What Happens When Air is Drained from a Hot Air Balloon with a Solid Shell?

[sophiecentaur]

does it weigh much less without the weight of any air pushing down on it?

The air is pushing against it from all directions and on all of its surface (down, up and sideways). The net force will always be positive (upwards) unless there is a vacuum. Fluid pressure always acts in this way, when there is a gradient of the gravitational field. In a space capsule (well away from the Earth), there would be no net bouyancy force because the pressure would have the same value everywhere..

 

[insightful]

Fluid pressure always acts in this way, when there is a gradient of the gravitational field.

Is it not more a function of the gradient of the fluid pressure?

 

[sophiecentaur]

I should have said " gradient of gravitational potential" . Sorry.
That would produce the pressure gradient.

 

[Buckleymanor]

(Whew, tough crowd.)

Okay, you hang a deflated Mylar balloon from a scale that reads 100g. You blow it up with ambient air to 1.001 atm to produce a roughly 1m diameter sphere. What does the scale read?

The scale will read 100g - the weight of the roughly 1 diameter sphere of air.

 

[nasu]

A sphere of air with 1 m diameter, at atmospheric pressure "weights" something more than 0.5 kg and not 100g.
But the indication of the scale is indeed close to 100 g. The exact value will depend on the density of the air inside.

 

[insightful]

I get 100.7g give or take depending on temperature and altitude (ambient air density).

 

[Zetan]

The air is pushing against it from all directions and on all of its surface (down, up and sideways). The net force will always be positive (upwards) unless there is a vacuum. Fluid pressure always acts in this way, when there is a gradient of the gravitational field. In a space capsule (well away from the Earth), there would be no net bouyancy force because the pressure would have the same value everywhere..

Thanks. That explains why objects are still pushed upwards even at the bottom of the deepest ocean with tons of water above them.

When they play around with water in space capsules, with no gravity and the water forms a blob, is that because the air pressure inside the capsule is pushing on the water from all around it?

Found this video about water in a vacuum on youtube:

 

[Buckleymanor]

A sphere of air with 1 m diameter, at atmospheric pressure "weights" something more than 0.5 kg and not 100g.
But the indication of the scale is indeed close to 100 g. The exact value will depend on the density of the air inside.

Is that when the volume of air is weighed in a vacuum ie if you weighed when surrounded by air the reading will be zero plus the density of air in the sphere.

 

[sophiecentaur]

A sphere of air with 1 m diameter, at atmospheric pressure "weights" something more than 0.5 kg and not 100g.
But the indication of the scale is indeed close to 100 g. The exact value will depend on the density of the air inside.

I used to do a great demo in School, with a 2l round bottomed flask. Weigh it first and then suck the air out with a vacuum pump and weigh it again. It is easy to show the difference in weight with a good lab balance. The message used even to get so some of the bored students that 'Air has Weight'.

 

[A.T.]

When they play around with water in space capsules, with no gravity and the water forms a blob, is that because the air pressure inside the capsule is pushing on the water from all around it?

Water also has surface tension.

 

[sophiecentaur]

Water also has surface tension.

yes. And when the pressure in all directions is the same, you will get a (wobbly) sphere.

 

[Zetan]

yes. And when the pressure in all directions is the same, you will get a (wobbly) sphere.

As with an air bubble in water then...

 

[Zetan]

Just to make sure I've got this - does an object placed on a set of scales in a vacuum weigh more than it does in "normal" air pressure, because air is fluid and is pushing on the object from all directions including upwards?

 

[Zetan]

Water also has surface tension.

Thanks, it looks like I need to read up on surface tension to, just to see what causes it! There are so many branches to go off on

 

[sophiecentaur]

As with an air bubble in water then...

Actually, no (at least not on earth. An air bubble in water experiences more upward force than downward force - hence the upthrust. In space, the lack of upthrust has a serious effect on how water behaves when heated. There is no natural convection so the liquid around the heating element will boil and turn to a bubble of vapour but it will stay there and not be moved upwards by the net-upward pressure of the water around it. LIkewise, a candle does not burn well because there is nothing to make the hot gases rise. The flame will just die out because no fresh O2 gets to the wick.

 

[sophiecentaur]

Just to make sure I've got this - does an object placed on a set of scales in a vacuum weigh more than it does in "normal" air pressure, because air is fluid and is pushing on the object from all directions including upwards?

Yes. There is no buoyant force in a vacuum.
Surface tension is due to the mutual attraction of the molecules in a liquid. At the surface, they are not attracted to the air so the net force on each molecule is 'inwards' which pulls the surface flat.

 

[Strong Eagle]

Folks, if I get the OP, he is essentially asking what is the lifting capacity of a balloon which has a perfect vacuum instead of being filled with hydrogen or helium. Assuming one could create a shell capable of sustaining the vacuum without collapsing, the vacuum wins... for a given volume of balloon, the vacuum "filled" one would have a stronger tendency to rise than a gas filled one.

But, not by much. It would be very hard to design a structure capable of sustaining the vacuum without a massive collapse that would be lighter than the difference between the lift of the vacuum and the lift of hydrogen or helium.

