Collapsing Can

The collapsing can demo is one I loved seeing for the first time when I was at school, although my teacher used a tin with a screwed down lid which took a little more time to cool down. In some ways I prefer the version using a can with a screw lid because the additional waiting time makes for an even more dramatic “collapse”. Doing the demo with a drink can is of course far cheaper (and I think, more reliable as it doesn’t depend on the lid being screwed down properly) and I suspect this is why the approach we use in our video has become far more widespread in schools.

I like the demo a lot but, as I hope we’ve managed to convey in the video, I think we need to be careful how and why we use it in our lessons. This is a really fantastic demo for using the Predict, Observe, Explain (POE) approach as the explanation of what’s going on is not entirely straightforward – there are a couple of things relating to the behaviour of particles and the action of forces that need to be considered and this can lead to some really interesting discussion with students, providing they’re familiar with the relevant concepts.

We’ve suggested in our video that the collapsing can demo can be used in conjunction with another demo, as a way of “scaffolding” (I really hope I’ve used that term correctly – I think this may be the first time I’ve used it in writing in this context).

Once you’ve done the demo live in class, you’ve got the perfect justification for showing your students this video of a rather more spectacular demonstration of the same physics at work:


Get Set Demonstrate logoThis film was produced for the Get Set Demonstrate project. Click through for teaching notes, and take the pledge to perform a demonstration to your students on Demo Day, 20th March 2014.

6 thoughts on “Collapsing Can”

  1. I always like the collapsing can demo – it challenges even the most able kids to use the particle models they already know to come up with an explanation. I’ll try it with the vacuum fountain in future too – that’s a handy extra tool to help with the particle theory models.

  2. Beautifully presented and shot. Thank you. FYI pressure using a rotary pump like that in that small volume after 15 seconds is likely to be ‘a few’ millibar – i.e. less than 1% of atmospheric pressure.

  3. For a few years now I’ve been using the collapsing can as one of five ‘amazing pressure demos’ that A2 students have to explain in terms of kinetic theory. They like it (it goes bang), they can show their friends (beer cans and a camping stove at a festival), and if they’re really good they can explain it too in terms of the kinetic theory! Good point about it being a long chain of argument, I’ll definitely include the water fountain next year as its a more obvious demo.
    BTW, the other demos I use are (1) the inverted tumbler of water with a piece of card over the top (very difficult to explain using kinetic theory, I do it as an example) (2) the poor man’s magdeburg hemispheres (one student pushes together a pair of rubber sink plungers, then attempts to pull them apart) (3) a small sealed balloon inside a bell jar connected to a vacuum pump and (4) the ‘boiled egg into the bottle’ trick, done with a conical flask and nothing more than a trough of ice water and a trough of very hot water.
    My favourite for sheer theatricality (and ease of explanation) is the egg into the bottle. There’s no sudden bang, but a long hard squeeze instead.

  4. Do you plan to make more of these? I often feel that non specialists struggle out of their subject area and would benefit from this sort of resource. Written instructions are great but a video helps so much more. What would be good to see is video accompanying the practical physics website so that each experiment had an accompanying video – a massive project!

    Pupils do experience the effect outside of the classroom, every time they use a straw they create a region of low pressure and form their own internal water fountain but very few will think of it being to do with difference in pressure, just a suck.

  5. Hi Alom,
    Great video and nicely explained demonstration. I really like the addition of the YuoTube video at the bottom with the imploding tank. I think it really shows how the same principle works on a much bigger scale. Kids can crush a can with their hand, but crushing a whole tank of those dimensions is another story 🙂
    Very nice, thanks!
    Alessio.

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