Heat Transfer

1
Heat Transfer

• How does the energy move from a hotter to a
  colder object?
• Three mechanisms
• Conduction
• Convection
• Radiation

2
Conduction

• Stir your hot soup with a metal spoon
• Pretty soon you need a pot holder because the end
  of the spoon you are holding gets hot
• This is heat transfer by conduction
• Energy travels up the spoon from the end in the
  hot soup to the end in your hand

3
Conduction

• We sense the movement of energy by the increasing
  temperature
• This means the atoms and molecules have higher
  average kinetic energy
• Primarily occurs by the movement of electrons in
  the material
• The more easily the electrons can move, the
  better the conduction

4
Conduction

• Metals have some electrons that are very loosely
  bound to the atoms in the material
• These electrons can move easily and can rapidly
  pick up additional kinetic energy
• Metals are good conductors
• Wood and plastic dont have loosely bound
  electrons, so they are poor conductors

5
Conduction
6
Conduction

• Air is a poor thermal conductor
• If you stand in the sun on a cold winter day and
  are shielded from the wind, you stay pretty warm
• Snow is a poor conductor, while water is better
• Makes igloos a useful as a house

7
Convection

• A phenomenon in fluids
• Instead of having energy moved by successive
  collisions of electrons, atoms and molecules, the
  fluid itself is set into motion called a current
• These moving fluid currents are convection

8
Convection
9
Convection

• When the radiator heats the air, it becomes less
  dense and rises
• Cool air moves in to replace the air that rose
• This generates the air flow
• So radiators dont need a fan to stir the air and
  to distribute heat throughout a room
• The rising air cools until its density matches
  that of the surrounding air

10
Convection

• We take advantage of the cooling that occurs
  during an expansion
• We make refrigerators and air conditioners
  operate by forcing gas under pressure through a
  small hole and expanding it into an empty space

11
Convection

• Explains why breezes come from the ocean in the
  day and from the land at night

12
Radiation

• Energy carried by electromagnetic waves
• Study waves later in detail
• Light, microwaves, radio waves, x-rays
• Wavelength is related to vibration frequency

13
Radiation
14
Radiation

• Every object is emitting electromagnetic waves
  regardless of temperature
• Things we can see from their own radiation are
  very hot to have energy emitted in the visible
  region of the spectrum
• Most things emit primarily in the infrared
• Night vision goggles, etc.

15
Radiation

• Things also absorb radiation
• If they didnt, they would run out of energy to
  emit
• Good emitters are also good absorbers
• Equilibrium established between emission and
  absorption
• When something cant equilibrate, it gets hotter
  or colder

16
Radiation

• Interior of a car on a sunny day
• Sunlight comes in as visible light
• Seats and interior are much cooler so they
  radiate in the infrared instead of visible
• Glass in the windows blocks infrared so energy
  cant get out
• Car interior heats up!

17
Radiation

• A good absorber reflects very little energy
• Think about dark pavement
• A poor absorber reflects a lot of energy
• Think about snow that doesnt melt in sunshine
  even though 1400 watts/meter2 are hitting it

18
Radiation

• At night, objects receive no input energy from
  the sun
• But, they are warmer than outer space, so they
  continue to radiate energy
• Thus, they cool off
• Can we make ice in the desert without a
  refrigerator?

19
Newtons Law of Cooling

• Rate of cooling of an object is proportional to
  the temperature difference between an object and
  its surroundings
• Works both ways, cooling and heating
• Rate of heating also depends on the temperature
  difference