Wikipedia:Reference desk/Archives/Science/2012 April 29

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April 29

How do I mimic static over the cellphone?

In some situations, I feel the need to mimic increasing static on a phone before disconnecting. It needs to sound convincing, so what are the best methods? Thanks. --68.102.29.129 (talk) 02:26, 29 April 2012 (UTC)

Stand outside, hold phone into wind. --Mr.98 (talk) 02:53, 29 April 2012 (UTC)

This isn't always convenient when the day is calm, and when I'm in a bad position to go outside and hold the phone into the wind. Are there other methods just as good? --68.102.29.129 (talk) 03:36, 29 April 2012 (UTC)

Modern digital communication rarely injects Gaussian noise into the audio channel. A dropped call often occurs without warning, and the call simply disconnects. If you're seeking to realistically simulate a call interrupted by poor reception, simply disconnect at a random time. If you actually want to play "static" over the microphone for some other reason, consider recording some white noise and playing it in the background using an audio player. Nimur (talk) 04:10, 29 April 2012 (UTC)

Well, there is comfort noise. DMacks (talk) 09:01, 29 April 2012 (UTC)

I don't know about cell phones in particular, but in general simply blowing directly onto a microphone tends to produce a surprising amount of noise, unless you're using a dead cat or something. Red Act (talk) 04:40, 29 April 2012 (UTC)

The best trick might be to hang up while you yourself are talking, reducing the possibility that the other party will think you hung up on them. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:52, 29 April 2012 (UTC)

I agree - digital cellphones don't often get noisy before dropping out, but what they often do is chop up / interrupt your voice a few times before disconnecting. You could simulate this by rapidly placing and removing your finger over the microphone hole while continuing your talking. On most modern phones, it's only a single hole 0.5 to 1 mm diameter. If you do this more than once or twice to the same caller, they'll will catch on to what you are doing though. Wickwack121.221.228.248 (talk) 05:34, 29 April 2012 (UTC)

A Fibres in Frog Sciatic Nerve

Hello. Are the A-alpha fibres of the frog sciatic nerve the only component of A fibres that are myelinated? Thanks in advance. --Mayfare (talk) 18:20, 29 April 2012 (UTC)

I don't know the answer but a google snippet suggested that this paper might have some information. SpinningSpark 10:54, 30 April 2012 (UTC)

Gravity

Physicists variously talk about gravity as an element of space-time, as gravity waves, and as gravitons. Are these related in some way or are they three different views of gravity? — Preceding unsigned comment added by Gstrom47 (talk • contribs) 20:19, 29 April 2012 (UTC)

Gravitational waves and viewing gravity as the curvature of spacetime are part of the same model of gravity, general relativity. Gravitons are an attempt to extend quantum field theory to gravity, which is a different approach in that general relativity doesn't deal with quantum mechanics at all. Red Act (talk) 21:02, 29 April 2012 (UTC)

On the other hand, I don't want to give the impression that general relativity and gravitons are completely unrelated. Two ways in which gravitational waves and gravitons are related are that both travel at the speed of light, and they both have the same symmetry: if you rotate either a gravitational wave or a graviton by 180 degrees, it will wind up looking the same as before the rotation. It's just that one involves a quantum field and the other involves a classical (nonquantum) field, very similar to the relationship between electromagnetic waves and photons. Red Act (talk) 21:53, 29 April 2012 (UTC)

They are all closely related. A graviton is a quantum of gravitational wave energy, and Feynman diagrams with virtual gravitons are mathematically related to the field equations of general relativity in much the same way that a Taylor series is related to the function it approximates. -- BenRG (talk) 00:48, 30 April 2012 (UTC)

Agree with all of the above. General relativity is a classical field theory and so gives rise to gravitational waves which are analagous to electromagnetic waves in classical electromagnetism. If you try to develop a quantum field theory version of general relativity (which is one of the possible routes to a quantum theory of gravity) then gravitational waves are replaced by gravitons, in the same way as photons replace electromagnetic waves in quantum electrodynamics. So gravitational waves and gravitons are two different ways of describing the same phenomena - one classical, one quantum. However, in contrast to electromagnetism, we have not yet directly detected gravitation waves (although there is some indirect evidence for their existence) or gravitons. Gandalf61 (talk) 10:53, 30 April 2012 (UTC)

