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Vapor Pressure In Closed Cylinder

  1. Aug 29, 2016 #1
    Hi There, I am trying to understand if I have a vacuum in the below scenario or simply just really low pressure.

    A pump supplies a pipe with a constant supply of water at one end (Point A) the water discharges several kilometers away at some lower elevation (Point B). At "Point A" a valve closes and seals. I assume that the water level in the pipe begins to drop, however, at some point it stops. At this instance have I created a Vacuum at point A? Or is the pressure at Point A simply below atmospheric pressure but above 0 pressure (by some small amount) due to the pressure created by water vapor?

    The Engineering side of me sees the pipe as a cylinder and the fluid level acting as a piston, and as the piston lowers it creates a vacuum in the pipe, but I feel as if this is a poor way to look at the situation.
     
  2. jcsd
  3. Aug 29, 2016 #2

    Borek

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    Staff: Mentor

    Closed space above anything is (if allowed to reach the equilibrium) saturated with the vapors of whatever is present. Technically even in the steel container evacuated of air there exist an iron vapor (although the pressure is so low we will assume it is just a vacuum).
     
  4. Aug 29, 2016 #3
    So to be more clear about it. I would have a very low pressure at or near 0 bar. But I would not have a negative pressure (I.e below 0 bar) Correct ?
     
  5. Aug 29, 2016 #4

    Borek

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    There is no such thing as a negative pressure (unless you mean a relative pressure, which doesn't make sense here).

    The pressure you will see depends on the temperature and doesn't have to be low (guess what it will be at the boiling point), you can easily check the value in the water vapor pressure tables.
     
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