Wikipedia:Reference desk/Archives/Science/2010 July 29

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July 29

Turbojets

The article on turbojets seems to indicate that they are more efficient than turbofans above Mach 2, but it doesn't really give a clear answer. Is that true? --The High Fin Sperm Whale 00:48, 29 July 2010 (UTC)

According to this chart (in the Jet Engine article) a turbojet will be more efficient (measured in Isp) than a turbofan starting at about Mach 1.5. I suspect this is because the turbojet will have a higher average exhaust velocity than the turbofan. A turbofan exhausts a large volume of lower velocity air, but since the thrust has to do with the difference between airspeed and exhaust velocity, the lower exhaust velocity means lower thrust at a high airspeed. The turbojet exhausts a smaller volume of high velocity air, so it maintains higher thrust at a high airspeed.

anonymous6494 02:14, 29 July 2010 (UTC)

Thanks, but the chart does not show turbojets. Thanks anyways. --The High Fin Sperm Whale 02:31, 29 July 2010 (UTC)

For these purposes, you can think of a turbojet as a low-efficiency turbofan - so on that chart, they will lie just below turbofans. For example, the engine that powered the infamous Concorde will have an Isp of around 3000 s at cruising speeds (just over Mach 1). You can verify this for yourself by converting the thrust specific fuel consumption in to specific impulse (the equations are given in our article for a variety of units). In this case, Olympus 593 engines have a SFC of ~1.2 lb/lbf·h - so convert by (3600/1.12), yielding Isp around 3000 s at cruise speed of Mach 1. In reality, the distinction between a turbofan and a turbojet is a semi-subjective distinction about what counts as "low pressure" bypass. There are many engines that could pass for either description. Compare the cartoon diagrams of turbofans to turbojets - as you can see, the big difference is expansion ratio of the bypass flow - and there is not a clear demarcating line between the two. In the extreme case, of course, we can clearly see that a large amount of ducted air is not "jet-like", and you can keep making the bypass ratio higher and higher... at some point you stop bothering with a nacelle and end up with an unducted turboprop (even more efficient! but much slower). Nimur (talk) 03:50, 29 July 2010 (UTC)

What is the lightest/least dense non-porous solid?

So if Aerogel is the least dense porous solid, which non-porous solid has the lowest density? —Preceding unsigned comment added by 71.163.18.235 (talk) 01:51, 29 July 2010 (UTC)

Lithium is the lightest element that's solid at room temperature at 0.53 g/cm³. I don't know whether there are any less dense compounds out there - but I suspect not. SteveBaker (talk) 02:02, 29 July 2010 (UTC)

If we can freeze non-solid-at-RT things, solid hydrogen I think is probably around 0.07 g/cm³. Or at least that's what hydrogen says the liquid is at the melting-point, so probably close? The article specifically about the solid has few physical properties. DMacks (talk) 02:38, 29 July 2010 (UTC)

It's 0.088 g/cm^3 which is the lowest of all known crystalline solids. Dragons flight (talk) 02:44, 29 July 2010 (UTC)

Could you add that with cite to the solid hydrogen, and maybe also hydrogen articles? DMacks (talk) 06:18, 29 July 2010 (UTC)

For this one, we might have to define "porous". Everything would have exactly the same density if it were not for the void-spaces. It just so happens that some solid materials have void-spaces on molecular scales that are defined by crystal lattices; some solid materials have void-spaces on macro-crystalline or polycrystalline scales; some solid materials have void-spaces that are large enough to be filled by air or gas; and so on. One could conceivably build a sturdy steel shell and pump out the air from its interior: would it be fair to call that a low-density material? After all, there would be nothing but steel in it; nothing would be "diluting" its volume or mass. And it wouldn't really be porous. How small of a pore is required before you call a material "nonporous?" Nimur (talk) 04:05, 29 July 2010 (UTC)

It should also be noted that an aerogel is not a material per se. It is a colloid; which is to say it is a type of mixture. As a mixture, it is a combination of multiple materials; aerogel being a mixture of the solid matrix and air. The density of a mixture is dependent not only on the properties of the components, but the proportions in which they are mixed. Pure substances generally have well-defined densities, for example look at the infobox of iron or or sodium chloride or sucrose. However, not being a pure substance, one cannot define a specific density for aerogel in general, only for specific formulations of aerogel. --Jayron32 06:42, 29 July 2010 (UTC)

