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Conceptual question, fluid pressure in a barometer

  1. Dec 31, 2015 #1
    1. The problem statement, all variables and given/known data

    I took a stab at the following question in a book:

    "In one observation, the column in a mercury barometer has a
    measured height h of 740.35 mm.The tem-
    perature is -5.0 C, at which temperature the density of mercury r is
    1.3608 10^4 kg/m^3 .The free-fall acceleration g at the site of the barom-
    eter is 9.7835 m/s^2 .What is the atmospheric pressure at that site in pas-
    cals and in torr (which is the common unit for barometer readings)?"

    And I got the right answer, but it left me wondering why it was the right answer.

    2. Relevant equations

    [tex]p = p_0 + \rho g h[/tex]

    3. The attempt at a solution

    I used the equation above with the obvious inputs except that I assumed p0 was 0 because I basically couldn't think of anything else to do. To within a very small error I got the same answer as the back of the book so I'm guessing I did this correctly, but is this correct or is there something I'm missing?
     
  2. jcsd
  3. Dec 31, 2015 #2

    SteamKing

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    In a mercury barometer, what is p0 over the mercury column inside the barometer itself?
     
  4. Dec 31, 2015 #3
    In some odd coincidence I just finished watching a YouTube video in which the presenter says the pressure at the top is 0. I wasn't actually aware of that but I guess that makes good sense and answers the question! Thanks. :)
     
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