h a l f b a k e r yBone to the bad.
add, search, annotate, link, view, overview, recent, by name, random
news, help, about, links, report a problem
browse anonymously,
or get an account
and write.
register,
|
|
|
You may be familiar with Maxwell's Demon (no relation, by the way).
This hypothetical machine consists of a gas-filled chamber divided by
a partition. Teeny weensy gates in the partition open when they see
a fast-moving gas molecule approaching from the left, to let it
through to the right;
they likewise open when they see a slow-
moving molecule approaching from the right. The result is that all
the hot molecules wind up on the right, all the cold ones on the left,
and you have "free" energy. Weird.
Weirdness, however, has never been a barrier to MaxCo, who are
proud to announce the MaxMobile Demonic Lifting Surface. The DLS
consists of a thin sheet of silicon supported on a carbon fibre frame.
When the light catches it, you will see a shimmering iridescence
which, on closer inspection, turns out to be caused by a fine regular
array of microscopic sliding doors in the surface. They have been
produced by standard MEMS techniques, as have the tiny sensors
adjacent to each trapdoor, on either side of the surface.
When a sensor detects a gas molecule approaching from below the
surface, it closes the adjacent microscopic sliding door, blocking the
molecule's progress and causing it to rebound, transferring its
momentum to the wing and hence giving it a teeny weeny kick
upwards. When a molecule is detected approaching the surface from
above, however, the sliding door opens, allowing the molecule to
pass through unimpeded and avoiding the downward kick which it
would otherwise have imparted to the wing.
Naturally, opening and closing each trapdoor, and sensing the gas
molecules, can be made arbitrarily low-energy events, as is the case
in Maxwell's Demon. The MaxCo Demonic Lifting Surface closely
approaches these lower limits, resulting in an energy consumption of
microwatts per square metre of lifting surface, whilst acheiving a
lift approaching the theoretical maximum of 15 pounds per square
inch.
differences-between-how-a-theorist-and-an-experimentalist-solve-a-problem
http://talklikeaphy...st-solve-a-problem/
OK, not so short... [csea, Apr 04 2009]
Earth Sea-level Atmospheric mean-free-path
http://amsglossary....?id=mean-free-path1
"About 0.1 micron" [csea, Apr 04 2009]
Brownian pump or wing
Brownian_20pump_20or_20wing#1075482000
[xaviergisz, Apr 19 2009]
Please log in.
If you're not logged in,
you can see what this page
looks like, but you will
not be able to add anything.
Destination URL.
E.g., https://www.coffee.com/
Description (displayed with the short name and URL.)
|
| |
//whilst acheiving a lift approaching the theoretical maximum of 15 pounds per square inch//...until the wing collapses in on itself [+] |
|
| |
Would a 'one way' style physical system not be more implementable than billions of sensors and trap doors? A molecular one-way flap? |
|
| |
//microscopic sliding door// = "msd"
I'm having a bit of trouble with the scale of this. |
|
| |
Aren't DMD mirrors on the order of 1 micron = 10,000 Angstroms in size, whereas gas molecules are on the order of 1-10 Anstroms? |
|
| |
Also, what sort of molecular velocities are we talking? How fast must each "msd" sense, open and close? |
|
| |
[-] waiting on a bit more elaboration. |
|
| |
I suspect that MEMS valve are several orders too large and slow to be able to effectively filter fast particles as described. |
|
| |
A second issue (as if one were needed) would be that the power requirement of an individual MEMS valve would far exceed the effective 'power' gained by filtering fast gas particles. |
|
| |
If that weren't enough, the rate of flow would be so slow that without containment, any pressure delta created would disperse before it could be effectively utilised. |
|
| |
Guys, read up on Maxwell's Demon. The point is that, from a
physics point of view, it all seems OK. The energy needed to
slide aside a door can be arbitrarily small. The "one way flap"
thing won't work because the molecule has to hit the flap to
open it, and in the end you still transfer momentum. |
|
| |
//They have been produced by standard MEMS techniques//
Which means what, exactly? ;o)
From a layman's perspective (for which read "total thicky"), I like the theory but have several doubts.
