Wikipedia:Reference desk/Archives/Science/2011 September 28

= September 28 =

Zero-point energy
Could zero-point energy be used for clean, limitless power generation? --70.134.53.27 (talk) 01:28, 28 September 2011 (UTC)
 * No. Dauto (talk) 02:02, 28 September 2011 (UTC)


 * No, please read the definition of zero point energy. Plasmic Physics (talk) 02:48, 28 September 2011 (UTC)
 * In case it's not immediately obvious, specifically the section on Claims in Pseudoscience.Vespine (talk) 06:17, 28 September 2011 (UTC)

Rechargeable batteries
I took the wrapping off several sets of batteries and now I can't tell which ones are rechargeable (and some of them are) and which ones are not. Not one of them says "rechargeable". I think the Energizer Lithium AA 03 2023 batteries are the rechargeable ones, but I can't seem to find anything on Google that says they are or they aren't, just lots of specs about their uses in cameras. Anybody out there know? Thanks Bielle (talk) 03:04, 28 September 2011 (UTC)


 * Having kept looking at batteries in general, I have concluded that if they don't say "rechargeable" writ large, then they are not. Thus, the Energizer Lithiums are not rechargeable. Comments appreciated. Bielle (talk) 03:15, 28 September 2011 (UTC)


 * The chemistry in rechargable batteries is very different than  the chemistry  of other batteries. Attempting  to recharge a non-rechargable battery involves unfun events like "leakage" and "overheating" and "explosion".  Don't do it.  Every rechargable battery is always clearly and unambiguously labeled as such, so if  it doesn't say "rechargable"  in a giant freindly font on the side of the battery, don't stick it in a recharger.   -- Jayron  32  03:37, 28 September 2011 (UTC)


 * Lithium-ion batteries are rechargeable, but Lithium batteries like Energizer Lithium aren't. -- BenRG (talk) 07:20, 28 September 2011 (UTC)


 * Lithium is a violently excitable substance if it is abused. A lithium fire would probably ruin your day. It cannot be put out by common extinguising substances such as water or carbon dioxide. Roger (talk) 09:26, 28 September 2011 (UTC)


 * In other words, say hello to the new battery-sized hole in your table. And to the huge blackened areas in your floor and ceiling. And I bet that any smoke entering your lungs will be completely healthy and won't leave you coughing for weeks. --Enric Naval (talk) 10:31, 28 September 2011 (UTC)


 * As a further note, the only common rechargable AA batteries are NiMH, NiCd and NiZn. Of these, NiMH are what you're most likely to encounter, NiCd are rare nowadays for various reasons including them containing cadmium, their general low capacity, the memory effect etc. NiZn are still relatively new and should be used with care since their nominal voltage is fairly high, may be too high for some devices. NiMH and NiCd batteries can usually be charged in the same chargers but NiZn need their own chargers. You can get rechargable Lithium-ion batteries (there are actually several kinds of rechargable lithium ion batteries) in AA size equivalent, more commonly called 14500 but these are not intended to be used with most devices accepting AA batteries because of the much higher voltage. These need special chargers and aren't really intended for the consumer market (although are popular in some circles) and instead are generally used for packs and in devices where they aren't intended to be removed, and you will not find Duracell ones. Because of the risks, I wouldn't recommend them if you don't know what you're doing. As has been noted, lithium batteries should be treated with care, even primary ones. Nil Einne (talk) 12:41, 28 September 2011 (UTC)

Normally if batteries are not rechargeable they will say "do not recharge" on them in small print somewhere. I have plenty of batteries which are rechargeable but don't explicitly say "you can recharge this battery" on them 82.43.90.142 (talk) 09:54, 28 September 2011 (UTC)

