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

The Electromagnetic Spectrum Rainbow

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

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

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 )

Courtesy of PHET

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

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

Insulation Well insulated windows Poorly insulated windows What makes the difference ?

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

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

(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 W/m∙K conductive losses ~ 100 W Double glass window with argon gap  thermal conductivity W/m∙K conductive losses ~ 65 W 4 x wider argon gap ~ 16 W argon Reduces losses by factor 200 !

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

(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

(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 !

(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

Insulation w trapped air or argon:

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

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

IPCC 2007

Muir Glacier, August 1941

Muir Glacier, August 2004

North Pole

Computer models Predicting the future IPCC 2007

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

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

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

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 …