7.3 Clothing, Insulation 7.3 Clothing, Insulation and Climate and Climate

New ideas for New ideas for today:today:

• Thermal radiation• Emissivity• Insulation and Climate

The Electromagnetic The Electromagnetic SpectrumSpectrum

RainbowRainbow

0 degree Kelvin

surface of sun 6,000 K Visible light

lava1,200 K

Red light

Body temperature309 K

infrared light

Universe2.7 K

microwavesabsolute zero

Objects at different Objects at different temperatures emit temperatures emit electromagnetic electromagnetic radiation.radiation.

Black Body:Black Body:

Object that emits Object that emits radiation but does not radiation but does not reflect radiation. It reflect radiation. It absorbs all incoming absorbs all incoming radiation!radiation!

IR radiationIR radiation

The Blackbody SpectrumThe Blackbody Spectrum

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

Blackbody radiationBlackbody radiation

The Stefan-Boltzmann LawThe 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 T4 APower = emissivity × Stefan-Boltzmann constant × temperature4 × surface area

is the Stephan Boltzmann constantwith value 5.67 x 10–18 J / (s m2 K4)

Courtesy of PHET

Emissivity, eEmissivity, eThe efficiency with which an object

emits or absorbs energyRanges 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 cubeLeslie cube

Clicker questionClicker questionWhich fleece should you wear to stay warmest at night?

(A) Black (B) White

InsulationInsulation

• Well insulated windows

• Poorly insulated windows

What makes the

difference ?

Ways to lose thermal energyWays to lose thermal energy

• Conduction (glass to air on surface)

• Convection (remove air layer on surface!)

• RadiationT

window

Convectioncurrents

Heat flowTw=

200C

atmosp

here &

surface

TA&S=

-10oC

radiation

(I) Reducing Conductive (I) Reducing Conductive LossesLosses

• 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∙

separation of distancearea difference etemperaturk flowheat

(I) Reducing Conductive (I) Reducing Conductive LossesLosses• 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 !

Window design with argon Window design with argon gapgap

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 stripBimetallic strip

(II) Reducing Convection (II) Reducing Convection LossesLosses

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

Convectioncurrents

(III) Reducing Radiation (III) Reducing Radiation LossesLosses

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 (III) Reducing Radiation LossesLosses

Solution:

cover inside surface of glasswith indium-tin-oxide (ITO)

ITO is transparent to visible lightbut a mirror for infrared light!

visible light

Infrared light

Thermos bottlesThermos bottles

Insulation w trapped air or Insulation w trapped air or argon:argon:

Earth as a GreenhouseEarth as a Greenhouseradiation from the sun enters the atmospherethe emissivity for visible light is small

the energy from theSolar radiation heats atmosphere + surface The emissivity for infrared

is larger than for visible light some infrared reflected back some escapes to space

atmosphere

Taverage ~ 15oC

surface

Taverage ~ -18oC

space

Changing the emissivityChanging the emissivityradiation from the sun enters the atmospherethe emissivity for visible light is small

the energy from theSolar radiation heats atmosphere + surface increasing the emissivity (eg. by

adding CO2 or methane to the

atmosphere) would change thesurface (greenhouse) temperature

atmosphere

Taverage ~ 17oC

surface

Taverage ~ -18oC

space

IPCC 2007

Muir Glacier, August 1941

Muir Glacier, August 2004

North Pole

Computer models

Predicting the futureIPCC 2007

See you next class!

For next class: Read Section 8.1

1 m = 109 nm1 m = 1,000,000,000 nm

1 nm = 10–9 m1 nm = 0.000000001 m

700 nm 550 nm 400 nm

A nanometer is very smallA nanometer is very small

Visible Light (approx):Visible Light (approx):

Blackbody spectrum: universeBlackbody 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 atomsExpansion of volume and

outer dimension of object

heat expansion depends on

material …