Funny balloon thought experiment

[belliott4488]

... For the sake of argument, let’s just consider a hypothetical liquid such as hypothetical vacuum pump oil that does not boil/does not vaporize at any low pressure. ...

Let’s start at atmospheric pressure at the upper surface, say 15 psia for the sake of argument, and the balloon is at the top of the container. ...

As we pull the balloon down 2 feet, we might think the balloon shrinks due to increased head. But if it did, we’d need to come up with that volume somewhere else in our system (ie: We may think there's an additional volume = V_v (vacuum volume)).

-> If a vacuum space formed above the liquid, and assuming Bernoulli’s still holds (which it does) then the surface pressure would be 0 psia, ...

In other words, if the surface pressure drops instantly from 15 psia to 0 psia, because there's a vacuum there, we have a sudden change in pressure ALL THE WAY DOWN to the bottom of the container.

Okay, that was a pretty good set-up - well thought-through - but I see a critical step where I think your reasoning goes astray. Not surprisingly, it's right where you say "the surface pressure drops instantly from 15 psia to 0 psia". Discontinuous changes like that are usually a sign that something needs be looked at carefully, in case you're side-stepping a process that must be considered in more detail (sometimes it's okay, and the change is just due to a simplifying assumption that does not affect the issue being investigated).

First, where does the 15 psi pressure come from at the start? We might consider that there's a tiny amount of gas at this pressure above the fluid (I know you didn't, but just bear with me for a moment). In that case, it's clear what happens when the balloon is drawn down - the level of the fluid drops, causing the volume of the tiny amount of gas to increase, its pressure decreases, Bernoulli applies, and the rest is as you've described. The final state is such that the pressure of the tiny amount of gas has been reduced to whatever value is necessary to balance all the equations.

Now, what if there is no gas at all, other than what's inside the balloon, as I believe you had in mind? Well, if there is 15 psi of pressure on the surface of the fluid, the only thing that can be exerting this pressure is the top of the container where it contacts the fluid. It cannot exert such a pressure unless it is less than perfectly rigid, which means that as the balloon is lowered and the level of the surface of the fluid drops, the top of the container will flex downward, exerting less pressure (think of it as an extremely stiff spring). It will do this until it reaches its mechanical equilibrium point, i.e. it is exerting zero pressure, after which time the vacuum appears.

This might seem like an unnecessarily arcane detail - we don't usually worry about the sides of a sealed vessel flexing - but if you insist that there was 15 psi on the surface of the fluid at the start, then I believe you must take this into consideration.

 

[Doc Al]

Now, what if there is no gas at all, other than what's inside the balloon, as I believe you had in mind? Well, if there is 15 psi of pressure on the surface of the fluid, the only thing that can be exerting this pressure is the top of the container where it contacts the fluid. It cannot exert such a pressure unless it is less than perfectly rigid, which means that as the balloon is lowered and the level of the surface of the fluid drops, the top of the container will flex downward, exerting less pressure (think of it as an extremely stiff spring). It will do this until it reaches its mechanical equilibrium point, i.e. it is exerting zero pressure, after which time the vacuum appears.

This might seem like an unnecessarily arcane detail - we don't usually worry about the sides of a sealed vessel flexing - but if you insist that there was 15 psi on the surface of the fluid at the start, then I believe you must take this into consideration.

The way I see it is that the pressure applied by the top (and sides) of the container compresses the water ever so slightly. But the assumptions that the container is rigid and the water is incompressible mean that the change in volume is microscopic and can be ignored. So "for all practical purposes" the water level (and balloon volume) won't change until that top pressure is reduced to zero. (It doesn't happen instantly.)

 

[Q_Goest]

Okay, that was a pretty good set-up - well thought-through - but I see a critical step where I think your reasoning goes astray. Not surprisingly, it's right where you say "the surface pressure drops instantly from 15 psia to 0 psia". Discontinuous changes like that are usually a sign that something needs be looked at carefully, in case you're side-stepping a process that must be considered in more detail (sometimes it's okay, and the change is just due to a simplifying assumption that does not affect the issue being investigated).

