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Thermal Insulators - Science whizz needed

Discussion in 'Primary' started by Flybee, Dec 31, 2011.

  1. I'm going to be teaching unit 4C Keeping Warm. It says that metal is a bad thermal insulator but when I did this previously the kids got the result that tin foil kept a beaker of water the hottest. Thinking about it, foil refects the heat back in even though I know it is also conducting it away.
    Also, the hottest take away you can get is always Chinese food which comes in foil containers. It's always much hotter than pizza or chips in card or paper wrapping. I expect I'm being really thick about this but please let me know what the rest of you think. Could tin foil keep beakers of water warmer than card?
     
  2. Starsarah

    Starsarah New commenter

    Im going to give you my science teacher opinion on this but is probably a bit higher level for your Year 4s (Im just guessing)

    A conductor is a material which allows thermal energy to pass through it.
    An insulator is a material which does not allow thermal energy to pass through it.

    Shiny, light objects are the best conductors.
    Black, matte objects are the worst conductors.

    In theory the tin foil would take the thermal energy away from the beaker as the energy passes from the hot water to the cold tin foil. BUT as the temperature of the foil increases (ie has more thermal energy) the water will become cooler, as it has lost thermal energy. This means that the thermal energy will pass back from the tin foil to the water. Thermal energy always travels from Hot to Cold.

    Where as in the card case the thermal energy would have no where else to go, in theory would stay in the water.The temperature should stay the same as the energy can not escape.

    There are obvious flaws in the design of the experiments that would affect your results. Unfortunately the experiments that match this piece of work don't often match the theory and lead to massive misconceptions which are hard to over turn.

    Just look at the Snowman in a coat problem and every child will tell you that the snowman will melt. In fact he will stay as he is.

    Hope that has helped!
     
  3. I agree that the experiments that often go with this topic often don't match the theory.
    My most successful attempt at it has been using plastic vending-machine type cups with lids and getting the children to wrap them completely in different possible insulators - newspaper, bubble wrap, fur fabric, cotton wool etc. In this way, you can steer away from metals.
    Each cup is wrapped in a piece of material and secured with an elastic band (keep the pieces of material the same size for a "fair test") with a circle of the fabric cut out and placed over the lid. Pre-cut a small hole in the lid and fabric circle, and a thermometer or digital temp. probe can be kept in the container for the duration of the experiment.
    I had reasonably reliable results with this last time - the thick cotton wool (the type from a roll) and fur fabric kept the water hotter for noticeably longer than the newspaper.
     
  4. joedoggyuk

    joedoggyuk New commenter

    Yes, it could. The water would leak through the card. Which is why curry sauce is kept in tin foil, but chips are wrapped up in newspaper.
    Cardboard is a good insulator because it has pockets of air trapped inside. Air, being a gas, does not conduct heat energy well. If the cardboard gets wet the air pockets are filled with water; water conducts well.

    This is another possible reason why the tinfoil could be keeping the water warm. There are three ways heat energy moves: conduction, convection and radiation.
    The tin foil would loose heat energy faster by conduction than paper, but the paper would loose more heat energy by radiation than the tinfoil because the tinfoil is shiny.
     

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