Chapter 22: heat transfer

1
Chapter 22 heat transfer
2
Do Now 4/28

• 1.) Does heat flow from warmer to cooler
  substances or from cooler to warmer substances?
• 2.) What happens to the molecules of a substance
  when the temperature increases?

3
Conduction

• Molecules in a warm substance have a lot of
  kinetic energy (they collide more frequently).
• Molecules in cool substances do not have a lot of
  kinetic energy.
• When heat is transferred from a warmer substance
  to a cooler substance, energy of molecules is
  also transferred. The molecules of each
  substance interact with each other. The fast
  moving molecules of the warm substance bump into
  the slower moving molecules of the cool
  substance, increasing the speed of the molecules
  (and ultimately the temperature) of the cool
  substance.

4
Conduction

• The conduction of heat from a warmer substance to
  a cooler substance means that there is an
  interaction between molecules.
• Metals are good conductors of heat because heat
  flows quickly through them.
• Which is a better conductor of heat metal or
  wood?
• Metal feels colder because it is a better
  conductor. The metal takes more heat energy out
  of your hand than the wood does, making the
  temperature of your hand lower.

5
Poor conductors are good insulators

• Wood, wool, straw, paper, cork, and Styrofoam are
  all poor conductors of heat. But all of these
  objects are good insulators because they delay
  the transfer of heat.
• A Styrofoam cup will keep coffee hot because it
  doesnt allow much heat to be transferred out of
  the coffee and into the cup.
• Air is a poor conductor of heat heat is
  transferred through air at a very slow rate.
• Snow is also a poor conductor and a good
  insulator. Snow slows the escape of heat from
  the earths surface. It prevents the heat from
  escaping too rapidly.

6

• A blanket is a good insulator because it slows
  the transfer of your body heat to your
  surroundings. A blanket does not provide you
  with heat it just delays the transfer of heat
  from your body to other materials.
• No insulator can completely prevent heat from
  escaping. An insulator just reduces the rate at
  which the heat is transferred.
• Heat is the only thing that can be transferred,
  cold cannot be transferred.

7

• 1.) If you hold one end of a metal bar against a
  piece of ice, the end in your hand will soon
  become cold. Does cold flow from the ice to your
  hand?
• 2.) Why does snow last the longest on the roof of
  a well-insulated house?
• 3.) You can stick your hand into a hot pizza oven
  for several seconds without harm, whereas youd
  never touch the metal insides for even a second.
  Why?

8
Convection

• Another means of heat transfer is by the movement
  of the warmer substance.
• When heat is transferred by means of convection,
  molecules of a substance (usually liquid or gas)
  move in the form of a current and take their
  energy with them.
• Warm air rising is a perfect example of heat
  transfer by convection

9

• Density is the measurement of mass per unit of
  volume.
• The first cube has more mass per unit of volume.
  More mass is confined to the same space, which
  means that it is more dense. When a liquid or
  gas expands, there is less mass per unit of
  volume, as in the second picture, so the liquid
  or gas will be less dense.
• As warm air or water expands, it becomes less
  dense and rises

10
Convection

• A desk lamp has a hole near the top of the metal
  lampshade. How do these holes keep the lamp
  cool?

11
Do Now 4/29

• 1.) If you stick a metal rod in a snow, the end
  in your hand will soon become cold. Does cold
  flow from the snow to your hand? Explain.
• 2.) Give an example of a good conductor of heat.
  Why is this a good conductor?
• 3.) Give an example of a good insulator. Why is
  this a good insulator?

12
Why Rising Warm Air Cools

• Warm air rises. As it rises, it expands.
• When air expands, it cools. When air is
  compressed, it warms.
• Warm air molecules move fast, cool air molecules
  move slowly. When a warm air molecules bumps
  into a cool air molecule, the speed of the warm
  air molecules slows down. When a fast molecule
  collides with a slower one, the speed is
  decreases.
• As air expands, fast molecules collide with slow
  molecules and the speed of all molecules
  decreases. This causes the air to cool.

13
Radiation

• The heat from the sun passes through miles of
  space to reach earth through radiation.
  Radiation allows heat to transfer through
  nothingness. No direct contact is needed like
  conduction, and no fluid or gas is needed, like
  convection.
• Energy or heat that is transmitted by radiation
  is called radiant energy.
• All electromagnetic waves are examples of radiant
  energy.

14
Radiant Energy may be absorbed

• Black objects are good absorbers of radiant
  energy, especially the range of radiant energy we
  call light.
• Good absorbers are also good emitters. Dark
  objects are good emitters of radiant energy,
  which means that they give off radiant energy.
• Shiny objects are poor emitters of radiant
  energy. They dont absorb and dont give off
  much heat.
• Which would you rather your coffee or hot tea be
  placed in to keep it hotter for longer a mug
  with a blackened surface or a mug with a shiny
  mirror-like surface?

15

• Which would you rather your coffee or hot tea be
  placed in to keep it hotter for longer a mug
  with a blackened surface or a mug with a shiny
  mirror-like surface?
• The shiny surface wont absorb and emit much heat
  from the coffee, so the coffee will stay hotter
  for longer.
• Is it more efficient to paint a heating radiator
  black or silver?
• Dull black so that the contribution of radiation
  is increased.

16
Do Now 4/30

• 1.) You can hold your fingers beside a candle
  flame without harm, but not above the flame.
  Why? (Hint It may have something to do with
  convection!)
• 2.) Why does expanding air cool? Explain in
  terms of colliding molecules.
• 3.) Give an example of a poor emitter of radiant
  energy.
• 4.) Give an example of a good absorber of radiant
  energy.

17
Journal 4/30

• 1.) Describe heat transfer by conduction. Give
  an example of heat being transferred by means of
  conduction.
• 2.) Describe heat transfer by convection. Give
  an example of heat being transferred by means of
  convection.
• 3.) Describe heat transfer by radiation. Give an
  example of heat being transferred by means of
  convection.
• 4.) Do cold objects have heat energy?

18
Newtons Law of Cooling

• An object will cool at a faster rate if it has a
  larger temperature difference with the
  surrounding temperature.
• Newtons Law of Cooling states
• The rate of cooling of an object is proportional
  to the temperature difference between the object
  and its surroundings, or ?T.

19
Newtons Law of Cooling

• Newtons law of cooling also holds for heating.
• If an object is cooler than its surroundings, its
  rate of warming up is also proportional to ?T
  (the temperature difference between the object
  and its surroundings).
• Frozen food will warm up faster in a warm room
  than in a cold room, because there is more of a
  temperature difference in the warm room.

20
Newtons Law of Cooling

• 1.) Which will undergo the greater rate of
  cooling a red-hot poker in a warm oven or a
  red-hot poker in a cold room (or do both cool at
  the same rate?)
• 2.) Will a soda can cool just as fast in the
  refrigerator as it will in the freezer? Explain.
  

21
Global Warming

• The earths atmosphere is transparent to solar
  radiation radiation that comes from the sun.
• The surface of the earth absorbs this radiant
  energy and reradiates some of it back into space
  at longer wavelengths. This energy that is
  reradiated by the earth is called terrestrial
  radiation.
• Gases in the atmosphere such as carbon dioxide
  and water vapor trap this terrestrial radiation
  and prevent it from being sent out into space.
  The radiant energy that is trapped makes our
  earth warmer.

22
What could happen?

• Rising sea levels increases the amount of
  evaporation, which means slightly increased
  snowfall in polar regions of earth.
• These large white areas of snow and ice reflect
  more solar radiation, leading to a significant
  drop in global temperature since our planet is no
  longer being warmed by the trapped radiant
  energy.