Air Pressure and Coke Cans

June 6, 2008 — Leave a comment

I recently did a piece on measuring the speed of light using your microwave. Well here is some more physics you can play with in your kitchen. This time let’s create a vacuum and then use it to crush something. I like crushing things. Don’t we all?

What you will need:

  • A regular drinks can
  • A pot of cold water big enough to submerge the can
  • A pair of tongs
  • A kitchen hob (gas or electric is fine).

What to do:

Now you have to be careful with this one. The tongs have to be good or you’ll burn yourself. If you’re a child reading this, then make sure someone supervises you while doing this experiment. Reading though all the instructions before you start out is vital. I recommend having a couple of attempts, so maybe have two or three cans ready. So let’s begin:

Whilst you are filling up the pot of water why not drink the coke or whatever is in your drinks can. We don’t need any of the contents for this experiment, just an empty can. Once it is empty, rinse it out and place about two tablespoons of water in the can.

Now take your tongs and get a firm hold on the can. Hold it over the kitchen hob. We need to boil the small amount of water we have put in the can. This won’t take long and you’ll know when it’s worked because you’ll see steam coming out of the hole at the top of the can. Let it steam for a minute or two to be sure the water has all boiled.

Now here’s the cool bit. Keeping the can in between the tongs, take the can directly from the hob and dunk it, upside down, into the pot of water. The can will instantly and violently be crushed! It will happen very quickly so be ready. When I did it, it made a loud smacking sound as it went under water. I did it twice because I missed it the first time!

What is happening?

There is some great physics going on in this simple experiment. When you heat up the can and boil the water inside, the can fills with steam and pushes out all the air. Then when you dunk the can into cold water, the steam quickly condenses into water and there is no air pressure inside the can to support it. The can cannot resist the forces pushing on all sides from the water and air above it. Therefore it is crushed instantly!

Balloons:

Air pressure is also at work in balloons. When you blow air into a balloon you are artificially increasing the air pressure inside it and the rubber skin expands outward, forced by the force of the air molecules bounding around inside it.

You can ‘crush’ balloons by dipping them into liquid nitrogen. This condenses the air inside into a liquid and the balloon goes flat as a pancake. Here can see a video of a balloon that has been dunked into liquid nitrogen thawing out. The air boils back into a liquid and the balloon re-inflates. We filmed this last year in our first year undergrad physics lab.

Enjoy playing with air pressure and feel free to send me any images of your crushed cans!

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