The Drinking Straw, Vacuum and Hydrostatics

In summary, the conversation discusses the concept of water level stabilization in a vertical pipe with the top end closed and a pressure of 1000 psia at the bottom. The experts calculate that the pressure can support a column of water up to approximately 2310 ft. However, there is a struggle in understanding how the water level can drop 2690 ft in a vacuum created at the top. The experts also mention the use of a mercury barometer as an example of how a closed tube and atmospheric pressure can support a column of liquid.
  • #1
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I am struggling with a mental model of what happens with the water level in a vertical pipe with the top end closed (say by a valve). Given:

Pipe height = 5000 ft
Pipe connected at bottom end to a chamber of pressure = 1000 psia
water density = 62.4 lb/ft^3

Given a liquid full pipe (5000 ft of water inside), where will the water level stabilize to (remember, top end of the pipe is closed and you've got a 1000psia pressure at the bottom of the pipe)?

I've gone through some other thought processes (like why the water does not drain out when you've got your thumb on top of a liquid full drinking straw), but will hold back on those so as to not contaminate or lead-on your comments and thoughts.
 

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  • #2
If I understand what you're asking: 1000 psi = 144,000 lb/ft^2, so that much pressure can support a column of water of height h = P/D = 144,000/62.4 = 2,310 ft.
 
  • #3
Doc Al said:
If I understand what you're asking: 1000 psi = 144,000 lb/ft^2, so that much pressure can support a column of water of height h = P/D = 144,000/62.4 = 2,310 ft.

I agree that the pressure can support a water column height of approximately 2310 ft if the top end was open: h = 2.31ft/psi * (P2 - P1) = 2.31*(1000 - 14.7) = 2274 ft. However, with the top end closed, doesn't the falling water level "pull" a vacuum at the top? Using a simple hydrostatic calculation, h=2.31*(1000 - 0), we get 2310 ft. But, I struggle with how the water level can drop 2690 ft once the vacuum has formed at the top, i.e. let's say the liquid full column drops 1 ft and pulls a vacuum (agreed that this is not a perfect vacuum), can the water level continue to drop and this vacuum space just "expand"?
 
  • #4
If the top end were open, you'd get a different answer: you'd have to include the downward pressure of the atmosphere (or whatever else the top of the column were exposed to) on the column, which will reduce the height of water that the upward 1000 psi could support.

Vacuums don't "suck" or create negative pressure. There are only two forces on the column of water when the top is closed and a vacuum is created: its weight and the upward pressure due to whatever's producing the 1000 psi at the base. The vacuum does not exert a force on the water.
 
  • #5
This is easier if you have ever seen a real mercury-in-glass barometer.
The top of tube is sealed, the height of mercury in the column is held up by the weight of atmosphere pushing on the open pool of mercury at the bottom.
If the pressure drops (bad weather) there is less force to hold up the column and the height decreases - create a vacuum in the sealed top of the tube.

This is also why with an ordinary 'suction' pump you can't suck water up a height greater than the height of a column of water supported by air pressure (around 10m). As DocAl says it's not the vacuum pulling, it's the air pushing, all you are doing by sucking is removing the air from the top which was pushing it back down.
 
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  • #6
mercury barometer

Good example. Here's a diagram and brief discussion of how a mercury barometer works: Mercury Barometer
 
  • #7
We had a really nice one outside my lab.
Somehow it had managed to survive there for 100 years without killing anyone until the safety people found it - then it was the end of the world.
 
  • #8
Based on your question and the drawing, I would have to assume that there is some type of barrier at the bottom of the water column that separates it from the 1000 psia pressure initially, correct?

If so, are you asking how far the fluid column will drop if the barrier is suddenly removed?

If you are, then the fluid column would only drop a negligible amount. You can take a plastic straw and a cup of water and put your thumb over the end of the straw once it's full and then hold it up. You'll see that the water doesn't fall out. Same principle here.

This assumes that the fluid column is in a tube and that the tube is strong enough not to collapse, and you are starting from a static condition.
 
  • #9
stewartcs said:
If you are, then the fluid column would only drop a negligible amount. You can take a plastic straw and a cup of water and put your thumb over the end of the straw once it's full and then hold it up. You'll see that the water doesn't fall out. Same principle here.
Not if the "straw" is 5000 feet long! Atmospheric pressure alone will support a 34ft column of water, not a 5000 ft one.
 
  • #10
Yeah I know, after I posted that and then thought about it for a second, I realized that!
 

1. What is the history behind the creation of the drinking straw?

The first known drinking straw was created by the Sumerians in 3000 B.C. and was made from a hollowed-out reed. In 1888, Marvin Stone patented the first paper drinking straw, and in 1937, Joseph Friedman created the flexible straw we know today.

2. How does a vacuum work?

A vacuum is created when all the air is removed from a sealed container. This absence of air creates a low-pressure environment, causing objects to be pushed towards the area of lower pressure. This is why liquids are able to be sucked up through a straw.

3. What is the principle behind hydrostatics?

Hydrostatics is the study of the behavior of fluids at rest. It is based on the principle that pressure in a fluid acts equally in all directions, and the pressure at any given point is dependent on the weight of the fluid above it. This explains why objects float in water and why water rises up a straw when we suck on it.

4. Can a straw work in space?

No, a straw cannot work in space because there is no air to create a vacuum. Without the pressure difference between the inside and outside of the straw, it would be impossible to suck liquid up through the straw.

5. What are some other uses for a drinking straw?

Besides being used for drinking, straws have many other practical and creative uses. They can be used as pipettes in science experiments, as makeshift tools for art projects, and even as tiny planters for seedlings. Some people also use straws to help keep cords and wires organized and untangled.

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