The nature of water pressure?

[IslandHopper]

hey, loosebits ....

heated air does not necessarily exert more pressure ... this only happens if it's volume stays the same ... if a given volume of air is allowed to expand as you add energy to it, it will NOT exert greater pressure.

You should have "degreed" in Physics, rather than Chemistry

 

[Drew Sailbum]

The various gas laws are commonly taken as completely accurate, but an underlying assumption explains why these break down in certain circumstances. The faulty assumption is that molecule to molecule collisions can be ignored and only collisions with the container need be considered.

In a liquid, molecules are packed so closely that most collisions are molecule to molecule. Similarly, when gases are under extreme pressure and/or at very low tempertures (ie when they are near the condensation point and close to transitioning to a liquid state), the gas laws start to break down.

 

[WarmWaterDiver]

But the "breakdown" can be addressed using pseudocritical relationships - it still behaves as a fluid, it's just that liquid and vapor phases then become indistinguishable. Transportation of fossil fuel fluids are going on all the time under such pressures, and sometimes even at cryogenic temperatures too (liquified natural gas being one case in point). So, the behavior can still be measured and predicted.

Going back to hydraulic head though - here's something that may help make more sense loosebits. A 100 foot tall vertical water filled soda straw and a 100 foot tall municipal water tank exert exactly the same pressure at the base (same elevation reference point for "bottom"). Similarly, a 100 foot tall soda straw filled with air and a 100 foot tall municipal water tank that is empty of water but open to the atmosphere will again register the same pressure at the base.

Water is pretty much a Newtonian fluid and can be treated as virtually incompressible. Its density doesn't really significantly increase with depth - but the hydraulic head exerted does. Air, on the other hand, is a gas and very compressible (thankfully for this sport), and so its density does increase with decreasing elevation (or decreases with increasing elevation). The fact that water is virtually incompressible is why it is selected for "hydrotesting" media. Only a small amount need escape to lower the internal pressure. However, if one uses a compressible fluid for pressure testing, and a failure occurs, the resulting release is orders of magnitude higher.

Hope that helps!

 

[loosebits]

heated air does not necessarily exert more pressure ... this only happens if it's volume stays the same ... if a given volume of air is allowed to expand as you add energy to it, it will NOT exert greater pressure.

Yes I know that, I was assuming constant volume.

If it wasn't, a barometric altimeter wouldn't work.

The density of the air at sea level is higher than at 50,000 ft. Using a kinetic model of pressure (impacts from molecules), this also explains why altimeters work.

What I still don't quite understand is the whole weight-of-the-column idea. Why does the weight of the column apply force from all sides?

Here is why I think it is not the weight of the column for air: if I am take a rigid container and open it up at 1 atmosphere, the container will fill with air at 1 atm. Now before I let the air in the container, everyone is telling me that the pressure is caused by the weight of the column. Now, inside the container, isn't the pressure the same? If I take the container 50,000 up, isn't the pressure in the container still 1 atm? Now everyone would probably agree that the pressure is being caused by the molecular imacts without gravity being a factor. Doesn't it seem a coincidence that the pressure exerted due to the weight of the air column (before the air was in the containter) is exactly the same as the pressure exerted without the weight of the air column? Hmmmm.

The pressure of a gas is determined by the denisity (and temperature if constant volume) of the gas - if there are more molecules available to strike a surface, the pressure will be greater. The density of the air at sea level is higher because of the *weight* of the column packing it down so we can say that the weight of the column causes pressure but it does so by changing the denisty of the gas.

Since water in incompressible, the mechanics must be different and I am fully willing to accept the who weight-of-the-column idea except I don't understand with that explanation why the force comes from all directions.

I needed to say that the pressure of the water column increases the DENSITY, and lowers the VOLUME of any given air space at that depth.

You kind of hit on the whole density thing but *how* does the water column exert this pressure? I can capture a can of air at 1 ATM, take it into a vacuum and I still have a can of air at 1 atm. If I capture a bottle of water at 5 atm and take it to sea level, I no longer have a bottle of water at 5 atm. This is why I say that the nature of the pressure of the two fluids are different.

