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NATS 101 Lecture 4 Radiation

What is Temperature? Microscopic View Energy due to random jiggling of molecules Related to average molecular speed; 500 m/s (=1100 mph) at room temperature for air

Modes of Heat Transfer Williams, p. 19 Latent Heat

Take Home Concepts Heat-Energy transfer due to temperature differences Four modes of heat transfer Conduction – molecule to molecule Convection – transport of fluid Radiation – electromagnetic waves Latent Heat – energy of phase changes

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.

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” (3x10 5 km/sec) in a vacuum.

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 AmplitudeFrequency

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

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

Electromagnetic Spectrum WAVELENGTH Danielson, Fig Wavelengths of Meteorology Significance

Emitted Spectrum White Light from Flash Light PurpleGreen Red Emitted radiation has many wavelengths. Prism (Danielson, Fig. 3.14)

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

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

Wien’s Law Relates the wavelength of maximum emission to the temperature of mass MAX = (0.29  10 4  m K)  T -1 Warmer Objects => Shorter Wavelengths Sun-visible light MAX = (0.29  10 4  m K)  (5800 K) -1  0.5  m Earth-infrared radiation MAX = (0.29  10 4  m K)  (290 K) -1  10  m

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  10 4  m K)  T -1 Temperature of glowing tungsten filament T= (0.29  10 4  m K)  ( MAX ) -1 T= (0.29  10 4  m K)  (1.0  m) -1  2900K

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  Wm -2 K -4 )  T 4 E=2  2  2  2=16 4 times Sun Temp: 6000K Earth Temp: 300K Aguado, Fig. 2-7

How Much More Energy is Emitted by the Sun per m 2 Than the Earth? Apply Stefan-Boltzman Law The Sun Emits 160,000 Times More Energy per m 2 than the Earth, Plus Its Area is Mucho Bigger (by a factor of 10,000)!

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).

Radiative Equilibrium (cont.) When the energy absorbed equals energy emitted, this is called Radiative Equilibrium. The corresponding temperature is the Radiative Equilibrium Temperature.

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  10 4  m K)  T -1 Stefan-Boltzmann Law: total energy emission E= (5.67  W/m 2 )  T 4

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

Reading Assignment Ahrens Pages Problems 2.7, 2.9, 2.10, 2.11, 2.12