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Question regarding water physics / please help

  1. Jan 17, 2012 #1
    This will probably be obvious to most of you so my apologies in advance. Some classmates and I got into a discussion about how an automatic feeder like this (http://i.ebayimg.com/t/Auto-Refill-...QCGk~$(KGrHqEH-C0EtrFJ6qq-BLdHF((cVg~~_35.JPG ) would work. When thinking about it I had guessed that it may be similar to hydraulics where the high pressure of the column would be countered by a large surface area in the pool (where a classmate had suggested atm pressure would be part of the opposing force here).

    Other classmates suggested that when the water level is low enough air can displace water (essentially creating a vacuum in the bottle). Can anyone shed light on the actual physics at play here?

    Your time is much appreciated.
    Last edited: Jan 18, 2012
  2. jcsd
  3. Jan 17, 2012 #2


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    I can't see the photo, but assume that the column is sealed. The weight of the water would result in the water moving downwards somewhat until the pressure differential of 1 atm outside at the surface of the water and the lower than ambient pressure at the top of the column results in an equal and opposing upwards force on the water, preventing further flow.
  4. Jan 18, 2012 #3

    sorry about that, hopefully that works better

    maybe I didn't understand the response (and you answered it) but I'm still unsure what is preventing the entire discharge of the bottle. Is it merely pressure such as in a hydraulics equation (F1A2 = F2A1) or is it a vacuum created in the bottle?
  5. Jan 18, 2012 #4


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    The vacuum created in the bottle is the source of the pressure difference, since the outside pressure remains at 1 atm. When the water level in the bowl drops below the opening in the bottle, then water will fall out and air will be drawn into the bottle, until the water level in the bowl rises above the opening in the bottle, preventing air from flowing inwards, and then the water moves down slightly resulting in a partial vacuum in the bottle.
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