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Heat

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Q: How many gorillas does it take to screw in a lightbulb? ... (ceiling fans help this. process a lot) Phys 100, How Things Work. Lecture 9, Heat & Energy Flow ... – PowerPoint PPT presentation

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Title: 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.

13
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
14
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

15
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
16
Stefan-Boltzmann spectrum
  • Colder means less
  • radiation
  • And lower energy
  • radiation
  • i.e. longer wavelength

17
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.
18
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.
19
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 .

20
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
22
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

23
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
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