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

2. The Electromagnetic Spectrum Rainbow

3. 0 degree Kelvin 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 absolute zero Objects at different temperatures emit electromagnetic radiation. Black Body: Object that emits radiation but does not reflect radiation. It absorbs all incoming radiation! IR radiation

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

5. The Stefan-Boltzmann Law This amount of power that a surface, which has an emissivity of e, a temperature of T and a surface area of A, radiates. P = e  T 4 A Power = emissivity × Stefan-Boltzmann constant × temperature 4 × surface area  is the Stephan Boltzmann constant with value 5.67 x 10–18 J / (s m 2 K 4 )

6. Courtesy of PHET

7. 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) Leslie cube

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 Conduction (glass to air on surface) Convection (remove air layer on surface!) Radiation T window Convection currents Heat flow T w = 20 0 C atmosphere & surface T A&S = -10 o C radiation

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 Double glass window with argon gap  thermal conductivity 0.016 W/m∙K conductive losses ~ 65 W 4 x wider argon gap ~ 16 W argon Reduces losses by factor 200 !

13. 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 Bimetallic strip

14. (II) Reducing Convection Losses 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 Convection currents

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

17. Insulation w trapped air or argon:

18. Earth as a Greenhouse Earth as a Greenhouse radiation from the sun enters the atmosphere the emissivity for visible light is small 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 atmosphere T average ~ 15 o C surface T average ~ -18 o C space

19. Changing the emissivity Changing the emissivity radiation from the sun enters the atmosphere the emissivity for visible light is small the energy from the Solar radiation heats atmosphere + surface increasing the emissivity (eg. by adding CO 2 or methane to the atmosphere) would change the surface (greenhouse) temperature atmosphere T average ~ 17 o C surface T average ~ -18 o C space

20. IPCC 2007

21. Muir Glacier, August 1941

22. Muir Glacier, August 2004

23. North Pole

24. Computer models Predicting the future IPCC 2007

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

26. 1 m = 10 9 nm 1 m = 1,000,000,000 nm 1 nm = 10 –9 m 1 nm = 0.000000001 m 700 nm 550 nm 400 nm A nanometer is very small Visible Light (approx):

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

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 …