Lava Lamp Sun
When Edward Craven Walker invented the Lava Lamp he had to make sure that his immiscible liquids – the molten wax and the water – were almost identical in densities. At the same temperature the wax is slightly less dense than the water. When heated by the bottom lamp, the wax becomes less dense than the water and rises up. When the wax is no longer near the heater it cools, contracts, becomes slightly less dense than the water and sinks. Craven Walker fine tuned the wax with different oil based additives and fine tuned the water with salts. The exact recipe took him over a decade to get right and is a closely guarded secret. However there are lots of recipes on the web.
An alternative are glitter lamps which appear to contain small pieces of aluminised Mylar. Glitter lamps have the advantage of starting from cold and are much easier to time as there is a constant stream of particles in the rising cell. Glitter lamps have a much faster flow rate than lava lamps. You need to be close to them to keep track of the bits as they rise.
An advantage of lava lamps is that the experiment can be done with only one lamp and a set of stopwatches per class.
These lamps are in effect a heat engines, converting the heat into kinetic energy. They require a temperature gradient between the bottom and top of the lamp. At KS4 students will meet similar gradients to explain Gravitational and Electrical potential energy.
In the convection cells of the Sun the surface is at 6100oC whilst the bottom of the convective zone, 200,000 km below is 200,000oC. Strangely it’s not the heat gradient of the cell that is the major driving force of the current. Instead, as the top of the cell is exposed to the vacuum of space it radiates heat and cools rapidly. The material contracts and becomes far denser than the underlying material through which it rapidly falls. Glitter lamps show this effect well. Once hot, when switched off they will continue to circulate as the liquid in contact with the glass walls is cooled and sinks.
The cells give a granular appearance to the surface of the Sun. Cells tend to be 1-2 thousand km in diameter. Most cells last for 5 – 20 minutes. At any one time there are about 4 million on the surface of the Sun. Each cell has a bright centre of hot, up welling plasma and a surrounding darker ring of falling cool material. The difference in temperature is in the order of 500oC.
One problem with investigating convection currents for assessed practicals at KS4 is that the input energy cannot be varied unless a dimmer unit is used. An alternative would be to use the lava lamp as an introduction and then go onto more conventional convection experiments using beakers and Bunsen burners or hotplates.
There are many random effects inside lava lamps. Students will need to decide on criteria for which cells they measure and which they do not, without introducing bias.
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