Thermal Energy Transmission - Radiation • Wavelength and frequency • Wien’s Law • Power/Intensity Unit 05 Thermal Radiation - Slide 1 Electro-magnetic waves How far you go Wavelength Velocity = = How long it took you v f * Period T Unit 05 Thermal Radiation - Slide 2 Example 5.1 Wave Length The frequency of the country music station “US 99” is 99.5 MHz (99.5 X 106 cycles/sec). What is the wavelength of its radio wave? The frequency of blue visible light is around 620 X 1012 Hz. What is the wavelength of its radio wave? Remember! All electromagnetic waves have the same speed! v=c=3X108 m/sec Unit 07 Electricity - Slide 3 Higher frequency Longer waves Enough energy to damage 60Hz =5X106m cells Thermal Radiation – Unit 05, Slide 4 How much energy via radiation? Stefan’s constant Power * * Area*T 4 Heat per time (Watts) Emissivity Stefan-Boltzman’s Law = 5.67 X 10-8 W/(m2-K4) -> amount absorbed (1 for black) Unit 07 Electricity - Slide 5 Everything Emits Radiation! As long as T = 0! Thermal Radiation – Unit 05, Slide 6 Example 5.2: Power from the sun What is the rate of energy released from the sun via radiation? • Tsun = 5887K • sun~1 8 • rsun=7X10 m P * * Area*T 4 Surface area of Sphere = 4r 2 (on formula sheet) Unit 05 Thermal Radiation - Slide 7 Example 5.3: Temperature of the Earth We just calculated that the sun puts out 4.14X1026 Watt of power via radiation. How much of that power hits the earth? 6 The radius of the earth is rearth=6.4X10 m. 11 The earth is rorbit=1.5X10 m from the sun. A) 1.9X1017Watts B) 0.9X1017Watts C) 2.4X1012Watts P * * Area*T 4 D) 3.4X1011Watts Unit 05 Thermal Radiation - Slide 8 Example 5.3: Temperature of the Earth We just calculated that 1.9X1017Watts of power hits the earth via radiation. 30% is reflected by the atmosphere, so the earth absorbs 1.3X1017 Watts If the same amount of power must leave the earth, what should the temperature of the earth be? Surface area of Sphere = (on formula sheet) A) 410K B) 350K 24 P * * Area4*rT6 C) 260K earth~1, rearth=6.4X10 m D) 150K Unit 05 Thermal Radiation - Slide 9 Wien’s Law Why is earth not freezing all the time? Wien’s Law – peak depends on T 2.9mmK peak T • out is longer than in • earth is smaller for longer • “Green house effect” Unit 05 Thermal Radiation - Slide 10 Example 5.4: Peak The temperature of the sun is 5887K. What is the peak wave length, peak, of the radiation beginning transmitted? What is its frequency of the wave, f ? What is the period of the wave, T ? What is the velocity of the wave? Easy! All electromagnetic waves have the same speed v=c=3X108 m/sec Unit 05 Thermal Radiation - Slide 11 Higher frequency Longer waves Enough energy to damage cells Thermal Radiation – Unit 05, Slide 12 Wien’s Law Thermal Radiation – Unit 05, Slide 13 Example 5.5: Peak The temperature of the earth is around 287K. What is the peak wave length of the radiation beginning transmitted? Unit 05 Thermal Radiation - Slide 14 Higher frequency Longer waves Enough energy to damage cells Thermal Radiation – Unit 05, Slide 15 Question We just determined that the peak wave length of the electromagnetic waves coming from the earth peaks at around 4.9mm. What is the velocity of this wave? A) 4.9 m/sec B) 4.9 mm/sec C) 3X108m/sec D) 4.9 X 108m/sec E) 1/4.9 mm/sec Thermal Radiation – Unit 05, Slide 16 Example 5.6: Do you glow? Do you emit light? The temperature of your skin is around 98.6oF. What is the peak wave length of the radiation you transmit? Unit 05 Thermal Radiation - Slide 17 Question The temperature of your skin is around 98.6oF. What is the temperature of your skin in Kelvins? A) 0K B) 98.6K C) 37K D) 310K E) 522K Unit 05 Thermal Radiation - Slide 18 Question The temperature of your skin is around 98.6oF. What is the peak wavelength of the light you emit? A) 2.9X10-3m B) 2.9X10-6m C) 3.1X10-6m D) 6.2X10-6m E) 9.4X10-6m Unit 05 Thermal Radiation - Slide 19 Longer waves Higher frequency Enough energy to damage cells Thermal Radiation – Unit 05, Slide 20 Look for heat losses? Thermal Radiation – Unit 05, Slide 21 Thermal Radiation – Unit 05, Slide 22 Heat Loss at my house Thermal Radiation – Unit 05, Slide 23 Colder at center Colder at frame Older Window Modern Window (double pane) (triple pane) Thermal Radiation – Unit 05, Slide 24 Embarrassing item #1 8” of concrete (e.g. uninsulated basement wall) has R ~ 1.35 Brand new zillion Exposed concrete dollar window basement wall R~5.1 R~1.35 Thermal Radiation – Unit 05, Slide 25 Embarrassing item #2 2” of wood (e.g. uninsulated band board + siding) has R ~ 2.2 (Note: Exterior Shot) Really BIG brand new zillion dollar window R~5.1 2”X10” Bandboard + siding R~2.2 Insulated Wall R~13 Exposed concrete basement wall R~1.35 Thermal Radiation – Unit 05, Slide 26 Embarrassing item #3 A wood-on-wood connection makes a very poor air seal (Note: Photos taken during 5Pa vacuum air tightness test) Cold air pouring into basement at joint Cold air pouring into kitchen at joint between band board and foundation between wall frame and floor wall Thermal Radiation – Unit 05, Slide 27 Embarrassing item #4 Insulation doesn’t always stay where you put it and in the condition you put it there New insulation fell during sheetrock Insulation damaged by installation? past water leak Thermal Radiation – Unit 05, Slide 28.