HEAT ENERGY 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 t www.worldofteaching.com for more free powerpoints
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HEAT ENERGYHEAT ENERGYWhat 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 TemperatureHeat and TemperatureThe temperature of an object tells us how
HOT it isMeasured in degrees Celsius - °CIt 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 CoolingHeating 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
HEATHEATTotal amount of molecular energy in a
substance.
TemperatureTemperatureMeasures speed of molecules (average
speed of molecules in a substance)
Intermolecular forcesIntermolecular forces
Forces between molecules in a substance that determine the phase of matter
Which phase would have the greatest intermolecular force?
Phase Change diagramPhase Change diagram
Heating and Cooling cont…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 risesCup of water at 20 °C in a room at 10°C loses heat energy and cools down – its temperature will fall.
HEAT ENERGYHEAT ENERGY•Energy transfer
Conduction Convection
Radiation
ConductionConductionHeat is transferred through a material by
being passed from one particle to the nextHeat transferred by direct contactParticles 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…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
togetherConduction does not occur very quickly in
liquids or gases
ConductorsConductorsMaterials that conduct heat quickly are
called conductorsAll metals are good conductors of heatCopper is a very good conductor of heatPans for cooking are usually made with a
copper or aluminium bottom and plastic handles
Insulators/poor conductorsInsulators/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
ConvectionConvectionTakes 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
currentsOccur 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 CurrentsConvection CurrentsHot liquids and gases expand and rise
while the cooler liquid or gas falls
1. Hot air rises
2. Goes across
3. Then down
4. And across
Convection cont…Convection cont…The sun can cause large convection
currents - WINDSDuring 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.
RadiationRadiationTransfer 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
EmittersEmittersHotter objects emit (give out) heatDifferent surfaces emit heat at different speedsA dull black surfaces loses energy more
quickly – it is a good radiatorA bright shiny or white surface is a poor
radiatorMarathon runners need to keep warm at the end
of races, covering in shiny blankets reduces radiation and therefore heat loss.
Emitters of heatEmitters of heat
Bright shiny canPoor radiator
Dull black canGood Radiator
AbsorbersAbsorbersCooler objects absorb (take in) heatSubstances absorb heat at different speedsDull, black surfaces absorb heat quicklyBright, shiny surfaces absorb heat slowlyIn 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 sun’s rays