Wikipedia:Reference desk/Archives/Science/2012 February 25

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February 25

Hermitian condition in Dirac notation

A Hermitian operator satisfies

$\int \operatorname {d} x\left[(A\psi )^{*}\psi \right]=\int \operatorname {d} x\left[\psi ^{*}A\psi \right]$

How do you write this in bra-ket notation? --superioridad ^(discusión) 02:01, 25 February 2012 (UTC)

Is this homework? You need to use the fact that $(A\psi )^{*}=\psi ^{*}A^{*}\!$. -- BenRG (talk) 02:24, 25 February 2012 (UTC)

This is not homework. So is it then just $\left\langle \psi \right|A^{*}\left|\psi \right\rangle =\left\langle \psi \right|A\left|\psi \right\rangle$? --superioridad ^(discusión) 02:42, 25 February 2012 (UTC)

Yes. Though you could also write $(A|\psi \rangle )^{*}|\psi \rangle$ for the left hand side. You don't have to use $(A\psi )^{*}=\psi ^{*}A^{*}\!$, it just makes it look nicer. If this were homework they would probably want the nicer looking version. -- BenRG (talk) 02:47, 25 February 2012 (UTC)

Electric trains and rails

Since electric trains put the return current to the running rails, is there any danger of shock if someone touches the running rails with a train nearby? Does the return current go to the ground directly under the track, or does it travel along the running rails? 169.234.111.180 (talk) 02:37, 25 February 2012 (UTC)

Yes, there is danger. See Third rail. RudolfRed (talk) 04:33, 25 February 2012 (UTC)

Assuming there's a third rail present, that would be dangerous. Inadequately-bonded running rails might present a danger, but I don't know what difference in potential might normally be present between the running rails and ground. Probably not much, if any, given that people walk over tram rails all the time in wet weather, or across rails at station platforms. I have no definitive information to offer, though. Acroterion (talk) 04:39, 25 February 2012 (UTC)

So just to clear things up, the current from the third rail/overhead lines is picked up by the train, and is returned through the wheels of the train, to the rail, and then to the ground; and the current doesn't have to return to the power station through the running rails, right? 169.234.111.180 (talk) 04:57, 25 February 2012 (UTC)

From this paper, it sounds like most modern systems (DC at least) do use the running rails to return the current, which can lead to dangerous shocks, but the lines are equipped with special devices which measure the potential between the rails and the earth, and open a connection if the potential gets dangerously high. Smurrayinchester 11:30, 25 February 2012 (UTC)

In DC traction systems, it is a lot harder to keep the current in the rails and out of the earth, than it is for AC, but it is still the aim. DC systems are a nightmare for causing corrosion in any metal (rails, pipes, buildings, whatever), and causing all manner of problems and safety issues in nearby AC and low voltage systems. DC Traction is a very old and obsolete system devised when the only way to vary the speed of an electric traction motor without punishing loss of efficiency was the Ward-Leonard motor-generator-motor system http://wiki.riteme.site/wiki/Ward_Leonard_motor_control_system. As soon as magnetic amplifiers were developed (1930's to 1950's) AC could be used at lower cost, and DC traction became obsolete, much to the relief of engineers everywhere, and the distress of litigation lawyers. From the 1960's onwards, semiconductor control (TRIAC/SCR) was available, giving extremely high reliability and efficiency with AC power. Keit60.230.195.53 (talk) 12:50, 25 February 2012 (UTC)

DC traction is certainly an "old" system, dating back to about 1895, but hardly an "obsolete" one, since many of the larges urban rail systems use DC, and plan to retain them for the indefinite future. See List of current systems for electric rail traction. Agreed, AC is the typical choice for new electrification, especially in less dense population areas and for long distance transportation. The simple regenerative braking of DC allows a train slowing down or going down a grade to power a different train on the system which is accelerating or going up a grade, rather than just dumping the energy into resistor banks on the train or into heating up brake linings. AC regeneration is far more complicated.Edison (talk) 23:02, 26 February 2012 (UTC)

