yhmfan
Posts: 573
Joined: Tue Feb 17, 2004 2:44 pm

A Physics Question

It's been years since my high school physics. I wonder if someone can tell me what's the flaw in my logic:

An hollow sphere which has been vacuumed out of air should float because:

1. Air weighs something.
2. a Vacuumed space with no air weighs less
3. If a sphere which is made out of very light material is emptied of air, the average density of it would be less than the sarrounding air and, therefore, it should float up!!

I will now go think of ways to spend my unimaginable wealth for coming up with the answer to the world's aviation energy cost crisis!!
If at first you don't succeed, skydiving is not for you

Fly2HMO
Posts: 7187
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RE: A Physics Question

 Quoting Yhmfan (Thread starter): 1. Air weighs something. 2. a Vacuumed space with no air weighs less 3. If a sphere which is made out of very light material is emptied of air, the average density of it would be less than the sarrounding air and, therefore, it should float up!!

The problem I see is finding material that would actually be less dense than air. It would also have to be rigid for it to maintain a sufficient surface area for the surrounding air to have something to push against.

Klaus
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Joined: Wed Jul 11, 2001 7:41 am

RE: A Physics Question

Your reasoning is perfectly valid as far as it goes – the problem is that a vacuum container (resisting external pressure) is much harder to make as light as a gas container (resisting internal pressure).

Submarines are much, much heavier built than airships for the same reason. The need to have much thicker walls negates the theoretical advantage of the vacuum.

Hydrogen or helium gas has the advantage of being less dense than air even when their internal pressure is higher so a thin layer of foil or fabric is sufficient to contain it.

For a vacuum you'd need far thicker and heavier material.

DocLightning
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RE: A Physics Question

 Quoting Yhmfan (Thread starter): 1. Air weighs something. 2. a Vacuumed space with no air weighs less 3. If a sphere which is made out of very light material is emptied of air, the average density of it would be less than the sarrounding air and, therefore, it should float up!!

That is completely correct.

 Quoting Klaus (Reply 2):Your reasoning is perfectly valid as far as it goes – the problem is that a vacuum container (resisting external pressure) is much harder to make as light as a gas container (resisting internal pressure).

Helium has an approximate molecular weight of 4 (a helium atom is also a helium molecule). Nitrogen gas, which is the primary component of air, has a molecular weight of 28 (because a molecule of nitrogen has two atoms). (Air is also about 21% oxygen, which has a molecular weight of about 36 [another diatomic gas]). But a given number of helium atoms at a given temperature will occupy the same volume at the same pressure as the same number of nitrogen (or oxygen) atoms. So that means that one liter of helium gas at standard temperature and pressure weighs a lot less than a liter of air at standard temperature and pressure, even if they contain the same number of molecules.

So the pressure inside a helium balloon is only slightly higher than the pressure outside. Thus, the helium balloon needs very little tensile strength at all.

To make a completely evacuated sphere with current materials, you would need to make it resistant to the 14.7 PSI found at sea level.

You'd need to use really fancy materials with a high compressile strength. The best way would actually be to cross-brace the sphere with spars, rather than making it fully hollow. You'd probably have to use exotic carbon-based materials like diamond, or very light metals like magnesium (corrosion at least would not be a problem in a vacuum). The ultimate issue is whether the mass of such a structure would be greater than or less than a helium-filled sack of the same volume. Either way, even if you could somehow evacuate a standard hot air balloon and not have it collapse, it wouldn't offer much more lift than if it were filled with helium or hydrogen (which has an molecular weight of ~2)
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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yhmfan
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RE: A Physics Question

Thank you very much guys for perfectly clear answers.
If at first you don't succeed, skydiving is not for you

luckyone
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RE: A Physics Question

 Quoting DocLightning (Reply 3):a helium atom is also a helium molecule

Not to be nitpicky but no, a He atom is not a He molecule   To be a molecule requires at least two molecules and a covalent bond. You are correct, obviously, about its atomic number as He is a noble gas.

DocLightning
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RE: A Physics Question

 Quoting Luckyone (Reply 5): Not to be nitpicky but no, a He atom is not a He molecule Smile To be a molecule requires at least two molecules and a covalent bond.