Thanks for all your help. I did some checking in with electronics stores today and they all, without exception, agree with Jayron: if it is rechargeable, it will say so; if it does not say so, do not try recharging for all the reasons shown above. I am concerned that 82.43.90.142's response is backwards, and may be dangerous, though perhaps this varies from country to country. Bielle (talk) 02:37, 29 September 2011 (UTC)

solar wind movement and direction
solar wind is the current of ejected particles from sun corona, those particles have 250~750 km/s velocity , which is further than sun gravity field escape velocity (180km/s).In addition the particles obey electromagnetic field rules which make them to have spiral rotation , suppose any particle (for example proton)how will it move at its way in interstellar space?akbarmohammadzadeIRAN--78.38.28.3 (talk) 03:36, 28 September 2011 (UTC)
 * Presumably, once past the heliopause, the wind will join the general interstellar medium where the particle's motions are effected by the prevailing magnetic field. Regards, RJH (talk) 22:10, 28 September 2011 (UTC)

hydostatic pressure and core of stars
can the diffrence between liquid and plasma matter change our ideas about the pressure and density and temperature of core of stars??akbarmohammadzadeIRAN--78.38.28.3 (talk) 04:06, 28 September 2011 (UTC)
 * I don't think so. They are only related in the sense that plasma can sometimes behave like a type of liquid. The current, widely-accepted stellar model holds that the cores of hydrogen fusing stars are entirely composed of plasma matter. At the millions of Kelvin needed for nuclear fusion of hydrogen, how can it be otherwise? Ergo, the properties of liquid matter don't appear directly relevant. Regards, RJH (talk) 21:55, 28 September 2011 (UTC)


 * Remember, there is pressure also to contend with. Plasma is not a state of matter, at least it is not distinct from solids, liquids, or gases. After all, you do get solid plasmas, liquid plasmas, and the more familiar gaseous plasmas. Plasmic Physics (talk) 22:06, 28 September 2011 (UTC)
 * Well, if plasma is not a state of matter, then clearly the first sentence of the Plasma (physics) article is in error. Regards, RJH (talk) 22:19, 28 September 2011 (UTC)
 * Yes, that is quite clear indeed. The misconception of plasma as a fourth state of matter is quite widespread, but has no basis in reality. Even if one were to dismiss ionic liquids as not being liquid plasmas because they derive their plasma-like quality from anions rather than electrons, there's still e.g. liquid metallic hydrogen which forms at tremendous pressures, and which derives its "plasma-like" quality from electrons, and as such is not actually "plasma-like" at all, but a clear example of liquid plasma. And as if that's not enough, metals in the process of conducting electricity fit the exact description of a solid plasma, to the point where the electron oscillations due to the current are literally called "plasmons".
 * So no, plasma is not a state of matter; there's gaseous plasma, which most people are familiar with, but there's also liquid plasma, and even solid plasma. 2A01:799:CE0:9000:AA7E:EAFF:FEDE:FF94 (talk) 23:22, 17 January 2024 (UTC)


 * Read the rest of my comment. Plasmic Physics (talk) 00:03, 29 September 2011 (UTC)
 * I did. No matter. RJH (talk)

Jet Sound
When the fighter aircraft pass over our heads they make such terrible sound, but why does the big Jumbojet(or Airbus) don't make so much noise though it has more and far bigger engines, when it is landing or taking off near us at airport etc, though it is quite close. 124.253.129.113 (talk) —Preceding undated comment added 04:15, 28 September 2011 (UTC).
 * Supersonic aircraft like jet fighters create a sonic boom when they break the sound barrier. Subsonic aircraft like jumbo jets don't.  -- Jayron  32  04:24, 28 September 2011 (UTC)


 * The planes don't always break sound barrier. Breaking sound barrier is a momentary phenomenon that last a fraction of a second...most of the time they are traveling below speed of sound 124.253.129.113 (talk) —Preceding undated comment added 05:02, 28 September 2011 (UTC).


