Trying to understand some basics about Gravity

[Jackamus]

I'm new to this forum and I'm not sure if I have chosen the correct subject heading.
I watched this Youtube video

showing how 2 helium filled balloons sank in a vacuum chamber. The presenter used this to prove that helium would sink in a vacuum. I think he was wrong. It was the balloons that sank because they were heavier than the helium. I suggest that the helium released from the exploded balloon expanded to fill the vacuum but I do not know why. If this is correct then some of the helium molecules would rise acting against gravity. Am I correct?

 

[DrClaude]

It was the balloons that sank because they were heavier than the helium.

But the balloons are filled with helium, and they were floating before the air was removed.

I suggest that the helium released from the exploded balloon expanded to fill the vacuum but I do not know why

Gas diffuses because the molecules keep bouncing against each other and the everything around them (such as the container walls). That process is very fast.

If this is correct then some of the helium molecules would rise acting against gravity.

Yes, some molecules are going up, as there is enough thermal energy for them to fly upwards. This is no different than throwing a ball.

 

[Jackamus]

Thank you Dr Claud for taking the trouble to explain.
But the balloons are filled with helium, and they were floating before the air was removed.
Yes I agree - the weight of the balloon plus the weight of the helium floated in air but when the air is removed the balloon being heavier than the helium sank taking the helium with it. I suspect that the balloon would have sank quicker if there was no helium inside just as the burst balloon did. That is why I asked what would happen if the helium was released?

Yes, some molecules are going up, as there is enough thermal energy for them to fly upwards. This is no different than throwing a ball.
Where does this thermal energy come from and how does it defy gravity? How do you compared that with throwing a ball? Surely a thrown ball rises because it has a greater than gravity force applied to it.

 

[Ibix]

Am I correct?

Sort of. The balloons float in air for the same reason a ship floats on the sea - a volume of air the same shape as the inflated balloon weighs more than the balloon and helium. When you reduce pressure the amount of air in a given volume reduces - so eventually the balloon and helium weighs more than the same volume of (low pressure) air, and the balloon sinks.

You are also correct that the helium from the burst balloon fills the tank, just like air does normally. But this doesn't mean that helium rises against gravity, any more than air filling the tank normally means that air rises against gravity - it just expanded because it was no longer being squashed in by the balloon. If you had enough helium you could cover (e.g.) the moon with a helium atmosphere open to space at the top just like our oxygen/nitrogen atmosphere.

To put it another way, if you had a tank 100km tall and evacuated it, then pumped in helium until you had atmospheric pressure at the bottom, you would still have vacuum at the top because the helium pools at the bottom. It just takes a very tall tank to be able to see it without a balloon to contain the helium.

 

[Ibix]

Where does this thermal energy come from and how does it defy gravity?

The sun keeps the atmosphere warm. If the sun went out, the atmosphere would cool and eventually liquify and we'd have an ocean of liquid oxygen and nitrogen, until that eventually cooled enough to freeze. Everything is subject to gravity.

Surely a thrown ball rises because it has a greater than gravity force applied to it.

No. A thrown ball has no force acting on it once you've let it go, but continues to rise.

 

[Jackamus]

Sort of. The balloons float in air for the same reason a ship floats on the sea - a volume of air the same shape as the inflated balloon weighs more than the balloon and helium. When you reduce pressure the amount of air in a given volume reduces - so eventually the balloon and helium weighs more than the same volume of (low pressure) air, and the balloon sinks.

You are also correct that the helium from the burst balloon fills the tank, just like air does normally. But this doesn't mean that helium rises against gravity, any more than air filling the tank normally means that air rises against gravity - it just expanded because it was no longer being squashed in by the balloon. If you had enough helium you could cover (e.g.) the moon with a helium atmosphere open to space at the top just like our oxygen/nitrogen atmosphere.

To put it another way, if you had a tank 100km tall and evacuated it, then pumped in helium until you had atmospheric pressure at the bottom, you would still have vacuum at the top because the helium pools at the bottom. It just takes a very tall tank to be able to see it without a balloon to contain the helium.