The Dirac sea perhaps. Cuddlyable3 (talk) 17:59, 29 July 2010 (UTC)

If you could get enough Di-positronium together and crystallize it you would get something about 1000 times less dense than solid hydrogen. However it is too radioactive with too short a life for this to happen in our current technology. Graeme Bartlett (talk) 10:23, 30 July 2010 (UTC)

Airport Runways

I would like to know if Airport Runways are built in one direction taking Earth's rotation into consideration. Are they built East-West? —Preceding unsigned comment added by Shyamlal777 (talk • contribs) 02:17, 29 July 2010 (UTC)

I don't think that the Earth's rotation needs to be taken into consideration, since the atmosphere spins with it. --The High Fin Sperm Whale 02:20, 29 July 2010 (UTC)

A trip to any reasonably-sized airport would disprove "Airport Runways are built in one direction". And even single-strip airparks would be likely to disprove "Are they built East-West?". They're built at whatever directions space allows and that is viable for prevailing weather conditions. Our runway article talks about it in a whole section about orientation. Airplanes care a lot about wind, which can blow in any arbitrary direction regardless of the earth' rotation, though there may be some places that almost always have wind in a certain direction. Weather systems are certainly affected by the earth's rotation on a large scale, but not "rotation -> that's how the breeze goes in general". Rockets and other things that are not relying on air motion for their own motion and that need to move in a certain direction relative to the earth could rely on the rotational boost. DMacks (talk) 02:29, 29 July 2010 (UTC)

Right. Most places have a prevailing wind, actually, and airplanes can take off and land more easily facing into the wind than facing with it or sideways to it. Looie496 (talk) 03:44, 29 July 2010 (UTC)

SImple land geography is a big factor too. Runways have to go where hills and mountains aren't. HiLo48 (talk) 04:28, 29 July 2010 (UTC)

I'm a pilot, and I can assure you that there is no set way that a runway must be oriented. DMacks, Looie496, and HiLo48 are all correct. They are built with the local environment, "normal" winds, etc in mind. Not all runways are the same. Some have [slight] hills in them, so they have a pronounced dip or crest in the middle, some are longer, some are shorter, some are wider, some are narrower, some can handle more weight, and there are some where you take off and land in one direction because there's a mountain on one end. It really depends almost entirely on what the runway is to be used for and where it is. Falconus^{p t c} 05:06, 29 July 2010 (UTC)

Indeed. The airfield just down the road from me (LEIG) is planning on extending the runway: the main consideration was where they could buy the land from (although it's also one of those one-way runways with a biggish hill at one end). Physchim62 (talk) 05:30, 29 July 2010 (UTC)

Other then what's already been said, I think the consequences of the flightpath the runway will impose will also get consideration. For example a runway which means the planes will be largely taking off over water or flat, uninhabited land would generally be preferred over one which results in planes over taking off or landing over houses (meaning when height is not a concern) not just for safety reasons but because planes tend to be noisy particularly when low flying Nil Einne (talk) 07:27, 29 July 2010 (UTC)

Landing flight paths over uneven terrain are avoided if possible because signal reflections can distort the radio guidance beams. See the article Instrument landing system#Limitations and alternatives. Cuddlyable3 (talk) 17:44, 29 July 2010 (UTC)

About CAN networking

Dear Sir, i have a fiat car wit me some days back its stop working , some engineer came and said some one is putted cut in the main network line(CAN Network) bcoz of that its stop start working,now they have rectify that error.Now i want to know that whether dis cut can produce any error in future nd what kind of cut should i make in the networking so that my sub woofer(Amplifier) work properly .

