Heat energy
اسلاید 1: HEAT 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 heatedVisit www.worldofteaching.com for more free powerpoints
اسلاید 2: Heat 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
اسلاید 3: Heating and CoolingIf an object has become hotter, it means that it has gained heat energy.If an object cools down, it means it has lost energy
اسلاید 4: 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.
اسلاید 5: HEAT ENERGYEnergy transfer Conduction Convection Radiation
اسلاید 6: ConductionHeat is transferred through a material by being passed from one particle to the nextParticles 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
اسلاید 7: 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
اسلاید 8: ConductorsMaterials 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
اسلاید 9: Insulators/poor conductorsMaterials that conduct heat slowly or poorly are called insulatorsGlass, 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 airA fridge has insulation material round it to keep it cold – reduces amount of heat conducted to inside from the warmer room
اسلاید 10: ConvectionTakes place in material where particles can move around inside the material, i.e. liquid or gasThe 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
اسلاید 11: Convection CurrentsHot liquids and gases expand and rise while the cooler liquid or gas falls1. Hot air rises2. Goes across3. Then down4. And across
اسلاید 12: 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.
اسلاید 13: RadiationTransfer 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 radiationHeat transfer by radiation does not need particles to occur and is the only way energy can be transferred across empty spaceINFRA-RED RADIATION
اسلاید 14: EmittersHotter 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.
اسلاید 15: Emitters of heatBright shiny can Poor radiatorDull black can Good Radiator
اسلاید 16: AbsorbersCooler 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
اسلاید 17: AbsorbersShiny, bright can Poor absorberDull black can Good absorber
اسلاید 18: Key WordsHeatRadiationConvectionConductionColdInsulatorConductorTemperatureEmitterAbsorberTransfer
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