Hot Air Rising

[Kayra]

Time for the science newb to make a fool of himself :)

 

Shouldn't the buoyancy of wood in water be using the same principle as the buoyancy of a gas?

 

I have a hard time visualizing the tempurature as being relevant except that it affects the density of gases.

 

It strikes me that buoyancy comes from the fact that gravity is present in a gradient, pulling downward. In a fluid (air, water, etc) at any point there is an omni directional force pushing in all directions. the higher you go, the lower that omni directional force is, based on the amount of gravity acting on the fluid. if you look at the block of wood (or parcel of air viewed as a system) then the bottom of it has greater pressure then the top of it. this wil act to move the object "Upwards" until the forces above and below the object are equal.

 

I am sorry I do not have the math skills to explain what I visualize. I can not see how the speed of the molecules (and therefore how far they will travel??)will have any effect on bouyancy beyond the change in the density of the material. If that were the case, I do not understand how a balloon could work.

[Tormod]

Welcome to Hypography, Kayra. You're not making a fool of yourself. I think it is a beautiful explanation.

[arkain101]

I was wondering something that can relate to hot air rising.

 

If you fill a container with helium it may float or become very light.

 

From what I have seen, when you create a vacume in a container it does not take on the same rising effect.

 

Would a vacume act like a less dense gas? If not, what is going on here.

[Pyrotex]

I was wondering something that can relate to hot air rising. If you fill a container with helium it may float or become very light. From what I have seen, when you create a vacume in a container it does not take on the same rising effect. Would a vacume act like a less dense gas? If not, what is going on here.

You are correct! Ding! Ding! Ding! Give the man a prize!

 

Vacuums are always made in rigid (and heavy!) containers, so you would not notice the "rising effect", unless you weighed the container with air, then with a vacuum and got the difference. Helium or hydrogen are the nearest thing you can have to a vacuum, but they still occupy volume; a vacuum does not.

 

Here is something I and a friend observed at a Christmas "company party" a few years ago. Helium balloons with strings nearly covered the ceiling of the hotel ballroom, which was about 16 feet high. There were hardly any air currents that near the ceiling. Every now and then, a balloon would SLOWLY, SLOWLY fall away from the ceiling. It barely moved at first, then as it fell, it picked up speed, until it was falling at a pretty good clip when it hit the floor. The balloon had no "rising effect" left at all.

 

The question is, what causes this strange behavior? Assume that the balloons contain Helium and have NO leaks.

[arkain101]

In response to your post,

 

I assume the air in the room became very warm, especially near the cieling, causing the density of the air to decrease, which will in turn affect the helium to act heavier. hmm but there is probably a different explanation.

 

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A vacume container obviously weighs less than when it has air.

 

I understand it has to be also built very strong because of the immense pressure.

 

Say you could design a triangulated honey comb style container that was very light and very strong, but also contained alot of room for volume.

 

If it would float with helium in it, would it float with absolutly nothing in it, and if so worse or better?

[Pyrotex]

I assume the air in the room became very warm, especially near the cieling, causing the density of the air to decrease, which will in turn affect the helium to act heavier. hmm but there is probably a different explanation.

...Say you could design a triangulated honey comb style container that was very light and very strong, but also contained alot of room for volume. If it would float with helium in it, would it float with absolutly nothing in it, and if so worse or better?

You are nearly 100% correct. The air at the ceiling was much warmer than the air at people level. Yes, the warmer air would itself be less dense, BUT, the heat would affect Helium even more. making it FAR less dense, and therefore bouyant. If the balloon fell even a few inches, the temperature drop would contract the Helium, lose even more bouyancy, and fall even more and faster. This process would accellerate until the balloon hit the floor.

 

In response to your question. Yes, your structure would float even better with a vacuum than with Helium. The reason is simple. The volume would be the same in either case. But Helium does have some mass, the vacuum has none. The density of Helium would be very low, but the density of the vacuum would be zero.

[Pyrotex]

...If it would float with helium in it, would it float with absolutly nothing in it, and if so worse or better?

I just remembered. Your question has been answered in a round-about way. But using hot air instead of Helium or a vacuum.

 

Build a spherical geodesic dome, a shell that is sufficiently rigid. build it big, say a kilometer in diameter. On the outside, glass panes. On the inside, nothing except "balconies"--ledges running around the circumference. Seal the sphere except for a small exhaust hole and heat up the air inside. You have a hot air balloon. Let it get to max altitude, then open up the bottom. Solar radiation trapped by the glass will keep the temperature inside at, say, 70 F. But at 30,000 feet, the temp outside will be around 0 F. The structure will float indefinately, with no more input of heat!!!! The key is to make it large enough.

 

You could fly helicopters up through the hole in the bottom and land on the ledges inside the shere. However, you would have to wear pressure suits.

[CraigD]

Helium balloons with strings nearly covered the ceiling of the hotel ballroom, which was about 16 feet high. There were hardly any air currents that near the ceiling. Every now and then, a balloon would SLOWLY, SLOWLY fall away from the ceiling. It barely moved at first, then as it fell, it picked up speed, until it was falling at a pretty good clip when it hit the floor. The balloon had no "rising effect" left at all.

 

The question is, what causes this strange behavior? Assume that the balloons contain Helium and have NO leaks.

I don’t think its valid to assume that the balloons have no leaks.

 

Discarding this assumption, and assuming that the balloons at your party were the ordinary, stretchy, latex kind, the explanation is that helium, a small, non-reactive, single-atom molecule, escaped through the slightly porous latex. Since the balloons also contained ordinary air – mostly nitrogen, a larger molecule less able to escape through the latex, the balloons lost only some of their diameter, giving the appearance that they were not leaking any gas at all.

 

You can confirm this with the following experiment. At your next party, get a few of the (slightly) more expensive, un-stretchy aluminized mylar balloons (sometimes called a “foil balloon”, and available most places that sell the latex kind). You should notice that, even though they are filled from the same tank, they take much longer to drop from the ceiling – a week or more, in my experience. The aluminum film is less porous than the latex (hardly at all – most of the leakage appears to come from the crimped, valved, or glue-sealed neck of these balloons), so the helium takes longer to escape.

[Qfwfq]

I don’t think its valid to assume that the balloons have no leaks.

Especially considering how "leaky" helium is! Pyrotex doesn't convince me 100% either. There are too many relevant factors to say for sure. If the heating is gradual enough that the helium also warms and the balloons still let it expand easy enough, you'd have to compare the PVT charts of the two gasses to predict the effect.

 

Dear oh me, what confusion here in this thread! :surprise:

 

The original question: If you heat the air in a rigid container, its density won't change but pressure will increase instead. When uncontained air in one place is heated, pressure remains the same as surrounding air so it expands intstead. Therefore it rises by Archimedes' force.

 

Kayra, good intuition! :gift: Look up the story of Archimedes running naked through the streets of Athens shouting "Eureka!".