Heat Transfer

1
Heat Transfer

• Physics 1010
• Dr. Don Franceschetti
• March 30, 2004

2
Heat Transfer

• Remember that heat is a form of energy.
• Heat transfer occurs by
• Conduction
• Convection
• Radiation
• Heat transfer always acts to reduce temperature
  differences

3
Conduction

• Energy transfer from atom to atom. Loose
  electrons also contribute
• Thermal Conductivity
• Best conductors are metals silvergtcoppergtaluminum
  gtiron
• Insulators include wool, wood, straw, paper,
  cork, air, styrofoam

4
You can walk on hot coals!

• Wood is a very good insulator, even when hot
• You can also stick your hand in a hot oven, but
  dont touch the side.
• Snow is also a good insulator

5
Convection

• Fluids transport heat as they move in response to
  density changes.
• Important in weather. Air expands as it rises,
  and it cools as it expands.
• As air expands molecules are more likely to
  collide with molecules receding from them.

6
At the seashore
7
Radiation

• Energy can pass through empty space in the form
  of electromagnetic radiation (light, radio waves,
  microwaves)
• The peak frequency is directly proportional to
  the absolute temperature

8
Emission of Electromagnetic Energy

• Everything emits radiant energy at a rate
  proportional to its area and the fourth power of
  the absolute temperature.
• Everything absorbs energy, too.
• Good emitters are good absorbers

9
Cavity Radiation Spectrum
10
Reflection of Radiant Energy

• Absorption and reflection are opposites.
• Best absorber doesnt reflectis black.
• Pupil of eye allows light to enter with very
  little reflection
• Snow melts slowly in sunlight because its white

11
Earth and Sun

• Day Suns 6000K emits a lot more than Earths
  300K or so.
• Night Earth has net loss of heat
• An object left out in the open at night radiates
  energy into space and receives much less in
  return.
• Good conductors draw heat from the ground
• Outer space has a temperature of 2.7K!

12
Newtons law of cooling

• Rate of cooling proportional to temperature
  difference.
• Wind chill, adjusts for effect of convection.

13
Greenhouse Effect

• Much of the Suns radiation is absorbed by the
  Earths Surface. It is at high frequencies to
  which the atmosphere is transparent.
• The Earth radiates more at lower frequencies
  which is absorbed and re-emitted by water vapor
  and carbon dioxide in the atmosphere.
• Earth would be at 18 degrees C were it not for
  the greenhouse effect.

14
Greenhouse
15
Solar Power

• Solar constant 1.4 kw/m2
• In U. S. average over all time is 0.18 kw/m2
• Enough for home heating/cooling falls on roof of
  typical house.
• Electricity generation is a challenge

16
Thermos bottle (Dewar Flask)

• A double-walled glass container with a vacuum
  between the walls. Glass surfaces are silvered
• No conduction through vacuum
• No convection through vacuum
• Radiation reflected through walls.

17
Chapter 16 Review

• Questions 1,2,3,6,8,9,10,13,15,18,20,24,29,32
• Exercises 2,4,10,11,12,21,22,25,34,41
• Problems 3,6

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Review

• Q16.6 Why are materials such as wood, fur,
  feathers and even snow good insulators?
• Q16.7 Does a good insulator prevent heat from
  getting through it, or slow its passage?
• Q16.9 How does buoyancy relate to convection?

19
Review

• Q 16.10 What happens to the volume of air as it
  rises? What happens to its temperature?
• When an air molecule is hit by an approaching
  fast-moving molecule, does its rebound speed
  increase or decrease? How about when it hits a
  receding molecule?