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

1. Nov 14, 2012

### kakuta

Hi guys, sorry if the question seems too basic or simple, but i cant 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 !

2. Nov 14, 2012

### JustinRyan

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: Nov 14, 2012
3. Nov 14, 2012

### kakuta

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

4. Nov 14, 2012

### Staff: Mentor

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

5. Nov 14, 2012

### kakuta

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?

6. Nov 14, 2012

### mikeph

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.

7. Nov 14, 2012

### Staff: Mentor

Yes. This is exactly what is happening. As MikeyW pointed out, this is called hydrostatic equilibrium.

8. Nov 14, 2012

### kakuta

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!

9. Nov 14, 2012

### JustinRyan

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. Nov 17, 2012

### kakuta

Thanks!
Another ques, if u guys dont mind,
Why dont we count in the liquid weight/phg when counting the pressure in a basic hydraulic system?

11. Nov 17, 2012

### mikeph

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. Nov 17, 2012

### kakuta

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 dont we add the pressure difference(counted using phg) + transmitted pressure onto the large piston?