The Collapsing can experiment

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This experiment has the "woah!" factor as it demonstrates the stregnth of air pressure as you crush an ordinary drinks can.
It is mainly suitable for third years studying Pressure and forces.

Apparatus

  • An empty Aluminium drinks can (Like Coca Cola)
  • A bowl or large trough of water
  • Bunsen burner
  • Something to safely hold a hot can - eg., tongs

Safety
This experiment should be demonstrated by the teacher or technician. It is suggested that a perspex screen should be erected between the demonstrator and audience. Safety Goggles should be worn by demonstrator.

What to do

Collapse2.jpg
  • Put about 1 cm depth of water (no more) into the can. Hold the can over the gas flame until the water starts to boil.
  • You’ll feel the boiling and see wisps of steam coming out of the top of the can. Let the water boil for a minute or so.
  • Remove the can from the flame and immediately turn it upside down and dip the open end into the bowl of water.
  • A fraction of a second later there will be a loud bang, and the can will collapse.
Cokecan.JPG

The Explanation
The air pressure inside the can is equal to the air pressure outside it. The thin metal of the can has equal forces pressing on it from either side, and experiences no overall force.

When you boil the water in the can, the water evaporates to form steam. As it does so it gets a lot larger – the small amount of water in the bottom of the can could fill it many times over with steam. As the water boils, more and more steam is produced, which pushes the air out of the can, until the can is completely full of steam. The steam exerts the same pressure inside the can as the air does outside, so the can remains unharmed…so far.
When you dip the can into the bowl of water, the steam is cooled and rapidly condenses back into liquid water. As a liquid, it no longer exerts the same pressure as the air outside. In fact, when the steam has all condensed, the inside of the can is nearly a vacuum. There is now no pressure inside the can to resist the air pressure outside it, and the air instantly crushes the can
--Ssmith 15:06, 6 February 2007 (GMT)


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