Both you and Red Act suggest that gravitons replace gravitational waves in quantum field theories. That seems very misleading to me. "Graviton" is just another word for "gravitational wave" in quantum field theory. It's equally true that in quantum theory sound is replaced by phonons. Anything involving vibration gets quantized. That's no call to invent new terms for everything, much less to treat them as new concepts. -- BenRG (talk) 17:29, 30 April 2012 (UTC)

Your QFT knowledge is much stronger than mine, but my understanding is that assuming that gravitons exist, then gravitational waves of the strength for example being searched for by LIGO et al can basically be viewed as a coherent state of many gravitons. However, although the existence of gravitons would imply the existence of gravitational waves, the converse is not true; gravitational waves are predicted to exist (by GR) independently of whether gravity actually quantizes into gravitons. To put it another way, if LIGO and even more powerful detectors continue to fail to find gravitational waves, that would be a blow to GR in a way that, say, a theoretical result in QFT showing that gravitons can't actually exist would not be. So gravitational waves and gravitons are at least different in that regard. Would you agree with that assessment? Red Act (talk) 18:48, 30 April 2012 (UTC)

Actually, I stand by "replaced". A classical wave is a solution to a system of PDEs such as the wave equation, Maxwell's equations or the Einstein field equations. It assumes that space, time and other properties of the system such as energy and momenta are continuous and take on definite values, and our knowledge of their values is limited only by the sensitivity of our instruments. A quantum model is fundamentally different. It involves probability waves that are superficially similar to the waves in the classical model, but we cannot observe them directly due to wave function collapse. What we observe are quanta. Only when a large number of quanta are observed does the classical wave picture emerge at a macroscopic level. Gandalf61 (talk) 14:05, 1 May 2012 (UTC)

Why were the X-15 rocket-powered aircrafts retired?

Why were the X-15 rocket-powered aircrafts retired? 82.31.133.165 (talk) 21:09, 29 April 2012 (UTC)

As the X indicates, they were experimental aircraft, never intended to be useful for any practical purpose. They were intended to test a concept, and once that concept was proven, there was no longer much point in flying them. Looie496 (talk) 21:19, 29 April 2012 (UTC)

Indeed. Compared with other experimental aircraft, X-15 had a remarkably long career (over 10 years, 199 flights, on three airframes). X-15 research fed directly into the space shuttle's glide phase design (and I imagine into classified hypersonic control projects like MIRVs). -- Finlay McWalterჷTalk 21:24, 29 April 2012 (UTC)

As for rocket-powered aircraft, in general, they suffer from a rather short flight time, which makes them less flexible (you can't circle the runway for long, waiting for a chance to land). A large turn radius is another problem. StuRat (talk) 21:31, 29 April 2012 (UTC)

The X-15 was a military research vehicle. The USAF's plan was to incrementally develop space transport - there should have been a successor to the X-15. However, the response of the US Govt to the success of Sputnik and other Russian space projects was to decide that a) a "space race" was necessary, and the USA must win it, and b) it must be done with a civilian effort. This was so that that orbiting altitudes and space did not becaome a military domain. Thus the military programme was terminated, and NASA created, with von Braun in charge. von Braun was partial to large "brute force" vertically launched disposable rockets, rather than re-usable flyable planes. Ratbone58.170.164.197 (talk) 14:58, 30 April 2012 (UTC)

That makes for a nice story, but the first X-15 flights didn't occur until after NASA was created, and the last of the 199 flights was more than a decade after Sputnik started the space race. Of the 12 people to ever fly an X-15, five were NASA personnel, including the first man on the moon Neil Armstrong. Yes, the successor vehicle was terminated in favor of the heavy lift rockets that came to represent the space race; however, it is too simplistic to say that X-15 was killed simply because it was military. The X-15 was originally proposed and created as a joint operation between the USAF and the civilian NACA program, which later became NASA. As a joint USAF / NASA project, it enjoyed a far longer life than most X- series experimental flight vehicles. Dragons flight (talk) 15:25, 30 April 2012 (UTC)

Nowhere did I say the X-15 was killed off - I only said the military programme (ie X15B, Dynasoar etc) was terminated and there was no successor when there should have been. It is misleading to say the X15 did not fly until NASA was created, without further qualification. It takes much much longer to design and build a air/space vehicle than it does to create a govt agency. Planning for the X-15 goes back to at least 1955, probably even further in meetings, discussions and the like not in the public domain. Nobody thought of creating NASA until 1958. Pres Eisenhower did not sign the enabling law into effect until late 1958, and Russian space achievements were the driver. Ratbone124.182.4.180 (talk) 02:55, 1 May 2012 (UTC)