 

[loosebits]

WarmWaterDiver, I think you may have an idea of what's confusing me. I have no problem with the idea that the water pressure on my finger at the end of a straw is due to the weight of the water in the straw (afterall, my finger is what's holding it aginst gravity and keeping it from falling out of the straw). What I don't understand is why does a body suspended in the column feeel pressure from all sides. Where is the force pushing from the bottom comming from?

 

[Don Burke]

WarmWaterDiver, I think you may have an idea of what's confusing me. I have no problem with the idea that the water pressure on my finger at the end of a straw is due to the weight of the water in the straw (afterall, my finger is what's holding it aginst gravity and keeping it from falling out of the straw). What I don't understand is why does a body suspended in the column feeel pressure from all sides. Where is the force pushing from the bottom comming from?

Thats basic hydraulics. A fluid under pressure exerts force in all directions. That's why I can jump on the waterbed and launch my cat.

 

[jonnythan]

Yes I know that, I was assuming constant volume.



The density of the air at sea level is higher than at 50,000 ft. Using a kinetic model of pressure (impacts from molecules), this also explains why altimeters work.

What I still don't quite understand is the whole weight-of-the-column idea. Why does the weight of the column apply force from all sides?

I assure you, it is due to the weight of the air directly above you. You are literally holding up the column of air above you, and the air on the sides of you is getting squeezed into the space you occupy by the air above it. It is most definitely gravity. Gravity pulls the air molecules to the surface, and something has to hold them up. All that air weighs quite a bit - it weighs 14.7 pounds per square inch, or almost 1 ton per square foot.

The pressure is quite literally from molecule impacts.. and the more dense the gas is for a given temperature, the more impacts there are. But why is it more dense at the surface? Gravity.

Here is why I think it is not the weight of the column for air: if I am take a rigid container and open it up at 1 atmosphere, the container will fill with air at 1 atm.

That's right. As soon as you close it, the container itself is holding up the column of air above it. As long as the container is open, it's the air inside that's holding up the column of air.

Now before I let the air in the container, everyone is telling me that the pressure is caused by the weight of the column. Now, inside the container, isn't the pressure the same? If I take the container 50,000 up, isn't the pressure in the container still 1 atm? Now everyone would probably agree that the pressure is being caused by the molecular imacts without gravity being a factor.

Like I said, as soon as you close the container, it's the solid container itself that's holding up the air. Once you close that internal system, it's not affected anymore by the air above and around it. The internal pressure is 14.7 psi absolute because of the density of the gas inside.... but the external pressure is 14.7 psi because of the weight of the air above it. Therefore there is a net pressure gradient of 0.

Once you take the container up to 50,000 feet, the external pressure drops because there's less air above it being pulled down to the surface. The internal pressure remains the same.

Doesn't it seem a coincidence that the pressure exerted due to the weight of the air column (before the air was in the containter) is exactly the same as the pressure exerted without the weight of the air column? Hmmmm.

It's not a coincidence. Gravity directly causes the density of the air to be higher at the surface, because of the cumulative effect of the weight of the air above it. The air you "capture" in the container is at the same density, and hence, absolute pressure.

The pressure of a gas is determined by the denisity (and temperature if constant volume) of the gas - if there are more molecules available to strike a surface, the pressure will be greater. The density of the air at sea level is higher because of the *weight* of the column packing it down so we can say that the weight of the column causes pressure but it does so by changing the denisty of the gas.

Since water in incompressible, the mechanics must be different and I am fully willing to accept the who weight-of-the-column idea except I don't understand with that explanation why the force comes from all directions.

The mechanics are actually absolutely the same. Any body that takes up space in water is quite literally "holding" up all of the water above it.

Imagine you have a 30 foot long straw filled with water. You put your finger on the bottom to hold the water in. If the straw end has a surface area of 1 square inch, You're going to have to exert almost 15 pounds of force with your finger to hold it up. Guess what? The water weighs a total of almost 15 pounds.

Now imagine you have a giant aquarium tank with water 30 feet deep, but the opening on the bottom is still one square inch. You'll still have to exert almost 15 pounds to hold the water in.

Now think of it like this - every single square inch of glass on the bottom of that 30 foot deep tank is exerting 15 pounds upwards to hold the water up. Does that make it more clear?