Assuming that the lifting surface starts on the ground, where are the upwards travelling molecules coming from in order to provide the initial lift?
Also, once you get the thing into the air, how many upwards travelling molecules would be required in order to sustain lift?
Furthermore, at what altitude would dispersion of upward travelling molecules preclude further lift?
And finally, and purely to send you scurrying off to do more mind-numbing calculations, although each individual door slide may have a negligible energy consumption, how many door slides would be required per second, say, in order to maintain altitude and at what energy cost? |
|
| |
//how many door slides would be required per second, say, in order to maintain altitude and at what energy cost?//
Perhaps the demon is in the detail. |
|
| |
So, this is a vehicle which gets it lift from Brownian motion? |
|
| |
No, it gets its lift from regular air pressure. If I hold a
sheet of material horizontally, it's being bombarded by air
molecules from both sides. Hence, air pressure on each
side is equal, and there is no net force. The sliding doors
just allow the surface to dodge the molecules bombarding
from one side, giving a pressure of zero (in the ideal case).
Hence, the net force available will be the same as the
ambient atmospheric pressure, or about 15 pounds per
square inch at sea level. |
|
| |
At 30,000 feet, air pressure is about 4psi. Hence,
maximum available lift is still 4 pounds per square inch of
lifting surface. An A380 Airbus has a maximum takeoff
weight of about 700 tons or 1.5 million pounds, requiring
about 375,000 square inches of Maxwell Demon wing area
to remain airborne. The conventional wings of the Airbus,
in contrast, have an area of about 1.3 million square
inches. |
|
| |
Incidentally, the same system turned sideways will give
thrust, of course. |
|
| |
//The energy needed to slide aside a door can be arbitrarily small.// From where does this energy come? |
|
| |
Why, from an arbitrarily small battery, of course. |
|
| |
It will need to be a rather large one if it is make that 700 ton plane fly. (do Airbus know about these 700 ton planes?) What's the wingspan - half a mile each side? |
|
| |
[xenzag] I respectfully refer you to my previous posts. An
Airbus weighs about 700 tons (max take-off weight). The
demonic lifting area needed is about 1/3rd the actual wing
area of an Airbus. Arbitarily small means arbitrarily small. |
|
| |
Be sure you tie a string to this thing before you take it outside. To my mind, it would be a buoyant as a vacuum balloon. Unlike the vacuum balloon it would not encounter air resistance as it accelerated upwards. |
|
| |
//An Airbus weighs about 700 tons (max take-off weight// sp. "560" |
|
| |
If Maxwell just turned his apparatus verticle then he could have taken advantage of the principle that heat rises against gravitation, and then cools off. If he took advantage of solar heating then he could have invented the sterling engine. Maybe he could have taken advantage of bouyancy too...but no, he had to go on talking about his demons and trying to make an equilibrium point diverge from itself, a real something-for-nothing kind of guy that Maxwell. |
|
| |
And now this, MaxwellBuchanan wants to make a lifting surface that has air pressure on only one side and a vaccuum on the other via a demon that adds and subtracts the length of the wing spans. Not even a bird though, since a bird expells energy too. |
|
| |
Note that you can't put anything on top of (or in front of, if using it for "horizontal thrust") the device or it ceases to work. |
|
| |
You could hang something below it, and use it like a balloon. It would have to be far enough below it to allow air pressure to equilibrate around the lifting surface. |
|
| |
the antigravity substance in First Men to the Moon was called Cavorite, I think. This miraculous surface needs some better moniker than "lifting surface." |
|
| |
Quite so, quite so. The game here, as with the original
Maxwell's Demon, is to find the flaw in the argument, not
worry about the maintenance crews. In effect, this idea is
simply a restatement of Maxwell's Demon but with the
effect turned into lift rather than into the creation of a
temperature differential, thereby making it more graphic
and posing a greater explanatory challenge. |
|
| |
Perhaps it would help to consider an analogue. Imagine
everything on a larger scale. Replace the air molecules
with golf balls rattling around inside a large box. Place
inside the box a horizontal sheet covered with sliding
shutters that can either open to allow a ball through, or
