Download Heat Transfer Powerpoint 1/6/15

If you put a hot cup of coffee into a
refrigerator, would “cold” transfer
from the fridge to the coffee, or
would “hot” transfer from the coffee
to the fridge?
Hmmmm….
Here is how it works:
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Heat Energy is what makes the coffee hot.
Since the inside of refrigerator has a lower
temperature than the coffee, heat energy
travels from the coffee to the refrigerator,
following the temperature gradient.
The temperature gradient is kind of like the
slope of a hill --- the steeper the hill, the faster a
ball will roll down it….
In a similar way, heat will leave the coffee faster
in the beginning and at a slower and slower rate
as the coffee cools.
Now that we know which way
heat energy travels (from hot to
cold), how does it actually
travel?
There are three types of heat transfer:
CONDUCTION
• CONVECTION
• RADIATION
•
Conduction
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Conduction is transfer
through direct contact.
On a molecular level, hotter
molecules are vibrating faster
than cooler ones.
When they come in contact,
the faster moving molecules
“bump into” the slower
moving molecules and heat is
transferred!
This is how heat is
transferred to your finger if
you touch a hot stove!
Convection
When fluids are
heated, currents are
created.
 This is because the
individual molecules
that come in contact
with a hot surface
expand, become less
dense, and rise. (this
is how hot air
balloons work!)
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When this happens, other molecules
circulate down and take their place,
and a cycle is established.
An example of this can be observed
in the air currents that are created in
a room with a radiator against one
wall.
The air in contact with the radiator
rises, moves across the ceiling to the
far wall, sinks, and then comes back
to the radiator across the floor.
Radiation
 Radiated heat energy
travels as an
electromagnetic
wave.
 Electromagnetic
waves travel at the
speed of light, which
is 300,000,000
meters per second
(186,000 miles per
second).
Sometimes these waves are in
the visible part of the spectrum,
like when something is “red
hot.” You can see how hot it is,
but you can also feel it from a
distance, as your skin absorbs
the energy.
Question: Do you think that
radiated heat needs a medium
to travel through like heat
transferred by convection does?
NO!
Electromagnetic
waves do not need
a medium to travel
through, although
they can travel
through many
substances.
 If this wasn’t true,
we wouldn’t feel the
heat of the Sun
here on Earth!
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Q.) When you put a teapot on
the stove to boil water, which
of the three kinds of heat
transfer can be observed?
A.) Actually, all three!
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First, there is conduction between the burner and the teapot, and then
conduction between the teapot and the water molecules that are in
direct contact with the teapot.
Next, there is convection in the water as the heated molecules of water
from the bottom of the teapot rise and spread their heat energy to the
cooler molecules above them through direct contact. This convection
current also pushes cooler molecules of water down to the bottom
where they come in contact with the heated bottom of the teapot.
While all of this is occurring, heat energy is being radiated in all
directions from the from the burner and is absorbed by other objects.
You can feel this energy if you stand too close!
Conduction, Convection and Radiation