Presentation on theme: "P1b(i) Keeping Homes Warm"— Presentation transcript:
1 P1b(i) Keeping Homes Warm P1b(i) Keeping Homes WarmYou will learn about:The insulating properties of airHow energy is transferred by conduction, convection and radiationHow insulation reduces energy loss
2 InsulationArctic explorers need to keep warm in their environment otherwise they would freeze.Air is trapped within the coat. Air is a really good insulator.Usually heat moves from where it is hot to where it is cold.This does not happen in the coat.Air stops that heat from escaping.So the explorers stay warmAll insulators work in the same way.Insulators prevent the heat from moving from hot to cold.
3 Conduction www.PhysicsGCSE.co.uk The thermal energy transfers into kinetic energy. The particles start to vibrateEnergy is passed on when the particles touchWhen an object is heated the particles inside it start to vibrate more quickly.)))The thermal energy is easily passed on if the particles can touch one another.HeatAbove shows a gas. The particles are spread out which means that conduction is difficult.On the right shows a solid.In a solid the particles are all touching so conduction is much easierThe arrows show the paths the heat can conductRemember: Conduction only occurs if the particles are touching…There are no particles in a vacuum so conduction does NOT occur here
4 Convection www.PhysicsGCSE.co.uk Remember: Convection only occurs in liquids and gasesOnce the heat source is removed, convection no longer occursWhen gas is heated it expands. So the same mass now occupies a much larger volume. Therefore its density decreases. Recall that density = 𝑚𝑎𝑠𝑠 𝑣𝑜𝑙𝑢𝑚𝑒Away from the heat source, the air has less thermal energySo less thermal energy can transfer into kinetic energySo the less dense air rises upwardsThe air particles do not vibrate as muchAs the particles spread out, the air becomes less dense than the surrounding airThe air becomes more dense than the surrounding air and sinks downwardsThe air particles start to vibrate: the thermal energy transfers into kinetic energyThe air gets heated back up again and the cycle starts again. This is called a CONVECTION CURRENT. This convection current transfers energyThe fire heats the air above itHeat source is here
5 Radiation www.PhysicsGCSE.co.uk Infra-red radiation from the Sun can be reflected off shiny mirrors to a focus. The focal point gets really hot and this can be used to make electricity.This solar heater sits on a roof. The pipes are black. Dull, black rough surfaces are very good absorbers of infra-red radiation. So they warm up quicker and keep the water inside them warmer for longerRemember keyword here: AbsorbNot attract… black is not magnetic!!We feel the heat from the Sun. Therefore heat can pass through a vacuum (space) via radiation. For radiation to occur particles are not needed. Radiation is a wave!Infra-red radiation travels really fast: the speed of light: 300,000,000 m/s !!
6 Conduction, Convection and Radiation into context Conduction, Convection and Radiation into contextHomes lose too much heat… here are some ways to reduce the heat loss…Cavity Wall Insulation: this foam contains lots of trapped air so conduction is reduced. Heat cannot pass on by touching very well as air is a gas. Furthermore, a convection current cannot exist so cavity wall insulation prohibits convection too. They also have shiny surfaces to reflect the heat back into the houseLoft Insulation: Warm air moves upwards by convection to the roof. If the roof is not insulated then the base of the loft warms up and a convection current of warm air moves upwards and escapes through the roof tiles via conduction.Double glazing: Contains a space between two panes of glass which may be a vacuum or perhaps filled with argon gas. These are both excellent insulators. So heat cannot move from hot to cold therefore retaining the heat in the house.Shiny foil behind radiators: These reflect the infra-red radiation back into the room rather than allowing it to conduct through the wallsCarpets: Contain micro-fibres that trap air. Air is a good insulator so heat cannot escape
7 Heat transfer in context again! CONDUCTIONCONVECTIONSlowly the handle heats up. The handle particles closest to the heat source heat up first. Then the particles touching those and so on. Like a Mexican wave the heat transfers particle by particle until it reaches the hand.The water is hotter at the bottom. A current is made in the water moving heat from hot to cold. If the gas ring was turned off then heat can no longer flow in this wayRADIATIONHeat is transferred from the gas ring in the form of an electro-magnetic wave. It needs no material to flow.
8 Quick QuestionsTwo snowmen are built. One wears a coat and one does not. Which one melts first? Why?Why does a vacuum not transfer heat by convection?Why is wood a better conductor of heat than air?Which types of heat transfer does cavity wall insulation prevent?
9 Quick QuestionsTwo snowmen are built. One wears a coat and one does not. Which one melts last? Why? The one wearing a coat. The coat traps the heat in (the snowman is warmer compared to the air) so the heat cannot conduct through the coat. The snowman is well insulated!Why does a vacuum not transfer heat by convection? Convection needs particles AND a heat source. A vacuum contains no particles so the heat cannot move this way. It only transfers by radiation through a vacuum.Why is wood a better conductor of heat than air? Wood is a solid. Air is a gas. The particles of a solid are much closer than a gas. Therefore heat can transfer better through a solid as less kinetic energy (vibrations) is necessary.Which types of heat transfer does cavity wall insulation prevent? All of them! Conduction (air pockets trap the air and stop the heat moving - insulation), Convection (air pockets stop a convection current occurring) and Radiation (shiny surface reflects the heat back into the house).