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NATS 101
Lecture 5Radiation
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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.
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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
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Why Electromagnetic Waves?
• Radiation has an Electric Field Component and a Magnetic Field Component– Electric Field is Perpendicular to Magnetic
Field
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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 (or more BB’s per second) and with higher wave amplitude to bigger BB’s
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Electromagnetic Spectrum
WAVELENGTH
Danielson, Fig. 3.18
Wavelengths of Meteorological Significance
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Emitted Spectrum
White Light from Flash Light
Purple GreenRed
•Emitted radiation has many wavelengths.
Prism
(Danielson, Fig. 3.14)
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Plank’s Law: Emitted SpectrumEnergy from Sun is spread unevenly over all wavelengths.
Wavelength
En
erg
y E
mit
ted
Emission spectrum of Sun
Ahrens, Fig. 2.7
Planck’s Law
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Planck’s Law and Wien’s Law
The hotter the object, the shorter the brightest wavelength.
Danielson, Fig. 3.19
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Wien’s Law
Relates the wavelength of maximum emission to the temperature of mass
MAX= (0.29104 m K) T-1
Warmer Objects => Shorter Wavelengths• Sun-visible light
MAX= (0.29104 m K)(5800 K)-1 0.5 m• Earth-infrared radiation
MAX= (0.29104 m K)(290 K)-1 10 m
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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= (0.29104 m K) T-1
• Temperature of glowing tungsten filament
T= (0.29104 m K)(MAX)-1
T= (0.29104 m K)(1.0 m)-1 2900K
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Stefan-Boltzmann’s (SB) Law
• The hotter the object, the more radiation emitted.
• Double the temperature Total emitted radiation increases by a factor of 16!
• Stefan-Boltzmann’s Law
E= (5.6710-8 Wm-2K-4 )T4
E=2222=16
4 times
Sun Temp: 6000K
Earth Temp: 300K
Aguado, Fig. 2-7
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How Much More Energy is Emitted by the Sun per m2 Than the Earth?
• Apply Stefan-Boltzman Law
• The Sun is 160,000 Times More Energetic per m2 than the Earth, Plus Its Area is Mucho Bigger!
-2 -2 -4
-2
-2
48
8 4
48
4 544
(W m ) W m K
W mW m
(5.67 10 )
(5.67 10 ) (5800 )5.67 ( )( 10 ) 290
(5800 ) 1.6 1020(290 )
Sun
Earth
E T
E KKE
KK
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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).
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Radiative Equilibrium (cont.)
• When the energy absorbed equals energy emitted, this is called Radiative Equilibrium.
• The corresponding temperature is the Radiative Equilibrium Temperature.
• Concept is analogous to a bathtub with the faucet running and the drain unplugged. 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 at an equilibrium level.
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Modes of Heat Transfer
Williams, p. 19
Latent Heat
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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= (0.29104 m K) T-1
• Stefan-Boltzmann Law: total energy emission
E= (5.6710-8 W/m2 ) T4
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Key Points
• Radiative equilibrium and temperature
Energy In = Energy Out (Eq. Temp.)
• Three modes of heat transfer
Conduction: molecule-to-molecule
Convection: fluid motion
Radiation: electromagnetic waves
• Heat transfer works to equilibrate temperature differences
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Reading Assignment
• Ahrens
Pages 34-42
Problems 2.10, 2.11, 2.12
