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7.3 Clothing, Insulation and Climate

7.3 Clothing, Insulation and Climate. New ideas for today: Thermal radiation Emissivity Insulation and Climate. Rainbow. The Electromagnetic Spectrum. IR radiation. Objects at different temperatures emit electromagnetic radiation. Black Body:

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7.3 Clothing, Insulation and Climate

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  1. 7.3 Clothing, Insulation and Climate New ideas for today: • Thermal radiation • Emissivity • Insulation and Climate

  2. Rainbow The Electromagnetic Spectrum

  3. IR radiation Objects at different temperatures emit electromagnetic radiation. Black Body: Object that emits radiation but does not reflect radiation. It absorbs all incoming radiation! surface of sun 6,000 K  Visible light lava 1,200 K  Red light Body temperature 309 K infrared light Universe 2.7 K  microwaves 0 degree Kelvin absolute zero

  4. Blackbody radiation The Blackbody Spectrum The wavelength and intensity of electromagnetic waves from a black body depend only on itstemperature

  5. The Stefan-Boltzmann Law P = e  T4 A This amount of power that a surface, which has an emissivity of e, a temperature of T and a surface area of A, radiates. Power = emissivity × Stefan-Boltzmann constant × temperature4 × surface area  is the Stephan Boltzmann constant with value 5.67 x 10–18 J / (s m2 K4)

  6. Courtesy of PHET

  7. Leslie cube Emissivity, e The efficiency with which an object emits or absorbs energy Ranges from e=0 to e=1 e is low (near 0) For white, shiny, or clear surfaces (poor emitter / absorber) e is high (near 1) For black surfaces (good emitter / absorber)

  8. Clicker question Which fleece should you wear to stay warmest at night? (A) Black (B) White

  9. Insulation • Well insulated windows • Poorly insulated windows What makes the difference ?

  10. Ways to lose thermal energy window Convection currents • Conduction (glass to air on surface) • Convection (remove air layer on surface!) • Radiation atmosphere & surface T radiation Heat flow TA&S= -10oC Tw= 200C

  11. (I) Reducing Conductive Losses • Heat flow by conduction is given by thermal conductivity, k: • Thermal conductivity is a material property: Argon 0.016 W/m∙K Air 0.025 W/m∙K Glass 0.8 W/m∙K Copper 380.0 W/m∙K

  12. (I) Reducing Conductive Losses • Glass window  thermal conductivity 0.8 W/m∙K conductive losses: ~ 3200 W • Double glass window with air gap  thermal conductivity 0.025 W/m∙K conductive losses ~ 100 W argon • Double glass window with argon gap  thermal conductivity 0.016 W/m∙K conductive losses ~ 65 W 4 x wider argon gap ~ 16 W Reduces losses by factor 200 !

  13. Bimetallic strip Window design with argon gap Wide argon gap can reduce heat loss from conduction by factor ~ 200 ! Challenge: heat expansion between glass and frame tends to break argon seal

  14. (II) Reducing Convection Losses Convection currents The gap design already does the trick: The Argon in the gap remains stationary and the heat absorbed in the argon cannot be carried away through convection currents! argon

  15. (III) Reducing Radiation Losses Room temperature ~ 290 K • infrared radiation glass is black for infrared light, e ~ 0.92 • Glass absorbs radiation and can re-emit radiation to the outside !

  16. Thermos bottles (III) Reducing Radiation Losses Solution: cover inside surface of glass with indium-tin-oxide (ITO) visible light Infrared light ITO is transparent to visible light but a mirror for infrared light!

  17. Insulation w trapped air or argon:

  18. Earth as a Greenhouse space radiation from the sun enters the atmosphere the emissivity for visible light is small atmosphere Taverage ~ -18oC the energy from the Solar radiation heats atmosphere + surface The emissivity for infrared is larger than for visible light • some infrared reflected back some escapes to space Taverage ~ 15oC surface

  19. Changing the emissivity space radiation from the sun enters the atmosphere the emissivity for visible light is small atmosphere Taverage ~ -18oC the energy from the Solar radiation heats atmosphere + surface increasing the emissivity (eg. by adding CO2 or methane to the atmosphere) would change the surface (greenhouse) temperature Taverage ~ 17oC surface

  20. IPCC 2007

  21. Muir Glacier, August 1941

  22. Muir Glacier, August 2004

  23. North Pole

  24. Predicting the future Computer models IPCC 2007

  25. For next class: Read Section 8.1 See you next class!

  26. A nanometer is very small 1 m = 109 nm1 m = 1,000,000,000 nm 1 nm = 10–9 m1 nm = 0.000000001 m Visible Light (approx): 700 nm550 nm 400 nm

  27. Blackbody spectrum: universe Cosmic Microwave Background Radiation (Universe 13.7 billion years old now) Bob Wilson and Arno Penzias: Nobel Prize, 1978

  28. Volume expands w/ increasing Temperature: Makes sealing windows challenging! Higher temperature: • Increasing thermal motion • Increasing separation between atoms • Expansion of volume and outer dimension of object heat expansion depends on material …

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