Question about the difference of pressure between two point in a liquid

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Discussion Overview

The discussion revolves around the concept of pressure differences in a liquid, particularly in a cylinder filled with water. Participants explore how pressure affects fluid movement, the principles of hydrostatic equilibrium, and the behavior of water in hydraulic systems. The conversation includes theoretical and conceptual aspects of fluid dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why water does not flow from a region of higher pressure (B) to a region of lower pressure (A) in a static cylinder, despite the pressure difference.
  • Another participant suggests that if a tube is inserted into the cylinder at B and bent upwards, water would flow to A due to the height difference, indicating that pressure differences can drive fluid movement under certain conditions.
  • Some participants discuss the concept of static equilibrium, noting that the pressure difference supports the weight of the liquid and prevents upward acceleration of the liquid at B.
  • There is a mention of hydrostatic equilibrium, where the pressure increases with depth due to the weight of the liquid above.
  • One participant raises a question about the movement of water to fill empty spaces, suggesting that this could explain upward flow to tanks when there is no obstruction.
  • Another participant discusses the role of liquid weight in hydraulic systems, questioning why it is not explicitly counted in pressure calculations.
  • There is a clarification that the term pgh in pressure calculations accounts for the weight of the liquid per unit volume.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of water under pressure differences, with some emphasizing static equilibrium and others exploring fluid movement dynamics. The discussion remains unresolved regarding the specific conditions under which water flows in hydraulic systems and the implications of pressure differences.

Contextual Notes

Participants reference concepts such as hydrostatic equilibrium and pressure calculations in hydraulic systems, but there are unresolved questions about how these principles apply in different scenarios, particularly regarding fluid movement and pressure transmission.

Who May Find This Useful

This discussion may be useful for individuals interested in fluid dynamics, hydrostatics, and the principles governing hydraulic systems, including students and professionals in physics and engineering fields.

kakuta
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Hi guys, sorry if the question seems too basic or simple, but i can't seem to grasp the concept about pressure here.

say, a cylinder filled with water;
------------
°A


°B

------------

at point A, the liquid pressure is lower than it is at point B ,
as there's a difference in pressure between the two region, why wouldn't the water at region B flows up to region A due to its higher pressure ?

from what I've read in my reference book,
water from a high region (reservoir ) flows down to a lower region(ground level) is able to flow back up to water tanks (above ground lvl) at our houses due to the high pressure at lower region when water flows down from reservoir at a great height,
in accordance to p=ρhg

have i got pressure and force mixed up ?

why wouldn't water at the bottom pushes upward in a cylinder/bottle due to greater pressure under it ?
at the same time, why would water flow upwards to water tanks located above the ground lvl?
is it because of the great height difference between reservoir and ground level hence the great pressure?
how does water remain still in a cylinder with all that pressure difference?

sorry again for my grammar and phrasings , and thanks in advance for all the help !
 
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If you put a tube through the cylinder wall at B, sealed it and bent it upward to the top of the cylinder, the water would flow up the tube, to A and up to water level.

This is how your water can flow upward, because the water level (at the reservoir) is higher than your tanks.

Water is moving around in the cylinder but any water that may flow from A to B, will quickly be replaced by more water.
 
Last edited:
What about water flowing from B to A seeing how the pressure at B is greater than at A which causes a movement due to the difference in pressure? Does something like that happens in water?
Until it reaches state of equillibrium or some sort
 
kakuta said:
What about water flowing from B to A seeing how the pressure at B is greater than at A which causes a movement due to the difference in pressure? Does something like that happens in water?
Until it reaches state of equillibrium or some sort

The pressure difference just supports the weight of the liquid between A and B. The system is in static equilibrium.
 
Chestermiller said:
The pressure difference just supports the weight of the liquid between A and B. The system is in static equilibrium.

In other words, pressure in liquid is present to support the weight of the liquid above it hence the pressure increases when the depth increases as weight of water increase? Is it safe to say that way?
 
There may be a pressure difference, but the liquid at B does not accelerate upwards because there is no resultant force. The force due to a pressure difference is balanced by the weight of the liquid under gravity. This is the hydrostatic equilibrium.
 
kakuta said:
In other words, pressure in liquid is present to support the weight of the liquid above it hence the pressure increases when the depth increases as weight of water increase? Is it safe to say that way?

Yes. This is exactly what is happening. As MikeyW pointed out, this is called hydrostatic equilibrium.
 
However water moves to fill up empty spaces/air as there is no weight of liquid obstructing it , hence able to flow upwards to the tank?

Ahhh think that clears up for me, thanks guys!
 
kakuta said:
What about water flowing from B to A seeing how the pressure at B is greater than at A

Yeah sorry I said that backward.
The inherent motion of the the water molecules in the cylinder is random. They can move from B to A but there is no "current" from higher to lower pressure in the cylinder.
 
  • #10
Thanks!
Another ques, if u guys don't mind,
Why don't we count in the liquid weight/phg when counting the pressure in a basic hydraulic system?
 
  • #11
I'm not sure what you mean when you say "count". The pg term in pgh is essentially the weight of liquid per unit volume, so it is counted in that way.
 
  • #12
Say in a hydraulic system, the pressure transmittted by a small cylinder is 50pa , hence 50pa is transmittted to the large cylinder. However there's a difference in height between the piston of small cylinder and the large cylinder,
The larger cylinder being at the shorter end.
In that case, why don't we add the pressure difference(counted using phg) + transmitted pressure onto the large piston?
 

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