# Homework Help: Barometer homework problem

1. Nov 20, 2008

### Tidus

1. The problem statement, all variables and given/known data
A thin tube, sealed at both ends, is 1.00 m long. It initially lies horizontally, with the middle 10.0 cm containing mercury, and the two ends containing air at standard atmospheric pressure:

See picture in attachment

If the tube is now turned to a vertical position, by how much will the mercury be displaced, if the process is isothermal? You may assume that no gas passes from one side of the mercury column to the other.

2. Relevant equations

P1f = hpg + P2f p=density of mercury , h=length of mercury column
P1V1 = P2V2
V1 + V2 = 0.9

3. The attempt at a solution

Tried solving simultaneous equations but got bogus answers

#### Attached Files:

• ###### mercury.jpg
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2. Nov 20, 2008

### Staff: Mentor

Re: Barometer

Say the mercury is displaced X cm. How much does the pressure change on each side? How much of a pressure difference is needed to support the mercury?

3. Nov 21, 2008

### Tidus

Re: Barometer

(P2 + hpg)(0.45-x) = (0.45 +x)P2 ?

4. Nov 22, 2008

### Staff: Mentor

Re: Barometer

Not sure how you got that expression. Do it step by step. Both sides start with the same volume (proportional to their length, which is initially 0.45m) and the same pressure Pa.

So what's the new pressure of the air on the side that compresses by distance X? And the side that expands?