| NATS 101 Lecture 5 Radiation |
| 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 donŐt need matter in order to propagate; they move at the Ňspeed of lightÓ (3x105 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. | ||
| 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, butÉ | ||
| 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 BBŐs per second | ||
| Electromagnetic Spectrum |
| Emitted Spectrum |
| Emitted Spectrum |
| WienŐs Law |
| The hotter the object, the shorter the brightest wavelength. |
| WienŐs Law |
| Relates the wavelength of maximum emission to the temperature of mass | |
| lMAX= (0.29«104 mm K) « T-1 | |
| Warmer Objects => Shorter Wavelengths | |
| Sun-visible light | |
| lMAX= (0.29«104 mm K)«(5800 K)-1 @ 0.5 mm | |
| Earth-infrared radiation | |
| lMAX= (0.29«104 mm K)«(290 K)-1 @ 10 mm |
| WienŐs Law |
| What is the radiative temperature of an incandescent bulb whose wavelength of maximum emission is near 1.0 mm ? | |
| Apply WienŐs Law: | |
| lMAX= (0.29«104 mm K) « T-1 | |
| Temperature of glowing tungsten filament | |
| T= (0.29«104 mm K)«(lMAX)-1 | |
| T= (0.29«104 mm K)«(1.0 mm)-1 @ 2900K |
| Stefan-BoltzmannŐs (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-BoltzmannŐs Law | ||
| E= (5.67«10-8 Wm-2K-4 )«T4 | ||
| E=2«2«2«2=16 | ||
| 4 times | ||
| 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. |
| Modes of Heat Transfer |
| Key Points |
| Radiation is emitted from all objects that have temperatures warmer than absolute zero (0 K). | ||
| WienŐs Law: wavelength of maximum emission | ||
| lMAX= (0.29«104 mm K) « T-1 | ||
| Stefan-Boltzmann Law: total energy emission | ||
| E= (5.67«10-8 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 34-42 | |
| Problems 2.10, 2.11, 2.12 |