Two metal cans per set are required for this practical, one needs to be painted in thick matt black paint on the outside. Acrylic paint is good for this as it is waterproof and quick drying.
The other can needs the label taking off and cleaned so it has a shiny metal outside surface.
Both cans should be filled to the same level with hot water and a thermometer placed in each. At set intervals, the water should be stirred with the thermometer and the temperatures taken. After several minutes it should be clear that the black can cools quicker. The shiny can is a poor radiator and so should contain hotter water than the black can after several minutes.
Equipment required (per set):
- Matt black painted can
- Shiny metal can (both need to be the same size and shape)
- 2 x thermometers or dataloggers with thermal sensors
An infrared thermometer may be used during this experiment to show how much heat is escaping from the outside of each can during cooling.
This practical has real life applications - kettles mainly have shiny metallic surfaces so they don’t radiate (and therefore do not lose) too much heat through their surfaces. Cooling fins at the back of refrigerators are usually dull black in colour in order to radiate as much heat away as possible. Photographs of these may be required to show pupils the practical applications of the science behind this experiment.
The same two cans can also be used to measure how much thermal radiation they absorb (see Fig 2). Pour the same amounts of cold water into each can and insert a thermometer. Put both cans either in direct sunlight or equally near a desk lamp or heater.
The dull black surface is a good absorber of heat and so the water in the can should be warmer than in the shiny can over a few minutes. The shiny can is a poor absorber as it reflects much of the heat energy away.