1 NATS 101 Lecture 4 TR Radiation Selective Absorption 2 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. 3 Modes of Heat Transfer Heat-Energy transfer due to temperature differences Three modes of heat transfer Conduction molecule to molecule Convection transport of fluid Radiation electromagnetic waves (This lecture) Latent Heat energy of phase changes 4 Latent Heat Energy associated with phase of matter. Must be either added to or taken from a substance when it changes its phase. To turn liquid water into solid ice, must remove energy from the liquid water. To turn liquid water into vapor, must add a lot of energy to the liquid water. 5 Latent Heat Take 2 Polarity Williams, p 63 Ice =>Liquid =>Vapor Takes energy from environment Vapor =>Liquid =>Ice Emits energy to environment 540 cal/gm 80 cal/gm 6 Modes of Heat Transfer Williams, p. 19 Latent Heat 7 New Business Radiation Selective Absorption and Emission 8 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 do not 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 adjacent peaks. How much: Amplitude or distance between crests and valleys. 9 Wavelength Distance btw peaks Amplitude Height of crests/troughs Frequency # crests per unit time 10 Why Electromagnetic Waves? Radiation has an Electric Field Component and a Magnetic Field Component Electric Field is Perpendicular to Magnetic Field 11 Photons Can also think of radiation as individual packets of energy or PHOTONS. Electron volts In simplistic terms, radiation with shorter wavelengths corresponds to photons with more energy (i.e. more BBs per second) and with higher wave amplitude (i.e. bigger BBs) 12 Emitted Spectrum White Light from Flash Light PurpleGreen Red Emitted radiation has many wavelengths. Prism (Danielson, Fig. 3.14) 13 Electromagnetic Spectrum Danielson, Fig WAVELENGTH Wavelengths of Meteorological Significance 14 Planks Law: Emitted Spectrum Energy from Sun is spread unevenly over all wavelengths. Wavelength Energy Emitted Emission spectrum of Sun Ahrens, Fig. 2.7 Plancks Law 15 Plancks Law and Wiens Law The hotter the object, the shorter the brightest wavelength. Danielson, Fig. 3.19 16 Wiens Law Relates the wavelength of maximum emission to the temperature of mass MAX = (0.2910 4 m K) T -1 Warmer Objects => Shorter Wavelengths Sun-visible light MAX = (0.2910 4 m K)(5800 K) -1 0.5 m Earth-infrared radiation MAX = (0.2910 4 m K)(290 K) -1 10 m 17 Wiens Law What is the radiative temperature of an incandescent bulb whose wavelength of maximum emission is near 1.0 m ? Apply Wiens 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 18 What is Radiative Temperature of Sun if Max Emission Occurs at 0.5 m? Apply Wiens Displacement Law 19 Stefan-Boltzmanns (SB) Law The hotter the object, the more radiation emitted. Double the temperature: Total emitted radiation increases by a factor of 16! Stefan-Boltzmanns Law E= (5.6710 -8 Wm -2 K -4 )T 4 E=2222= Sun Temp: 6000K Earth Temp: 300K Aguado, Fig. 2-7 20 How Much More Energy is Emitted by the Sun per m 2 than the Earth? Apply Stefan-Boltzman Law The Sun is 160,000 Times More Energetic per m 2 Than the Earth, Plus Its Area is Much Bigger! 21 How Much More Energy is Emitted by the Sun than the Earth? Apply Stefan-Boltzman Law 22 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). 23 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 open. 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 it is said to be at an equilibrium. Clicker Instructions 1.Turn on clicker 2.Make sure youve entered in your UA NetID correctly 3.Join the class: MULLEN 4.When a question comes up, enter the answer on your key pad. 5.You will have a certain number of seconds to enter an answer, as indicated on the presentation 6.When you successfully send your answer, you will see a received message on the device. 7.Can change your answer as long as time has not run out. Participation Scoring + Attendance WARNING: IF YOU DONT ENTER ANY ANSWER THEN YOU WILL BE RECORDED AS ABSENT, SO ALWAYS ENTER SOMETHING. SEE SYLLABUS POLICY FOR ADMINSTRATIVE DROP POLICY. Always get one point for entering an answer, whether right or wrong An additional point if you get the right answer FOUR POSSIBLE FATES OF RADIATION: 1.Transmitted 2. Reflected 3. Scattered 4. Absorbed The atmosphere does ALL of these Transmitted: Radiation passes through object SUNLIGHT GLASS WINDOW TRANSMITTED SUNLIGHT Reflected: Radiation turned back MIRROR SUNLIGHT REFLECTED SUNLIGHT Scattered: Path of radiation deflected SUNLIGHT FROSTED GLASS SCATTERED SUNLIGHT Absorbed: Radiation transferred to object Blackbody: a perfect absorber and emitter of radiation in equilibrium, with no reflection or scattering. BLACK BOX SUNLIGHT Radiative equilibrium: Absorption = Emission (Kirchoffs Law) SUNLIGHT (SHORTWAVE) INFRARED (LONGWAVE) EMISSION BLACK BOX A Grey Body = Not all radiation absorbed How the atmosphere behaves SUNLIGHT (SHORTWAVE) INFRARED (LONGWAVE) EMISSION GREY BOX SOME TRANSMISSION OF SUNLIGHT THROUGH BOX 33 Why Selective, Discrete Absorption/Emission? Life as we perceive it: A continuous world! Atomic perspective: A quantum world! Gedzelman 1980, p 103 34 Energy States for Atoms Electrons can orbit in only permitted states A state corresponds to specific energy level Only quantum jumps between states can occur Intervals correspond to specific wavelengths of radiation Hydrogen Atom Probability States Gedzelman 1980, p 104 Hydrogen Atom 35 Energy States for Molecules Molecules can also rotate, vibrate, librate But only at specific energy levels or frequencies Quantum intervals between modes correspond to specific wavelengths Gedzelman 1980, p 105 H 2 O molecule H2O Bands H2O Bands 36 Selective Absorption The Bottom Line Each molecule has a unique distribution of quantum states! Each molecule has a unique spectrum of absorption and emission frequencies of radiation! H 2 O molecule Williams, p 63 37 Humans are Selective Absorbers Danielson, Fig 38 Absorption Visible ( m) is absorbed very little by atmosphere UV (shorter than 0.3 m) is absorbed by O 2 an O 3 Infrared (5-20 m) is selectively absorbed by atmosphere H 2 O & CO 2 are strong absorbers of IR Little absorption of IR around 10 m atmospheric window MODTRAN 5 (D. Archer) Visible Akin to Ahrens, Fig. 2.9 IR UV 39 Total Atmospheric Absorption Visible radiation ( m) is not absorbed Ultraviolet radiation (