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Vacuum in a jug

  1. Aug 29, 2005 #1
    If you have a five gallon jug of water with a 1/2 inch spout, and it is laid on its side, because there is no other opening in the jug it empties as air spills into the spout and replaces the water, but if the jug would have another 1/2 inch spout on it, now on top of the laid down jug,
    Would any of the air still enter through the first spout or would 100% of it now come from the other spout, although probably a simple answer, I'm unsure because I've never took a physics class, but I would assume that 100% would now come from the other opening.
  2. jcsd
  3. Aug 29, 2005 #2


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    The air outside the jug would be pressing on both spouts,
    about the same pressure. But the water would (when deep)
    would be going so fast out the exit hole (over 1 m/s)
    that "essentially none" of the air would diffuse upstream,
    until the jug was almost empty.

    try it, with food coloring in the exit stream -
    especially right next to the wall of the spout. Air isn't much faster.
  4. Aug 30, 2005 #3
    what if this changed?

    If the secondary spout on top had a resevoir of water pressing down on it, would the sucking in of this water hold back the air from going into the first spout?
  5. Aug 30, 2005 #4


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    If there's a hole in the top, the pressure at the bottom of the jug (at the first spout) no longer decreases rapidly as the water leaves (since there's never a vacuum made at the top). So the water just flows smoothly out, provided the hole in the top is big enough. (Eventually, if it's air coming in the top, the speed of water flow out the spout will slow, since the water depth decreases.)
  6. Aug 30, 2005 #5
    thanx to lightgrav

    I originally posted in as 2112 because I couldn't access my account, thanks for the solutions.
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