Thanks —Preceding unsigned comment added by Kuks222 (talk • contribs) 02:46, 29 July 2010 (UTC)

The CAN-bus is an automotive network technology. Think USB for a car and it's a close approximation. As such, the repair shouldn't lead to errors down the line. Presumably they replaced a cable and, unless that cable is cut again, that's not a part that's likely to fail any more than USB cables go bad (which is to say, rarely). Of course, a future cut would produce the same problems. As for the subwoofer, as far as I know, car stereos don't use CAN-bus to carry audio signals. They'd be analog speaker wire rather than digital CAN-bus. — Lomn 03:54, 29 July 2010 (UTC)

I'm wondering if the person who installed the subwoofer cut the CAN bus wire, thinking it was the audio line and trying to re-route it to the subwoofer amplifier. In that case, you should make them pay for any repairs necessary. Nobody should have touched your CAN bus to install an amplifier. If the CAN bus is damaged, depending on your particular FIAT model, there may be serious safety concerns operating the vehicle - some engine control units and safety features like brake or airbag controllers may use the CAN for communication. Most cars have a separate CAN bus for critical vehicle systems and "passenger-compartment" things like radios and DVD players; but the information provided is insufficient to know what has been cut. Have a mechanic you can trust analyze the problem, and make whomever cut the wire pay for it. Nimur (talk) 04:13, 29 July 2010 (UTC)

iitmadras

Hi I am from Hyderabad. I want to do my MTech from IITs but my aggregate is less than 60% in my BTech. But in IIT Madras website I saw the minimum qualification is a degree and GATE score http://gate.iitm.ac.in/mtechadm/gelig.php Can I get the admission if my GATE score is good?? —Preceding unsigned comment added by 115.109.194.75 (talk) 12:09, 29 July 2010 (UTC)

why do stocks prices move during the day even when there is no new information!

why is it that even in boring times with no new information, multibillion dollar companies' stock prices fluctuate at least a few percentage points continuously every second or few, all day, every day? For me, it would make more sense for them to stay in a flat line for a few day (or hours, if a piece of news comes unexpectedly) then jump to the new price based on the new information and stay in a flat line until it jumps to a new price due to the next piece of news. But this never, ever, ever happens even within a single day (for the largest companies) let alone for days or weeks. Why is that? You don't get a different price at the gas station every fifth of a second, why do you on the stock market? 84.153.183.42 (talk) 16:31, 29 July 2010 (UTC)

The price does not depend only on the single company, it also depends on the industry group. It also depends on the relative risk between that company and other companies, and between that company and bonds. So other things change, which causes a relative change, which moves the price, i.e. other stuff might suddenly be slightly better or worse. Ariel. (talk) 16:37, 29 July 2010 (UTC)

so? Information changes in the real world second by second, but the gas price isn't updated at the pump that fast. What's the difference? 84.153.183.42 (talk) 16:39, 29 July 2010 (UTC)

There isn't a single, efficient market for gas, there are lots of different gas stations. If I want to buy a share in a company, I will buy the cheapest share on offer. If I want to fill my gas tank, I go to the most convenient gas station and pay whatever they are charging (within reason). --Tango (talk) 16:46, 29 July 2010 (UTC)

Gas prices do actually change every second, it's just the gas stations refuse to adjust the price because it would make people nuts. They do earn and loose money on the changes! They hate volatility for this reason. It's even worse for them - they may buy a truck full of gas for a high price, and the next day prices are lower, and they loose money on all the gas they are storing. And they reverse too, and they hope it works out long term. Ariel. (talk) 17:00, 29 July 2010 (UTC)

There is a constant influx of new information: all the buy and sell orders that are going on. Friday (talk) 16:43, 29 July 2010 (UTC)

Yeah, the price is determined by what people have recently traded it for. When the price fluctuates that means there have been trades and since different people have slightly different ideas about what the stock is worth the price will change. What people are paying in turn affects what other people think about the value. There is also new information all the time in the sense that lots of things can subtly affect the market and people are working very hard to analyze the incoming data and update their valuations. When the stock of some other company goes up, when a new trade agreement gets signed, when a new oil reserve is discovered, that can affect the value, even if the company has little to do with these other industries. There are things happening in the world all the time. Rckrone (talk) 17:13, 29 July 2010 (UTC)