We now seem to be touching on the subject of buoyancy. If what you say is correct why then isn't there a layer of helium at the extremities of the atmosphere since helium rises in air?
According to your 100km tall tower the helium is static with a vacuum at the top.
But earlier you said that the helium molecules would disperse to fill the vacuum in the tank!
What would happen if you pumped in some air at the bottom of the tower? Would it simply push the helium a little higher up the tank still leaving a vacuum at the top? If so why doesn't the helium molecules disperse to fill the remaining vacuum as stated earlier?
If helium molecules can migrate to fill a vacuum then I assume that air molecules will do the same so why then do we still have an atmosphere?But earlier you said that the helium molecules would disperse to fill the tank!

 

[Dale]

Surely a thrown ball rises because it has a greater than gravity force applied to it.

Remember that forces cause acceleration. So to determine the force you cannot look only at the velocity (going up) but you must also look at the acceleration (going up and getting slower means accelerating down).

The presenter used this to prove that helium would sink in a vacuum. I think he was wrong.

Remember, we are talking about forces, so it isn’t about sinking or rising. It is about acceleration.

In a gas you cannot easily track the individual gas molecules to directly see their acceleration. Instead, you need an average measurement that is related to the acceleration: the pressure. The difference in pressure between the top and the bottom is directly related to the acceleration of gravity for the individual molecules.

Although the balloon sinking in the vacuum chamber is cool, it is not necessary. What would have been better is to measure the pressure at the top and the bottom of the helium and show that there was a difference in pressure. This pressure difference is due to the force of gravity acting on the helium.

If helium molecules can migrate to fill a vacuum then I assume that air molecules will do the same so why then do we still have an atmosphere?

We have an atmosphere precisely because gravity does affect a gas, just like any other object.

 

[Jackamus]

Remember that forces cause acceleration. So to determine the force you cannot look only at the velocity (going up) but you must also look at the acceleration (going up and getting slower means accelerating down).

Remember, we are talking about forces, so it isn’t about sinking or rising. It is about acceleration.

In a gas you cannot easily track the individual gas molecules to directly see their acceleration. Instead, you need an average measurement that is related to the acceleration: the pressure. The difference in pressure between the top and the bottom is directly related to the acceleration of gravity for the individual molecules.

Although the balloon sinking in the vacuum chamber is cool, it is not necessary. What would have been better is to measure the pressure at the top and the bottom of the helium and show that there was a difference in pressure. This pressure difference is due to the force of gravity acting on the helium.

We have an atmosphere precisely because gravity does affect a gas, just like any other object.

We have an atmosphere precisely because gravity does affect a gas, just like any other object.

Yes but as previously stated gas molecules will migrate to fill a vacuum chamber at ground level under maximum gravity. That means some of the molecules will rise against gravity. I think it is a density/buoyancy issue. The forces generated both upwards and downwards are dependent upon the density of a substance relative the density of its environment.

 

[Ibix]

If what you say is correct why then isn't there a layer of helium at the extremities of the atmosphere since helium rises in air?

Alcohol is about 4/5ths the density of water, and a container of alcohol would float on water assuming the walls weren't too heavy. But beer does not separate into a thin layer of alcohol on top and water underneath. The same thing is happening here - helium in the atmosphere mixes with the air, but a container of helium floats.

According to your 100km tall tower the helium is static with a vacuum at the top.
But earlier you said that the helium molecules would disperse to fill the vacuum in the tank!

They will fill a small tank, but if you have a sufficiently sensitive barometer you will find that the pressure is lower at the top than the bottom. With a taller tank, the pressure difference at top and bottom is larger. With a tall enough tank (and I do mean kilometers tall) the pressure is zero at the top. Simple altimeters use this principle to measure height above sea level - they are just barometers.

What would happen if you pumped in some air at the bottom of the tower? Would it simply push the helium a little higher up the tank still leaving a vacuum at the top?

You'd have an oxygen-nitrogen-helium mix instead of pure helium. Since you've increased the gas pressure at the bottom you'd have some gas higher up the tower than you did before, and you would find a more helium-rich mixture higher up the tower, but it would just be a mix of the gases.

helium molecules can migrate to fill a vacuum then I assume that air molecules will do the same so why then do we still have an atmosphere?

Because the atmosphere is held down by gravity, the same way a helium atmosphere would be. Some does boil off over time, though (not enough to worry about).