 * Jumbo jets have high-bypass turbofan engines, which derive most of their thrust from pumping cold air through the bypass ducts and out the exhaust nozzle; this is for better efficiency at medium-high speeds (500-600 knots or so), but it also muffles the roar of the hot gases from the combustion section of the engine. Jet fighters, on the other hand, have straight turbojet engines that derive most if not all of their thrust from the hot gases; these produce more power at very high speeds (> Mach 1), but obviously don't have the same muffling effect from the cold air. 67.169.177.176 (talk) 05:32, 28 September 2011 (UTC)
 * The above is totally correct. If it's worth mentioning, this is not an accident, noise consideration is a considerable factor in commercial jet engine design. If anything, apart from absolute performance, fighter jets are, in complete contrast, designed to shock and awe. Vespine (talk) 06:15, 28 September 2011 (UTC)
 * Also, military jets sometimes use afterburning which makes them EVEN LOUDER! One civilian airliner with very loud engines was Concorde; her Olympus engines had been developed from those of a supersonic jet bomber. Alansplodge (talk) 17:47, 28 September 2011 (UTC)
 * I have always assumed that stealth aircraft are fairly quite though. Am I correct?  Googlemeister (talk) 18:18, 28 September 2011 (UTC)
 * Not as a general statement; it depends on what kind of stealthing the aircraft is concerned with. For instance, a helicopter operates a low altitudes where being noticed by foot soldiers can be a threat.  Thus, stealthing a helicopter reasonably includes sound-deadening measures.  Stealth aircraft operating at high altitudes, on the other hand, won't care about sound output to nearly that extent.  If they're quiet, it's more likely as the result of a happy design accident (perhaps aforementioned high-bypass turbofans are good for stealth purposes due to heat of exhaust) than intentional design methodology. &mdash; Lomn 18:50, 28 September 2011 (UTC)
 * I was going to say a simiar thing that noise is not generally a factor on "stealth aircraft" but our article on the state of the art F-22_Raptor seems to disagree. It says The aircraft was designed to be less visible to the naked eye; radio, heat and noise emissions are equally controlled.  Vespine (talk) 23:31, 28 September 2011 (UTC)
 * I'll provide a "yes, but" to that. Reading the source document, there is but a token generic statement ("To make a stealthy aircraft, designers had to consider... muffling noise....").  That's it.  The rest of that article discusses relevant stealth features in detail -- radar signature, visual signature, emcon, and heat signature.  There is zero mention of noise reduction as an actual meaningful objective of F-22 design.  I note also this image, from this F-35 acoustics test report, notes that the F-22 is the noisiest fighter in the modern US arsenal at both minimum and military power.  The F-35, itself a stealth design, is also relatively loud.  Finally, note that the acoustics study is for purposes of noise exposure to maintenance personnel and the general public around military airfields.  It's not a combat consideration at all.  &mdash; Lomn 03:18, 29 September 2011 (UTC)


 * Planes in supersonic flight produce a sonic boom continuously. It is not a momentary effect when they "cross the barrier". The sonic boom is highly directional, however, so an observer at a fixed location on the ground only hears it briefly, but observers everywhere along the plane's path will hear it. It's true, though, that military jets do not always flight faster than the speed of sound, particularly when traveling over populated land.--Srleffler (talk) 19:02, 30 September 2011 (UTC)

Although the noisiest aircraft in the world seems to be a propeller plane, the Tupolev Tu-95: "Its blades, which rotate faster than the speed of sound, according to one media source, make it arguably the noisiest military aircraft on earth,[2] with only the experimental 1950s era Republic Thunderscreech turboprop powered American fighter design as a likely rival." Count Iblis (talk) 23:51, 28 September 2011 (UTC)
 * See Republic XF-84H: "Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down." Alansplodge (talk) 00:58, 29 September 2011 (UTC)
 * Which is a lot more than what the noise from the Tupolev can do. 67.169.177.176 (talk) 01:40, 29 September 2011 (UTC)


