|MadSci Network: Physics|
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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