Why does the liquid stay in the straw?

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Discussion Overview

The discussion centers around the phenomenon of liquid staying in a straw when a finger is placed over the top, exploring concepts related to pressure and fluid dynamics. Participants examine the mechanics of air pressure, equilibrium, and the behavior of fluids in a confined space.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that the liquid stays in the straw due to the pressure of outside air pushing up against it, but they express confusion about how pressure changes when the straw is sealed with a finger.
  • One participant notes that when the finger is placed on the straw, the air pressure above the liquid drops significantly, while the pressure from below remains at approximately 1 atm.
  • Another participant questions why the liquid does not move upwards towards the finger when the air pressure above decreases, and why it does not fall out when the straw is inverted.
  • A later reply corrects an earlier assumption, stating that the pressure does not drop to zero but remains in equilibrium at 1 atm from both above and below.
  • Some participants discuss the implications of air pressure and gravity, suggesting that if both pressures are equal, the net force should be gravity, which would pull the liquid down.
  • One participant introduces the concept of creating a low-pressure area when the water starts to drop, leading to an imbalance in forces acting on the liquid.
  • Another participant mentions that if there is air in the straw, the water level does not rise as much as the straw due to the expansion of air as pressure decreases.
  • Mathematical relationships involving pressure, height, and density are referenced to explain the behavior of the liquid in the straw.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanics of pressure and fluid behavior, and there is no consensus on the precise dynamics at play. The discussion remains unresolved with multiple competing interpretations of the phenomenon.

Contextual Notes

Participants acknowledge that the behavior of the liquid is influenced by factors such as air pressure, gravity, and the compressibility of air, but the specific conditions and assumptions underlying their claims are not fully resolved.

itsthemac
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This might sound trivial, but I've always been curious as to why exactly when you put your finger on the top of a straw in a drink and lift it up, the liquid stays in it. We're studying pressure and fluids right now in my physics class, and I have a general idea that it has to do with the pressure of the outside air pushing up on the liquid in the straw. What I don't understand is exactly how to describe the whole thing. Shouldn't the pressure of the air in between the liquid and your finger be the same as it was before you put your finger on the straw? Does that pressure actually change at all just from enclosing it? And if it doesn't change, then wouldn't the same forces be acting on the liquid that were acting on it before you put your finger on it? thanks in advance
 
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The air pressure comes from the fact that there's the equivalent of 15 pounds per square inch air pushing down due to the air pressure. When you put your finger on the straw, you've cut off the liquid in the straw from all but a few inches of air (whatever sits between your finger and the liquid). The air pressure from the top drops to almost nothing whereas the air pressure from the bottom is still the ol 1atm pushing up.
 
Pengwuino said:
The air pressure comes from the fact that there's the equivalent of 15 pounds per square inch air pushing down due to the air pressure. When you put your finger on the straw, you've cut off the liquid in the straw from all but a few inches of air (whatever sits between your finger and the liquid). The air pressure from the top drops to almost nothing whereas the air pressure from the bottom is still the ol 1atm pushing up.

But if the air pressure at the top drops to almost nothing, shouldn't the water in the straw move upwards towards your finger a little bit the second you put your finger on the straw? I would also think that this would mean that if you tipped the straw upside-down (while keeping your finger on it), that the water would fall towards your finger. But this doesn't happen.
 
itsthemac said:
But if the air pressure at the top drops to almost nothing, shouldn't the water in the straw move upwards towards your finger a little bit the second you put your finger on the straw? I would also think that this would mean that if you tipped the straw upside-down (while keeping your finger on it), that the water would fall towards your finger. But this doesn't happen.


There's air between your finger and the water, it can't be crushed anymore than if it were in a balloon!
 
itsthemac said:
But if the air pressure at the top drops to almost nothing, shouldn't the water in the straw move upwards towards your finger a little bit the second you put your finger on the straw? I would also think that this would mean that if you tipped the straw upside-down (while keeping your finger on it), that the water would fall towards your finger. But this doesn't happen.

Wait wait no no, I didn't think that one through. The pressure doesn't drop to 0, it stays the same. Except now it's in equilibrium being pushed from below at 1atm and from above at 1atm.
 
Pengwuino said:
Wait wait no no, I didn't think that one through. The pressure doesn't drop to 0, it stays the same. Except now it's in equilibrium being pushed from below at 1atm and from above at 1atm.

but if they're both at 1 atm, then shouldn't the force caused by the pressure pushing up on the liquid from the bottom and the force caused by the pressure pushing down on the liquid from the air in between the liquid and your finger cancel out? which would mean that the net force acting on the liquid in the vertical direction would just be the force of gravity? which should just pull it down and out of the straw?
 
itsthemac said:
This might sound trivial, but I've always been curious as to why exactly when you put your finger on the top of a straw in a drink and lift it up, the liquid stays in it. We're studying pressure and fluids right now in my physics class, and I have a general idea that it has to do with the pressure of the outside air pushing up on the liquid in the straw. What I don't understand is exactly how to describe the whole thing. Shouldn't the pressure of the air in between the liquid and your finger be the same as it was before you put your finger on the straw? Does that pressure actually change at all just from enclosing it? And if it doesn't change, then wouldn't the same forces be acting on the liquid that were acting on it before you put your finger on it? thanks in advance

This is a good puzzle.

First, let's say there is *no* air space- the water completely fills the straw. Then, you can pull the straw up about 400 inches (atmospheric pressure is 408 inH2O = 760 mmHg = etc.) before a small vapor bubble will appear in the top: that's how Torricelli's experiment worked.

The key concept is that the pressure in the water anywhere along the length of the straw is given simply by the height. Once the pressure gets to zero (or the vapor pressure of water), the water will vaporize. Drawing the column up further will only make a larger region of vacuum.

Now we do this again, but there is a small region of air already present. There's no substantive difference between this case and the above- the pressure in the air bubble will go down as the straw is raised, but the water column will rise for the same reason it did before. Conversely, the pressure in the bubble will increase if the straw is submerged. The size of the bubble will change as the straw is raised and lowered because the pressure changes and air is compressible.
 
itsthemac said:
which would mean that the net force acting on the liquid in the vertical direction would just be the force of gravity? which should just pull it down and out of the straw?
The moment the water starts to drop, it creates a low pressure area at the top. The force on the water is no longer in equilibrium. There is 1atm pushing up, but none pushing down. This is enough to compensate for gravity.
 
Note, if there's air in the straw, then the water level doesn't quite rise as much as the straw, because the air in the straw expands as it's pressure decreases. It's a very small amount in the case of a straw, but if this were a huge tube, 20 meters tall, there would be a large decrease in pressure and a bit increase in expansion of the air.

The pressure of the water inside the straw at the same height as the top of the water outside the straw is 1 atm, and the pressure above that point decreases according to the formula, pressure(height) = 1 atm - ( density (of the water) x gravity (eg, 9.8m/s^2) x height (above 1 atm point) ).
 

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