[PPT]HEAT ENERGY - Quia · Web viewHEAT ENERGY What is HEAT? Form of energy and measured in JOULES...

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

AbsorbersAbsorbers

Shiny, bright canPoor absorber

Dull black canGood absorber

Key WordsKey Words

HeatRadiation

ConvectionConduction

Cold

Insulator

Conductor

Temperature

EmitterAbsorber

Transfer