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[Phys-L] Re: electricity



Actually it is not a bad experiment. It does demonstrate that air has
weight, but there is certainly a bit of extra that goes into the
concept. There is a greater density of air in the balloon, so one is
in a sense weighing the extra air. Whether of not you wish to go into
the extra concepts is a matter of what level the students are at.

One could take an empty gas cylinder, and then pump air into it at
really high pressure. By weighing it before and after you can easily
also demonstrate that air has weight. But it is just a high tech
version of the 2 balloon experiment.

The 2 balloon experiment is only problematic because of the buoyant
force. You are actually measuring the difference between the weight
and the buoyant force.

John M. Clement
Houston, TX



Since I have just discovered that our 'weighing air using two
balloons and
a meter stick' experiment isn't accurate (apparently it only shows
that
air under pressure weighs more than air that isn't), does anyone
know of
any simple experiments/demonstrations that can show kids that air
has
weight? Or is it something we have to take on faith?

Thanks for your help-- I know my requests aren't for the usual
'deep'
topics discussed on the list, but I really do appreciate it.

Alison Innes
Education Coordinator
Bell Homestead National Historic Site
519-756-6220