First, where does the 15 psi pressure come from at the start? We might consider that there's a tiny amount of gas at this pressure above the fluid (I know you didn't, but just bear with me for a moment). In that case, it's clear what happens when the balloon is drawn down - the level of the fluid drops, causing the volume of the tiny amount of gas to increase, its pressure decreases, Bernoulli applies, and the rest is as you've described. The final state is such that the pressure of the tiny amount of gas has been reduced to whatever value is necessary to balance all the equations.

Now, what if there is no gas at all, other than what's inside the balloon, as I believe you had in mind? Well, if there is 15 psi of pressure on the surface of the fluid, the only thing that can be exerting this pressure is the top of the container where it contacts the fluid. It cannot exert such a pressure unless it is less than perfectly rigid, which means that as the balloon is lowered and the level of the surface of the fluid drops, the top of the container will flex downward, exerting less pressure (think of it as an extremely stiff spring). It will do this until it reaches its mechanical equilibrium point, i.e. it is exerting zero pressure, after which time the vacuum appears.

This might seem like an unnecessarily arcane detail - we don't usually worry about the sides of a sealed vessel flexing - but if you insist that there was 15 psi on the surface of the fluid at the start, then I believe you must take this into consideration.

An air bubble or gasseous volume of any sort is not needed at the top of the container. Consider for example, a perfectly rigid container filled with a perfectly incompressible liquid. Now we put this 15 psia balloon in, just under the surface and cap it so there's no air or gas of any kind inside. What's creating the 15 psia at the surface? Well, the balloon is! If we imagine taking out 1 molecule of liquid, the balloon volume increases by some infintesimal amount along with a corresponding infintesimal decrease in pressure. The entire system is completely stable for minor changes in the container's volume. There's no need to hypothesize about an extremely stiff spring. We can make the container infinitly rigid and the liquid infinitely incompressible. There's no details here that need clarification because of the rigidity of the container nor the incompressibility of the liquid. The balloon's ability to accommodate small changes in volume can account for that. As the balloon is pushed down, the pressure at the upper surface changes gradually because the balloon's volume is able to take up any hypothesized miniscule variations in pressure at the upper surface.

 

[Phrak]

A very interesting phenomena. It would be nice to see a simple experimental arrangement As a balloon in a sealed, constant volume container is pulled downward, the pressure in the balloon remains nominally constant and the pressure of the container drops.

As long as the pressure at the top of the container remains above vapor pressure, is this correct by you, Q Goest?

 

[Phrak]

Nice probelm Physical1, to keep so many (myself included) scratching their heads!

The result is very counterintuitive. How could a little bubble pushed to the bottom of a container cause a large change in pressure?

It might be more intuitive to consider the following. You have a large rigid container 32 feet deep. It's filled with pure water and caped, without any air, at one atmosphere of pressure. Someone kicks the container loosening a bubble loose from a crack in the bottom. It rises, but doesn't significantly expand. The pressure rises to twice atmospheric pressure to keep the bubble at it's original volume. I find this scenario more fathomable.

Very small bubbles shouldn't have the same effect, due to the compressibility of water and expansion of the container.

 

[Q_Goest]

A very interesting phenomena. It would be nice to see a simple experimental arrangement As a balloon in a sealed, constant volume container is pulled downward, the pressure in the balloon remains nominally constant and the pressure of the container drops [as] long as the pressure at the top of the container remains above vapor pressure, is this correct by you, Q Goest?

Yes, that's correct. There are other phenomena that will affect this however, such as disolved gas and the liquid's bulk modulus, but what you're saying here is essentially correct.

 

[Q_Goest]

It might be more intuitive to consider the following. You have a large rigid container 32 feet deep. It's filled with pure water and caped, without any air, at one atmosphere of pressure. Someone kicks the container loosening a bubble loose from a crack in the bottom. It rises, but doesn't significantly expand. The pressure rises to twice atmospheric pressure to keep the bubble at it's original volume. I find this scenario more fathomable.

