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:




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




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!)
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.
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!
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!



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