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Vacuum question

  1. Jan 4, 2012 #1
    Hi this is probably very easy for you guys, but I can't figure this out. Heated air rises right? Then why is it that when I put a bowl of soup in the microwave and heat it in a plactic Tupperware-like container, then remove and cover it -- does a vacuum like effect occur? It would seem to me tha the hot, steamy air would want to push the cover off. Instead it sucks the cover down. Why?
  2. jcsd
  3. Jan 4, 2012 #2


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    Once you remove it no more air is added. As the air cools down the volume reduces (ideal gas law) - so it sucks down.
  4. Jan 4, 2012 #3


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    This is the same reason as to why airbeds can be completely inflated at first but seem to loose air throughout the night. I strangely worked this out in a semi-dream state recently (before looking it up to confirm it). I went away for the weekend and stayed at a friend's on the Friday, his house was well heated and the airbed was fine all night. On Saturday I stayed at a different friend's on the same bed but this friend's house was much colder, halfway through the night I woke up on a partially deflated bed. In my half-dream state I concluded that if it had a puncture it would have deflated all the way and that the inbuilt pump must heat the air it pumps through, thus because of the cold air it had seemed to deflate.
  5. Jan 5, 2012 #4
    That's what I thought at first, but I think a larger reason for the effect is that as it cools, the steam inside the container condenses into water. The volume of water is something like 1000x less than steam, so it greatly reduces the pressure inside the container.

    When the soup is heated, the fact that hot air rises plays a small part trying to lift the lid. However, a larger part is played by the increased pressure inside, again due to water turning into steam.
  6. Jan 5, 2012 #5


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    I suspect both factors (water condensation and air cooling) play a role. I'll leave it to others to quantify.
  7. Jan 5, 2012 #6


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    I believe that you are completely correct. This is analogous to the standard physics experiment in which water is boiled in a large thin-walled tin can (big gas cans are idea) and then sealed. As the steam condenses, the can implodes (slowly)
  8. Jan 6, 2012 #7
    You guys are awesome. Thanks. Now I will try to figure out what the composition of the soup (density) verses water content is required to create x lbs. of vacuum pressure and whether there is a limit or maximum force that can be created.
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