Very nice! You're quite right, the bubble at the bottom of an incompressible liquid inside an infinitely rigid container rises in that container at a constant volume. Regardless of how high the container is (it can be infinitely tall) then assuming there's no vacuum space at the top of the container, the bubble will not increase in volume as it rises.

 

[Phrak]

Very nice! You're quite right, the bubble at the bottom of an incompressible liquid inside an infinitely rigid container rises in that container at a constant volume. Regardless of how high the container is (it can be infinitely tall) then assuming there's no vacuum space at the top of the container, the bubble will not increase in volume as it rises.

Somebody oughta tell Cyrus. He left on an early flight.

 

[Dadface]

Yes, that's correct. There are other phenomena that will affect this however, such as disolved gas and the liquid's bulk modulus, but what you're saying here is essentially correct.

So let us now consider the dissolved gases, the bulk modulus, the hysteresis properties of the rubber,the speed of descent, etc etc.Just kidding.

 

[pallidin]

So, a bowling ball dropped into a swimming pool will cause the water level to rise.

However, is the water-level rise slightly less when the bowling-ball is just 1-ft underneath the surface versus the ball being at, say, 12-ft underneath the surface?

 

[physical1]

I would start out with a system under 34 feet so I can build something on my stairways in my first home I just got (mortgage free, baby!). Well I may be able to make a system over 34 feet by going out windows with some tubes or pipes.

It is interesting to consider systems 500, 1000, or even 10,000 metres high too, even if the experiment can not be done right away - it would be nice to predict what would happen in those cases.

If water started boiling then it will also take up lots of space - causing... the balloon to collapse even more, and maybe even causing a sterling engine to spin since water needs to extract something in order to boil and gain that molecular movement...

Ultimately what would be interesting is if someone, maybe even myself, could invent a device based on this experiment.

I suspect I was having troubles getting water to boil in a syringe due to lacking a trigger air bubble or a mineral/impurity. I have now read that to get water boiling in a syringe, one needs to snap back the syringe and let things move around a bit, or let a dissolved air bubble pop out of place. Maybe water can therefore be "super steam" before it boils, similar how water can be super cooled and not turn into ice!

I got reading about how water surface actually has a sort of vapor existing above it (water liquid is therefore not just a liquid, to be anal retentive about it - it is a bit both a gas and a liquid in liquid state - therefore it is not a binary 0 or 1). Then I started looking into mercury barometers and they refreshed my memory that in fact even mercury has vapor sitting above it. I can see why some of the quacks are worried about mercury fillings in teeth so much.

I am going to try the balloon experiment with some simple materials. From the day I thought this up, I had planned to take a trip to the dollar store and purchase the materials required.

Some other simpler experiments I have done in the mean time:

1. fill up a 2 litre plastic p.e.t. pop bottle with water
2. squeeze pop bottle and let some water go out the top. Hold the squeeze still.
3. put hand palm tightly on the bottle opening, sealing it off
4. release the "squeeze"
5. bottle stays deformed in the squeezed state

Why wouldn't there be a vacuum space at the top of the container that is formed. Because the bottle "gives way" first before the vacuum space even has a chance to form. The vacuum is transmitted throughout the bottle, and takes the least path of resistance - which is holding the most flexible part of the bottle in deformed imploded state. If there was a balloon in the bottle, I suspect it would latch on to the balloon and pull it. Actually, this is all incorrect technically - since vacuum does not "suck". But you understand what I mean - suction is just an abstraction anyway. (Actually I could try and educate housewives that vacuums do not suck up dirt... forget it ;-) first I need to find one to prey on)

That is why I had trouble seeing how a balloon would just collapse under hydrostatic pressure in a sealed system - because the balloon can expand and contract according to how much vacuum is in the entire system. I think the balloon would more easily expand back to its original size than say a vacuum space being created of vapor/nothing. Why? For the same reason that a plastic pop bottle deforms under vacuum. So the balloon, to me just couldn't change size due to battling itself. I could be wrong and I am not happy until the experiments are completed!

