The nature of water pressure?

[Big-t-2538]

Now, since density of water is not a factor (constant), the cause of the pressure is different. You can have air pressure in a weightless environment but you cannot have water pressure. In a gas, gravity -> density -> pressure via molecular impacts, in a liqued gravity -> pressure, the force from molecular impacts can be ignored (extremely low RMS speeds).

I'm now following you on the reason for water pressure comming from all directions, that's what I never got about the whole weight of the column idea - why it pushes from all directions.

I'm not following your confusion...in a column of water....pick any point....the pressure at that point is exactly density*gravitational constant*height of the liqiud. Why would it matter what direction you are looking....the fluid is all around that point, thus the pressure exerted at that point comes from all directions.

 

[loosebits]

I can fill one balloon with helium and an identical balloon with argon and the pressure and temperature will be identical while the density will be quite a bit different.

The whole density argument requires like gasses. If you want to talk about comparing seperate gasses, we need to look at a numberic density, not a weight denisty (a helium atom weighs much less than an argon atom). Instead of Kg/L, we need to talk about # of molecules/L (moles/L). I'm sorry, I should have stated that. In all my posts where I mention density, understand that I mean numeric density.

Don, the pressure exerted by a gas IS caused by molecular impacts. This is where the gas laws come from. If I increase the number of molecules in a space, more are going to hit a surface in any given time causing more total force, if I speed up the molecules (by heating them) in a space, each impact has a greater force. Either way the pressure increases. This is why I can capture air at 1 atm and it will always be at 1 atm (assuming constant temp) no matter what the surrounding external pressure is. I let gravity compress it for me to 0.045 moles/L (a mole of molecules is 6.02 * 10^23 molecules, see Avogadro's Number, a mole of any gas takes up 22.4 L of space at 1 ATM, 20 deg C) and then stick it in a bottle. However, I cannot store a liqued at pressure by capturing it at depth and sticking it in a bottle because numberic density (moles/L) does not contribute to the pressure.

 

[loosebits]

Wrong. If the container is absolutely rigid (in theory impossible), the depth gauge will read the same depth no matter where you take it. Water *does* compress, but on such an incredibly small level it's irrelevant.

Huh??? You're saying that if I fill a bottle of water at 99 feet and bring it to the surface, the pressure inside the bottle is still 4 atm absolute???? Think about this for a second, take a weak coke bottle to 100 feet and open it up, let in that high pressure water. If the pressure of the water in the bottle were to remain the same on the way to the surface, the bottle would explode!! This obviously doesn't happen. However, this would happen if I filled the bottle with air at 100 feet with my regulator (is that what you were talk about and I misunderstood?). This is the reason for my conjecture that the nature of water pressure is different than that of a gas, gas pressure can be *stored*, water pressure cannot. If the mechanisims of the two pressures were the same, this would not be possible.

 

[Big-t-2538]

Huh??? You're saying that if I fill a bottle of water at 99 feet and bring it to the surface, the pressure inside the bottle is still 4 atm absolute???? Think about this for a second, take a weak coke bottle to 100 feet and open it up, let in that high pressure water. If the pressure of the water in the bottle were to remain the same on the way to the surface.

O.K...I think I see what you're missing.

Let's assume you close that bottle at 100' full of water, and it's a rigid container (All external forces do not effect said container). Let's say this bottle is a Sphere with a diameter of 1 foot. What is the pressure inside the sphere that's now full of water?

It no longer matters where you take this sphere, the pressure in the sphere will remain the same.

 

[Don Burke]

This is why I can capture air at 1 atm and it will always be at 1 atm (assuming constant temp) no matter what the surrounding external pressure is.

incorrect Blow up a balloon at sea level and take it to one thousand feet. It is no longer 1atm.

I let gravity compress it for me to 0.045 moles/L (a mole of molecules is 6.02 * 10^23 molecules, see Avogadro's Number, a mole of any gas takes up 22.4 L of space at 1 ATM, 20 deg C) and then stick it in a bottle. However, I cannot store a liqued at pressure by capturing it at depth and sticking it in a bottle because numberic density (moles/L) does not contribute to the pressure.

Actually, a rigid container sealed at depth will continue to hold (or attempt to hold) the pressure.

You've built a house of cards based on density change and it caves in when the density change is not available. There's a message there. Your initial premise is flawed.

 

[Don Burke]

You're saying that if I fill a bottle of water at 99 feet and bring it to the surface, the pressure inside the bottle is still 4 atm absolute????

That is correct.