Competent electrical engineers never design systems so that significant current returns to the power station or substation via the earth - earth current only occurs during certain fault conditions. This is because earth current causes a wide range of problems, including interferance and hazardson telephone lines (even if overhead and not buried) and hazardous conditions on metal pipelines. The rails thru which the locomotive return current flows are connected at regular interavls to a return cable system, which "drains" off the current. In the overhead feed system, transformers are installed each couple of km or so to force the return current out of the rails. With overhead systems, it is not in the least dangerous (electric shock wise) to touch the rails - of course being run over by the train is another matter. With 3rd rail systems, yes - touch the 3rd rail (which is the energising rail), and you can die. Fortunately 3rd rail systems are not very common in most countries, due to performance issues, problems with interference to other services due to the necessarily low voltage & high current, the need to securely fence off and gate the railtrack, and residual serious safety issues (what happens if the train breaks down? Usually passengers get fed up after a while and want to get off - Don't ever do that) 3rd rail systems are used most often in coutries like Britain, whose focus of safety and reliability, due to cultural standards & industrial history, is lower. In countries with a greater focus on performance and safety (eg Australia), 3rd rail feeding is not allowed. Keit60.230.195.53 (talk) 05:55, 25 February 2012 (UTC)

I think that you will struggle to find evidence that there is a lower focus on safety and reliability in Britain than Australia. The main issue with both the original electrification of urban railways in Britain and converting the existing network is bridges and tunnels - the cost of rebuilding these would be enormous. Mikenorton (talk) 14:50, 25 February 2012 (UTC)

There might be other reasons than "a greater focus on performance and safety" for Australia not having the third rail systems found in the US and Britain, such as not having dense urban areas needing electrification of rail systems as early on. The assertions sounds a bit chauvinistic. As for side effects of the current used to power rail systems, I agree that DC leakage promotes corrosion, but AC causes inductive interference with phone and other signal systems. "Competent engineers" realize induction is also a problem with AC powered trains, especially with thyristor or other solid state controls, since the rails are used as returns and transposition is not the simple affair it is with overhead power transmission systems. Edison (talk) 23:02, 26 February 2012 (UTC)

There's heaps of evidence that reliability and safety is less a priority in Britain than in Australia, for cultural and historical reasons. One of the more well known examples known to electrical and electronic engineers in Australia is colour TV. Britain was first, Australia didn't go colour until 1974. There was immense pent up demand. To meet it some companies decided to import British-made sets. These didn't last long in the market - they were rubbish compared to Australian and Japanese sets - too many faults, too many safety compromises, too expensive to repair due to rough construction & old-fashioned circuitry. One set, "Decca", shortly after market release in Australia, was banned by the Authorities, because not only was it liable to give electric shocks if only minor wear & tear defects occurred, it had a transformerless half-wave rectifier power supply, causing DC in your house earth, which causes corrosion and earth system failure in the street MEN earthing system. A truely disgusting thing - yet it was a major volume seller in Britain. Those of us of my generation remember British cars, before they were driven out of the Aust market when Japanese designs began to be imported and made locally in Australia in the 1960's. British cars in comparison to Jap and American - sourced cars were cheap, nasty, unsafe, had high failure rates, and poor finishes that didn't last. Before WW2, Britain had a huge export market. After WW2, little exported. Why? Because by pre-war standards, British products were on a par, but during WW2, other countries used their experience in making high quality war materiel to improve quality, but Britain was economically damaged, and resumed with pre-war standards and expectations. Another good example: motorcycles: In the 1960's Japanese bikes (Honda, Yamaha, etc) became available in Australia. That was the end of British bikes (BSA, Triumph). British bikes had poor finish, high failure rates, and vibrated. Jap bikes had high standard of finish, better handling and brakes, very low failure rates, and were smooth. I could go on and on with many many more examples. Australia has, ever since WW2, been economically much stronger than Britain, with a higher standard of living. That has allowed Australia to spend the money to do things right. Yes, it cost a lot of money to put in Overhead feed AC rail traction. Australia just went ahead and did what was necessary for a reliable safe system. Keit60.230.195.53 (talk) 16:19, 25 February 2012 (UTC)