 

[Jackamus]

Thanks Ibix.
Alcohol is about 4/5ths the density of water, and a container of alcohol would float on water assuming the walls weren't too heavy. But beer does not separate into a thin layer of alcohol on top and water underneath. The same thing is happening here - helium in the atmosphere mixes with the air, but a container of helium floats.

What decides if two liquids, say alcohol and water, stay mixed or separate?

 

[russ_watters]

If helium molecules can migrate to fill a vacuum then I assume that air molecules will do the same so why then do we still have an atmosphere?

The box is very small compared to the atmosphere. So for example if you throw a ball up hard in your house it would just hit the ceiling and come back down and you wouldn't be able to see the parabolic trajectory that it has if you throw it up outside.

 

[russ_watters]

What decides if two liquids, say alcohol and water, stay mixed or separate?

Density and miscibility.

 

[Dale]

That means some of the molecules will rise against gravity.

”Rise against gravity” describes the velocity, not acceleration. If you want to understand forces you must focus on acceleration.

I think it is a density/buoyancy issue. The forces generated both upwards and downwards are dependent upon the density of a substance relative the density of its environment.

You are not wrong, but what you are missing is that buoyancy is itself due to gravity. The buoyant force on an object is $\rho g V$ where $\rho$ is the density, $V$ is the displaced volume of fluid, and $g$ is the gravitational acceleration. If $g=0$ then there is no buoyancy

 

[Ibix]

What decides if two liquids, say alcohol and water, stay mixed or separate?

As I recall, it depends on the relative sizes of the molecules and their polarisability. Probably other things too. Gases are much lower density than liquids, though, so you'd have to find some bizarre and extreme conditions to get anywhere where immiscibility could even be a possibility for gases.

This isn't an entirely accurate picture of what's going on in the atmosphere, but I think it's not too bad. Think of a gas as just a lot of tiny little balls bouncing around. They pick up energy from colliding with the Earth or the walls of whatever container they're in, which are warmed (directly or indirectly) by the Sun, so they never stop bouncing. At room temperatures and pressures they are typically bouncing around at a few hundred meters per second (around ten times faster than the fastest fastballs) and there are just as many going in one direction as the other. Some of them are going upwards. Because they start ten times faster than a fast ball (and that's the average molecule), they can go a hundred times as high (energy being proportional to speed squared), but they slow down and eventually stop and come back down just like a ball - they aren't going fast enough to escape Earth's gravity (you need to be doing 11km/s for that).

So if you release some helium atoms into a small empty tank they spread out across it at a few hundred meters per second and start bouncing off the walls. But if the tank is really, really, tall, the ones going straight up are turned back by gravity before they reach the top. So in a lab-sized experiment like the one in the video the helium fills the tank because the atoms aren't appreciably slowed before they bounce off the top of the tank. But in a hypothetical kilometers-tall tank, the atoms never reach the ceiling because of gravity.

What's missing from that model is interactions between atoms. That means I'm talking about a so-called "ideal gas" rather than a real one, but that's not a terrible approximation for the atmosphere.

 

[Jackamus]

So where do helium molecules collect when rising through the atmosphere? So what I'm asking is what forces are acting on an atom/molecule, in the presence of a vacuum, that they get overcome by gravity?
As I understand it helium is said to escapes into space. Is this true?

 

[Jackamus]

”Rise against gravity” describes the velocity, not acceleration. If you want to understand forces you must focus on acceleration.

You are not wrong, but what you are missing is that buoyancy is itself due to gravity. The buoyant force on an object is $\rho g V$ where $\rho$ is the density, $V$ is the displaced volume of fluid, and $g$ is the gravitational acceleration. If $g=0$ then there is no buoyancy

In a nutshell I am questioning gravity. I am trying to understand how it was established in the first place. Before Newton how did scientists view the phenomena? I guess that up until Newton Archimedes ruled OK.

 

[DaveC426913]

As I understand it helium is said to escapes into space. Is this true?

In virtually undisturbed space, helium would simply rise until it sat on top of the heavier air (more accurately, mixed in with the top layer of rarified air). But the solar wind tends to rip away these light, exposed layers, and that's how the helium is lost to space.

 

[Jackamus]

Density and miscibility.

Can it be said that substances that are of low density are likely to be miscible and vice verse?

