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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 Latent Heat Williams, p. 19
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
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. Radiation - Any object that has a temperature greater than absolute zero (0 K), emits radiant energy. The radiation is in the form of waves that have both electric and magnetic properties. Thus, they are called electromagnetic wave. The waves are produced by electric charges moving. This means of energy transfer does not involve matter, but instead electro-magnetic waves that travel at 300,000 km/sec. This is the major method of transferring energy from the sun to the earth, and at this rate it takes about 8 minutes for radiation to travel from the sun to the earth.
Electromagnetic Waves Two important aspects of waves are: What kind: Wavelength or distance between peaks. How much: Amplitude or distance between peaks and valleys. The picture of electromagnetic waves passing through the atmosphere is much like that of the waves on the surface of the water. There are two important aspects about the waves: 1. What kind of wave is it?: Wavelength or distance between the peaks of the waves. 2. How big is it?: Amplitude or distance between peaks and valley. When more radiation is emitted, the amplitude increases. Wavelength Amplitude Frequency
Why Electromagnetic Waves? Radiation has an Electric Field Component and a Magnetic Field Component Electric Field is Perpendicular to Magnetic Field Radiation has an Electric Field Component and a Magnetic Field Component 1. What kind of wave is it?: Wavelength or distance between the peaks of the waves. 2. How big is it?: Amplitude or distance between peaks and valley. When more radiation is emitted, the amplitude increases.
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 Not to confuse you, but I just wanted to touch on the fact that sometimes it is convenient to think of radiation in terms of particles or discrete packets of energy or “photons.” When doing this remember the relation, photons with shorter wavelengths have more energy. That is why, in spite of the fact that the sun emits more visible photons than UV photons, the UV photons have much more energy than visible photons and are thus more dangerous to humans than visible photons. Although visible photons can harm a person in large doses, i.e. looking directly at the bright light.
Electromagnetic Spectrum Wavelengths of Meteorology Significance The electromagnetic waves can occurs with any size of wavelength. In meteorology, however, the unit of measure commonly for the EM waves is the micrometer. Danielson, Fig. 3.18 WAVELENGTH
Emitted Spectrum Emitted radiation has many wavelengths. White Light from Flash Light Purple Green Red Prism (Danielson, Fig. 3.14)
Emitted Spectrum Energy from Sun is spread unevenly over all wavelengths. Emission spectrum of Sun Energy Emitted Ahrens, Fig. 2.7 Wavelength
The hotter the object, the shorter the brightest wavelength. Wien’s Law Danielson, Fig. 3.19 The hotter the object, the shorter the brightest wavelength.
Warmer Objects => Shorter Wavelengths 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
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
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= (5.6710-8 Wm-2K-4 )T4 E=2222=16 4 times Sun Temp: 6000K Earth Temp: 300K Aguado, Fig. 2-7
How Much More Energy is Emitted by the Sun per m2 Than the Earth? Apply Stefan-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)!
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= (0.29104 m K) T-1 Stefan-Boltzmann Law: total energy emission E= (5.6710-8 W/m2 ) T4
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 30-42 Problems 2.7, 2.9, 2.10, 2.11, 2.12