Yes. Reported stock prices are snapshots of continual trading activities. The only way to get a prolonged flat "noise free" price would be for nobody to trade in that stock. Gas (petrol) stations that have electronic price displays can use them to play "gotcha" knowing that different market segments refuel at characteristic times e.g. commercial vehicles on weekdays, private vehicles at weekends. However changing the retail price too often would be counterproductive as bad PR. Cuddlyable3 (talk) 17:31, 29 July 2010 (UTC)

Also, consider some investor who holds 10,000 shares of BoringCo. It has had no news for a month and will have no news for another month. But that investor is also investing in and monitoring ExcitingCo, which you can find 20 news stories on every day. When news on ExcitingCo makes the investor think the shares are going to go up, he may sell 5,000 of his BoringCo shares to invest the money in ExcitingCo instead; and when ExcitingCo announces all their iPads are overheating, the investor may dump it all and invest the money back in safe old BoringCo. Multiply this by many investors and there's plenty of price fluctuation, though all this buying and selling of BoringCo has nothing really to do with news coming out of BoringCo. Comet Tuttle (talk) 18:08, 29 July 2010 (UTC)

This is really a question about human behaviour related to financial greed. That also applies where computers are assisting because humans have programmed the computers. It's all about people trying to gain the tiniest advantage, and then gain financially from it by multiplying that tiny advantage thousands of times. HiLo48 (talk) 19:49, 29 July 2010 (UTC)

Dog repellent

I need to keep dogs away from a part (not very large) of my garden and I'd like to find out if there are some substaces whose smell acts like a deterrent/repellent for them. Obviously not something armful. The best would be a "homemade" substance relatively odourless for humans but as disgusting as possible for dogs. --151.51.156.20 (talk) 18:27, 29 July 2010 (UTC)

I have seen some products sold, though I cannot recall the company. --Chemicalinterest (talk) 19:37, 29 July 2010 (UTC)

Suggestions I've heard include lemon slices, or ammonia, but the latter can make it pretty unpleasant for humans too. And I've had a beagle that decided he liked to eat citrus fruit, along with the million other things beagles typically found attractive. It can depend on the breed of dog, and what it is in that part of your gardent that could attract them. Scent hounds can be attracted to something that seems heavenly to them but quite unobservable by humans. Any human created product has to overcome a scent we can't even detect, quite a big ask. A bit of trial and error may be needed. Can't do a wire fence? HiLo48 (talk) 19:59, 29 July 2010 (UTC)

If the dogs are yours and it's acceptable to have them wear shock collars while outside, you could also bury a wire around the forbidden area and make an invisible fence. Comet Tuttle (talk) 20:06, 29 July 2010 (UTC)

And if the dog is not yours—or you just want to keep things simple—just build a visible fence. --Mr.98 (talk) 13:39, 30 July 2010 (UTC)

Double S(not T)P in one day

If the pressure of the atmosphere gradually increased to double its normal pressure within a day, what would be the effects on human life and industry? --Chemicalinterest (talk) 20:36, 29 July 2010 (UTC)

I think it would still be safe to breath, SCUBA divers breath standard air at that pressure all the time.

The boiling point of water would go up by about 30 degrees C. So your spaghetti would cook a lot faster.

Fires would probably burn hotter and stronger with twice as much oxygen per unit volume of air. You can imagine that both these things would effect large industrial processes like power generation. Not all of them would be able to adapt. APL (talk) 21:10, 29 July 2010 (UTC)

If the boiling point went up, wouldn't spaghetti take longer to cook and not cook faster? ~AH1^(TCU) 00:52, 2 August 2010 (UTC)

Climbing Mount Everest would be a walk in the park. HiLo48 (talk) 21:13, 29 July 2010 (UTC)

Hmm I don't know about your last point. I still think it will be hard to climb 20,000 feet even though you could breathe easily. --Chemicalinterest (talk) 21:49, 29 July 2010 (UTC)

True, but, as mountains go, Everest is apparently quite an easy one to climb. You can basically walk up most of it. The cold, wind and lack of oxygen are the main problems, rather than the physical act of climbing that high. --Tango (talk) 00:15, 30 July 2010 (UTC)

You'd need to put more helium in your Zeppelin, but once you did that it would have greater carrying capacity. APL (talk) 22:14, 29 July 2010 (UTC)