Any neutrally buoyant body or object that's 30 feet deep in the water is experiencing about 15 pounds per square inch of weight from above. However, the water below is pushing back up at 15 psi, and that's why it stays in place. Just like the chair you're on right now. You're exerting 150 pounds (or whatever) onto the seat... but the seat is pushing back up with exactly 150 pounds.

You kind of hit on the whole density thing but *how* does the water column exert this pressure? I can capture a can of air at 1 ATM, take it into a vacuum and I still have a can of air at 1 atm. If I capture a bottle of water at 5 atm and take it to sea level, I no longer have a bottle of water at 5 atm. This is why I say that the nature of the pressure of the two fluids are different.

The difference in the "capturing" is due to the differences in compressibility, that's all. As soon as you close the system (screw the top on the bottle), you've taken away the effects of the weight of the water above and moved them to the container instead.

I hope I've explained this well enough. The key is closing the system and compressibility. I also hope the aquarium example explained a bit too.

Now here's a question for you. Take a clear light canister to depth, open and empty. At 33 feet drop a depth gauge in it and close it. Take it to the surface.

What happens?

 

[Don Burke]

Here is why I think it is not the weight of the column for air: if I am take a rigid container and open it up at 1 atmosphere, the container will fill with air at 1 atm. Now before I let the air in the container, everyone is telling me that the pressure is caused by the weight of the column. Now, inside the container, isn't the pressure the same?

If it was empty, the pressure would be 0psia.

If I take the container 50,000 up, isn't the pressure in the container still 1 atm?

If you are talking about before the air was let in, it would be 0psia. If you equalized at sea level, the rigid container would keep the pressure at 1 atm.

Now everyone would probably agree that the pressure is being caused by the molecular imacts without gravity being a factor.

I wouldn't. The pressure is due to the mechanical stress on the gas inside.

Doesn't it seem a coincidence that the pressure exerted due to the weight of the air column (before the air was in the containter) is exactly the same as the pressure exerted without the weight of the air column? Hmmmm..

Not really. I can drive my truck onto a spring and the spring will compress. If I put a clamp on the spring to hold the compression, I can drive my truck off without the spring recovering. If I then put the clamped spring in a drawer, it doesn't mean my truck didn't compress the spring.

The pressure of a gas is determined by the denisity (and temperature if constant volume) of the gas - if there are more molecules available to strike a surface, the pressure will be greater. The density of the air at sea level is higher because of the *weight* of the column packing it down so we can say that the weight of the column causes pressure but it does so by changing the denisty of the gas.

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 weight of the column causes the pressure which changes the density of the gas. The density change doesn't cause the pressure change.

Since water in incompressible, the mechanics must be different and I am fully willing to accept the who weight-of-the-column idea except I don't understand with that explanation why the force comes from all directions.

That's basic hydraulic theory.

You kind of hit on the whole density thing but *how* does the water column exert this pressure? I can capture a can of air at 1 ATM, take it into a vacuum and I still have a can of air at 1 atm. If I capture a bottle of water at 5 atm and take it to sea level, I no longer have a bottle of water at 5 atm. This is why I say that the nature of the pressure of the two fluids are different.

See above truck and spring example.

 

[loosebits]

It's not a coincidence. Gravity directly causes the density of the air to be higher at the surface, because of the cumulative effect of the weight of the air above it. The air you "capture" in the container is at the same density, and hence, absolute pressure.

Okay, we agree. Air pressure is caused by the density of air. The density of air is caused by gravity. Indirectly, air pressure is caused by gravity (using density).

To answer your question, the pressure gauge will read the same as the one clipped of on your BC. Didn't I already say that?

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.

 

[jonnythan]

Okay, we agree. Air pressure is caused by the density of air. The density of air is caused by gravity. Indirectly, air pressure is caused by gravity (using density).

To answer your question, the pressure gauge will read the same as the one clipped of on your BC. Didn't I already say that?

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.

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.

The density of the air is only a side effect. It's basically incidental. If air were incompressible, you'd still have air pressure because of gravity, just like you do with water. The mechanism is absolutely identical. The weight of the air column pushes in all directions, just like the weight of the water column pushes in all directions. That one is compressible and the other is not is what throws you, and it shouldn't. Gravity causes both air and water pressure, and it's because of the weight of the air above you or the weight of the water above you. That's it.