Impact of Atmospheric Pressure on the Water in a Tank and a Pipe

  • Thread starter tomtomtom1
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  • #1
tomtomtom1
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TL;DR Summary
Impact of Atmospheric Pressure on Water in a Tank and Pipe
Hello all

I was hoping someone could help with understanding how fluids level out under atmospheric pressure. For example:-

Below is a picture of a tank of water with a closed door at the bottom, the door leads to an inclined pipe that is closed off at the end there is another pipe connected vertically.

I have drawn arrows in red to represent the atmospheric pressure which gets greater due to depth and I have drawn arrows in blue to represent the water pressure which also gets greater due to depth.

one.JPG




Now if i opened the green door and allowed the water to flow, but let's say for now the water did not rise up the tube then my diagram would look like:-


two.JPG





The question is why does the water rise up the tube and stop rising until the water in the tank and the water in the fluid are at the same level?

What i think is correct is that; as the water in the tank flows down into the pipe, the water level in the tank decreases which means that the force due to the pressure in the tank decreases but the atmosphere pressure in the tank increases.

But i am struggling to understand why the water would rise UP the tube when there is more atmospheric pressure pushing down?

Can anyone explain?

Thank you.
 
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Answers and Replies

  • #2
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Is the water flowing out the pipe while this is going on, or is the water blocked at the lower end after the water fills the pipe?
 
  • #3
tomtomtom1
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The pipe is blocked at the lower end.
 
  • #4
22,331
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OK. So down low, right below the vertical tube, do you think that the water pressure is very high or do you think it is not very high? (For example, do you think that about 10 m below the water surface in a lake, the water pressure is much higher than near the surface, or not very high?)
 
  • #5
tomtomtom1
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OK. So down low, right below the vertical tube, do you think that the water pressure is very high or do you think it is not very high? (For example, do you think that about 10 m below the water surface in a lake, the water pressure is much higher than near the surface, or not very high?)

I would say that it is high because the water flowing in from the tank is making it high.
 
  • #6
22,331
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I would say that it is high because the water flowing in from the tank is making it high.
That's the reason it rises into the vertical tube. The air pressure doesn't increase nearly as fast as water pressure with depth because air density is very low.
 
  • #7
Bandit127
Gold Member
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The pressure at the bottom of the empty tube is greater than atmospheric pressure, by about 7 arrows. (Atmospheric pressure can be ignored because it is bearing on both the water in the tank and the water in the tube).

The additional pressure in the tube will push water up the tube and as the level in the tube goes up the pressure at the top of that water reduces due to the height and the pressures will tend to balance out. When the level in the tube is the same as the level in the tank you have equilibrium.

It is how water level sight glasses work. You could put one in your tube to measure the level of water in the tank.
https://inspectapedia.com/heat/Steam_Boiler_Sight_Glass.php

What would happen if you blocked the top of the tube before you opened the "door" (valve) in the tank?
 

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