Heat Transfer
Getting from here to there
…Let me count the ways
• Recall from the last chapter that HEAT
transfers from on object to another until
their TEMPERATURES are equal.
• There are three ways for heat to transfer
from one region to another.
– Conduction
– Convection
– Radiation
Conduction
• Let’s say you have a metal spoon that you leave
in a pot of boiling water for several minutes.
When you grab the spoon, will you burn your
hand? Why?
• Now you try taping a second spoon to the
handle of the first and you leave it for a few more
minutes. What will happen now when you go
grab the second one?
• Then you give up on metal spoons and change
to a wooden spoon. Would you still burn your
hand if you leave the wooden spoon in the water
for several minutes? Why or why not?
Understanding conduction
• What is was difference between the cases
in the previous slide?
• In the first spoon, heat was conducted
within the material
• With the two spoons, heat easily was
conducted from the first to the second
because they were in contact with one
another.
What conduction is
• Conduction is heat transferred from
particle to particle within an object or
between two objects in contact.
• If heat transfers easily within an object,
that object is a good CONDUCTOR
• If heat does not transfer easily within an
object, that object is a good INSULATOR
Why conduction occurs
• Metals are good conductors.
• Remember atoms? Remember protons and
electrons? Which ones are in the nucleus and
which are outside?
• Conduction occurs because metals have
electrons that they can lose easily.
• When one end of a metal rod is heated, the
atoms in that end gain energy. The electrons in
that region gain enough energy to drift away
from their atoms and bounce into other atoms
and electrons.
• This drift is the conduction of heat.
Impromptu conductor/insulator quiz
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Steel
Snow
Wood
Water
copper
Fiberglass
Gold
Now pick four of these and give examples of
how they are used as a thermal insulator or
conductor.
A check for understanding
• I need some volunteers…
Convection
• Remember that conduction was the
transfer of heat (energy) from molecule to
molecule within a substance.
• CONVECTION is heat transfer where the
molecules themselves move around.
• Examples
– Air currents
– Boiling water
Why does convection happen?
• What do we know happens whenever
something is heated?
• It expands
• If you have two objects of the same mass
but different volumes, what can we say
about their densities?
• What happens when you have fluids of
different densities?
• So if you have warm air, what does it do?
Why does convection happen?
• So if warm air or water rises, whole
volumes of air or water will physically
move around.
• Think about this and make sure you
understand the difference between this
and conduction.
Candle/Test tube demo?
• If the equipment is ready…
Examples of convection to discuss
• We are now going to break up into groups
of 6. Each group will have 5 minutes to
come up with and explanation for one of
the following phenomena:
– Land breeze/sea breeze
– Why does boiling water ‘roil’?
– If you light a match on earth, it burns. If you
light a match on a space station, it snuffs out
quickly. Why?
– Why can you stay warm in an igloo even
though it’s made of cold snow?
Check for understanding
• Once again, we need a few volunteers…
• Someone has to explain why you can put
your hand 2 inches away from the side of
a candle flame for as long as you want,
but can’t put your hand 2 inches above a
candle flame for the same length of time.
Radiation
• IMPORTANT NOTE: Heat radiation is
NOT the same thing as radioactivity.
• Radiation in this case is conduction of heat
by electromagnetic waves.
• Give some examples of electromagnetic
waves.
• What is the difference among these?
Wavelength
http://www.qrg.northwestern.edu/projects/vss/docs/media/Communications/wavelength.gif
Electromagnetic Radiation
http://www.antonine-education.co.uk/physics_gcse/Unit_1/Topic_5/em_spectrum.jpg
Okay, so here’s the deal…
• Pretty much all objects continually give off heat
by radiation.
• This radiation is at mixture of different
wavelengths
– The cooler the object, the longer the wavelengths
– The warmer the object, the shorter the wavelengths
• Hotter everyday objects emit radiation in the
INFRARED (pronounced “infra-red”) range
• Let’s look at some videos…
• Everyone write down one thing they noticed in
each video
Discussion of videos
• What did you notice about the videos?
Somebody get a rope…
• K, I need some more volunteers for a
demo…
One last bit about radiation…
• An object does two things to different
amounts when it encounters radiated heat:
Absorb and reflect
• Absorption is when an object takes in the
radiated energy
• Reflection is the opposite. The radiated
heat is not taken in.
• Think about things that absorb and reflect
light. What do you notice about these
things?
Final check for understanding
• Once again with the volunteers