Think about this for a second, take a weak coke bottle to 100 feet and open it up, let in that high pressure water. If the pressure of the water in the bottle were to remain the same on the way to the surface, the bottle would explode!!

If the pressure isn't there, why does the bottle expand?

This obviously doesn't happen.

Actually, it does. In fact, I wouldn't bet against a Coke bottle being able to hold 44psig. That carbon dioxide can be some pretty energetic stuff.

However, this would happen if I filled the bottle with air at 100 feet with my regulator (is that what you were talk about and I misunderstood?).

Air and gas will do the same thing. The difference is that compressed gas is the gift that keeps on giving. When testing pressure systems, liquid is normally used so that a rupture results in a short spurt of liquid, not an ongoing, meat slicing, jet of gas. In the Coke bottle example, the slight stretch of the bottle is the only result you will see.

This is the reason for my conjecture that the nature of water pressure is different than that of a gas, gas pressure can be *stored*, water pressure cannot. If the mechanisims of the two pressures were the same, this would not be possible.

The pressure can be stored. I can't make it any plainer.

Liquid and gas use the same laws of fluid dynamics.

Using your molecular impact theory to explain this requires changing the laws of physics. It is being misapplied in this case.

 

[loosebits]

incorrect Blow up a balloon at sea level and take it to one thousand feet. It is no longer 1atm.Actually, a rigid container sealed at depth will continue to hold (or attempt to hold) the pressure.

You've built a house of cards based on density change and it caves in when the density change is not available. There's a message there. Your initial premise is flawed.

Yes, I know and agree with the whole balloon thing. The wall of the balloon is not rigid so it will expand. The pressure inside the balloon will always equal ambient.

Is Don and Big-T saying that water pressure can be stored????? This is nonsence, if it weren't you would be warned not to drink any *high pressure* water at depth because your bladder could rupture on the way up (leading to the 2nd rule of diving, always pee continuously).

If I fill a rigid container at depth (regardless of shape), a pressure gauage inside the container will always read ambiant pressure!!! Can't we think of people as containers of fluid? If my fluids are at 1 atm at the surface and are still 1 atm at depth, I would be crushed.

Numerical density (and temperature) being the direct cause of air pressure (regardless of how that density was *formed* be it by gravity or an air compresser) is fundemental to the gas laws. Knowing the numerical density and temperature of a gas, I can actually calculate the pressure of the gas (the ideal gas law, p = n/V * RT, n/V being number of molecules per volume - numerical density, R being a costant and T being the temperature absolute).

All of the other gas laws can be simply derived from the ideal gas law (also know as the equation of state for ideal gasses).

Don, you are confusing filling a container of water at depth and a container of air at depth. Water brought up in a rigid container from 5000 feet will not be under pressure. How do they bring up specimin bottles from extreme depth? Do theu have to put in extremely strong containers? When they open the lid, what happends? Does all of that high pressure water escape. Make sure that when you a are pumping water out of a well that you don't get the high pressure water from the bottom of the well

I'm done arguing the whole bringing up water from depth, if anyone believes the pressure inside the container is constant and doesn't always reflect ambient pressure, I won't be able to convince them without the doing a test - take a soft walled container (such as a balloon) to 33 feet, let some water in it (no air), tie it off and then come to the surface, has the balloon expanded to twice its size? No, it hasn't therefor the water pressure can't be stored (try it with air, you will find that the balloon has expanded).

 

[Big-t-2538]

Is Don and Big-T saying that water pressure can be stored????? This is nonsence, if it weren't you would be warned not to drink any *high pressure* water at depth because your bladder could rupture on the way up

1 - Yes
2 - No it wouldn't.

What is the difference in volume for a change in pressure for a liquid compared to a gas.

Like Don mentioned, when you bring the coke bottle to the surface, it will stretch a marginal amount, as a matter of fact, it will probably go un-noticed....where as if it were a GAS, it would expand a significant amount more....enough to rupture or blow the cap off, whichever comes first.

Maybe that is the point you are missing.

 

[loosebits]

Urgh.

 

[Uncle Pug]

Hahahahahahah... you guys are funny.

Water isn't pressurized (of consequence) at any depth. It transfers pressure but it isn't pressurized. If you enclosed water in a rigid container at 100' it wouldn't be any more pressurized than the water you enclosed at 10'.

Now if you included a little bubble of gas while getting your 100' water that gas would be pressurized. If that little bubble of gas happened to be in the bourdon tube of an SPG then the SPG would maintain its reading of 100' until the gas was allowed to expand and fill a larger space, thus changing the shape of the bourdon tube.