Preventing water damage when building a building

Is it necessary, since the building still doesn't have roof, or is it not, since it's just concrete? XPPaul (talk) 22:12, 29 April 2012 (UTC)

If the builders get to the point where there are things inside the building that would be damaged by water before the roof is finished, then can cover the top with plastic sheeting or something similar (I have seen that on building sites). I think it's normal to finish the roof before you start to work on the interior, though, so it would just be concrete and bricks - a bigger problem is if you are working on the roof of an existing building, then you some kind of temporary protection. --Tango (talk) 22:34, 29 April 2012 (UTC)

Just to make things clear, a bare reinforced concrete frame of a building in construction won't get damaged by direct water contact?XPPaul (talk) 23:09, 29 April 2012 (UTC)

Well, the concrete needs a dry period to set. After that, the concrete itself should be OK, but if there's no drainage, water could accumulate and undermine the foundation, causing it to fail. Rebar can also rust, if sitting in water. This could also cause the concrete to fail. I seem to recall a walking bridge over a highway failing that way a few years back. Then filler in low-quality concrete may also be damaged by water. Chunks of iron, for example, will also rust there. Most of these effects would require a lot of time and water, though, so one or two rainstorms won't be a problem. StuRat (talk) 23:51, 29 April 2012 (UTC)

There can be a problem with timber that has too much moisture if it is going to be enclosed and unable to dry out. SpinningSpark 23:23, 29 April 2012 (UTC)

Niagara falls into dam

Is it theoretically possible that the Niagara Falls could be turned into a hydroelectric dam? Also, how long will it take until the Great Lakes drain into Lake Ontario? 64.229.204.143 (talk) 22:46, 29 April 2012 (UTC)

Anywhere with a significant flow of water could be turned into a hydroelectric generator. They don't usually use waterfalls, though - it's easier to create your own drop using a dam. There would be a lot of engineering problems with using the Niagara falls, and they would also lose all the tourism. I'm not sure what you mean about the Great Lakes - they aren't draining out, there is always water being added from rivers that drain into them. --Tango (talk) 23:04, 29 April 2012 (UTC)

I don't think a flow of water is sufficient. You also need a canyon, so your dam can be like the plug in a bathtub. To build a dam on flat ground would require that you build the entire "bathtub", which would cost more than it would be worth. In the case of Niagara Falls, I believe the part above the falls is fairly flat. StuRat (talk) 23:42, 29 April 2012 (UTC)

No canyon necessary, but a slow flow (at low pressure) would not generate electricity very efficiently, so a reasonable height difference is usually preferred. The advantage of using a canyon (or similar geographical feature) is that a large reservoir can be maintained at reasonable cost, so that generation can continue in dry weather. Dbfirs 06:52, 30 April 2012 (UTC)

If you mean no canyon is necessary for hydroelectric power generation, I agree, since this is currently done at Niagara. However, if you mean that a large dam can be constructed economically without a canyon, which is what this question is about, then I disagree. StuRat (talk) 15:13, 30 April 2012 (UTC)

No, I wasn't suggesting that a large dam would be economic without some sort of natural valley, so we are in agreement, except on what the question is about. My point was that only a height difference is important, not a dam or canyon. Dbfirs 07:25, 1 May 2012 (UTC)

The title and first sentence would seem to indicate that the question was about a dam, not other forms of hydroelectric generation. StuRat (talk) 07:30, 1 May 2012 (UTC)

... but it doesn't say "large dam". Anyway, we agree on the essentials, and that generation is separate from damming. Dbfirs 07:10, 2 May 2012 (UTC)

The land around Niagara Falls is pretty flat, although there are escarpments, like the Niagara Escarpment, which is why there's a waterfall there at all. Also there is a canyon, or at least a gorge, below the falls, extending to about Lewiston, NY (Google Maps terrain shows it here, [1]). It would probably be possible, in theory anyway, to build a dam near Lewiston, creating a reservoir backing up to the falls. It's easy to think of various reasons why this hasn't been done, and won't be. Instead they built "off-river" reservoirs. Pfly (talk) 01:33, 1 May 2012 (UTC)

Also, for more on hydroelectric dams with minimal reservoirs, see Run-of-the-river hydroelectricity. Pfly (talk) 01:41, 1 May 2012 (UTC)