 * The thrust of an engine is equal in magnitude to the change in momentum it imposes on the air. The change in momentum is the mass flow of air times the change in air's velocity. Since the minimum energy required for that grows quadratically with the change in speed and only linearly with mass flow (kinetic energy is m * v^2 / 2), the most fuel-efficient way to generate a given thrust is with a high mass flow (= high bypass ratio) and a smaller change in air velocity. Fuel efficiency is of high concern to airliners, so this is exactly what they have done. It's a nice side effect that a flow with a smaller velocity tends to make less noise. --145.94.77.43 (talk) 11:26, 2 October 2011 (UTC)

Question about GRAVITY?
Galileo was first to demonstrate that all objects fall at the same rate in the absence of an atmosphere. As it is said that the earth and the apple fall toward each other but apple looks a lot to falls to the earth as compared to the falling of earth toward the apple which is so tiny to be detected.

Let's imagine earth is a homogeneous sphere therefore in the following cases, what would be the direction of direction [falling] of earth in the absence of all other gravitational attraction including atmosphere?

1- If two apples start falling simultaneously from ANTIPODEAN trees from same altitude.

2- If two different masses [say one apple and other big asteroid] start falling simultaneously from same ANTIPODEAN altitude.

Also, would gravity "g" of two equal planets cancel each other if placed on each other?68.147.43.159 (talk)Eccentric Khattak#1 —Preceding undated comment added 04:55, 28 September 2011 (UTC).


 * Presumably this is a continuation of your question listed under "Question about the Galileo Statement"? ←Baseball Bugs What's up, Doc? carrots→ 05:01, 28 September 2011 (UTC)


 * I think there is some very fundamental goof here. He says apple and earth fall towards each other. The apple is OK, it falls i.e. it moves in a straight line towards earth, but does earth move towards apple ? I think no, as it is already "falling" - it is moving around the sun - that is technically falling, no ?. Now, how can something move towards two different directions at same time ? 124.253.137.182 (talk) —Preceding undated comment added 05:49, 28 September 2011 (UTC).
 * Different vectors. Any object in space is moving along a path that's the "sum" or "net effect" of its vectors (there may be a different word for that, but I can't think of it just now). Earth and moon orbit around a common point. That point orbits around the sun. The sun presumably orbits around the center of the Milky Way Galaxy. And the Milky Way is presumably heading along an approximately straight line away from wherever the Big Bang occurred. So the earth, viewed in isolation and relative to all these different forces, would be taking a very interesting path. As noted in the earlier section, the falling apple has a theoretical effect, but so small as to be negligible. ←Baseball Bugs What's up, Doc? carrots→ 05:52, 28 September 2011 (UTC)


 * Fair answer except for the moment where you talk about a place where Big Bang happened since Big Bang did not happen in a place. Big Bang is an expansion of the whole universe, not an event within the universe with specific time and space coordinates. Dauto (talk) 14:03, 28 September 2011 (UTC)
 * Are you saying the Big Bang is still going on? Also, is it possible to extrapolate all the galaxies back to a point? ←Baseball Bugs What's up, Doc? carrots→ 23:57, 28 September 2011 (UTC)
 * Yes, Metric expansion of space started with the Big Bang and has continued for the last 13,730 million years, though Dark energy might be the main cause now. It is possible to extrapolate back to a "point" but that "point" is now everywhere in the universe.    D b f i r s   19:58, 29 September 2011 (UTC)


 * Again you are essentially setting up a Three body problem. What don't you understand? It's fairly simple. In scenario 1, given "perfect conditions, the earth will stand perfectly still, pulled equally by the two apples. For 2, assuming the asteroid is more massive then the apple, then it will have more of a gravitational effect on the earth then the apple, however unless the mass of the asteroid is a meaningful fraction of the mass of the earth, the effect will still be insignificant. I don't actually understand what you mean by Also, would gravity "g" of two equal planets cancel each other if placed on each other? No, if your adding their masses, their gravity would also add up, not cancel out. Note, the gravity would NOT double with a doubling of the mass, it's one of those inverse square law situations. Vespine (talk) 06:09, 28 September 2011 (UTC)
 * The force on an object due to gravity does double when the mass is doubled. This means that the acceleration due to gravity remains the same. Depends exactly what the OP meant, though, which isn't clear. Grandiose (me, talk, contribs) 09:42, 28 September 2011 (UTC)


