Heat Energy on the Move

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					          Heat Transfer


Heat is another word for thermal energy.

Heat energy always moves from warm to
     cold, until the system reaches
         equilibrium (balance).
             Heat Transfer
Heat can be transferred (moved) 3 ways:

   conduction: direct contact between
  molecules.
  convection: up-and-down movement of
 fluids (liquids and gases) due to differences in
 temperature and density (hot fluids rise ,
 cold fluids sink ).
 radiation: electromagnetic (EM) waves that
travel through space, and transfer their energy
as heat when they reach an object.
                Everyday Examples:
CONDUCTION
 the metal spoon in your hot chocolate gets hot
after a while
 a cold can of soda gets warm after it’s been out of
the refrigerator for a while
CONVECTION
 the attic of your house is always warmer than the
basement.
 drafts of cold air come in at the bottom of the door,
not the top.
RADIATION
 solar panels collecting and storing solar energy
 dark clothes are hotter on a sunny day than white
clothes.
   Conductors & Insulators

 If a material allows heat to pass through it
quickly, it is called a good conductor.
             (Examples: metal, glass)

 If a material does not allow heat to pass
through it quickly, it is called a good insulator.
           (Examples: plastic, wood)

                                                      QuickTime™ and a
                                            TIFF (Uncompressed) decompre ssor
                                               are neede d to see this picture.
                        Heat Transfer by
                          Conduction




• Fast-moving, high-energy molecules
collide with the slow-moving, low-energy
molecules, transferring their energy (heat)
to them.
        Heat Transfer by Convection
• When molecules absorb heat,
they move faster, spread out
and rise because they are less
dense.
• When molecules lose heat,
they slow down, come closer
together and sink because they          http://www.wisc-
                                        online.com/Objects/View
are now more dense.                     Object.aspx?ID=SCE304


 This is why we say “Heat rises, cold   http://www.echalk.co.uk/S
sinks.” It’s actually more accurate     cience/physics/convection/
to say “High-energy molecules rise,     convection.html
low-energy molecules sink,” but
that takes way too long to say.
      Heat Transfer by Radiation
• Radiation moves energy without any help from
particles of matter.
• The Sun’s energy radiates (travels in waves)
through space to reach Earth.
• Energy that travels by radiation is called
electromagnetic (EM) waves.
Light is one kind of EM radiation we can see, but
visible light is just one tiny part of the EM spectrum.


                            QuickTime™ and a
                  TIFF (Uncompressed) decompress or
                     are needed to see this picture.
 Radiation




http://www.spaceflight.esa.int/i
mpress/text/education/Heat%20
Transfer/Radiation.html
Electromagnetic (EM)spectrum




                                 micrometer




As the wave’s wavelength decreases (shortens),
   the amount of energy it carries increases.
                 Photo of infra-red radiation
                    (heat) coming from a cat
Although we
can’t see it,
the heat we
feel on our
skin when we
stand in the
sun or put our
hands over a
hot stove
is caused by
infrared
radiation,
another type
of EM
radiation.
The 3 Kinds of Heat Transfer




 http://www.wisc-online.com/Objects/ViewObject.aspx?ID=SCE304
All 3 kinds
Fire transfers heat energy 3 ways:




                 QuickTime™ and a
        TIFF (Uncompressed) decompre ssor
           are neede d to see this picture.
                Convection

Convection currents: currents in fluids
(liquids or gases) that transfer heat as warm
fluids rise and cold fluids sink.


            http://www.spaceflight.esa.int/i
            mpress/text/education/Heat%20
            Transfer/Convection%20in%20
            a%20Liquid.html
                Convection
              Currents in the
                  Earth’s
               atmosphere
                       Convection currents in the earth




Convection       Convection currents in the atmosphere &
                                                  mantle



Currents in
the Earth’s
  mantle
       Most of our weather is the result of
     convection currents in the atmosphere.




convection
Forced-Air Heating Systems– fuel is burned in a
furnace and a fan circulates the heat in the house




 Which method of heat transfer is being used here?
Which of the following best explains why
the end of a spoon sticking out of a cup
of hot water also gets hot?


The hot water causes a chemical reaction to
occur in the spoon.
 The heat from the hot water is conducted
through the spoon to the handle.
The hot water heats the air surrounding the
upper part of the spoon.
The hot water causes a physical change in the
spoon handle.
When it is very hot in his room,
Stuart makes it cooler by opening
both the top and the bottom of his
window. Which statement best
explains why this works?

 Air comes in the top to cool the ceiling and in the
bottom to cool the floor.
 Cooler air comes in the bottom and warm air goes
out through the top.
 Warmer air goes out through both the bottom &
top.
 It does not make any difference because if there is
no wind, the air will not come in.
  In which situation has heat energy
    moved mainly by conduction?

A radiator heated with hot water warms a room.

The person sitting closest to the blazing fire finds
it necessary to move farther from the heat.

The handle of a cast iron skillet being used to
cook something becomes too hot to touch.

The red-hot coil of an electric space heater is
used to keep a small room warm.
Which BEST
explains how
the food on the
stove is being
heated?
 conduction
 convection
 interaction
 radiation
Which of the following is an example
  of heat transfer by convection?

 A hot wood stove heats an entire
room.
 The handle of a pot that is being
heated becomes warm.
 A teaspoon in a glass of hot tea
becomes warm.
 The coils of a toaster become red.
What is the process by which
heat energy gets to Earth from
          the Sun?



conduction
radiation
subduction
convection
Patti used a steel wrench to work on her bicycle
on a hot, sunny day. She left the wrench on the
hot concrete for an hour, then returned, to find
the wrench very hot to touch. What is the BEST
explanation for how the wrench got so hot?




Heat transferred from the wrench to the air.
Heat transferred from the bicycle to the wrench.
Heat transferred from the wrench to the concrete.
Heat transferred from the concrete to the wrench.

				
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