What if we considered the balloon as "part of the container". I.e. consider it an "easily deformable portion of the container", similar to how a p.e.t. plastic bottle is deformable under vacuum. Different way of thinking about it. In the p.e.t pop bottle the container gives way fairly easily to any sort of vacuum. A balloon is even more sensitive and gives way easily.

Is it a matter of a fight for the path of least resistance. In a non rigid container, pop bottle "gives way" easier than water vaporizes/boils into a vacuum. In a rigid container a balloon trapped inside does what? Similar? Is the intra molecular bond strength quite strong, more strong than another path of lesser resistance (deforming something sitting in the container.. and if we can transmit throughout the container according to Pascal, what gives way first?)

Honestly, I am about to stop making any more hypothesis(es). I have had enough! I must experiment now. Experiments to me are a generally a waste of time. I would much prefer to have it done in the mind. But in this experiment the mind only seems to lie, cheat, and mislead. Begin the experiments ASAP.

I try to add some humor to my posts so I do not come off as a total jerk. Sorry for any flames/heat that this post has caused. Cheers to your p.e.t. pop and sparkling water bottles. Bonk!

 

[physical1]

A very interesting phenomena. It would be nice to see a simple experimental arrangement As a balloon in a sealed, constant volume container is pulled downward, the pressure in the balloon remains nominally constant and the pressure of the container drops.

If one places a turbine in the system, then he could extract power from the change of pressure? How does a conservation of energy apply? Something cools down? Temperature? Stealing energy from the atmosphere with the help of gravity, giving Tesla a big spin in his grave?

Disprove what I say - I encourage it.

It takes energy to push the balloon down - but sorry folks - I have a valve in mind. A valve can switch the hydrostatic pressure off and on, controlling the "height" of the system immediately. The balloon does not need to be drawn down with energy. It can be placed at a low level, and the water above it can be connected to this lower level container through a valve. The valve let's massive amounts of hydrostatic pressure be applied to the balloon immediately on an instant. To "recharge" the depressurized water, one might open the container to the atmosphere. The valve that connects to the lower portion of the system where the balloon is sitting is closed off before recharging. This isolates the balloon area from the recharging portion of the container: the upper water (hundreds of litres).

When recharging the low pressure water, the atmosphere pressure finds its way into a low pressure zone. Water and atmosphere find equilibrium. The upper portion container is sealed after recharging, and the valve is opened again to let the hydrostatics contact the balloon in the lower portion of the system.

The balloon is tied on a short string to a weight, to not float up, yet exposing its edges to hydrostatic pressure as much as possible. (does a balloon have edges? hehehe)

Above could be missing some important science that causes this system to fail, form icicles and frost, overheat, or simply "not work at all". On the other hand, refridgerators, heaters, pressure driven turbines, weather changing tools, and an endless amount of items for humanity could be created if this has any potential at all (a few puns on potential here).

Pressure is stolen from atmosphere potential? Any temperature changes or environmental effects?

Tesla where are you? Roll 3 times if you heard me, and get a few humans to prove me wrong please.

 

[Cyrus]

Somebody oughta tell Cyrus. He left on an early flight.

I'll read through Q_Goest's reply when I get some time later in the week or two.

 

[physical1]

What makes nature choose the container to become lower pressure, and not the balloon become higher pressure? Does pressure naturally want to decrease rather than increase?

For the balloon to become higher pressure in the constant volume, it would have to become hotter. Does nature not heat balloon because heat would have to be added by some external source, and cannot be just extracted spontaneously from the surrounding water? Does it not have any reason to want to heat up, or it simply can't due to thermodynamics laws?

The high pressure water molecules are ramming against the balloon which is transmitting through rubber, which rams against the air molecules. Are the air molecules affected at all by this ramming of water molecules against the balloon shell.

 

[EEstudentNAU]

If one places a turbine in the system, then he could extract power from the change of pressure? How does a conservation of energy apply? Something cools down? Temperature? Stealing energy from the atmosphere with the help of gravity, giving Tesla a big spin in his grave?