 * With both planet's particles being present, the various forces these generate will be present too, but they can create a static equilibrium with forces opposed to each other. If you lived in the place where the two equal planets are joined together, the net force due to gravity is zero. See for instance Lagrangian point #L1.  In a similar situation, gravitational acceleration decreases in a tunnel or well with increasing depth, see Gravity of Earth.  Living between the two planets should be interesting, being nearly weightless and being able to easily move around but with a significantly greater rate of fall as you climbed out. The building materials on the surface would need to be progressively stronger too. The L1 position is unstable, so apples will fall towards the planet that is nearest, but one might become adapt at calculating the inertia needed to miss the surface in a fall (see Douglas Adams' novels, :-), for more detail). Once you reached either "end of the world" the gravity due to both masses will not be doubled on account of the one planet being further away. So locally, there is no doubling, but as you leave the system, the difference in the distances to each planet can become so insignificant that the gravitational acceleration due to both can be practically twice that of only one planet. --Modocc (talk) 17:54, 28 September 2011 (UTC)

faster than light question
Just to clarify the predominant understanding of traveling faster than the speed of light actually means I take it then that if I am traveling on the back of a light beam and a faster than light is coming up behind me that from my perspective it is in my past and when it passes me it is in the future while I remain in the present? --DeeperQA (talk) 07:37, 28 September 2011 (UTC)
 * In many ways, FTL travel introduces many real paradoxes which cannot be resolved by a simple explanation in the English language (or indeed any language). -- Jayron  32  13:20, 28 September 2011 (UTC)
 * You keep asking the same question. Do you expect to get different answers? Speed is relative - that is: different observers can see objects moving differently. For someone in a train, their luggage is just sitting their on the floor, not moving at all, but that same piece of luggage is observed to be moving by someone at the train station. It turns out that the rules for transforming speed between different observers is more complex than believed by 19th century physicists and an object seen moving faster than light by one observer will be observed moving backwards in time by another observer leading to severe paradoxes. The simplest and most logical way to avoid those paradoxes is to assume that faster than light travel is not possible. Dauto (talk) 13:32, 28 September 2011 (UTC)
 * Also, if you are traveling on the back of a light beam, you have no past and no future since your time stops due to an infinite time dilation factor. From the point of view of a photon, the universe is flattened into a 2-dimensional pancake and the photon is created and absorbed in the same spot simultaneously. Dauto (talk) 13:57, 28 September 2011 (UTC)
 * The universe is fucking awesome. --Goodbye Galaxy (talk) 17:01, 28 September 2011 (UTC)
 * No, its more than that. Richard Avery (talk) 17:21, 28 September 2011 (UTC)

Growth
Is muscular exercise helpful in increasing height in adolescents? — Preceding unsigned comment added by 1.39.146.152 (talk) 11:26, 28 September 2011 (UTC)


 * Yes (or more accurately peak bone mass), with proper nutrition, of course. Though ultimately, height is determined by genetics, being in the best possible health while still young can have significant effects that carry on to adulthood. Health problems in the developing years can also lead to stunted growth. Also see Skeletal development during childhood and adolescence and the effects of physical activity (Kemper, 2000) and Bone development in young people (IOF).--  Obsidi ♠ n   Soul   11:59, 28 September 2011 (UTC)


 * I can't see anything there that justifies answering this question with a yes. Nutrition does matter, but to my knowledge there is no evidence that muscular exercise affects height. Looie496 (talk) 16:27, 28 September 2011 (UTC)


 * I did explicitly say bone mass. While it does not (or at least only very slightly) affect bone length, surely you can agree that being a bit more healthy can make the difference? --  Obsidi ♠ n   Soul   16:53, 28 September 2011 (UTC)


