Lava Lamps

The basic laws governing the lava lamp are density and specific gravity. There is a thick wax (usually paraffin wax) in medium with a similar density (for example, a water-alcohol mix). At room temperature, the wax should be marginally less dense than the medium it is in. When the lightbulb at the bottom of the lava lamp is switched on, the wax begins to heat up. The wax molecules become more and more excited and so the wax expands, taking up more volume but remaining the same mass. Since density = mass/volume, as the volume increases, the density decreases. Eventually, the density of the wax decreases to a point at which it becomes less dense than the salinated water it is surrounded by. At this point, the wax begins to rise, much as an air bubble in water would, to the surface. This is where the long, conical shape of the lamp comes strongly into play. The lamp loses heat to the atmosphere via conduction. Due to the shape of the lamp, there is a temperature gradient across it – the bottom is warmer than the top which is relatively cool. As the wax rises, it comes into contact with the cooler liquid at the top and transfers heat to it. The wax therefore cools and becomes denser, reaching a point at which it is denser than the salinated water, and so sinks back down to the bottom of the lamp, ready to start the process again. There is a metal coil at the bottom of the lava lamps which has two functions. The first is to aid the transfer of heat to the wax. The second is to break the surface tension of the wax. Without the coil, the wax would move up and down as a single unit but with it, the wax rises in different parts, creating the mesmerising flow.

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