Pressure: Doubts & Explanation - Why Liquid Falls

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Discussion Overview

The discussion revolves around the mechanics of why liquid flows in a siphon, exploring the underlying principles and theories related to pressure differences and weight. Participants engage in clarifying concepts and challenging each other's explanations.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the relationship between atmospheric pressure and liquid flow, suggesting that the phenomenon occurs independently of atmospheric conditions.
  • Another participant proposes that the flow of liquid in a siphon is due to the weight of the water on one side outweighing that on the other side, using a hypothetical analogy with marbles to illustrate the concept.
  • There is a discussion about the role of pressure differences, with one participant asserting that the height of the liquid column is more significant than the volume of the containers involved.
  • Some participants express confusion about the reasons behind the flow, reiterating the question of whether pressure differences are the cause.
  • One participant emphasizes that the siphoning process ceases when the water levels on both sides equalize, indicating a lack of pressure differential at that point.

Areas of Agreement / Disagreement

Participants express differing views on the role of pressure versus weight in the flow of liquid through a siphon. There is no consensus on the primary cause of the phenomenon, as some focus on weight while others emphasize pressure differentials.

Contextual Notes

Participants have not fully resolved the assumptions regarding the influence of pressure and weight, and the discussion reflects varying interpretations of these concepts.

kent davidge
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Hi, I wonder if what I think is in according to the theory in Why the liquid falls. Check out the image I've uploaded. (sorry my bad english)
 

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No, this is nothing to do with atmospheric pressure (the same thing will happen in a vacuum) and the liquid will continue to flow until the surface in each container is at the same level (ie the depth in the lower container is greater).
 
The reason liquid flows in a siphon is because the weight of the water in the left side of the hose outweighs the water in the right side.

Imagine, instead of water, you had a long piece of string in the hose. At the right end of the string there is one marble tied to the string, at the left end, there are TWO marbles tied, weighing twice as much. Discounting friction, the string and marbles will slide into the lower bucket.
 
yes, I know that. But why it happens? Is it not because the difference between the pressure in each container?
 
kent davidge said:
Is it not because the difference between the pressure in each container?
No. Either side (or both) could be a 5 gal pail or an ocean. It would make zero difference.

What would make a difference is the height. A siphon falling 50 feet will be very powerful. A siphon falling 6 inches will be very weak, because there's very little pressure differential. That's also why - when the water level on both sides is equal - the siphoning process stops completely.

kent davidge said:
yes, I know that. But why it happens?
?? It happens because of the weight of water in the pipe.
 
good DaveC. I was typing my comment when you did your post. (sorry my bad english). Thank you.
 

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