Depends on your definition of molecule. For example, NaCl has no covalent bonds, but it is considered a molecule, even though individual molecules of NaCl are very rarely (read: never) found in nature. Also, one can define a molecule as the smallest component of a pure substance that has all the chemical properties of that substance. So for hydrogen gas, the smallest unit is H2, since H is a very reactive radical with different properties from H2. But for the noble gases, they only exist as individual atoms in nature, barring some very bizarre circumstances.

[Edited 2009-12-26 18:58:41]
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

FX772LRF
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Joined: Thu Apr 02, 2009 4:07 am

RE: A Physics Question

 Quoting DocLightning (Reply 6):Depends on your definition of molecule. For example, NaCl has no covalent bonds, but it is considered a molecule, even though individual molecules of NaCl are very rarely (read: never) found in nature. Also, one can define a molecule as the smallest component of a pure substance that has all the chemical properties of that substance. So for hydrogen gas, the smallest unit is H2, since H is a very reactive radical with different properties from H2. But for the noble gases, they only exist as individual atoms in nature, barring some very bizarre circumstances.

I'm pretty sure NaCl is considered a molecule because it has an ionic bond, correct?

Just my

-Noah
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luckyone
Posts: 2367
Joined: Thu Aug 21, 2008 1:50 pm

RE: A Physics Question

NaCl is an ionic compound, not a molecule

 Quoting DocLightning (Reply 6):Depends on your definition of molecule. For example, NaCl has no covalent bonds, but it is considered a molecule, even though individual molecules of NaCl are very rarely (read: never) found in nature. Also, one can define a molecule as the smallest component of a pure substance that has all the chemical properties of that substance. So for hydrogen gas, the smallest unit is H2, since H is a very reactive radical with different properties from H2. But for the noble gases, they only exist as individual atoms in nature, barring some very bizarre circumstances.

Not to sound elitist or "I-know-more-than-you-do" BUT, I do hold a degree in chemistry (whatever that's worth), so I think my definition/understanding of molecule vs. ionic compound and noble gas chemistry is pretty sound   An ion has no bond. It's an interaction, and a selfish one at that. A covalent bond IS a true bond. So technically (I hate that word, but am forced to use it here) NaCl has no bonds. It's found in a crystal lattice.

As a side note, Xe actually forms a lot of halides and oxides, and while they're not prevalent, they are found in nature, and are quite easy to synthesize in the lab

DocLightning
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RE: A Physics Question

 Quoting Luckyone (Reply 8): Not to sound elitist or "I-know-more-than-you-do" BUT, I do hold a degree in chemistry (whatever that's worth),

Great. I've got two degrees in molecular biology. We can both parrot what our professors taught us.

Oh, and the biological definition of a molecule is also different from a chemical one. For example, DNA is held together by hydrogen bonds, so a molecule of DNA would be two molecules by your definition.

Definitions are merely human constructs. We can argue back and forth and both be right and wrong at the same time.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

unattendedbag
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Joined: Wed Oct 08, 2003 10:35 pm

RE: A Physics Question

 Quoting Luckyone (Reply 5):To be a molecule requires at least two molecules and a covalent bond.

Shouldn't that read, "To be a molecule requires at least two atoms and a covalent bond."?
Slower traffic, keep right

DocLightning
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RE: A Physics Question

 Quoting UnattendedBag (Reply 10): Shouldn't that read, "To be a molecule requires at least two atoms and a covalent bond."?

He gotcha.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

luckyone
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Joined: Thu Aug 21, 2008 1:50 pm

RE: A Physics Question

He did indeed. Score one for the typo!

DocLightning
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Joined: Wed Nov 16, 2005 8:51 am

RE: A Physics Question

So I looked it up. The IUPAC (International Union of Pure and Applied Chemistry, for the rest of you   ) definition of a molecule is the smallest unit of a substance that has all the chemical properties of that substance and that it must be electrically neutral and contain at least two atoms. You=win, me=fail

However in kinetic gas theory (which is really what we're discussing) a "molecule" is defined as any particle of a gas, whether mono- or polyatomic. Me=win, you=fail.

To make it even worse, in biochemistry and molecular biology, the definition of a molecule gets even more nebulous. A single molecule of hemoglobin is actually a tetramer of two alpha and two beta chains, none of which are covalently linked to each-other. DNA is treated as a single molecule, even though the two strands are held together by hydrogen bonds. The two halves of the molecule are called "strands," and are not considered a complete molecule of DNA.

As I said, the atoms and molecules are what they are. What we choose to call them is ultimately unimportant. A rose by any other name still smells as sweet.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

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