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Air Mass?

  1. Nov 25, 2015 #1
    If E=mc^2, why is hot air lighter than cold air? Hot air has more energy and should thus have greater mass and therefore be heavier as weight = mg
  2. jcsd
  3. Nov 25, 2015 #2
    At the same pressure, hot air is less dense - there's less of it per volume.
    Aside from that, a number of molecules on a hot gas would be heavier than the same number of the same cold gas. But it would be very hard to measure the difference. For E=mc^2, you have an awful lot of E equating to very little m.
  4. Nov 25, 2015 #3
    So it's the fact that the air's volume is not constrained which allows it to expand it's volume and being less dense it rises above the colder denser air.

    I would assume then however that if two volumes of air were contact rained in equal sized separate vessels, the hotter air vessel would be infinitesimally heavier?
  5. Nov 25, 2015 #4


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    Staff Emeritus
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    Exactly. Given two containers of gas, identical in all respects except for temperature, the hotter one would have a tiny bit more mass than the cooler one.
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