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A temperature problem. Touch a piece of metal and a piece of wood at the same temperature. Which material feels warmer and why?. The metal feels cold and the wood feels warm. Metal is a good conductor and conducts the heat away from your hands, so it feels cold.
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A temperature problem Touch a piece of metal and a piece of wood at the same temperature. Which material feels warmer and why? The metal feels cold and the wood feels warm. Metal is a good conductor and conducts the heat away from your hands, so it feels cold. Wood is not a good conductor and does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.
What are thermal insulators? Some materials are very poor conductors of heat. These are called thermal insulators. Examples of materials that are insulators include plastics, wood, ceramics and air. Air becomes a very effective insulator when it is trapped and stopped from moving. This is how your clothes keep you warm – air is trapped between the fibres and so acts as an insulator. Other insulating materials, including polystyrene and loft insulation, use trapped air because it is so effective.
Charlie the Chef and his frozen chicken! Charlie forgot to take the chicken out of the freezer last night! Will his plan to defrost the chicken in time for lunch work?
How does a vacuum flask work? How is a vacuum flask able to keep hot drinks hot and cold drinks cold? 2. The plastic (or cork) lid is an insulator and the screw top prevents convection currents escaping from the flask. 1. There is a vacuum between two layers of glass or steel, which prevents heat leaving or entering by conduction. 3. The walls have silvery surfaces, which prevent heat leaving orentering by radiation.`
Heat loss from houses A thermogram shows the distribution of heat over the surface of a house. It highlights where heat is being lost. The white, yellow and red areas are the warmest, so these are the worst insulated parts of the house. The blue and green areas are the coolest, so these are the best insulated parts of the house. A poorly insulated house loses more energy and so costs more to heat. It also means that more pollution, particularly carbon dioxide, is created in order to heat the house.
Heat loss from houses Where is the most heat lost from a poorly insulated house? roof 20% windows 12% walls 36% doors 4% floor 28%
How can heat loss be reduced? Type of insulation Type of heat transfer stopped How insulation stops heat transfer Various types of insulation can prevent the loss of heat energy from a house. Use the following slides to complete this table showing how different types of insulation can prevent loss of heat energy from a house.
Can heat loss through windows be reduced? A lot of heat energy can be lost through windows. Double glazing adds an extra pane of glass. The air trapped between the two panes acts as an insulator and reduces heat loss by conduction. insulating layer of air side-view of double glazing The insulating effect of the gap can be improved by removing the air and creating a vacuum. The problems with double glazing are that it can be expensive and that it is difficult to break in emergencies without a special hammer.
How can heat loss through walls be reduced? Most outside walls have an empty space between the two layers of bricks called a cavity. convection currents in the cavity The air trapped in the cavity acts as an insulator and reduces any heat loss due to conduction through the bricks. foam insulation prevents convection currents Heat loss can also occur due to convection within the cavity. Plastic foam insulation can be pumped into the cavity to prevent this.
How can heat loss through a roof be reduced? Loft insulation contains trapped air and so forms an insulating layer between the loft and the rest of the house. This helps to reduce heat loss through the roof. The air warmed by radiators (and other heaters) is carried around a house by convection currents. heat loss The house becomes heated but, if there is no roof insulation, the warm air continues to rise. The heat eventually escapes through the roof and is lost due to conduction through the roof tiles. loft insulation
How can heat loss from radiators be reduced? A radiator produces infrared radiation. Some of this heat energy is absorbed by the wall that the radiator is attached to, and so the wall heats up. This can be a real problem on outside walls, where the heat energy absorbed by the wall can escape from the house. Heat loss from a radiator can be reduced by placing shiny foil between the wall and the radiator. The foil prevents heat radiation from reaching the wall by reflecting it back into the room. This method of insulation is very cheap to install.
How can heat loss through doors be reduced? A draught is the movement of air due to a convection current. A lot of heat energy can be lost from a house due to draughts escaping through gaps under doors and around windows. This is one of the easiest types of heat loss to prevent. Draught excluders are hairy or spongy strips that can be used to close the gaps around doors and windows. They prevent draughts escaping and so reduce heat loss.
How can curtains reduce heat loss? A draught is the movement of air due to a convection current. Warm air rises and is carried up towards the windows in a house by convection currents. This heat energy can escape through gaps around windows that are uncovered. Fitting curtains and closing them can prevent draughts leaving a house and so reduce heat loss. In addition, curtains are opaque and so radiated heat does not pass through them.
How does double glazing work? How does double glazing keep a house warmer? Double glazing keeps a house warmer because there is a layer of a__ between the panes of glass. ir Air is a poor c________, so it acts as an i_______. onductor nsulator The trapped air r_______ heat loss by c_________ from a house. educes onduction side-view of double glazing
What is payback time? cost of insulation saving each year payback time (in years) = Payback time is the time it takes for the cost of installing insulation to be equalled by the savings made from reduced energy costs. Example: Adding silver reflectors behind radiators costs £25 and saves £50 per year. payback time = 25 50 = 0.5 years (6 months)
Calculating payback time What is the payback time for these types of insulation? How heat escapes Cost of heat escaping per year Cost of insulation Payback time 3 years roof windows draughts walls £80 £40 £50 £100 £240 £3,200 £50 £500 80 years 1 year 5 years Why is double glazing popular if the payback time is so long?
Which insulation is most cost-effective? saving each year cost of insulation Cost-effectiveness = Cost-effectiveness is a comparison of the annual savings in reduced energy bills and the cost of insulation. Example 2: Example 1: Adding draught excluders costs £50 and saves £50 per year. Adding silver reflectors behind radiators costs £25 and saves £50 per year. Cost-effectiveness = 50/25 Cost-effectiveness = 50/50 = 2 = 1 So, the reflectors are more cost-effective than the draughtexcluders.
Glossary (1/2) • cavity wall – A wall made up of two layers of bricks with an air gap between them, which reduces heat lost by conduction. • cavity wall insulation – Plastic foam insulation that is pumped into the gap in a cavity wall to prevent heat lost due to convection in the cavity. • cost-effectiveness – A comparison of the annual savings in reduced energy bills and the cost of insulation. • double glazing – Two panes of glass with an air gap between them, which reduces heat lost by conduction. The air can be sucked out of the gap to create a vacuum. • draught excluder – A hairy or spongy strip that prevents draughts escaping through the gaps around doors and windows, and so reduces heat loss.
Glossary (2/2) • loft insulation – A material which contains trapped air and helps to reduce heat lost through the roof of a house. • insulator – A material that is a very poor conductor of heat and so can be used to reduce heat loss. • payback time – The time it takes for the cost of insulation to be equalled by the savings from reduced energy bills. • vacuum flask – A vacuum flask is a bottle with double walls separated by a vacuum. It reduces heat transfer and so keeps warm drinks warm and cold drinks cold.
