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How pressure is measured using a manometer

  1. Feb 10, 2013 #1
    Hello. I'm reading a physics book on my own and am learning about pressure measurements. I have a question with how pressure is measured using a manometer. Here is a link talking about it:

    http://www.upscale.utoronto.ca/PVB/Harrison/Manometer/Manometer.html

    My question is, why do points at the same height necessarily have the same pressure (as in Cases 2 and 3)? If the liquid is in equilibrium, why can't we say that the pressure at any point in the liquid is equal to the pressure at any other point?

    Thanks!
     
  2. jcsd
  3. Feb 10, 2013 #2
  4. Feb 11, 2013 #3

    Redbelly98

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    It can be explained by thinking about forces and free-body diagrams.

    From your link:
    Fig2.png
    Consider the water lying between the dashed lines on the right. It has three forces acting on it:

    1. Force due to air pressure at A, acting downward
    2. Force due to water pressure at B, acting upward
    3. Weight, acting downward

    Since this water does not move, it must have zero net force acting on it. Because there is a gravitational force acting on it, the pressures at A and B cannot be equal.
     
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