close to absorb its impact. The doors can be made to use
arbitrarily little energy: we can postulate frictionless
bearings and, if you like, they can be sprung so that the
energy needed to open them is recovered when they
spring back - the point is that there is no absolute
minimum for the energy needed to operate the doors. |
|
| |
The sensors can be, if you like, simple cameras - again,
there is no lower limit on the energy needed for them to
operate, in theory. |
|
| |
Now, if the sliding doors open to allow downward-moving
golf balls through, but close to block the path of upward-
moving balls, you will have net lift: the sheet will rise,
powered by the selected random movements of the golf
balls. |
|
| |
Now, imagine the whole system scaled down to work not
with golf balls but with marbles - it must still work. Then
with grains of sand. Then ultimately with bacteria, then
with air molecules. |
|
| |
Not so fast baby... once the energy from the impacts has been transferred from the particles, gravity will take over and the overall mass of the object will increase as the particle accumulation builds. It's a zero sum gain. |
|
| |
In the case of single sheet with gates, and no retention of particles, it won't work either because when the gates are open to allow a particle to pass through from above, another one will take the opportunity to pass through from below. |
|
| |
But Maxwell's Demon doesn't work, because it takes more energy to identify the speed of a particle than you could possibly gain from isolating it. |
|
| |
This wouldn't work because thermodynamics can only go back on itself at an extremely tiny scale (picometres), one far too small to create a workable machine. At any scale big enough to have surfaces/gates/sensors which aren't going to fall apart from Brownian motion, the net effect of the surrounding particles will average out to zero. |
|
| |
I am less curious about why it would not work than how a sheet of this stuff would behave. |
|
| |
any moment now the mighty plane will rise into the air, any moment....... any moment...... somebody give it a push! |
|
| |
//the overall mass of the object will increase as the
particle accumulation builds.// No, there's no
accumulation of particles, any more than air molecules
stick to the wing of a regular plane - they bounce off. |
|
| |
// when the gates are open to allow a particle to pass
through from above, another one will take the opportunity
to pass through from below.// Not so, assuming the gates
are small and work at a reasonable speed. The distances
between air molecules are huge compared to their sizes;
the odds of one coming the other way at the same time in
the same place are teeny weeny. |
|
| |
//it takes more energy to identify the speed of a particle
than you could possibly gain from isolating it.// Now we're
cooking on gas! That'll be it. |
|
| |
No need to identify speed with this apparatus. Location is what is relevant: top vs bottom. |
|
| |
//the odds of one coming the other way at the same time in
the same place are teeny weeny.// Actually the odds are
very much in favour of particles passing through in both
directions at the same time, otherwise there would be a
pressure differential above and below the surface and it
would simply rise unaided. (none of bones mine by the way;
in fact have this rotating, dark matter croissant - to quell the
autoboner) |
|
| |
A 1-way check valve for air particles would only work if there were a sufficient pressure differential between the bottom and the top of the wing. The concept can, however, be workable with 2 different phases such as air and rainwater, and floaties on the checkvalve which is floating in a tub of water.... |
|
| |
The sliding door is going to field molecules via
electromagnetism, right ? |
|
| |
D@%~ theoreticians! [link] |
|
| |
// Actually the odds are very much in favour of particles
passing through in both directions at the same time,//
Nope. In a volume gas at sea-level, molecules are very few
and far between; provided the gate is only a few times larger
than the molecule, and operates only when the molecule is
nearby, nothing else is going to pop through. If you used the
molecule itself to swing open a spring "trap door", you'd
defeat the purpose since the molecule would impart
momentum to the system. |
|
| |
1 cu ft of air at sea level = 28.316 litres
(22.4 litres of any gas at STP will contain an Avagadro's number, 6.02 x 10^23 particles or 1 mol of particles.) Assuming the air to be only N2 and O2, in 28.316 litres you have 28.316/22.4 = 1.264mol
1mol = 6.02 x 10^23 molecules,
therefore 1.264 mol = 7.61 x 10^23 molecules.
