Heat

1
Heat
How to warm your hands at the fire How to warm
the entire county Stoves and lamps
Q How many gorillas does it take to screw in a
lightbulb? A Only one gorilla, but a lot of
bulbs.
2
What does hot mean?

• Atoms move more
• In gases or liquids or solids
• Even in a solid they jitter

3
Three ways to transfer thermal energy

• You move heat by
• Conduction (direct transfer through materials)
• Convection (via flowing gas or liquid)
• Radiation (like light or radio waves)

4
What mechanism for heat transfer
5
Ouch thats hot!

• How to burn your fingers
• One end of object is heated and
• after a while the other end
• can get pretty annoying
• Heat is conducted through the
• material
• How much heat is conducted through a certain
  material?

6
Obviously it depends

• Best insulator is vacuum
• Next best is stagnant gas
• Then (nonmetal) material
• Worst is metal

7
Nice and warm in here

• Radiators, wood stoves and other passive heating
  systems use
• convection.
• Hot air is less dense and
• rises and circulates
• down as it cools
• (ceiling fans help this
• process a lot)

8
Canadians are so smart

• Toronto is the same latitude as Sioux Falls and
  Detroit.
• for you southerners that means it gets really
  cold
• BUT the Ontario Hydro office building has no
  heating system
• All heat comes from
• passive solar
• and computers, etc
• in the building

9
The most efficient clothes dryer

• What if electricity were absolutely free?
• It would cost nothing to dry the laundry
• Thanks to modern
• technology this
• amazing invention
• is available
• TODAY!

10
Radiation as the bad boy

• Night vision goggles
• aka IR goggles look at heat
• Windows are a problem!
• Heat leaks out as radiation in winter
• and pours in in summer

11
Taming the bad boy The lost art of awnings

• Sizes chosen to occlude sunlight
• in summer
• but to admit sunlight
• in winter.

12
More defeating the demons

• Block the convection along cold windows with
  valances
• or fit a quilted frame into the window.

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How much energy is radiated

• Stefan-Boltzmann equation
• Power energy/time e ? T4 A
• Surface area A at temperature, temperature T
• and emissivity e
• gives off energy in proportion to e, T4, and A
• The constant s is just to get the units right
• 5.7 ? 10-8 (J/s)/m2/K4 5.7 ? 10-8 W/m2/K4
• Emissivity is higher for darker objects!
• black, dull surfaces have e approaching 1
• shiny, white surfaces have e near 0

e 1
e 0
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Why are thermos bottles silver inside?

• Thermos is a double-wall container
• with vacuum between the walls.
• Mirrors reflect radiation to
• prevent heat transfer
• Insulation holds little air pockets
• that do not convect
• or conduct well
• The top prevents convection

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Shiny means low emissivity

• Reflects a lot of light
• from both sides
• Nothing goes through
• Emissivity measures
• how likely it is for
• radiation to escape

e 0
e 1
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Stefan-Boltzmann spectrum

• Colder means less
• radiation
• And lower energy
• radiation
• i.e. longer wavelength

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How much do YOU radiate??

• Power e A T4 ?
• e emissivity 0.5
• A surface area 2m2
• T temperature 300K
• s 5.7 ? 10-8 W/(m2 K)
• So
• Power 0.5 ? 2 ? (100000)2 ? 6 ? 10-8
• 600W

• 500 W
• 50 W
• 5 W
• 0.5 W

Well, the bulb filament is really much smaller
than you are, so you dont have to be so hot.
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Gruesome applications of radiation

• Hiroshima victim with radiation
• burns patterned by dark colors
• in the plaid kimono she wore
• on Aug 6, 1945, while
• walking her son to school
• (He was vaporized.)

Logic class IF this picture troubles you, THEN
be careful when you vote. EVERY TIME you vote.
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How inefficient can you get and still make ?

• Light bulbs are hard to
• beat for sheer waste
• About 3-5 of the energy
• is making light
• the rest is extra heat
• Kind of obscene that the
• bulb is the icon for
• a good idea .

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So install something that makes sense

• Fluorescent bulbs save a factor of 4 or 5 in
  energy
• So Use em

21
Which is hotter?

• 100 gm water
• or 100 gm steel?

DQ m c DT
DQ change in thermal energy m is mass c is
specific heat DT is change in tempurature
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If you dont believe in heat capacity

• Why is a 50F swimming pool a torture device and
• walking in autumn is perfection?
• Walking through 30F weather bare-handed is OK
• but putting your hand in ice-water is not much
  fun?
• Walking in summer on the desert is great,
• but walking in Florida is too much

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Take home messages

• Heat is kinetic energy of molecules and atoms
• Heat is transferred by
• conduction
• convection
• radiation
• Hot things radiate a lot more T4
• Heat capacity tells you how much thermal energy a
  chunk of material stores