HEAT ENERGY

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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. 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

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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

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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 30C -
  gains heat energy and heats up its temperature
  rises
• Cup of water at 20 C in a room at 10C loses
  heat energy and cools down its temperature will
  fall.

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HEAT ENERGY

• Energy transfer Conduction
  Convection Radiation

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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 fromthe
  HOT end the cold end

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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

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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

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Insulators/poor conductors

• 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

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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

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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
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Convection cont

• 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.
• Rising convection currents can be uses by glider
  pilots to keep their planes in the air and by
  birds to stay aloft.

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Radiation

• Transfer of heat directly form the source to the
  object by a wave, travelling as rays.
• Heat radiation is also known as
• 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

INFRA-RED RADIATION
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Emitters

• Hotter objects emit (give out) heat
• Different surfaces emit heat at different speeds
• A dull black surfaces loses energy more quickly
  it is a good radiator
• A bright shiny or white surface is a poor
  radiator
• Marathon runners need to keep warm at the end of
  races, covering in shiny blankets reduces
  radiation and therefore heat loss.

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Emitters of heat
Bright shiny canPoor radiator
Dull black canGood Radiator
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Absorbers

• Cooler objects absorb (take in) heat
• Substances absorb heat at different speeds
• Dull, black surfaces absorb heat quickly
• Bright, shiny surfaces absorb heat slowly
• In hot countries, people wear bright white
  clothes and paint their houses white to reduce
  absorption of energy from the sun.
• Petrol storage tanks sprayed silver to reflect
  suns rays

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Absorbers
Shiny, bright canPoor absorber
Dull black canGood absorber
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Key Words
Temperature
Cold
Insulator
Radiation
Heat
Transfer
Convection
Conduction
Conductor
Absorber
Emitter