The vast majority of UK electric stock that does not run underground uses overhead (Great Northern Electrics uses overhead and switches to 3rd rail underground - a matter of a few miles). Safety is a complex issue and the comparison between two countries can't be dealt with easily (there is some interesting background comparing DV/AC and UK US on Talk:Vacuum_tube). The sad fact is that the three pin system, for example, used in the UK and Australia (with different plugs) is being undermined by two-wire devices suitable for the US and mainland Europe, which are significantly less safe. Although the British manufactures post war certainly took a much smaller share of the world market, and quality was a problem (which was mainly about the Far Eastern improvement in quality, as you say) none of these things generalise that well. We see, for example, cycles in the competitiveness and quality of other producers, and we see an increase in the demands of standards bodies such as the IEEE regulations. Earth bonding requirements in the UK, for example, have changed radically in the last 50 years. Rich Farmbrough, 20:30, 25 February 2012 (UTC).

In Australia at least, if the plug does not have an earth pin, the appliance must conform to the requirements of "double insulated", and must display the symbal. This makes it more safe than an earthed appliance, not less. The reasoning for the 3-wire eath system is that if appliance metal work thru a fault becomes live, the user is protected from electric shock because the appliance metalwork is as low in voltage as anything else nearby. But this assumes that the appliance and the house wiring is not otherwise faulty, and the house is multiple earthed per code requirements. If this is not so, or the source is a portable generator (which makes earthing rather a theoretical exercise), the user can still be electrocuted. There are other, admittedly quite uncommon, circumstances too complex to explain here, where you can be electrocuted with earth-pin/3-wire systems that are not faulty. With double insulated appliances, it is virtually impossible for such faults to occur, and thus virtually impossible for users to be electrocuted. In any case, in Australia, the use of ELCB's for ALL wall outlets installed for portable or mobile appliances is mandatory. Lastly, IEEE is an American professional body for engineers (broadly similar to IET in Britain), and has nothing to do with the UK. Keit121.221.230.136 (talk) 00:36, 26 February 2012 (UTC)

The IEEE article seems to disagree with your characterization of it. 75.41.110.52 (talk) 21:31, 26 February 2012 (UTC)

Despite what you may have or have not misread or read in either my posts above or in any Wiki article, it is a fact that the IEEE, as an AMERICAN association of professionals, cannot and does not control how things are done in Britain (or any other country) any more than the BRITISH standards bodies can expect to control how things are done in America - that is the point I made. Basically, both are associations of professionals extablished to maintain professional standards by dissemination of information, however they differ in detail as to how they do that, and what they do. If you work in Britain, you will have to conform to any applicable British mandatory standards, which will in many aspects require you to violate some aspects of American standards. This does not mean either standard is wrong, as they will have been written in different contexts. The same with working in America - you will have to conform to NEC etc, not British standards. This is not to say you cannot use the standards of another country as a GUIDE to good practice, should a local or international standard to cover the subject not exist. Generally, if there is a local standard, comply with that, if no local standard comply with an applicable International (eg IEC) Standard, if no international standard, comply with an applicable voluntary industry code of practice, if no industry code, use a subject applicable standard or code from another country if in your profesional judgement it seems fit, if not, just use your best professional judgement. Keit124.182.21.71 (talk) 02:03, 27 February 2012 (UTC)

"more than 400,000 members in more than 160 countries, about 55% of whom reside in the United States" from the article with two references. If you have better references please improve the article. 75.41.109.190 (talk) 20:08, 27 February 2012 (UTC)

The data on membership is totally non-relevant to both the OP's question and the points I've made, however the reasons for the membership is very interesting. As a practicing consulting engineer based in Australia but with some limited international work, I am a member of both the IEEE and the IET. Neither have any "control" in the countries my customers are located in, but a) membership gives me access to their journals so I can keep current on scientific developments etc, and b) membership gives me some international credibility. The equivalent Australian institution is the IEAust. Keit124.182.55.62 (talk) 09:02, 28 February 2012 (UTC)

I removed the excess "E" from my post, and refer the hon. IP to the IEE/IET regs BS 7671. I note that these are also the standards for "Mauritius, St Lucia, Saint Vincent and the Grenadines, Sierra Leone, Sri Lanka, Trinidad and Tobago, Uganda, Cyprus". Also that they have a requirement for RCDs on all household sockets. Double insulation is no match for a properly earthed device, there is no reason, of course, that a properly earthed device can't also be properly insulated. Rich Farmbrough, 02:56, 27 February 2012 (UTC).

