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A Physics Question  
User currently offlineYhmfan From Canada, joined Feb 2004, 607 posts, RR: 0
Posted (4 years 7 months 1 week 2 days 22 hours ago) and read 1851 times:

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
13 replies: All unread, jump to last
 
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 1, posted (4 years 7 months 1 week 2 days 22 hours ago) and read 1849 times:



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.


User currently offlineKlaus From Germany, joined Jul 2001, 21418 posts, RR: 54
Reply 2, posted (4 years 7 months 1 week 2 days 22 hours ago) and read 1826 times:

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.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19419 posts, RR: 58
Reply 3, posted (4 years 7 months 1 week 2 days 21 hours ago) and read 1780 times:



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)


User currently offlineYhmfan From Canada, joined Feb 2004, 607 posts, RR: 0
Reply 4, posted (4 years 7 months 1 week 2 days 13 hours ago) and read 1686 times:

Thank you very much guys for perfectly clear answers.


If at first you don't succeed, skydiving is not for you
User currently offlineLuckyone From United States of America, joined Aug 2008, 2165 posts, RR: 0
Reply 5, posted (4 years 7 months 1 week 1 day 14 hours ago) and read 1583 times:



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  Smile 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.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19419 posts, RR: 58
Reply 6, posted (4 years 7 months 1 week 1 day 13 hours ago) and read 1571 times:

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]

User currently offlineFX772LRF From United States of America, joined Apr 2009, 675 posts, RR: 12
Reply 7, posted (4 years 7 months 1 week 1 day 13 hours ago) and read 1559 times:



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  twocents 

-Noah  wave 



Cleared to IAH via CLL 076 radial/BAZBL/RIICE3, up to 3k, 7k in 10, departure on 134.3, squawk 4676, Colgan 9581.
User currently offlineLuckyone From United States of America, joined Aug 2008, 2165 posts, RR: 0
Reply 8, posted (4 years 7 months 1 week 1 day 11 hours ago) and read 1539 times:

NaCl is an ionic compound, not a molecule  Smile

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  Smile 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  Smile


User currently offlineDocLightning From United States of America, joined Nov 2005, 19419 posts, RR: 58
Reply 9, posted (4 years 7 months 1 week 1 day 9 hours ago) and read 1521 times:



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.


User currently offlineUnattendedBag From United States of America, joined Oct 2003, 2323 posts, RR: 1
Reply 10, posted (4 years 7 months 1 week 1 day 8 hours ago) and read 1507 times:



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

I know nothing about chemistry.

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



Slower traffic, keep right
User currently offlineDocLightning From United States of America, joined Nov 2005, 19419 posts, RR: 58
Reply 11, posted (4 years 7 months 1 week 1 day 7 hours ago) and read 1502 times:



Quoting UnattendedBag (Reply 10):

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

He gotcha.  Smile


User currently offlineLuckyone From United States of America, joined Aug 2008, 2165 posts, RR: 0
Reply 12, posted (4 years 7 months 1 week 1 day 1 hour ago) and read 1467 times:

He did indeed. Score one for the typo!

User currently offlineDocLightning From United States of America, joined Nov 2005, 19419 posts, RR: 58
Reply 13, posted (4 years 7 months 1 week 11 hours ago) and read 1413 times:

So I looked it up. The IUPAC (International Union of Pure and Applied Chemistry, for the rest of you  Wink ) 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.  razz 

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.


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