Heat Transfer
(feel your tabletop)
Most of these slides were taken from Laura Lemay. and Michael McDowell
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Thermal Energy
• TOTAL energy of motion in
molecules of a substance
(therm=heat)
• hotterhigher T.E.
• biggerhigher T.E.
2
Temperature*
• Average amount of energy in motion
•Measured with a thermometer
hotterhigher
biggersame
3
HEAT energy transfers
from hotter
object to
COLDER object.
hot
Heat
cold
4
Three types of heat Transfer
1) Radiation: infrared radiation (not visible);
felt as heat
2) Conduction: transfer of heat by touch
(direct contact)
3) Convection: transfer of heat by
movement of fluid (liquid/gas)
5
First type
6
Do you remember the EM spectrum?
Radiation does not require a medium, but it can
travel through one, such as air.
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Radiation (infrared)
• Examples:
*Feel warmth on
face from sun=
solar radiation
* Warmth of a fire
* Heat off car engine
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Second type
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Conduction=touch*
• Examples:
* Hot sand on feet, feet on cold floor
* Touching hot light bulb
•
* Touching metal on hot seatbelt
*Stove to pan to handle
* ice in hand
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Third type
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Convection
* Convection in water
(INSIDE the pot)
•Convection oven
(INSIDE the oven)
12
More convection examples
* Holiday chimes
* Spinning Lamps
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Name ALL THREE EXAMPLES in your table:
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Type
Radiation
Conduction
Convection
Definition
Examples:
•infrared radiation
(not visible to
humans)
•felt as heat
•transfer of heat by
touch
transfer of heat by
movement of fluid
(liquid or gas)
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Next year:
Heat Transfer in the Troposphere
* convection
radiation
* CONVECTION is the dominant
heat transfer in the troposphere
(the air where we live)
conduction
What do we call convection of air in our atmosphere?
Answer: ____________
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Any questions?
• (The rest of the slides are extra.)
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Temp Conversions
• Pop test
• 1) 4 C --------------------- ______ F
• 2) 7 C ---------------------- ______ F
• 3) 14 F -------------------- ______ C
• 4) 0 C --------------------- ______ F
• 5) 21 F ------------------- ______ C
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Transfer of Energy
Energy Transfer
• Energy is transferred from hotter
object to colder object
• Examples?
– Cooking pasta
– Making ice
– Making popcorn
– Roasting marshmallows
Conduction
• Stir your hot soup with a metal spoon
• Pretty soon you need a pot holder
because the end of the spoon you are
holding gets hot
• This is heat transfer by conduction
• Energy travels up the spoon from the end
in the hot soup to the end in your hand
Conduction
• Transfer E as heat between particles in
direct contact
• Example
– Roasting marshmallow with wire
– Air molecules collide with wire
• Transfer energy to wire
• Particles in wire move faster, have more E
• E then transfer to hand, detect warmth
Conduction
• For example, a spoon in a cup of hot soup
becomes warmer because the heat from
the soup is conducted along the spoon.
• Fun fact: Have you ever noticed that
metals tend to feel cold? Believe it or not,
they are not colder! They only feel colder
because they conduct heat away from
your hand. You perceive the heat that is
leaving your hand as cold.
Conduction
Conduction Example
• Put your hand on the desk.
– How does it feel?
– Is it colder or is the heat transferring to your
hand through CONDUCTION?
• Popcorn in a pot
– The heat is transferred by direct contact from
the pan, to the oil, to the kernels of popcorn.
Convection
• Energy transfer through movement of
fluids at different temperatures (fluids liquids and gases)
• Examples
– Camp fire
• See soot, embers rise & swirl
• Rise upwards – warm air expands
– Why?
Convection
• Convection current – flow of fluid due to
heated expansion followed by cooling and
contraction
• Examples
– Wind current
– Boil water in pot
Convection
• You can hold your fingers beside the
candle flame without harm, but not above
the flame. Why?
Convection Examples
• Water…boiling
– What is happening?
• Hot air popcorn
– What is happening?
– The hot air transfers the heat to the cooler
kernels, and when enough hot air heats the
kernels they pop.
Convection
Convection
• Explains why breezes come from the
ocean in the day and from the land at night
Radiation
• Transfer of E by electromagnetic waves
• No movement of matter – will work in
vacuum or outer space
• No contact or movement of fluid
• Infrared, visible light, UV rays
• Example
– Fire emits radiation (infrared), skin absorbs E,
increase in temperature
Radiation
• Radiation can be absorbed and emitted
• Color
– Summer day: wear light or dark clothes?
– Winter day: wear light or dark clothes?
– Why?
Radiation Example
• Microwave Popcorn
– The kernels are heated by the radiation in the
microwave, and the kernels heat up, giving off
more heat to the kernels surrounding it and
making it "doubly warm."
Radiation
• Interior of a car on a sunny day
• Sunlight comes in as visible light
• Seats and interior are much cooler so they
radiate in the infrared instead of visible
• Glass in the windows blocks infrared so
energy can’t get out
• Car interior heats up!
Radiation
• Radiation: Electromagnetic waves that directly transport
ENERGY through space
• Sunlight is a form of radiation that is radiated through
space to our planet without the aid of fluids or solids. The
energy travels through nothingness! Just think of it! The
sun transfers heat through 93 million miles of space.
• Because there are no solids (like a huge spoon) touching
the sun and our planet, conduction is not responsible for
bringing heat to Earth. Since there are no fluids (like air
and water) in space, convection is not responsible for
transferring the heat. Thus, radiation brings heat to our
planet.
What type of heat transfer is
involved?
• Heating a room with a fireplace
• Egg cooking in a frying pan
• Roof of a house becoming hot
Let’s Review
• http://www.quia.com/pop/10875.html
• BBC Bytesize
http://www.bbc.co.uk/schools/gcsebitesize/
science/aqa/energy/heatact.shtml
• http://www.bbc.co.uk/apps/ifl/schools/gcse
bitesize/science/quizengine?quiz=heattest
&templateStyle=science