Incorrect - a double insulated appliance, which may have exposed metal work, SHALL NOT HAVE THIS METAL WORK EARTHED, as that defeats the whole idea of double insulation. If you earth it, you will reduce safety, not increase it, as earthing it allows the user to be electrified due to the house or whatever earth/nuetral wiring being at voltage above earth either due to faults or in circumstances where a good MEN system earth is not possible. Double insulation, as difined in standards, is, atleast with the Standards I am familaier with (eg Australian Standards) is not a matter of being "properly insulated" or good insulation, it is a set of construction and testing requirments that make an appliance fault that electifies exposed metal work, or metal parts that can be exposed without the use of a tool, virtually impossible. For example, the complete failure of any single insulation item shall not be possible or shall not enable such electrification. See my post on this above. Note: I have added the word "otherwise" which was inadvertantly left out on a previous post of mine. BS7671 would be a British Standard, not an IET publication. In any case I cannot access it without paying money, which I will not do just for a RefDesk debate - can you post the title of the standard and the relevant paras, should you feel I've got something wrong somewhere? Keit58.170.170.136 (talk) 03:36, 27 February 2012 (UTC)

The OP may be interested that in the New York City subway there are what looks like short snips of limp metal cable connecting all the expansion gaps in the regular rails. I've also seen things spaced at regular intervals on the running rails which appears to be where the power goes after passing through the train. You must be rather close to the train for all three rails to be electrified. I've heard on the news of people electrocuting themeselves on the third rail. However, I don't think I've ever heard of someone being electrocuted by the running rails, even being saved in the nick of time from the train by a Good Samaritan, which happens sometimes (you know, very large population, a crazy bum tries to kill themeselves, even other people). Unless you're under the influence of a substance it's likely intentional if you get electrocuted as the TR's covered on 5 out of 6 sides. (and Mythbusters has busted that you can get electrocuted from peeing on it) Sagittarian Milky Way (talk) 00:32, 28 February 2012 (UTC)

"Things spaced at regular intervals" may be the drain system I mentioned in my post near the start of this thread. The drain system is designed to take the return current out of the rails so that it doesn't normally try and return via the earth - that ensures that the return rails stay at very low voltage compared to earth, and you can't be electrocuted. However short snips of cable as described across the rail expansion gaps are required in any case to ensure end-to-end good rail conductivity within sections for signalling purposes - when the train is near, and passing a road crossing, it electrically shorts the rails together and this is detected by the signalling controller box to bring the barriers down & turn the lights and bells on, so that the cars & trucks halt to let the train thru. Keit124.178.178.1 (talk) 06:56, 28 February 2012 (UTC)

Extreme oxidation states

Is the +9 oxidation state (or any other oxidation state higher than +8) possible? (Talk:Potassium nonahydridorhenate suggests CoH₉, RhH₉ and IrH₉.) What about oxidation states lower than -4? Double sharp (talk) 07:09, 25 February 2012 (UTC)

The highest I've ever heard of is +8 (Xe(VIII)), but my ignorant mind is certainly not all-knowing. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 14:31, 25 February 2012 (UTC)

No idea about what's possible, but the Oxidation state article mentions +8 and –4 as the currently-known extremes. DMacks (talk) 15:50, 25 February 2012 (UTC)

Well, with enough energy and the right equipment you can zap any number of electrons from an atom (see Ionization energy) - so your maximum "oxidation state" there is +Z. However that's quite an artificial set-up - I guess you're thinking about states found in actual, stable molecules. LukeSurl ^{t c} 00:08, 26 February 2012 (UTC)

