Liquid-in-glass barometer
The figure at the top represents a liquid-in-glass barometer. There is liquid (in the demonstrated case water) in the base and in the tube and at the top of the tube there is either a vacuum or, as in the demonstrated case, a vacuum. At the top of the tube in the demonstration was an air valve.
Questions:
1. Why does the water in the tube not simply run down out of the tube?
- Atmospheric pressure holds it up against the force of gravity
2. What is the pressure at A?
- Ambient atmospheric pressure
3. What is the pressure at B?
- Atmospheric pressure plus the pressure due to the weight of the short column of water between A and B
4. What is the pressure at C?
- Same as at B, otherwise liquid would flow between B and C
5. What is the pressure at D?
- Same as at A, pressure at C minus pressure due to the weight of the short column of water between C and D
6. What is the pressure at E?
- Atmospheric pressure minus the pressure due to the weight of the water in the column between D and E
7. If we open the valve at the top and allow enough air into the tube to equalize the pressure above and below the valve, what will the pressure be at E?
- Atmospheric pressure, as the water will be forced out of the column so that the level of the water is the same inside and outside the tube
8. What would happen to the height of the water column if with no change in temperature the atmospheric pressure dropped over time?
- The pressure at point A (and thus points B, C, and D) would decrease, but for the pressure at D to decrease the weight in the column must decrease, which must occur by removing water from the column and lowering the water height
9. With a vacuum (no air) at the top of the column, how high would the water column have to be to balance the atmospheric pressure around sea level?
- 34 feet (10.3m), as the weight of a water column that tall has the same weight as a column of the same size of air spanning from sea level to the top of the atmosphere
10. How could the height of the column be changed if the barometer was used at the top of a mountain?
- The column would not need to be as tall since there is less atmosphere above you as you go higher in the atmosphere - the top of Mt. Rainier is almost halfway up through the mass of the atmosphere so the column would only need to just more than half as tall (just make sure your liquid does not freeze at colder, higher elevations)
11. Why would you want to have a vacuum above the liquid in the column?
- In that case the only thing measuring the air pressure (by balancing the pressure with its weight) is the liquid in the column, and assuming the amount in the column and physical characteristics of the liquid are known the measurements of the air pressure will be much more precise and not hampered by some unknown amount of air in the column
12. How can we make a barometer like this with a vacuum at the top of the column but that is not so tall?
- Use a more dense liquid - mercury (Hg) is 13.6 times more dense than water, so the column is 13.6 times shorter, which is about 2.5 feet (0.76m) rather than 34 feet (10.3m)