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

[Zetan]

Hello to everyone.
My first question on this forum (it was quite tricky thinking of a heading and prefix for this thread!):
Imagine for a moment that there is a hot air balloon which has a strong shell forming the balloons shape rather than one made from material and it weighs the same as a normal hot air balloon, What would happen if air was gradually drained from the balloon, would it rise?

 

[Drakkith]

It would rise as long as the balloon was lighter than the same volume of air, regardless of what's inside.

 

[Zetan]

Thanks, that's cleared that one up.

Another question: If an object was placed on a set of scales in a vacuum chamber and the air was removed, does it weigh much less without the weight of any air pushing down on it?

 

[256bits]

Thanks, that's cleared that one up.

Another question: If an object was placed on a set of scales in a vacuum chamber and the air was removed, does it weigh much less without the weight of any air pushing down on it?

The air removed from the vacuum chamber I suppose you mean, and not from the object.

You might be able to answer that question yourself with a little help.

When dropped from a height in air, which will reach the ground first:
a blown up balloon, or not-blown up balloon. If you put them on a scale, which weighs more?

If you then put them in a vacuum chamber, and drop them,:
which one hits the ground first. On a scale in the vacuum chamber, which one weighs more?

( You can cheat by looking up something called bouyancy )

 

[Drakkith]

Another question: If an object was placed on a set of scales in a vacuum chamber and the air was removed, does it weigh much less without the weight of any air pushing down on it?

I believe so.

 

[russ_watters]

Buoyancy.

 

[Buckleymanor]

I believe so.

Not so.
Do you weigh more on a scale when submerged in water than air.
Would you weigh more on a scale when submerged in air than a vacuum.

 

[Zetan]

The air removed from the vacuum chamber I suppose you mean, and not from the object.

You might be able to answer that question yourself with a little help.

When dropped from a height in air, which will reach the ground first:
a blown up balloon, or not-blown up balloon. If you put them on a scale, which weighs more?

If you then put them in a vacuum chamber, and drop them,:
which one hits the ground first. On a scale in the vacuum chamber, which one weighs more?

( You can cheat by looking up something called bouyancy )

Yes, I mean the air being removed from the vacuum chamber and not the object.

Funnily enough I recently watched a feather and hammer dropped side by side in a vacuum chamber and they both fell at the same speed and hit the ground together. I haven't seen two objects weighed in a vacuum chamber yet, would the blown up balloon would weigh more because of the extra weight of the air inside it?

 

[256bits]

Yes, I mean the air being removed from the vacuum chamber and not the object.

Funnily enough I recently watched a feather and hammer dropped side by side in a vacuum chamber and they both fell at the same speed and hit the ground together. I haven't seen two objects weighed in a vacuum chamber yet, would the blown up balloon would weigh more because of the extra weight of the air inside it?

That should tell you something.

In a vacuum, they both fall at the same rate.That's true.
And on the scale in the vacuum, the blown up balloon would weigh more due to the air inside.
So you are 2 for 2 - 100% so far.

( You didn't answer the first part when dropped in the air and weighed in the air, but since that's an everyday experience, so I'll give it to you. 4 for 4 )
( And no, we aren't going to worry about air friction. We will assume the friction is small and negligible )

So the question is :
Why does the blown up balloon weigh more than the non-blown up in a vacuum.
But weighs less than the non-blown up balloon , when weighed in air.

Have you looked up buoyancy yet?

 

[Zetan]

Not so.
Do you weigh more on a scale when submerged in water than air.
Would you weigh more on a scale when submerged in air than a vacuum.

Does that mean every object has a certain amount of buoyancy whilst it is submerged in air, depending on the amount of air it displaces, and in a vacuum it would weigh more without any buoyancy?

 

[A.T.]

Does that mean every object has a certain amount of buoyancy whilst it is submerged in air

If there is a pressure gradient in the air, for example due to gravity.

 

[Zetan]

That should tell you something.

In a vacuum, they both fall at the same rate.That's true.
And on the scale in the vacuum, the blown up balloon would weigh more due to the air inside.
So you are 2 for 2 - 100% so far.

( You didn't answer the first part when dropped in the air and weighed in the air, but since that's an everyday experience, so I'll give it to you. 4 for 4 )
( And no, we aren't going to worry about air friction. We will assume the friction is small and negligible )

So the question is :
Why does the blown up balloon weigh more than the non-blown up in a vacuum.
But weighs less than the non-blown up balloon , when weighed in air.

Have you looked up buoyancy yet?

Thanks, yes I've looked up buoyancy and displacement, it's forming a picture in my head gradually!

Hmm, in a vacuum, there isn't any buoyancy at all, there is nothing to "squeeze" the balloon upward through layers of less dense atmosphere. There is more density to the balloon with the air inside it, so it would weigh more on the scales in the vacuum?

Would the blown up balloon weigh less when weighed in air because it is displacing more air? I'm not sure if can see that yet because I think the inflated balloon would contain more air pressure than the outside atmosphere, which means it wouldn't be displacing enough air to make it buoyant?

 

[256bits]

because I think the inflated balloon would contain more air pressure than the outside atmosphere, which means it wouldn't be displacing enough air to make it buoyant?

Somewhat more air pressure, but most party balloons are quite buoyant you will have to admit.

 

[A.T.]

I think the inflated balloon would contain more air pressure than the outside atmosphere,

The content is irrelevant for the force of buoyancy.

 

[Zetan]

Somewhat more air pressure, but most party balloons are quite buoyant you will have to admit.

