Transcript NATS 101, Section 36 Lecture 5 Radiation by Dr. David Flittner

NATS 101
Lecture 5
Radiation
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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
Wavelengths of Meteorology Significance
Danielson, Fig. 3.18
WAVELENGTH
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Emitted Spectrum
•Emitted radiation has many wavelengths.
Prism
White
Light
from
Flash
Light
(Danielson, Fig. 3.14)
Red
Purple Green
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Emitted Spectrum
Energy from Sun is spread
unevenly over all wavelengths.
Emission spectrum of Sun
Ahrens, Fig. 2.7
Wavelength
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Wien’s Law
Danielson, Fig. 3.19
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The hotter the object, the shorter the brightest wavelength.
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
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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= (0.29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
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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 of 16!
• 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
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How Much More Energy is Emitted by
the Sun per m2 Than the Earth?
• Apply Stefan-Boltzman Law
E (W m-2 )  (5.67108 W m-2 K-4) T 4
ESun (5.67108 W m-2) (5800 K )4

EEarth (5.67108 W m-2) (290 K )4
4
(5800
K
)
4 1.6105


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(290 K )4
• The Sun is 160,000 Times More Energetic per m2
than the Earth, Plus Its Area is Mucho Bigger!
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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.
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Modes of Heat Transfer
Latent
Heat
Williams, p. 19
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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= (0.29104 m K)  T-1
• Stefan-Boltzmann Law: total energy emission
E= (5.6710-8 W/m2 )  T4
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Key Points
• Radiative equilibrium and temperature
Energy In = Energy Out (Eq. Temp.)
• Three modes of heat transfer due to
temperature differences.
Conduction: molecule-to-molecule
Convection: fluid motion
Radiation: electromagnetic waves
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Reading Assignment
• Ahrens
Pages 34-42
Problems 2.10, 2.11, 2.12
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