16 Heat Transfer
• how does heat move?
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Conduction through matter
Convection of matter
By emitting/absorbing Radiation
Homework:
RQ: 3, 6, 11, 20, 21, 25.
1
Conduction
• Heat conduction is the transmission of
heat through matter.
• most metals are excellent conductors
• Examples:
• silver 429 copper 401 aluminum 250
• iron 80
stainless steel 14
• wood, rubber 0.16
2
conduction
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heat current – energy/time
heat current = kADT/L
k = thermal conductivity
& DT = temperature difference, L below
3
Convection
• Convection – transfer through bulk motion
of a fluid
• Natural, e.g. warm air rises, cool falls
• Forced, e.g. water-cooled engine
4
Convection Videos
• Hot & Cold Water
• Smoke in Air
• Hot air in tube with candle
5
Radiation
• Radiation - objects emit infrared electromagnetic
radiation, which can be absorbed in another
object, heating it up.
• Ex. Stand by a brick house at night. You feel
heat but cannot see it.
6
Radiation Video
• Tv ontario cartoon
7
Solar Power
• Earth surface gets about 800 watts / m².
• Convection power turbine
8
Newton’s Law of Cooling
• rate of heat loss is proportional to the
difference in temperatures between the
body and its surroundings
• rate of heat-loss ~ DT
• Ex. Cooling of hot coffee
9
Summary
• Three modes of heat transfer, conduction,
convection, radiation
• Good conductors: Metals
• Poor conductors: Wood, air
10
Greenhouse Effect
• ‘dirtier’ air must be at higher temperature
to radiate out as much as Earth receives
• higher temperature air is associated with
higher surface temperatures, thus the term
‘global warming’
• very complicated model!
11
Real Greenhouse
• covering allows sunlight to enter, which
warms the ground and air inside the
greenhouse.
• the ‘house’ is mostly enclosed so the
warm air cannot leave, thus keeping the
greenhouse warm (a car in the sun does
this very effectively!)
12
What determines surface
temperature of earth?
• (Heat IN) Earth receives two radiative
heats:
1) direct sunlight
2) atmospheric radiative emission
• (Heat OUT)
Earth re-radiates heat out ~ T4.
• Surface temperature rises or falls to
balance the two inputs.
13