Yes. So the material of the balloon when inflated\spread out, is displacing more air than the non inflated one, making it more buoyant?

Am I right in thinking that as an object is stretched out, the closer the physical matter of the object becomes to the density of air, until it eventually matches the weight of air and achieves neutral buoyancy. Then if it is stretched out further it becomes lighter than surrounding air and is squeezed upward until it reaches air density that matches it?

 

[A.T.]

Yes. So the material of the balloon when inflated\spread out, is displacing more air than the non inflated one, making it more buoyant?

The volume of the object as a whole matters for the force of buoyancy.

 

[Buckleymanor]

Does that mean every object has a certain amount of buoyancy whilst it is submerged in air, depending on the amount of air it displaces, and in a vacuum it would weigh more without any buoyancy?

Yes.

 

[Zetan]

Right, think I've got it now! As a balloon is inflated and the skin stretches out, the actual skin itself displaces more and more of the surrounding air until eventually the air it has displaced weighs more than the balloons skin.

If the balloon skin were stretched out far enough and it displaced enough air, it would begin to be squeezed upwards towards thinner layers of air by the weight of the air around it, until it matched the weight of the air it was displacing and achieved neutral buoyancy?

 

[A.T.]

Right, think I've got it now! As a balloon is inflated and the skin stretches out, the actual skin itself displaces more and more of the surrounding air until eventually the air it has displaced weighs more than the balloons skin.

No. The balloon as a whole displaces more and more air.

If the balloon skin were stretched out far enough and it displaced enough air, it would begin to be squeezed upwards towards thinner layers of air by the weight of the air around it, until it matched the weight of the air it was displacing and achieved neutral buoyancy?

No. Balloons float when filled with gases of lower density than the surrounding air.

 

[Zetan]

No. The balloon as a whole displaces more and more air.No. Balloons float when filled with gases of lower density than the surrounding air.

Thank you.

Supposing a sheet of rubber were stretched out until it is volume displaced more air than the rubber itself weighed, would it float?

 

[256bits]

Thank you.

Supposing a sheet of rubber were stretched out until it is volume displaced more air than the rubber itself weighed, would it float?

Being "lighter than air" and buoyancy are not the same thing, even though related.

To float, the total weight of the balloon and the gas enclosed within the balloon together have to weigh as much as the air it displaces.
If the total weight is heavier, the balloon will sink to the ground.
If the total weight is lighter, the balloon will rise.

 

[A.T.]

Supposing a sheet of rubber were stretched out until it is volume displaced more air than the rubber itself weighed, would it float?

If you could actually increase the volume of the rubber by several orders of magnitude, so it's mass density becomes less than air's, then yes.

But this not what happens when you stretch rubber sheets. The stretched rubber's surface area increases, while the thickness decreases. So the rubber volume doesn't change much.

 

[Buckleymanor]

Right, think I've got it now! As a balloon is inflated and the skin stretches out, the actual skin itself displaces more and more of the surrounding air until eventually the air it has displaced weighs more than the balloons skin.

If the balloon skin were stretched out far enough and it displaced enough air, it would begin to be squeezed upwards towards thinner layers of air by the weight of the air around it, until it matched the weight of the air it was displacing and achieved neutral buoyancy?

Not

If you could actually increase the volume of the rubber by several orders of magnitude, so it's mass density becomes less than air's, then yes.

But this not what happens when you stretch rubber sheets. The stretched rubber's surface area increases, while the thickness decreases. So the rubber volume doesn't change much.

Don't imagine that even if you increased the volume by several orders of magnitude it would.The density of air within a balloon is always more than the surrounding air when inflated because the skin stretches so it pushes the air it surrounds into a smaller volume.

 

[A.T.]

Don't imagine that even if you increased the volume by several orders of magnitude it would.The density of air within a balloon is always more than the surrounding air when inflated because the skin stretches so it pushes the air it surrounds into a smaller volume.

He was talking about stretching a rubber sheet there, but of course it's completely unfeasible anyway.

 

[Zetan]

Many thanks for all of your replies, very helpful.

I came across this useful description whilst searching for information:
http://www.pbs.org/wgbh/nova/lasalle/buoybasics.html

 

[Buckleymanor]

Many thanks for all of your replies, very helpful.

I came across this useful description whilst searching for information:
http://www.pbs.org/wgbh/nova/lasalle/buoybasics.html

Yes it's basically Archimedes Principle which states a floating body when partially submerged in water displaces it's own weight.
So if you weighed the volume of water displaced it would be equal to the piece of wood or whatever else was floating.
In air though most objects don't float and if they do they don't normally stay part submerged.
What does happen to an object in air is that is denser than the air around it is that whatever volume of air it displaces there is an up thrust which is equal to the weight of air the object occupies.
So if you were able to remove the air which the object had displaced (occupied) and weighed it, that weight would equal the amount of up thrust or buoyant force the object experienced.

 

[insightful]

So, you have a folded up Mylar balloon. It weights X. You inflate it to 1m in diameter such that the internal pressure is still 1 atm. What does it weigh now?

 

[Buckleymanor]

So, you have a folded up Mylar balloon. It weights X. You inflate it to 1m in diameter such that the internal pressure is still 1 atm. What does it weigh now?

You can't inflate the Mylar balloon to have an internal pressure of still 1 atmosphere at best it would be 1 atmosphere + the weight of the Mylar balloon which surrounds the air when inflated.

 

[nasu]

What will you get when you add pressure to weight?

 

[insightful]

(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?

 

[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?