An old one I posted years ago: helium balloon

Originally posted by ua41
But, it rises on earth not due to gravity but due to buoyancy.
If you were on the moon, it would fall to the surface

It still comes down to gravity when on the surface of the Earth. What causes the buoyancy?
When you correctly answer that one, I have another poser.

Originally posted by sonhouse
It still comes down to gravity when on the surface of the Earth. What causes the buoyancy?
When you correctly answer that one, I have another poser.

We consider the air a fluid medium. When pressure is applied, it essentially pushes the fluid similar to sticking your hand down a cup a water. In this case gravity is the force applied. I'm not good with all the technicalities but I think there's a certain limit to the density of the object (in this case helium?) to float on the fluid so the pressure applied from such fluid is able to keep it afloat.

I realize my quoted comment was just being a stickler and I'm not necessarily clear cut on it myself 😛

Originally posted by ua41
We consider the air a fluid medium. When pressure is applied, it essentially pushes the fluid similar to sticking your hand down a cup a water. In this case gravity is the force applied. I'm not good with all the technicalities but I think there's a certain limit to the density of the object (in this case helium?) to float on the fluid so the pressure applied f ...[text shortened]... my quoted comment was just being a stickler and I'm not necessarily clear cut on it myself 😛

That describes the buoyancy but what is the exact mechanism?

Originally posted by sonhouse
That describes the buoyancy but what is the exact mechanism?

Acceleration? Or am I mixing terms up?

Originally posted by ua41
Acceleration? Or am I mixing terms up?

To the balloon that contains a gass lighter than air it would be like being suspended in water. Therefore if the ship was full of water and it accellerated the water would initially be forced backward. This in turn wound cause the balloon to act like a cork and want to float pushing the balloon forwards.

The accelerating rear bulkead of the rocket pushes the air molecules in front of it, which transmit the force to the molecules in front of them. The more massive a molecule is, the more energy it will need to accelerate to the speed of the bulkhead, and as the bulkhead is supplying energy at a constant rate, everything takes a while to get moving. Because the helium molecules in the balloon are lighter than the molecules of air, they get up to the bulkhead's speed more quickly than the air molecules do and therefore the balloon moves forward with respect to the air.

Because the various molecules in air have different molecular weights, the constant acceleration of the rocket will effect them all differently. Because oxygen molecules are slightly heavier than nitrogen molecules, the concentration of oxygen at the floor of the rocket will be be higher than the concentration at the ceiling. Carbon dioxide will tend to fall to the floor as it is the heaviest common molecule in air. If water vapour is present in the air, the water molecules will rise, as they are much less massive than both oxygen and nitrogen molecules; condensation may form at the forward bulkhead.

Originally posted by iamatiger
Because the various molecules in air have different molecular weights, the constant acceleration of the rocket will effect them all differently. Because oxygen molecules are slightly heavier than nitrogen molecules, the concentration of oxygen at the floor of the rocket will be be higher than the concentration at the ceiling. Carbon dioxide will tend to fa ...[text shortened]... massive than both oxygen and nitrogen molecules; condensation may form at the forward bulkhead.

That's a better explanation than your first one. It's the differential pressure gradient that gives helium the lift. So here is the poser:
Is there some condition, some planet with some kind of atmosphere that would NOT allow helium to rise? Outside of a totally helium atmosphere that is! In that case the pressure differentials would be exactly the same and the helium would not rise but what about other atmosphere/pressure combinations, can there be some such combination that would not let helium rise?

The He in a balloon will match the pressure of the surrounding atmosphere. Such a balloon might sink in a hydrogen atmosphere, but nothing else has a lower particle weight in gas form than helium that I know of.

The gas giant planets in our solar system have atmospheres that are mostly hydrogen plus a bit of helium (because they are massive enough to stop their hydrogen and helium from escaping into outer space). The balloon would sink in these planets' atmospheres.

Ok, assuming that, what if you had a helium balloon with an extremely light skin on the balloon but totally rigid, would that make a difference in its floatability? Also, would it be possible to make a floating foam using helium or hydrogen as the medium gas? What would be the smallest bubble size possible and still rise in the air? It seems obvious the lifting capability of some styrofoam fed with helium or H2 would be able to float with some individual cell size but how small a cell size could you get and still fly? I envision a foam filled with H2 or He that would inject inside a wing for lift capability that should hold helium or H2 longer than a bag.

Originally posted by sonhouse
Ok, assuming that, what if you had a helium balloon with an extremely light skin on the balloon but totally rigid, would that make a difference in its floatability? Also, would it be possible to make a floating foam using helium or hydrogen as the medium gas? What would be the smallest bubble size possible and still rise in the air? It seems obvious the lif ...[text shortened]... would inject inside a wing for lift capability that should hold helium or H2 longer than a bag.

As helium is not flammable, there is no particular danger in storing it in large impermeable bags and stuffing them inside whatever hollow body you want. You may find helium quickly escapes from styrofoam though, because its small molecule size lets it pass through plastic.

Originally posted by iamatiger
As helium is not flammable, there is no particular danger in storing it in large impermeable bags and stuffing them inside whatever hollow body you want. You may find helium quickly escapes from styrofoam though, because its small molecule size lets it pass through plastic.

We all know helium is not combustible, not even sure if it can react with ANYTHING.
But I don't think there is a material you can use that will hold helium for months or years at a time without replacement. The other problem is, helium is a national treasure kind of resource and should not be used for recreational balloons. Of course the only alternative is hydrogen and that would not be too cool at a birthday party given lit candles and such🙂
My thought was just to replace the lost He with H2 which can be made from electrolysis onboard the craft powered by PV cells. It would take a lot of H2 to convert that mixture to a flammable gas.

Originally posted by sonhouse
We all know helium is not combustible, not even sure if it can react with ANYTHING.
But I don't think there is a material you can use that will hold helium for months or years at a time without replacement. The other problem is, helium is a national treasure kind of resource and should not be used for recreational balloons. Of course the only alternative i ...[text shortened]... raft powered by PV cells. It would take a lot of H2 to convert that mixture to a flammable gas.

http://en.wikipedia.org/wiki/Helium_hydride_ion

Originally posted by AThousandYoung
http://en.wikipedia.org/wiki/Helium_hydride_ion

Interesting, but still it looks like it only happens from nuclear reaction byproducts, it would be difficult to make in the lab. Interesting that it is the strongest acid, if you could get enough of it to use. Kind of like Francium, sounds like (about one ounce of the stuff on the planet at any given time)