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Pre-Lunch Experiment: Fancy Flames

By Mark Steer

If you’re ready for some fancy physics at the dinner table then this is sure to raise a few smiles and plenty of voices. Set the tone for the meal by spraying hot wax across the table whilst trying to do another Christmas experiment.

What to do
  • Attach a candle securely the outer edge of a revolving platform, and I mean securely. Use one of those fancy revolving serving trays if you've got one, but a turntable will do just as well.
  • Cover the candle with a jam jar (also firmly attached) but leave a bit of space at the bottom for air to get in.
  • Spin, spin, spin.
  • Notice that the flame bends inwards towards the middle of the revolving platform.
  • Demand that everyone pays you at least one roast potato to buy an explanation.

What’s happened

Click to enlarge
It seems a bit counter-intuitive that the flame should point inwards; usually you would expect something being spun around to forced outwards. Well in fact that is what’s happening, but it’s air that’s being forced about and not the flame.

The candle heats some of the air inside the jam jar causing it to expand and become lighter. Therefore you get pockets of air in the jar which are hotter and lighter and pockets which are colder and heavier. The colder, denser air gets forced out to the edge of the jar by centrifugal forces, the hotter air being pushed to the inside.

This means that the pressure on the inner side of the jar is lower than on the outside, where the cold, dense air is. So the flame, seeking the path of least resistance, moves inwards where the air pressure is lowest.

There, what an excellent way of increasing your haul of roast potatoes.

Light Speed < Prev | See all | Next > Sprouts

This experiment has been adapted from Mick O'Hare's excellent book How to Fossilise your Hamster see more at www.newscientist.com/hamster.

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Hub image: Ivan Sanchez

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27 Jun 2011
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