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P1b(i) Keeping Homes Warm
P1b(i) Keeping Homes Warm You will learn about: The insulating properties of air How energy is transferred by conduction, convection and radiation How insulation reduces energy loss
Insulation Arctic 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 warm All insulators work in the same way. Insulators prevent the heat from moving from hot to cold.
The thermal energy transfers into kinetic energy. The particles start to vibrate Energy is passed on when the particles touch When 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. Heat Above 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 easier The arrows show the paths the heat can conduct Remember: Conduction only occurs if the particles are touching… There are no particles in a vacuum so conduction does NOT occur here
Remember: Convection only occurs in liquids and gases Once the heat source is removed, convection no longer occurs When gas is heated it expands. So the same mass now occupies a much larger volume. Therefore its density decreases. Recall that density = 𝑚𝑎𝑠𝑠 𝑣𝑜𝑙𝑢𝑚𝑒 Away from the heat source, the air has less thermal energy So less thermal energy can transfer into kinetic energy So the less dense air rises upwards The air particles do not vibrate as much As the particles spread out, the air becomes less dense than the surrounding air The air becomes more dense than the surrounding air and sinks downwards The air particles start to vibrate: the thermal energy transfers into kinetic energy The air gets heated back up again and the cycle starts again. This is called a CONVECTION CURRENT. This convection current transfers energy The fire heats the air above it Heat source is here
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 longer Remember keyword here: Absorb Not 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 !!
Conduction, Convection and Radiation into context
Conduction, Convection and Radiation into context Homes 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 house Loft 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 walls Carpets: Contain micro-fibres that trap air. Air is a good insulator so heat cannot escape
Heat transfer in context again!
CONDUCTION CONVECTION Slowly 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 way RADIATION Heat is transferred from the gas ring in the form of an electro-magnetic wave. It needs no material to flow.
Quick Questions Two 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?
Quick Questions Two 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).
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