# Homework Help: Conceptual question, fluid pressure in a barometer

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1. Dec 31, 2015

### Addem

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

$$p = p_0 + \rho g h$$

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. Dec 31, 2015

### SteamKing

Staff Emeritus
In a mercury barometer, what is p0 over the mercury column inside the barometer itself?

3. Dec 31, 2015

### Addem

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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