 * From my knowledge of developmental biology, increase in bone length is not caused by increase in muscle mass, and the diversion of resources to the muscles and premature bone closure by increased hormone levels might instead lead to stunting. If this is a serious question ask a specialized pediatrician.  There are far to many factors and risks to take comments here as above the level of nonsense. μηδείς (talk) 00:24, 29 September 2011 (UTC)


 * Ok now you're both putting words into my mouth. Read my original post again. First I never said anything about bone length. I said bone mass - i.e. bone strength, bone health. And I, again, explicitly said with proper nutrition and that it all depends on genetics in the end. And no, muscular mass is tied intimately (literally) to the bones they're attached on. You can not exercise any major muscle without affecting bone as well. And to the contrary, just as exercise has no effect on increasing bone length, neither does it stunt it. Unless we're talking about one of those crazy parents pushing ten-year olds into bodybuilding with steroids or malnourished kids working high-intensity jobs in sweatshops. I mean, jeez, why do you think we had PE classes? To stunt our growths? Nevertheless, I agree, talk to a pediatrician. Overenthusiastic and incorrect exercise can result in injuries that can stunt growth.--  Obsidi ♠ n   Soul   03:04, 29 September 2011 (UTC)

What manner of substance is this?
So, I was watching videos of shredders shredding things and found myself on this one, which shows large chunks of metal igniting and burning and popping and sparking whilst going through the machine. The video just calls it 'speciality metals' and there are a few guesses in the comments section - but does anyone here know for sure what the hell that stuff is? --Kurt Shaped Box (talk) 19:19, 28 September 2011 (UTC)
 * Looks a lot like magnesium to me; for liability purposes they may not want to name the metal, which is why they use the term "specialty metals". Magnesium is a fairly reactive metal, and it is flammable, even in pure CO2.  (google "magnesium and dry ice" for some fun videos).  The bright flashes look a LOT like magnesium when it burns, and grinding it up like that could easily produce enough heat via friction to ignite it.  -- Jayron  32  19:28, 28 September 2011 (UTC)
 * Oh, to work at the "magnesium shredder," "Nitrogen tri-iodide shredder," "dynamite shredder," or "TNT shredder!" Edison (talk) 04:30, 29 September 2011 (UTC)
 * I'd just settle for shredding propane tanks. Man, if I owned a big shredder, I'd be tempted to throw random objects in there, just to see what they look like whilst being ground up. I'd be going around looking at things and thinking 'will that shred?'. Hmmmm, I wonder if those guys could shred a (battle) tank whole? --Kurt Shaped Box (talk) 14:08, 29 September 2011 (UTC)

A high alkaline phosphate level
A high alkaline phosphate level of over 200 - can this have a relationship to lipo phosphates and also to lipodema- — Preceding unsigned comment added by 94.13.166.2 (talk) 21:44, 28 September 2011 (UTC)
 * Look at Alkaline phosphatase and . The articles do not mention a connection with Lipedema.  And what is lipo phosphate? If you interpreting a blood test result, you had better ask a doctor. Graeme Bartlett (talk) 12:48, 30 September 2011 (UTC)

Power required for levitation
Mass M is required to be levitated (stationary) in a gravitational field of strength g by pumping out a fluid of density rho over a cross-sectional area A (e.g. a ducted fan blowing out air). Assuming no losses, how much power is required?

