Resource 5: Keeping things cool
Background information / subject knowledge for teacher
Background information on insulation and keeping water cool
- Energy (heat) is transferred from hot objects to colder objects.
- Anything that is warmer than its surroundings becomes cooler by transferring energy to the surroundings (so they get warmer); anything that is cooler than its surroundings becomes warmer (and the surroundings become cooler) as energy is transferred from the surroundings to the object.
- If you leave something long enough, it will reach the same temperature as its surroundings.
- To keep a hot object hot, or a cold object cold, you have to slow or stop the transfer of energy (heat).
- Energy (heat) is transferred by conduction, convection and radiation.
- Metals are good conductors but plastics, or materials with lots of air gaps like foams or bubble wrap, are poor conductors (or good insulators).
- Using thermal insulation or insulating a hot or cold object means wrapping the object in a material which is a poor thermal conductor.
- The thicker the insulation, the better it works. Don’t leave gaps in the insulation.
- Heat (energy) is transferred through a fluid like air or water by convection currents. Convection currents rise above the heat source as the warmed air or liquid expands (because it is less dense than the air/water around it). As the air or liquid cools it becomes more dense again and sinks.
- Building designs can make use of convection currents to keep the building cool: to do this they allow warm air to rise through the building and escape from the top, drawing cool air in at the bottom.
- Shiny or white surfaces reflect most radiation, while black surfaces are the best absorbers and transmitters. To make use of solar heating you would use black surfaces to absorb as much energy as possible; to keep something cool, you use shiny surfaces (wrap them in foil, spray them with shiny paint) or white surfaces to reduce absorption.
- Vacuum flasks reduce heat transfer by all three mechanisms: the silvered layer reduces transfer by radiation, the vacuum means there is no air to allow losses by convection, and the insulating foam beneath the casing reduces losses by conduction.
- Thick coverings also help to keep cool things cool because it takes more energy to heat up the covering material. This is why buildings with thick stone walls stay cooler than buildings with thin wooden walls – there is more ‘stuff’ to heat up.
- Water has a high specific heat capacity, meaning it takes a lot of energy to raise its temperature, so it keeps the temperature steady for longer: rivers and lakes are slower to heat up than the surrounding land; leaving cold bottles in a bucket of water on the table keeps them cool longer than just leaving them on the table.
- When a solid melts, it absorbs energy from its surroundings but it stays at the same temperature until all the solid has turned to liquid. Changing from solid to liquid (or from liquid to gas) is called a ‘phase change’, and materials which need a relatively large amount of energy to melt are used in cooling jackets for transferring foods or medical supplies without refrigeration. (When used like this, or in the plaster of buildings to help keep rooms cool, they are called ‘phase change materials’ or PCMs for short.)
- When a liquid evaporates, it uses energy from the surroundings to do so, so we can use evaporation to help keep things cool: letting the water evaporate from your skin instead of using at towel to dry yourself makes you feel cooler, and wrapping a bottle in a damp towel helps to keep it cool for longer.