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Homework Help: Multifluid manometer

  1. Sep 8, 2010 #1
    1. The problem statement, all variables and given/known data
    This is not a homework, but an example in my textbook that I don't really understand.
    We need to find P1
    The website I took this picture from explains it but it looks too complicated. The teacher did it moving along the tube and it looked really easy but I didn't understand. He almost did it in his head without using the board.

    2. Relevant equations
    pbottom = ptop +ρgh
    the pressure at two points that have the same height are the same if they are connected with the same fluid.

    3. The attempt at a solution
    Find P1
    Here's my try:

    I don't know how to continue
    I know I need to relate PD to the other known quantities but I don't really see how.

    Another question: Isn't P1 = Pair ? Since 1 is at the boundary of air and water

    Thank you for your help

    Attached Files:

    Last edited: Sep 8, 2010
  2. jcsd
  3. Sep 8, 2010 #2


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    Staff: Mentor

    No idea what P1 is.

    I guess D means the point where the water and oil meet, B & C look well defined as well, but what are A & E?
  4. Sep 8, 2010 #3
    P1 is the point where air and water meet.

    the answer, if it helps, is:

    P1 = Patm - ρwatergh1 - ρoilgh2 + ρmercurygh3
  5. Sep 8, 2010 #4


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    Staff: Mentor

    Go from the outside - if you know pressure at C (Patm), can you calculate pressure at B?
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