Transcript Energy - Heat Transfer - Montgomery High School

KS4 Physics
Heat Transfer
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Contents
Heat Transfer
Conduction
Insulators
Convection
Thermal radiation
Summary activities
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Heat transfer
Heat is a type of energy called thermal energy.
Heat can be transferred (moved) by three main processes:
1. conduction
2. convection
3. radiation
During heat transfer, thermal energy always moves in the
same direction:
HOT
COLD
Heat energy only flows when there is a temperature
ooler area.
difference from a w______
armer area to a c______
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What is conduction?
How are the particles arranged in a solid, a liquid and a gas?
solid
liquid
gas
Particles that are very close together can transfer heat
energy as they vibrate. This type of heat transfer is
called conduction.
Conduction is the method of heat transfer in solids but
not liquids and gases. Why?
What type of solids are the best conductors?
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Conduction in non-metals
Graphite is a non-metal that is a good conductor of heat.
If you heat one end of a graphite rod, the heat travels to the
other end.
As the rod is heated, the atoms vibrate. These vibrations
make the adjacent atoms vibrate, and so on. This is how
the heat energy travels along the rod.
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Conduction in metals
The outer electrons of metal atoms are not attached to any
particular atom. They are free to move between the atoms.
When a metal is heated,
the free electrons gain
kinetic energy.
This means that the free
electrons move faster and
transfer the energy
throughout the metal.
heat
This makes heat transfer
in metals very efficient.
Insulators do not have these free electrons, which is
why they do not conduct heat as well as metals.
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Contents
Heat Transfer
Conduction
Insulators
Convection
Thermal radiation
Summary activities
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A temperature problem
Touch a piece of metal and a piece of wood at the same
temperature. Which material feels warmer and why?
The metal feels cold and the wood feels warm.
Metal is a good conductor and conducts the heat away
from your hands, so it feels cold.
Wood is not a good conductor and does not conduct the
heat away from your hands as well as the metal, so the
wood feels warmer than the metal.
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Insulators
Some materials are very poor conductors of heat. These
are called insulators.
Examples of materials that are insulators include plastics,
wood, ceramics and air.
Air becomes a very effective
insulator when it is trapped and
stopped from moving.
This is how your clothes keep
you warm – air is trapped
between the fibres and so acts
as an insulator.
Other insulating materials, including polystyrene and loft
insulation, use trapped air because it is so effective.
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Double glazing
How does double glazing keep a house warmer?
Double glazing keeps a house
warmer because there is a layer of
ir between the panes of glass.
a__
onductor so it acts
Air is a poor c________,
like an i_______,
nsulator .
educes heat loss
The trapped air r_______
onduction from a house.
by c_________
Side-view of
double glazing
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insulating layer
of air
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Charlie the Chef and his frozen chicken!
Charlie forgot to take the chicken out of the freezer last night!
Will his plan to defrost the chicken in time for lunch work?
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How does it insulate?
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Contents
Heat Transfer
Conduction
Insulators
Convection
Thermal radiation
Summary activities
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Heating fluids
Liquids and gases can behave in similar ways and so are
called fluids.
What happens to the particles in a fluid when it is heated?
heat
less dense
fluid
The heated fluid particles gain energy, so they move about
more and spread out. The same number of particles now
take up more space so the fluid has become less dense.
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What is convection?
Cooler regions of a fluid are
more dense than warmer
regions of the same fluid.
The cooler regions will sink
as they have the greatest
mass per unit volume.
The warmer regions will rise
as they have a lower density.
In effect, they float on top of
the denser, cooler regions.
This is how heat transfer
takes place in fluids and is
called convection.
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Convection current
hot water
cools
hot water
rises
cool water
sinks
cool water
warms
This is called a
convection current.
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Why is it windy at the seaside?
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Convection in a fridge
Why is the freezer compartment at the top of a fridge?
freezer
compartment
The freezer
compartment is at
the top because
cool air sinks.
The freezer cools
the air at the top
and this cold air
cools the food on
the way down.
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It is warmer at
the bottom of the
fridge.
This warmer air
rises and so a
convection
current is set up.
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Convection questions
Why does hot air rise and cold air sink?
Cool air is more dense than warm air.
The cool air ‘falls through’ the warm air and so the
warm air rises to the top.
Why are boilers placed beneath hot water tanks
in people’s homes?
Hot water rises.
When the boiler heats the water, and the hot water
rises, the water tank is filled with hot water.
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Conduction and convection – summary
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Contents
Heat Transfer
Conduction
Insulators
Convection
Thermal radiation
Summary activities
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The third type of heat transfer
Earth is warmed by heat energy from the Sun.
How does this heat energy travel from the Sun to the Earth?
?
infrared
radiation
There are no particles
between the Sun and the
Earth so the heat cannot
travel by conduction or by
convection.
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The heat travels to Earth by
infrared waves. They are
similar to light waves and
are able to travel through
empty space.
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Infrared waves
Heat can move by travelling as infrared waves.
These are electromagnetic waves, like light waves,
but with a longer wavelength.
This means that infrared waves act like light waves:
 They can travel through a vacuum.
 They travel at the same speed as light – 300,000,000 m/s.
 They can be reflected and absorbed.
Infrared waves heat objects that absorb them and so
can be called thermal radiation.
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Emitting thermal radiation
All objects emit (give out) some thermal radiation.
Some surfaces are better at emitting thermal radiation than
others.
worst emitter
best emitter
matt
black
white
silver
Matt black surfaces are the best emitters of radiation.
Shiny surfaces are the worst emitters of radiation.
Which type of kettle would cool down faster: a black kettle
or a shiny metallic kettle?
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Absorbing thermal radiation
Infrared waves heat objects that absorb (take in) them.
Some surfaces are better at absorbing thermal radiation than
others – good emitters are also good absorbers.
worst emitter
best emitter
matt
black
best absorber
white
silver
worst absorber
Matt black surfaces are the best absorbers of radiation.
Shiny surfaces are the worst emitters because they reflect
most of the radiation away.
Why are solar panels that are used for heating water
covered in a black outer layer?
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Radiation questions
Why are houses painted white in hot countries?
White reflects heat radiation and so keeps the
house cooler.
Why are shiny foil blankets wrapped around marathon
runners at the end of a race?
The shiny metal reflects the heat radiation from the
runner back in to their body, which stops the runner
from getting cold.
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Infrared radiation – true or false?
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Contents
Heat Transfer
Conduction
Insulators
Convection
Thermal radiation
Summary activities
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Glossary
 absorber – A material that takes in thermal radiation.
 conduction – The heat transfer mechanism in solids.
 conductor – A material that lets heat flow through it.
 convection – The heat transfer mechanism in fluids, which





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occurs because hot fluids are less dense than cold fluids.
emitter – A material that gives out thermal radiation.
free electrons – Electrons in a metal that are free to move
throughout the metal.
heat transfer – The flow of heat energy from a hotter area
to a colder area.
insulator – A material that does not let heat flow through it
and so is a very poor conductor of heat.
radiation – Heat energy transferred by infrared waves.
This method of heat transfer does not need particles.
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Anagrams
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Multiple-choice quiz
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