One final tidbit I can't help but mention--there have been some very grand plans envisioned for Niagara Falls. One of the wildest was described in The Human Drift by King C. Gillette--the guy who founded the Gillette company. He pictured a giant utopian city built right on top of Niagara Falls, rectangular in shape, 135x45 miles large, built out of porcelain, the only city on the entire continent, powered with electricity from the falls, and run by a "world corporation". Pfly (talk) 01:56, 1 May 2012 (UTC)

Kinda like Fallingwater, only much bigger. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:03, 2 May 2012 (UTC)

Niagra Falls is used for hydroelectric power generation - see Niagra Falls#Power. So much so that the amount of water the power plant uses is limited by treaty for fear of the Falls drying up to a trickle. SpinningSpark 23:28, 29 April 2012 (UTC)

According to our Niagara River page it was the very first river used for hydroelectricity in North America. There's a monument to Nikola Tesla on Goat Island right at the falls. In satellite imagery, like File:Niagara Falls SPOT 1314.jpg, you can see the two large reservoirs used for hydroelectricity generation (one in the US and one in Canada). Water is diverted from the river above the falls into the two reservoirs and then returned to the river below the falls, falling through large turbines to make electricity. In short, not only is Niagara Falls already used for making electricity, it is one of the most famous and historic examples of hydroelectric power. Pfly (talk) 23:36, 29 April 2012 (UTC)

As has already been noted, the Great Lakes are continually refilled with rain and snow melt. However, even if all precipitation in the Great Lakes watershed ended, the lakes still wouldn't "drain", as the lakes are deeper than the rivers connecting them (Detroit River, Saint Clair River, Niagara River, Saint Lawrence River). So, each lake would only drain to the depth of it's outlet river. From there, it would eventually evaporate. StuRat (talk) 15:19, 30 April 2012 (UTC)

(psst, also the St. Marys River!) Pfly (talk) 04:15, 1 May 2012 (UTC)

Thanks, couldn't remember the name of that one. There's also the Straits of Mackinac, although I'm not sure if that's shallower than Lake Michigan. StuRat (talk) 08:40, 1 May 2012 (UTC)

Also don't forget the Chicago River which was canalized from an inlet river to an outlet river. It is technically feasible to get from New Orleans to the mouth of the St. Lawrence via boat. Another way to put that is that the entire eastern portion of the U.S. is technically an island, as it is completely surrounded by water (though not all natural sources). --Jayron32 23:20, 1 May 2012 (UTC)

Kinda like Cape Cod, only much bigger. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:04, 2 May 2012 (UTC)

What are typical Power capacities for suburban Distribution Substations??98.225.64.151 (talk) 23:13, 29 April 2012 (UTC)

I live in Arizona, with major summertime Air-Con loads. I do know that my own neighborhood substation was built in 1985 on about 1 acre of land, and is fed by a 138kV line. I've asked my Utility for that substation's Power rating, but they won't tell me because of "security reasons." Thanks in advance to you Power engineers.

You are asking the wrong question and/or are asking a question, which if you had the answer, the answer would have no meaning for you. 138kV lines would generally be used to supply power at levels exceeding 20 MW (megawatt), up to 600 MW or more. This is a power level that is appropriate for entire districts up to significant cities. A substation requiring 1 acre is a huge substation. If you want to know if what feeds your house is adequate, you need to focus on the distrubtion transformer and/or RMU (Ring Main Unit, a type of switch) that feeds your street. This is typically about the size of one or two domestic fridges. In most countries, these are mounted up on poles if you have open wire distribution, or in roadside cabinets in underground cabling is used. In the USA, with undertground street cabling, the transformers & RMU's are often underground as well. Instead of asking what is the power rating, you coukld ask something like what is the site design power capacity, and what has been the load growth over (say) the last 10 years. But even this will be pretty meaningless, because there is a lot of substation reconfiguration, load transfer, and other upgrade work the power company can do, so that even if a substation appears to be at 100% capacity, there is likley nothing electricity consumers should be concerned about. In any case, substations at the level you are talking about are usually fully redundant - if there is a fault, the power company can changing switching and feed the loads from somethwere else, and they can switch loads as required to even up the loads on various interconnected substations. Wickwack121.221.89.56 (talk) 03:08, 30 April 2012 (UTC)

