MadSci Network: Physics

Re: Are vacuum flasks of varying sizes equally effective?

Date: Sun Aug 21 19:21:27 2005
Posted By: David Coit, Aerospace Engineer, Naval Air Warfare Center - Weapons Division
Area of science: Physics
ID: 1119970026.Ph

As far as conduction goes, it doesn't matter if you have an inch of vacuum
or a mile, conduction is not possible. There might be some differences in
the flasks that makes one slightly more effective than the other, however
it's really hard to say. I suggest, if you have access to two flasks of
different sizes, that you perform a little experiment to see if you can
measure a difference. You could start with both at room temperature and
pour some water from a pitcher that has been in a refrigerator for awhile
and use a thermometer to measure the temperature increase over time.

It might also be instructive to do a similar experiment, but start by
putting the water in the flasks, then letting them sit for awhile in the
refrigerator together, then taking them out and measuring the temperature
change over time.

I'm guessing the only major difference between flasks of different sizes
would be that the larger one takes longer to initially cool off, because if
you start with each at room temperature and pour a cold liquid into them,
the larger flask will have more energy to pass to the liquid initially.
After that they should be about the same. If you want to do a couple of
more experiments, repeat the ones above but with a fan blowing over the top
of the flasks. I'd expect to see the larger one approach room temperature
quicker in that case, since it will have larger surface areas for
convection to work on, but the difference still might not be measurable.

I hope this helps,
David Coit

[note added by MadSci Admin:  The vacuum in a thermos flask is not
perfect, so some conduction does occur.  Also, the inner vessel must
be supported somehow by the outer vessel, and conduction occurs
through the mechanical supports.  Even without conduction there
is transfer of energy by radiation ("blackbody").]

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