Transcript Heat - Haiku

HEAT ENERGY
What is heat?
• A form of electromagnetic energy, measured in
joules or calories.
• Molecules move about more or vibrate and take
up more room if heated—which is why things
expand if heated
• It is also why substances change from:
solids
liquids
gases when heated.
Temperature and Heat
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The temperature of an object indicates the average
kinetic energy of its molecules.
Temperature is measured in degrees Celsius - °C
Temperature is NOT the same as heat energy although
the two quantities are related.
e.g. a beaker of water at 60 °C is hotter than a bath of
water at 40 °C BUT the bath contains more joules of
heat energy
Heating and Cooling
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If an object has become hotter,
it means that it has gained heat energy.
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If an object cools down, it means it has
lost heat energy
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If an object cools down to absolute zero, the molecules
have ceased to move at all.
0th Law of Thermodynamics
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Heat energy always moves from:
WARMER object
COOLER object
A spoon at 20 °C in a cup of water at 30°C - gains
heat energy and heats up – its temperature rises
A spoon at 20 °C in a cup of water at 10°C loses
heat energy and cools down – its temperature falls.
Heat energy movement
•Energy transfer:
Conduction
Convection
Radiation
Conduction
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Heat is transferred through a material by being passed
from one molecule to the next.
No molecules move down the material. Heat energy is
passed on from molecule to molecule.
Molecules at the warm end move faster and this then
causes the next molecules to move faster and so on in
a domino effect.
In this way heat in an object travels from:
the HOT end
the cold end
CONDUCTION heat transfer
Occurs by the molecules hitting each other and
so transferring energy.
 Surprisingly the electrons are the key agents.
 Can happen in solids, liquids and gases,
 Happens best in solids-molecules are very
close together
 Conduction does not occur very quickly in
liquids or gases. (Remember test-tube demo.)
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Conductors
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Materials that conduct heat quickly are called conductors
All metals are good conductors of heat
Copper is a very good conductor of heat
Pans for cooking are usually made with a copper or
aluminium bottom and plastic handles
Insulators/poor conductors
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Materials that conduct heat slowly or poorly are called
insulators.
Glass, wood, plastic and rubber are poor conductors (good
insulators).
Nearly all liquids, including water, are poor conductors (good
insulators).
Gases, including air, are very poor conductors. So wool or a
winter ski jacket feels warm not because of the type of material
but because of its ability to trap a lot of air.
A bird fluffs its feathers so as to trap air, which is a good
insulator.
Starbuck double-walled mugs keep coffee warm because of the
trapped air. Think about it… steel is a good conductor of heat—
hence a very poor insulator.
A refrigerator has insulation material around it to keep it cold –
reducing the amount of heat conducted to the inside from the
warmer room.
Convection
Convection can only take place in materials where
molecules can move around inside the material, i.e.
a liquid or gas.
 The heat is carried by the molecules themselves
moving
Convection currents.
 Convection occurs because an area with warm
molecules expands and becomes less dense than
the cooler areas nearby. Hence the warm area rises.
Cooler molecules fall into the space left by the
warm molecules and a convection current is set up.
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Convection Currents
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Hot liquids and gases expand and rise
while the cooler liquid or gas falls
2. Goes across
3. Then down
as it cools
and contracts
1. Hot air rises –
it has expanded
4. And across
Convection currents illustrated
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As molecules are heated,
they expand and rise since
they are less dense.
When they cool, they
become more dense and
fall.
Since air expands and
contracts much more than
liquids, convection is much
more effective in air.
Convection in air
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The sun can cause large convection currents—WINDS
During the daytime the land warms up to a higher To than the
sea, because water has a much higher heat capacity (5x) than
the land. The warm air rises over the land and cool air falls
over the sea.
So we feel a cool sea breeze blowing in towards us as we
stand on the shore in the daytime.
At night, the breeze direction is reversed, as the water takes
longer to cool down than does land.
Convection currents on the beach
Radiation
The transfer of heat directly from the source
to the object by an electromagnetic wave,
travelling as rays.
 Heat radiation is also known as
Infrared Radiation
 All objects that are warmer than their
surroundings give out heat as infrared radiation
 Heat transfer by radiation does not need
molecules to transfer heat (as do conduction and
convection), but can radiate energy across empty
space.
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Emitters
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Hot objects emit (give out) heat.
Cool objects absorb (take in) heat.
Different types of surfaces emit heat at different
speeds.
A dull black surface loses energy more quickly – it
is a good radiator.
A bright, shiny or white surface is a poor radiator.
Marathon runners needing to keep warm at the end
of races are covered in shiny blankets which reduce
radiation and therefore heat loss by emission.
Emitters of heat
A bright shiny can 
Poor radiator
A dull black can 
Good Radiator
Absorbers
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Cooler objects absorb (take in) heat.
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Substances absorb heat at different speeds.
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Dull, black surfaces absorb heat quickly.
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Bright, shiny surfaces absorb heat slowly.
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In hot countries, people wear bright white
clothes and paint their homes white to reduce
absorption of infrared energy from the sun.
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Petroleum storage tanks are sprayed silver so as to
reflect the sun’s rays.
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Good emitters are also good absorbers of heat.
Absorbers
A shiny, bright can
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Poor absorber
A dull black can
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Good absorber
Key Words
Temperature
Infrared
Insulator
Radiation
Heat
joules
Transfer
Convection
calories
Conduction
Emitter
Conductor
Absorber