You'd get a lot more horsepower out of your car too - no need for a turbocharger! But even if humans could endure that - I'm sure that there would be abrupt and spectacular decline in animal and plant populations for one reason or another. It's hard to predict exactly what all of the details might be but I would guess that forest fires would be much more common - and insanely difficult to control, more gasses would dissolve into the oceans - and that could change their pH and cause all manner of other nastiness for oceanic life. It's pretty certain that it would lead to total disaster of one kind or another within a matter of months. SteveBaker (talk) 22:25, 29 July 2010 (UTC)

Be sure to check your tire pressure before trying out your new-found horsepower! You'll find it's nearly 15psi lower than it should be. APL (talk) 00:04, 30 July 2010 (UTC)

Not overnight, but eventually you'd get really big bugs. Most insects rely on diffusion (rather than an active circulatory system) to provide oxygen to their tissues and this is a key factor in determining their maximum sustainable size. Increasing the pressure of oxygen will allow insects to evolve large sizes. Dragons flight (talk) 22:43, 29 July 2010 (UTC)

As evidenced by the giant insect fossils found from times in the Earth's past where the proportion of oxygen was much higher, eg. the Meganeura. Not all scientists agree about the role of higher oxygen in allowing large insects to survive, though. (That article gives some details.) --Tango (talk) 00:15, 30 July 2010 (UTC)

Well that's convinced me not to follow through on my evil plan to double air pressure and thus rule the world. -mattbuck (Talk) 19:30, 30 July 2010 (UTC)

Major changes in weather and climate. The amount of solar heat escaping to space would be greatly reduced, so the Earth would get quite a bit hotter and wouldn't cool nearly as much at night. Looie496 (talk) 00:13, 30 July 2010 (UTC)

The troposphere would expand, making the tropopause higher, and thus producing taller thunderstorms, larger hail, more intense lightning, etc. Sea levels would likely initially fall due to higher surface pressure exerted downward (can anybody confirm?). The pressure gradient would likely be greater, making for stronger winds. ~AH1^(TCU) 00:52, 2 August 2010 (UTC)

Liquids are barely compressible, so a 1 atm higher pressure at sea level will have minimal impact on the ocean depth. Googlemeister (talk) 13:48, 2 August 2010 (UTC)

what type of endocytosis is this?

Surely it's not phagocytosis right? These are lung cancer a549 cells, not white blood cells. (Watch the gold particle in the upper left corner of the centre region.)

• File:First capture animation.gif

John Riemann Soong (talk) 22:12, 29 July 2010 (UTC)

Image converted to link. Animated gif was more than 2MB — that's rather large to include inline. TenOfAllTrades(talk) 23:34, 29 July 2010 (UTC)

helium,

How many pounds of helium is necessary to support an object with a weight of 200lbs,utilizing one square meter of material filled with the gas?The height of the object is six feet.What type of material should be utilized to constrain the pressure? The apposing force to the object is water. Thank you Sincerely, Tom Ambron email {email removed} —Preceding unsigned comment added by 75.82.85.84 (talk) 23:08, 29 July 2010 (UTC)

We do not respond by email here at the ref desk -- you'll just have to check back. DRosenbach ^{(Talk | Contribs)} 23:22, 29 July 2010 (UTC)

Do you mean that you wish to construct a balloon with 1 square meter of surface area? This will limit you to a maximum sized balloon based on the surface-area of a sphere (the most efficient volume-to-surface-area structure). If you use a balloon, the necessary consideration is "how many kilograms of helium are needed to inflate the balloon? If the material is inelastic, you can fill it with helium at atmospheric pressure (or the pressure of the water at the depths you are considering). Then you can use the ideal gas law to calculate the mass of helium that will inflate the volume at that pressure. Now the question is whether the resulting density of helium is sufficiently low to provide 200 pounds of buoyant force. I think you need to restate the question - it is not really clear what you're looking for. We can solve all kinds of equations, but when the requirements are unclear, such solutions may be inapplicable. Nimur (talk) 23:33, 29 July 2010 (UTC)

It's simply not possible. Forget the helium, even the mythical vacuum balloon couldn't do it. If you mean that the balloon is a cubic meter, the displaced air weighs only 2.6 pounds. If you mean that the envelope is made of exactly one square meter of material, then the displaced air is only ¼ pounds! A balloon, no matter what it's filled with, cannot lift more than the weight of the air it has displaced.