 

[Jackamus]

In virtually undisturbed space, helium would simply rise until it sat on top of the heavier air (more accurately, mixed in with the top layer of rarified air). But the solar wind tends to rip away these light, exposed layers, and that's how the helium is lost to space.

Surely all the top of the atmosphere is 'exposed'. What prevents the solar wind from ripping all the atmosphere away? How do we know that this happens?

 

[DaveC426913]

Surely all the top of the atmosphere is 'exposed'. What prevents the solar wind from ripping all the atmosphere away? How do we know that this happens?

Partly, the magnetosphere deflects and thus somewhat lessens the ability of the solar wind to strip away the top atmo. But also, the solar wind is not strong enough to strip away the denser layers against gravity.

 

[Dale]

In a nutshell I am questioning gravity. I am trying to understand how it was established in the first place. Before Newton how did scientists view the phenomena? I guess that up until Newton Archimedes ruled OK.

What do you mean by “questioning gravity”? Are you saying that an apple falls down based on some other force besides gravity, or are you saying that gravity affects apples but not gasses?

Again, buoyancy is based on gravity. You cannot have buoyancy without gravity. So if you think a gas is not affected by gravity then how do you explain buoyancy?

 

[Jackamus]

I'm trying to get back to basics and understand how gravity was quantified.

 

[Dale]

I'm trying to get back to basics and understand how gravity was quantified.

Gravity is quantified by dropping objects and measuring the rate at which they accelerate when they are under the influence of gravity alone.

Most of the challenge in such quantification is in making sure that the only force on the object is gravity. That is where the vacuum chamber comes in. Removing the air gets rid of buoyancy and drag.

You can take an empty balloon, drop it, and see how fast it accelerates in the vacuum chamber. This gives you the force of gravity on the balloon itself. Then, you can fill the balloon with helium and drop it in the vacuum chamber again. The difference in accelerations (filled vs unfilled) gives you the net force of the helium on the balloon. From that you can determine the net force of the balloon on the helium. The acceleration of the helium that is not due to the net force from the balloon is due to gravity.

 

[russ_watters]

Can it be said that substances that are of low density are likely to be miscible and vice verse?

No.

 

[russ_watters]

Surely all the top of the atmosphere is 'exposed'. What prevents the solar wind from ripping all the atmosphere away? How do we know that this happens?

It does happen, but how much depends on the velocity of the molecules. Since thermal energy is mostly kinetic, lighter molecules travel faster and are more likely to escape. Hence, not much hydrogen and helium in the atmosphere.

 

[PeroK]

I'm trying to get back to basics and understand how gravity was quantified.

Gravity is a universal force, but it's not the only game in town. In fact, it took the genius of Newton to see a simple underlying law of gravity through the complexity of various natural phenomena.

 

[snorkack]

As I recall, it depends on the relative sizes of the molecules and their polarisability. Probably other things too. Gases are much lower density than liquids, though, so you'd have to find some bizarre and extreme conditions to get anywhere where immiscibility could even be a possibility for gases.

Gases are defined by their lack of surface tension against their own vapour.
I suppose you probably can get immiscible gases when they are sufficient density. Which means sufficient pressure. (Because below their critical points, you can just define the as liquids, for which immiscibility is common).

 

[Drakkith]

So where do helium molecules collect when rising through the atmosphere?

They don't collect anywhere, because the atmosphere is a very chaotic, messy place for atoms and molecules. It's got a temperature gradient, updrafts, downdrafts, winds blowing in every direction, incoming solar radiation that does things to the top layers, and much more. All this has the effect of mixing the gases together instead of allowing them to separate into individual layers based on their densities.

It's like pouring a bunch of oil into a glass of water and then vigorously stirring the mixture. The stirring keeps the oil and water from staying separated.

 

[Daystar]

Regarding miscibility, one of the main considerations is the polarity of the molecules. For example water molecules are polar and tend to stick to each other. Oil molecules are not polar and generally will ot stick to water so the two liquids are immiscible. Most gas molecules are non polar so mixtures of gases are usually completely miscible.
Regarding gravity someone has already explained Newtonian gravity. If @Jackamus wants to understand Einsteinian gravity he could look at a book called "Black Holes"by Brian Cox and Jeff Foreshaw. It is a nice explanation linking relativity, gravity and black holes for non theoretical physicists. But there might be more appropriate books that address gravity only.