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A philosophical question about piston and vacuum

  1. May 21, 2010 #1
    Hello everybody,

    I have kind of a philosophical question about vaccum and piston. I know this does not work in the real world because, of imperfection in seals and lack of "real vacuum", but here it goes.

    If you have a tube which is closed in one end, open in the other end, and 100% rigid. You put a piston into it with a one way valve so that air can escape out of it. You push it to the end of the tube, so that there are very little air between the piston and the end of the pipe.

    If you now start to pull the piston out (perfect seal) what will happen? The pressure difference between the inside and outside of the piston will go towards 1 bar (we do this in normal pressure on earth). You pull it out far enough and the force you are using is dependent on the pressure difference. When the pressure difference has reached 1 bar, and the force is F, will you now be able to keep moving the piston outwards using the force F?

    Thanks,

    Frank
     
  2. jcsd
  3. May 21, 2010 #2

    Doc Al

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    Sure, why not? (Ignoring friction and all such complications.)
     
  4. May 21, 2010 #3

    Redbelly98

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    Also, the pressure difference doesn't just "go towards" 1 bar. It becomes 1 bar as soon as the piston loses contact with the closed end.
     
  5. May 21, 2010 #4

    Doc Al

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    I think he was assuming a little bit of air still trapped between piston and closed end to start with.
     
  6. May 21, 2010 #5

    Redbelly98

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    Ah yes, in that case the pressure would indeed approach 1 bar (or whatever the surrounding pressure is).
     
  7. May 21, 2010 #6
    Thanks, everyone. Me and a friend had a discussion about this, and we needed a closure :)

    Frank
     
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