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Thought experiment about pressure (intro physics)

  1. Apr 17, 2009 #1
    Thought experiment about pressure (intro physics) ..you have a U-shaped tube?
    that is open to the atmos on both the left and right side
    you fill it with water so the water level is the same on both sides.

    then you put a rubber stopper in one end (..the right end)
    (also the remaining air between stopper and water surface was somehow sucked out after you put the stopper in)

    next you somehow suck out some of the water from left side, so now the levels on each side are uneven. (right side remains right up to the stopper, and left side is halfway down)

    So... the pressure at the surface of the left side is 1atm right?

    But what is the pressure at the SAME LEVEL on the right side? If it is also 1atm please explain why, because its totally confusing me. thanks alot!!!!!!!!
  2. jcsd
  3. Apr 17, 2009 #2


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    The pressure down at the other side is the weight of water above that point.
    Since this is probably less than 1 atmoshere (unless you have a tube 9.8m tall) the other side is actually in compression, the weight of the atmosphere is forcing the water in the other side up - this is what most people think of as the vacuum 'sucking' the water in that side up.

    Once you get to a column more than 9.8m the weight of the water is greater than the weight of the atmosphere and you will get a (partial) vacuum at the top of the sealed water column.
    Then you have a barometer - the height of the water column tells you the air pressure. Generally you use mercury since 760mm is a lot easier to fit on your wall than 9.8m!
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