Title: Heat
1Heat
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.
2What does hot mean?
- Atoms move more
- In gases or liquids or solids
- Even in a solid they jitter
3Three 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)
4What mechanism for heat transfer
5Ouch 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?
6Obviously it depends
- Best insulator is vacuum
- Next best is stagnant gas
- Then (nonmetal) material
- Worst is metal
7Nice 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)
8Canadians 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
9The 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!
10Radiation 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
11Taming the bad boy The lost art of awnings
- Sizes chosen to occlude sunlight
- in summer
- but to admit sunlight
- in winter.
12More defeating the demons
- Block the convection along cold windows with
valances - or fit a quilted frame into the window.
13How 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
14Why 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
15Shiny 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
16Stefan-Boltzmann spectrum
- Colder means less
- radiation
- And lower energy
- radiation
- i.e. longer wavelength
17How 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
Well, the bulb filament is really much smaller
than you are, so you dont have to be so hot.
18Gruesome 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.
19How 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 .
20So install something that makes sense
- Fluorescent bulbs save a factor of 4 or 5 in
energy - So Use em
21Which 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
22If 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
23Take 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