19. Reading Assignment • Ahrens Pages 34-42 Problems 2.10, 2.11, 2.12

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