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Homework Help: Manometer U Tube

  1. Nov 13, 2011 #1
    The problem statement, all variables and given/known data


    The U-tube in the figure is sealed at both ends. It contains a gas in A, another gas in B, and mercury in C. The heights of the mercury in the two arms are as shown. The density of mercury is 13.6 g/cm3. If the pressure in A is 1 × 10^4 dyn/cm2, the pressure in B is

    The attempt at a solution

    4-7 = 3CM

    0.03m * 9.81 * 13600 kg/m^3 which = 4000 Pa which converts to 4x10^4 dyn/cm^2

    But the answer given was 5x10^4 dyn/cm^2

    I am not sure what I am doing wrong?

    Edit: 4x10E4 is the pressure difference. So I have to add A which is 1x10E4 and that gives B = 5x10E4.

    Could someone explain why I add A?
    Last edited: Nov 13, 2011
  2. jcsd
  3. Nov 13, 2011 #2
    One way to think about this problem is to calculate the pressure at the bottom of the U-tube. Look at the left U-tube column the pressure at the bottom is Pb+ hb*density*g that will equal the right U-tube Pa+ha*density*g

    It is easy to see why you must add Pa if you are measuring the pressures at the bottom of the U-tube
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