Disprove what I say - I encourage it.

It takes energy to push the balloon down - but sorry folks - I have a valve in mind. A valve can switch the hydrostatic pressure off and on, controlling the "height" of the system immediately. The balloon does not need to be drawn down with energy. It can be placed at a low level, and the water above it can be connected to this lower level container through a valve. The valve let's massive amounts of hydrostatic pressure be applied to the balloon immediately on an instant. To "recharge" the depressurized water, one might open the container to the atmosphere. The valve that connects to the lower portion of the system where the balloon is sitting is closed off before recharging. This isolates the balloon area from the recharging portion of the container: the upper water (hundreds of litres).

When recharging the low pressure water, the atmosphere pressure finds its way into a low pressure zone. Water and atmosphere find equilibrium. The upper portion container is sealed after recharging, and the valve is opened again to let the hydrostatics contact the balloon in the lower portion of the system.

The balloon is tied on a short string to a weight, to not float up, yet exposing its edges to hydrostatic pressure as much as possible. (does a balloon have edges? hehehe)

Above could be missing some important science that causes this system to fail, form icicles and frost, overheat, or simply "not work at all". On the other hand, refridgerators, heaters, pressure driven turbines, weather changing tools, and an endless amount of items for humanity could be created if this has any potential at all (a few puns on potential here).

Pressure is stolen from atmosphere potential? Any temperature changes or environmental effects?

Tesla where are you? Roll 3 times if you heard me, and get a few humans to prove me wrong please.

Could you draw a diagram of this?

 

[physical1]

Could you draw a diagram of this?

I will try make one soon, maybe even an animated drawing showing how a machine could work based on it. First I am performing some experiments to verify what actually happens in nature with the balloon. If I make a machine drawing up and it is all based on assumptions and bad science, it would be just crackpottery.

The experiments I did yesterday used a valve, custom connections, garden hose, and a p.e.t. 2L sparkling water bottle. Not the best setup since garden hoses and p.e.t. bottles are not so rigid. Still it brought back some results. Until I set up the rigid pipes though, I cannot say for certain what happens physically in nature. I will say, that it appears the balloon doesn't change size (significantly). The garden hose appeared to contract slightly and this threw everything off a bit. When I tried it in an open system situation the balloon contracted a massive and noticeable amount. With the closed system situation and a partly rigid container (garden hose and p.e.t. bottle), the balloon contracts a tiny amount and I think it is from the garden hose contracting.

If the balloon is not changing significantly in a partly rigid system (almost good enough) - then the container pressure must be decreased (similar to how water can hold static pressure, maybe in this case it holds negative static pressure below atmosphere), or the balloon temperature must increase in order to maintain equilibrium. Or, somehow the balloon just acts like a steel ball would (not affected by pressure) and I doubt this since the balloon air is still exposed via soft rubber surface. I have not measured temperature or exact pressure changes yet. Need more materials and time.

I would like to know why nature would choose one or the other and whether we can predict which one it usually or most definitely chooses. By one or the other, I mean either temperature of the balloon must increase to maintain constant volume and increase pressure, or the pressure of the container must drop. Is temperature increase in the balloon a no-no? Nature just wouldn't do that? Why? What law? What causes nature to choose one or the other and why do we know it is one and not the other? Some common behavior we know of?

One interesting tid bit to think about is - where would a "loss of container pressure" go to and come from? Was it transferred from the static pressure of water in the container (static pressure is 0, or atmosphere). And if so, where did this pressure disappear to? Would it be converted into molecular movements holding everything together in the container, a glue like strength? As the pressure of the container drops, the water molecules steal some "pressure" and convert it to intra molecular glue/quantum movements? If the pressure is stolen, it is no longer pressure, so pressure was converted again into WHAT exactly? Molecular stretching/binging/ramming/glue?

 

[Stan_Smith]

I do believe that waiting beside the 1000m tall container on Physical1’s workbench, is another can of worms. This particular can may be sprung as intense pressure causes condensation of the gaseous components of the air in the balloon.