Transcript heat energy - Parkway C-2

HEAT ENERGY
What is HEAT?
• Form of energy and measured in JOULES
• Particles move about more and take up
more room if heated – this is why things
expand if heated
• It is also why substances change from:
solids
liquids
gases
when heated
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Heat and Temperature
The temperature of an object tells us how
HOT it is
 Measured in degrees Celsius - °C
 It is NOT the same as heat energy although
the two quantities are related.
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Heating and Cooling

If an object has become hotter,
it means that it has gained heat energy.

If an object cools down, it means it has
lost energy
Heating and Cooling cont…

Heat energy always moves from:
HOT object
COOLER object
e.g.Cup of water at 20 °C in a room at 30°C gains heat energy and heats up – its temperature
rises
Cup of water at 20 °C in a room at 10°C
loses heat energy and cools down – its
temperature will fall.
HEAT ENERGY
•Energy transfer
Conduction
Convection
Radiation
Conduction
Heat is transferred through a material by
being passed from one particle to the next
 Particles at the warm end move faster and
this then causes the next particles to move
faster and so on.
 In this way heat in an object travels from:

the HOT end
the cold end
Conduction cont…
Occurs by the particles hitting each other
and so energy is transferred.
 Can happen in solids, liquids and gases,
 Happens best in solids-particles very close
together
 Conduction does not occur very quickly in
liquids or gases

Conductors
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 poor conductors,e.g., wool
feels warm because it traps a lot of air
A fridge has insulation material round it to keep it
cold – reduces amount of heat conducted to inside
from the warmer room
Convection
Takes place in material where particles can
move around inside the material, i.e. liquid or
gas
 The heat is carried by the particles themselves
moving
Convection
currents
 Occur because an area with warm particles
expands and becomes less dense than the
cooler areas nearby. The warm area rises.
Cooler particles fall into the space left by the
warm particles and convection current is set up

Convection Currents

Hot liquids and gases expand and rise
while the cooler liquid or gas falls
2. Goes across
3. Then down
1. Hot air rises
4. And across
Convection cont…
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The Sun can cause large convection currents WINDS
During daytime the land warms up more than the
sea. The warm air rises over the land and cool
air falls over the sea. So we feel a sea breeze.
(You will talk more about this in 8th grade)
Rising convection currents can be uses by glider
pilots to keep their planes in the air and by birds
to stay aloft.
Radiation
Transfer of heat directly form the source to the
object by a wave, travelling as rays.
 Heat radiation is also known as
INFRA-RED RADIATION

All objects that are hotter than their surroundings
give out heat as infra-red radiation
 Heat transfer by radiation does not need particles
to occur and is the only way energy can be
transferred across empty space

KEY WORDS

1-1 Heat: A form of
energy
– Heat
– Molecule
– Heat transfer
 Conduction
– Insulator
– conductor
 Convection
 radiation

1-2 Temperature and
Heat
– Kinetic energy
– Temperature
– Thermometer
– Celsius scale
– Kelvin scale
– Absolute zero