You may be of the common misconception that helium has negative weight and that the more you cram into a balloon the lighter it gets, I'm afraid that's not true. Balloon lift comes from buoyancy, the same force that causes boats to float. A common analogy is that wood floats, but if you pile enough wood onto a ship, it will eventually sink. APL (talk) 23:58, 29 July 2010 (UTC)

Is there any theoretical particle that exerts negative gravitational force with which we could fill the ballon? —Arctic Gnome (talk • contribs) 01:19, 30 July 2010 (UTC)

No, there is no such particle. Antigravity is firmly in the realm of fiction. As far as exotic matter with negative mass, there is currently no consistent explanation for how such a particle would behave; there is no need for such a particle to play any role in the standard model; and any speculative theories about negative mass or repulsive gravitational force is exactly that - speculative. Nimur (talk) 04:00, 30 July 2010 (UTC)

The anticharm quark exerts negative gravity. --142.104.53.238 (talk) 22:50, 1 August 2010 (UTC)

I don't understand that bit about water. Are you saying the object is underwater? Helium at above atmospheric pressure would be worse than helium at atmospheric pressure, since there is more helium taking up the same amount of space, so it will weight more while displacing the same amount of air/water. The absolute best buoyancy you could get with 1m² surface area would be a complete vacuum (you would need a strong container to hold a vacuum, which would therefore be heavy, but let's ignore that and assume a massless container). Such a vacuum-filled container submerged in water would be able to lift the weight of water displaced by the container. As Nimur says, the best shape is a sphere. The surface area of a sphere is $A=4\pi r^{2}$ and the the volume is $V={\frac {4}{3}}\pi r^{3}$. If we substitute A=1 into the first and rearrange, we get $r={\frac {1}{\sqrt {4\pi }}}$. We substitute that into the other formula and we get $V={\frac {4}{3}}\pi {\frac {1}{(4\pi )^{\frac {3}{2}}}}=0.094\mathrm {m^{3}}$. That is 94kg or 207lb, so you could just lift your object. If we're using helium at atmospheric pressure, rather than a vacuum (which is wise, due to the weight of material required to cope with the pressure), then we have to minus off the mass of that helium, which is about 16 grams (if I've calculated it right - I expected it to be more, but I don't know why), so not significant. If I misunderstood your mention of water and your object is actually just in air, then you don't stand a chance. --Tango (talk) 00:06, 30 July 2010 (UTC)

Instead of using the ideal gas law directly, under standard temperature and pressure, one mole of any gas has a volume of just over 22.4 litres. Nitrogen-14 has a molar mass of 28 grams, oxygen-16 is 32 grams, helium-4 (which is mono-atomic) is 4 grams. Therefore air (80% nitrogen, 20% oxygen approx), has a density of 28.8 grams / 22.4 litre. A helium-balloon has a buoyancy of 24.8 grams/22.4litre or 1.1 grams/litre.

As for the water analogy, imagine an (theoretical) advanced life form that lives on the sea bed. Their density is well above that of water, so they can't swim up. If they want to lift an object to the surface of the water then they can attach a balloon to the object. As long as the object+balloon is less dense that water then it will rise. If the balloon is filled with vegetable oil (900grams/litre) then there is 100grams/litre of lift in water. The balloon still has real mass, it's just that the density is less that water, so it will rise.

BTW, a wooden boat, full of wood, will float, but very close to the water. It will act like a solid wooden object. In fact they were used for demining sea areas during WWII, see Naval_mine#Mine_breaking.

CS Miller (talk) 03:38, 30 July 2010 (UTC)

Sure, but you couldn't keep piling wood on indefinitely. Eventually the weight of the wood would exceed the boat's capacity and the boat would sink. (Once the boat fills with water and the cargo itself becomes buoyant, the comparison to a balloon in air breaks down, of course.) APL (talk) 00:29, 31 July 2010 (UTC)