Heat Transfer Conduction Convection and Radiation (PowerPoint) by xuyuzhu

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									       Heat Transfer

Conduction, Convection, and
• Heat is thermal energy moving from one
  object at one temperature to an object at a
  different temperature.
       How is heat transferred?
• There are THREE ways heat can move.
  – Conduction

  – Convection

  – Radiation
• Heat is transferred from one particle of matter
  to another in an object without the
  movement of the object.

• Conduction=CONTACT
             Have you ever…
• Touched a metal spoon sitting in a pan of
  boiling water only to be surprised by hot HOW
  it is??
            Example of Conduction
• Think of a metal spoon in a pot of hot water being
  heated on an electric stove.
• The fast-moving particles of the hot electric coil (in the
  stove) collide with the slow-moving particles of the cool
• Because of these collisions, the slower particles move
  faster and heat is transferred.
• Then the particles of the pot collide with the particles
  in the water, which collide with the particles at one end
  of the spoon.
• As the particles move faster, the metal spoon gets
  hotter. This process of conduction is repeated all along
  the metal until the entire spoon is hot.
        Examples of Conduction
• A piece of metal in a forge glows red as heat is
  transferred to the metal from the forge.
Why does metal feel colder than wood, if they
are both at the same temperature?

   Metal is a conductor, wood is an insulator. The metal conducts the
   heat away from your hands, the wood does not conduct the heat
   away from your hands as well as the metal, so the wood feels warmer
   than the metal.
• Convection is the movement that transfers
  heat within the water.

• In convection, heat is transferred by the
  movement of currents within a FLUID (liquid
  OR gas)

• Convection=VENTS through air and liquid
             Have you ever…
• noticed that the air near the ceiling is warmer
  than the air near the floor?
• Radiation is the transfer of energy by
  electromagnetic waves.

• Radiation does NOT require matter to transfer
  thermal energy. (Passes through a vacuum.)

• Radiation=RADIATES (heat escaping the sun)
             Have you ever…
• sat too close to a campfire while cooking
  marshmallows– enjoying the warmth-- only to
  notice (when you move) that your skin is really
        Example of RADIATION
• You can FEEL the radiation from a bonfire from
  several meters away!
         Examples of Radiation
• Think of the sun’s radiation…it travels through
  MILLIONS of kilometers of empty space to
  reach us.

• Radiation from heat lamps

• A blacksmith feels radiation from his forge.
Radiation questions

  Why are houses painted white in hot countries?

  White reflects heat radiation and keeps the house cooler.

  Why are shiny foil blankets wrapped around marathon runners at the end of a

  The shiny metal reflects the heat radiation from the runner back in, this stops
  the runner getting cold.

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