It is a very reasonable and sensible question, but not one easily answered based only on the area of the substation and the incoming transmission voltage. 138kv lines differ in their current capabilities, based on conductor size, for instance. To figure out the answer to your question, one would need to know the transmission capability of the 138kv line or lines entering the substation, and the capabilities of the power system (generating stations or substations) supplying those 138kv lines. That represents the maximum power could be supplied to transformers at the distribution substation. The next questions is how many transformers of what megawatt rating are located at the distribution substation, to step the 138kv transmission down to 34kv, 12 kv or 4kv distribution. Other elements which could limit the power are any other things the current might pass through, such as current transformers, inductors, bus bars, switches, circuit breakers and wavetraps. A utility strives to make sure that some cheap element is not the limiting factor. Then one could consider the capabilities of the distribution lines leaving the substation. In the end, a maximum power capability could be computed, based not on the strongest but on the weakest link in the chain. One utility I am familiar with can have up to four 40 MVA (megavoltamp) transformers stepping down 138kv transmission from two incoming transmission lines to about 30 12 kv distribution lines, for a total of 160 MVA. I know of other substations in urban areas with four 50 mva transformers. Other utilities might have varying ultimate designs for their substations. (The megawatts would be somewhat less than the MVA due to the power factor of motors etc being less than one). A utility might start with one 30 mva 138kv-12kv transformer transformer in a substation. Edison (talk) 01:50, 1 May 2012 (UTC)

I agree with Wickwack - the "power rating" is not a question meaningfull or relavent to a consumer. However the rest of what Edison says is valid and supports Wickwack. It should also be noted that the effective power (or (VA) capacity of a substation may depend on its' switched configuration: E.g., due to internal bus limitations, if power is drawn from incomming feeds A and B and switched to supply distributions X and Y, while feed C supplies distribution W, the total capacity might be different if say distribution B is switched on to feed D. Particular distributions may be feed from more than one substation. Sometimes large consumers with critical functions (eg satelite tracking stations, hospitals) have feeders from two or more substations so that they have power no matter what accident or fault happens in either feeders or substations. Ratbone124.182.4.180 (talk) 03:10, 1 May 2012 (UTC)

Also, a utility typically builds in a LOT of redundancy, so that one or more transmission lines, transformers, bus ties, bus sections, and distribution lines can be out of service without having to drop load. They are not so optimistic as to assume everything will be working perfectly on a peak load day. A substation might be expected to serve only half the load it is nominally capable of, to allow for contingencies. Additionally, the "stiffness" or available fault current from a substation might have an impact on the amount of flicker all the other customers from a substation see when some industrial load such as a steelmaking arc furnace, or a very large motor is switched on and off frequently. This is one aspect of the power available besides the issue of how much current it could supply without burning something out (transmission line, transformer, etc). Edison (talk) 03:33, 1 May 2012 (UTC)

Leftover rabies?

So it seems 2 animals of some sort had a fight in my back yard last night. Bits of fur and whatnot laying about. So: IF one or more had rabies, AND IF said animal bled or drooled or whatnot on the ground, what danger would there be of being exposed by being in that area? Obviously I didn't roll around in the grass, but I'm just curious. Thanks in advance. — Preceding unsigned comment added by 71.62.103.2 (talk) 23:31, 29 April 2012 (UTC)

Pretty low, unless you intend to use the drool-covered fur to make a bandage for a cut on your arm. Even then you probably wouldn't get rabies (but might get some other infection). StuRat (talk) 23:39, 29 April 2012 (UTC)

This website has a fairly detailed description of what counts as exposure, and surprisingly (to me at least) it states that contact with blood of an infected animal does not count as exposure. Of course in your case there might be some saliva mixed in with the blood, so I still wouldn't go touching it. Vespine (talk) 23:54, 29 April 2012 (UTC)

I don't believe you can get rabbies by touching contaminated saliva or blood. Bites are obviously a different ball park. XPPaul (talk) 00:11, 30 April 2012 (UTC)

Actually you can should you have exposed wounds or get it on mucous membranes. See this from the CDC which gives nonbite criteria. Our article, Rabies transmission, mentions cave explorers being infected, although TTBOMK the route of transmission such as aerosols IMO was somewhat speculative although not unreasonable. To safeguard other scavenging animals such as pets from getting into the mess, I'd use a shovel to remove any large pieces and rinse the area down with a garden hose. Plus, avoid not tracking any of it in with your shoes. --Modocc (talk) 01:05, 30 April 2012 (UTC)

Yes the website I linked does explain that it is possible to be infected through "non-bite exposure" to saliva or CNS tissues, such as brain matter. As the above states, you would need to contact mucous membrane (which can be as simple as rubbing your eye) or a wound such as sa cratch or abrasion. Vespine (talk) 01:28, 30 April 2012 (UTC)