 

[Zetan]

Folks, if I get the OP, he is essentially asking what is the lifting capacity of a balloon which has a perfect vacuum instead of being filled with hydrogen or helium. Assuming one could create a shell capable of sustaining the vacuum without collapsing, the vacuum wins... for a given volume of balloon, the vacuum "filled" one would have a stronger tendency to rise than a gas filled one.

But, not by much. It would be very hard to design a structure capable of sustaining the vacuum without a massive collapse that would be lighter than the difference between the lift of the vacuum and the lift of hydrogen or helium.

That's what I wanted to know, thanks. It was thinking about why warm air rises that started me off. I suppose as the air inside of a hot air balloon warms up and expands it is expelled through the hole at the base until there is less weight of air inside?

 

[A.T.]

I suppose as the air inside of a hot air balloon warms up and expands it is expelled through the hole at the base until there is less weight of air inside?

It doesn't need to be expelled. Same weight with more volume means less density.

 

[Zetan]

Yes. There is no buoyant force in a vacuum.
Surface tension is due to the mutual attraction of the molecules in a liquid. At the surface, they are not attracted to the air so the net force on each molecule is 'inwards' which pulls the surface flat.

Is that why oil stays separate from water, the oil molecules not being attracted to the water molecules?

What creates the mutual attraction? Is the force similar to magnetism or something completely different? I suppose if it were similar to magnetism there would be degrees of attraction and repulsion between all things? Is there any attraction at all between air and water molecules for example?

 

[sophiecentaur]

What creates the mutual attraction?

The electric forces between the charges in adjacent molecules produce bonding between adjacent molecules. A molecule may (will) have neutral charge but the arrangement of the charges can mean that a nearby molecule will find itself closer to a net negative charges than to positive net charge. This is the basis of why liquids are liquids and solids are solids.
(This post is nowhere near enough as a proper description of the situation; it is just a hint in the right direction.)

 

[Zetan]

It doesn't need to be expelled. Same weight with more volume means less density.

Ah yes, thanks.
That must mean that when the balloon is inflated to the point where neutral buoyancy is achieved, the balloon material must be able to expand further for the volume to increase to achieve lift off then?

You'll be glad to hear that I am not a hot air balloon pilot!

 

[sophiecentaur]

You may find this interesting.

 

[russ_watters]

It doesn't need to be expelled. Same weight with more volume means less density.

Hot air balloons aren't very elastic: when you heat the air inside, some is expelled through the bottom and the volume remains roughly constant.

 

[A.T.]

Hot air balloons aren't very elastic: when you heat the air inside, some is expelled through the bottom and the volume remains roughly constant.

Yes, but during the initial inflating the volume increases. The point was that in general, density matters, not the total weight.

 

[Strong Eagle]

That's what I wanted to know, thanks. It was thinking about why warm air rises that started me off. I suppose as the air inside of a hot air balloon warms up and expands it is expelled through the hole at the base until there is less weight of air inside?

Essentially, that is correct but instead of thinking about "weight of air", think about density, the mass per unit of volume. Warm air has lower density than cold. And, if you've ever seen a hot air balloon being filled, you'll understand that air isn't actually expelled from the bottom. This is because the balloon is laid out on the ground empty and the propane heaters are started in order to fill the balloon

 

[russ_watters]

I don't think the precise sequence of the inflation (when, exactly, they start heating while filling) is critical to the question and can lead to misleading interpretation of the normal operation of the balloon:

When in normal flight, the volume of the balloon is fixed and the pilot controls the mass of the balloon by adding or removing air mass by heating it (removing mass) or alowing it to cool (adding mass).

This is different from most helium balloons, which have a fixed mass (over the short term) but a volume that changes with altitude. This means that in general the hot air balloon has a max altitude which it will reach and stabilize at, while the helium balloon may just rise until it over inflates and pops.

 

[Zetan]

It's probably my fault for not explaining exactly why I wondered whether a vacuum would have a similar lifting effect to warm air, as I didn't know about buoyancy and why less air mass would lift the weight of the entire balloon and basket, etc. It has ended up even more interesting though, learning about all sorts of other branches of the subject as well.
I originally wondered what the warm air was doing exactly and pondered on what would happen if you took "less air mass" to the extreme and removed all of it, eliminating air altogether (in an imaginary situation where it were possible to do that of course).
I have plenty to read about now though, thank you.

 

[Zetan]

Another question I have is (this might need a new thread?): If you have two balls exactly the same size, one made of lightweight foam and the other of steel, why do they fall at the same speed in a vacuum? When you hold them in each hand, why does it take less effort to hold one up than the other and if you caught them as they were falling?

 

[sophiecentaur]

Another question I have is (this might need a new thread?): If you have two balls exactly the same size, one made of lightweight foam and the other of steel, why do they fall at the same speed in a vacuum? When you hold them in each hand, why does it take less effort to hold one up than the other and if you caught them as they were falling?

The effect of gravity is to produce the same acceleration on all masses (assuming the masses are very small compared with the planet. That's because the weight force is mg and the acceleration that the weight force produces on a mass m is g. The m's cancel. If you are holding them, however, you are experiencing the weight forces, which will not be the same.