I get Power = (M g)^(3/2)/sqrt(2 rho A), which somehow is not quite what I was expecting. Can anyone verify (or correct) this? Regards, 86.179.118.99 (talk) 22:08, 28 September 2011 (UTC)

Clarification: In case unclear, I mean that the fan is attached to (part of) the mass M and is also hovering (as opposed to blowing upwards from the ground). 86.179.118.99 (talk) 22:16, 28 September 2011 (UTC)


 * If it helps, this is called a gravity burn. "Assume no losses"?  The entire thing is a loss!  No work is done on Mass M - it isn't moving - which means that 100% of the work is done on the exhaust-gas!  Nimur (talk) 22:38, 28 September 2011 (UTC)


 * I mean no losses to heat/noise/etc (other than after the fluid has escaped and done its business, obviously). For example, a real fan would not convert all of the energy it consumes into moving the air in the required direction, I assume. Thanks for the link to the article, but it doesn't really help to me verify that exact formula, as far as I can tell. 86.179.118.99 (talk) 23:21, 28 September 2011 (UTC)


 * I concur with your formula. It is a measure of the minimum rate at which downward kinetic energy must be imparted to the air in order to stay stationary in the limit that air is incompressible.  In practice, there will be corrections (potentially large ones) from the fact that air is a compressible fluid and the fact that the amount of energy used to operate any real fan will always be significantly larger than the amount of downward kinetic energy acquired by the air.  Dragons flight (talk) 01:38, 29 September 2011 (UTC)


 * Thank you! Dragons flight. Do have any feel for whether greater compressibility would lead to more power being required or less? 86.160.212.172 (talk) 02:45, 29 September 2011 (UTC)
 * Unless I'm making some gross error of intuition, wouldn't it be more? Think of what a propeller does in incompressible water vs what it does in air. Vespine (talk) 04:19, 29 September 2011 (UTC)
 * I dunno. There are viscosity and density to take into account too... Plugging the density of water versus air into my formula above makes a huge difference. 109.156.50.129 (talk) 13:36, 29 September 2011 (UTC)
 * Right, I see what you mean. Ok, so I still think the answer is more. Planes need more power to fly high in thin atmosphere which is more compressible, right? I wasn't sure if thin atmosphere counts as more "compressible" but I think by the definition in our Compressibility article, it does. Vespine (talk) 01:23, 30 September 2011 (UTC)
 * Air a high altitude would be more compressible, but air already has a compressibility factor of near 1, so the change is minor. See http://en.wikipedia.org/wiki/File:Compressibility_Factor_of_Air_250_-_1000_K.png for a chart.  As a side note, how do I change that chart since there is no such thing as a °K?  Googlemeister (talk) 13:20, 30 September 2011 (UTC)
 * Also, thin air is lighter, so, again, more power would be needed according to the above formula, even without considering compressibility. 86.177.108.144 (talk) 17:12, 30 September 2011 (UTC)

Cantenna
Normally, cantennas DIY projects require that you have a USB-WiFi stick + cable or some other way of connecting the cantenna to your WiFi circuity. However, would it be possible just to do a cantenna without that? Just imagine a normal catenna + some other improvised devise forwarding the signal to your laptop (without messing with the laptop). 22:55, 28 September 2011 (UTC)


 * You mean a repeater? 802.11 repeaters are commercially available; for example, D-Link DWL-900AP+ can operate as a WiFi repeater.  Nimur (talk) 23:44, 28 September 2011 (UTC)


 * I wanted actually an inexpensive solution for connecting a laptop too far away from the router. A directional antenna (cantenna wa my first thought). I'll try to find a DIY repeater. Quest09 (talk) 23:46, 28 September 2011 (UTC)


 * A (DIY) Parabolic trough might be useful here. But it has to be of the right size up to the mm. Wikiweek (talk) 00:08, 29 September 2011 (UTC)


 * I outright reject the usurpation of the term "Cantenna" for some upstart waveguide doo-hickey used with Wi-Fi. The word has a long and honorable history as a descriptor of a RF dummy load embodied by a 50 Ohm resistor in a gallon can of mineral oil, used to adjust an amateur radio transmitter, and able to dissipate a kilowatt. Can the upstart handle a kilowatt? Hunh? Edison (talk) 04:23, 29 September 2011 (UTC)


 * Times change. In the past a computer was as big as a shipping container, but, alas, we still use the same word for a completely different machine. Quest09 (talk) 16:30, 29 September 2011 (UTC)