As N2 and O2 are diatomic molecules, the number of atoms will be
2 x 7.61 x 10^23 = 1.52 x 10^24 atoms. That seems like a rather a lot me! |
|
| |
Yes, it is quite a lot. However, they are still very sparsely
scattered compared to the sizes of the molecules. Air is
basically a vacuum with a few dots in it. |
|
| |
Takes a deep breath and starts counting the dots before they can get away. |
|
| |
OK, swapping my experimentalist's cap for a theoretical bowler, it seems to me that the key to this approach at lift is the dimensions and spacing of the apertures ("door slides.") |
|
| |
Given an atmospheric mean-free-path of 0.1 micron [link], as an order of magnitude approximation, apertures of this size could work, but the efficiency of the lifting surface would be proportional to the porosity. You don't want spurious molecules bouncing off the slide structure. (If the doors slide, doesn't that limit the open porosity to <0.5?) |
|
| |
Also, I imagine there will be a tradeoff between material strength and surface thickness, unless bouncing off the sidewalls would average out to 0 horizontal force (?) |
|
| |
Anyone care to do a drawing? It's hard to continue this discussion without a chalk- or white-board. |
|
| |
//If you used the molecule itself to swing open a spring "trap door", you'd defeat the purpose since the molecule would impart momentum to the system.// |
|
| |
but the impact of the downward moving particles would not be as big as that of the upward moving ones so you would still get lift - wouldn't you? |
|
| |
And you could have something on top, just not all over on top, like someone sitting cross legged in the middle |
|
| |
//A FEW DOTS!? ARE YOU STARK RAVING MAD!? // |
|
| |
Yes, of course I'm stark raving mad - I have the paperwork
somewhere. BUT IT'S STILL A FEW DOTS. |
|
| |
OK, suppose you magnify air until the molecules are about
the size of a marble each. At sea level, there's about two
feet between the marbles. GEDDIT? |
|
| |
Also, I'm not quite clear what the relevance of your
percentages is - if it were 100% nitrogen, or 1% of each of
100 different gasses, would that make any difference
whatsoever? |
|
| |
//If the doors slide, doesn't that limit the open porosity to
<0.5?// Interesting question - but not necessarily. Imagine a
square mesh with a "door" covering every other square. Each
door could slide to stop a molecule or let one through over
the entire surface (bar the thickness of the mesh). You'd
only have a problem in the rare cases where two molecules
needed to be let through (or two needed to be blocked) in
adjacent 'cells'. |
|
| |
//there are quite a few gas molecules floating around in any
given volume of air.// Yes, there are. There are also quite a
few stars floating around in any given volume of galaxy, but
it's still more space than star. |
|
| |
//lased air// Ooooh - I like that! |
|
| |
Harvesting thermal energy from the air... interesting. |
|
| |
The only drawback to this idea is that it won't work, and
[MaxwellBuchanan] ought to learn a little basic quantum
mechanics. |
|
| |
However, I would politely suggest to him that he has
inadvertently stumbled across a possible way of harvesting
lift from turbulent air; the doors could be bigger (up to the
size of the smallest "cell" of turbulence) and could operate
at practical speeds. |
|
| |
Many thanks. I shall continue to fill a much-needed gap in
the spectrum of unimmeasurably cunning ideas with
unlimitless possibilities. |
|
| |
//unlimitless possibilities.// meaning there are limits? |
|
| |
not really your invention, unless you are considerably older than seems reasonable. |
|
| |
// you are considerably older than seems reasonable // |
|
| |
There is nothing 'reasonable' about any of the scions of the House of Buchanan. If you ever have the privilege of visiting Buchanan Towers, DON'T go into the attic. The portrats up there are truly terrifying, particularly the one of Sturton that's clearly done on papyrus with heiroglyphic labellng. .. |
|
| |
The papyrus to which you are referring is _not_ a
portrait. It is a topographic survey. |
|
| |