16 Heat Transfer

1
16 Heat Transfer

• how does heat move?
• Conduction through matter
• Convection of matter
• By emitting/absorbing Radiation
• Homework
• RQ 3, 6, 11, 20, 21, 25.

2
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

3
conduction

• heat current energy/time
• heat current kADT/L
• k thermal conductivity
• DT temperature difference, L below

4
Convection

• Convection transfer through bulk motion of a
  fluid
• Natural, e.g. warm air rises, cool falls
• Forced, e.g. water-cooled engine

5
Convection Videos

• Hot Cold Water
• Smoke in Air
• Hot air in tube with candle

6
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.

7
Radiation Video

• Tv ontario cartoon

8
Solar Power

• Earth surface gets about 800 watts / m².
• Convection power turbine

9
Newtons 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

10
Summary

• Three modes of heat transfer, conduction,
  convection, radiation
• Good conductors Metals
• Poor conductors Wood, air

11
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!

12
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!)

13
What determines surface temperature of earth?

• (Heat IN) Earth receives two radiative heats1)
  direct sunlight2) atmospheric radiative emission
• (Heat OUT) Earth re-radiates heat out
  T4.
• Surface temperature rises or falls to balance the
  two inputs.