atomic_fungus (atomic_fungus) wrote,
atomic_fungus
atomic_fungus

#3977: Density, and more floating spheres

Og suggested they could be made of glass--very thin glass--and he's not wrong. Glass is plenty strong under compression; but you'd end up with something as fragile as a soap bubble, and I'd want to be able to touch the things.

The glass shell of a typical incandescent light bulb, for example, is too thick; it weighs too much for the bouyancy of its internal vacuum to be evident. You'd need a precise scale to tell you that it weighs less when surrounded by air than its air-filled counterpart.

If you used the same amount of glass to contain, say, eight times the volume (doubling the diameter of the globe), you'd be better able to tell the difference...but it would be perilously fragile.

Which is not to say that it can't be done. This is garden variety glass we're talking about, and there are some interesting and exotic glasses. We've had the technology to make glass for more than a thousand years but we are still only scratching the surface of what can be done with the stuff; in fact we are now only just beginning to understand what is possible with it.

Of particular interest are the metallic glasses, which are (as the name suggests) metals which have a highly ordered glass-like molecular structure. They're much stronger than their crystalline metal counterparts, but they're not transparent.

As I was falling asleep last night I thought again about the ping pong ball, and realized that a partial atmosphere of hydrogen would be more advantageous than helium: the hydrogen molecule weighs half what a helium atom weighs. This is also part of the reason dirigibles used hydrogen; it just lifts better than helium does.

When you're building a lighter-than-air vessel of any kind you have to get your bouyancy from somewhere, obviously. In a hot air balloon, you heat the air in the gas bag, reducing its density while maintaining a slight positive pressure with respect to the surrounding air. In a blimp or dirigible you use a very light gas like helium or hydrogen--again, at a slight positive pressure--which similarly gives you a volume of gas with a lower density than that of the atmosphere. Either way, you get the bouyancy you seek.

But it's inefficient. Look at how big the gas bag has to be; a typical hot air balloon has a payload capacity of perhaps a thousand pounds (accounting for pilot, three passengers, fuel, and the gondola itself) and can travel about ten miles. A smaller volume of helium or hydrogen would do as well; a similar volume of helium would be able to lift more mass. (And if you heat the helium....)

"So?" You say. "Take your sphere and heat it. The gas inside will become less dense!" That will happen only if you can siphon off some of the gas inside; contained in a rigid body the gas cannot increase its volume, and so its density remains the same while its pressure rises with temperature. Hot air rises because it is less dense than cold air at the same pressure.

Maybe an aerogel is the only good way to get a sphere which is physically robust and which will float in air. But as far as I know we can't make an aerogels that's lighter than air. Maybe later.

* * *

Blizzard has announced that they're going to discontinue the real money auction house in Diablo III. Great; does this mean we can dispense with the online-only play and move the characters back to local storage? WTF.

* * *

I write on these topics because there is nothing in the news that is noteworthy. GOP postures over debt limit and Obamacare, blah blah blah, maniac shoots up unarmed people, blah blah blah, nothing new under the sun, blah blah blah etcetera. I'm just not feeling it and I have other things occupying my mind right now.

But because I am so uninspired, it's going to have to do. Sorry about that.
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