Yes, I am thinking about states that could (hypothetically) be found in a stable molecule. How stable would Ir(IX) probably be? (Co(IX) and Rh(IX) would probably be less stable, but what about Am(IX) and Mt(IX)?) Double sharp (talk) 04:34, 26 February 2012 (UTC)

This paper thinks Mt can have a +9 oxidation state: Himmel, Daniel; Knapp, Carsten; Patzschke, Michael; Riedel, Sebastian (2010). "How Far Can We Go? Quantum-Chemical Investigations of Oxidation State +IX". ChemPhysChem 11 (4): 865–9. doi:10.1002/cphc.200900910. PMID 20127784. Ratbone60.230.199.111 (talk) 13:17, 29 February 2012 (UTC)

Transistor question

for a transistor the current amplification factor is 0.8 ,when transistor is connected in CE configuration . calculate the change in the collector current when base current changes by 6mA? — Preceding unsigned comment added by 117.252.66.67 (talk) 15:23, 25 February 2012 (UTC)

No. You calculate it...it's your homework not ours. DMacks (talk) 15:49, 25 February 2012 (UTC)

Can you multiply 6 by 0.8?--92.29.192.13 (talk) 20:35, 25 February 2012 (UTC)

I know that this is just a homework problem, and the OP needs to run with the figures given, but does a current gain of 0.8 make any sense in a common emitter circuit? Transistor beta states: "It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications." In practice, would you ever see a CE current gain of less than unity? -- ToE 09:12, 26 February 2012 (UTC)

does the Q mean the alpha of the transistor insted of the beta?--92.29.200.31 (talk) 11:57, 26 February 2012 (UTC)

It took me a moment to realize that by "Q", you meant Q, not Q. -- ToE 12:19, 26 February 2012 (UTC)

The terms "alpha" and "beta" (although beta is still in common use) have been obsolete for decades - Alpha was a useful property with the very earliest of transistors. Yes, the current gain in CE mode (termed hfe) is usually much greater than unity (may be as much as 100,000 or more for very small "supergain" transistors, up to 1000 for small discretes intended for audio premaps), but it can be significantly less than unity for very high voltage switching power transistors. So, the question should probably be taken as written. Unless the correct standardised symbols are used (hfe in this case), it is impossible to be certain. Keit124.182.18.249 (talk) 12:32, 26 February 2012 (UTC)

What's the point of the "being" portion of Human Being

I know "human" refers to our genus, Homo, but I don't know what the point of the "being" is. It doesn't refer to our species, sapiens, so what's the point? I know you can refer to other animals in the genus Homo as human, but are they "Human Beings" as well? ScienceApe (talk) 19:29, 25 February 2012 (UTC)

Well, it's not a scientific term. I would hazard to say it is simply idiomatic — that is, the "being" doesn't mean much other than "I am implying this individual human has a soul of some sort," or something similarly meant to imply a notion of "dignity." The OED says that a "human being" is simply "a person, a member of the human race; a man, woman, or child." This implies to me that while you could drop the "being," it's that last part that matters — you're implying personhood, you're implying some specificity (man, woman, child), you're implying that they're alive (or invoking their formerly living status explicitly), you're implying something more than a pure zoological classification. The term dates back to at least the 17th century. --Mr.98 (talk) 19:36, 25 February 2012 (UTC)

Back to the days when adjectives were not usually used as nouns. "Human" has become a noun, but it was originally an adjective describing a type of creature, or being. We still sometimes hear the expression "alien being", but it's usually abbreviated to "alien". Same thing with human creatures or beings. "Creature" has a mainly pejorative use, so "being" is preferred. -- Jack of Oz ^{[your turn]} 20:52, 25 February 2012 (UTC)

Some folk think of human as an adjective only, like Latin humanus (although the OED cites substantive uses back to the 16th century), so they need to use "human being" for the noun (Latin homo). Deor (talk) 21:07, 25 February 2012 (UTC)

I will just note that the use of human as a noun goes back to the early 16th century. So it's not that recent, though of course usage patterns vary. Ngrams suggests using human as a noun was not uncommon but far less common than using "human being".[1][2] (just two variants I used to try and gauge relative frequency) --Mr.98 (talk) 21:43, 25 February 2012 (UTC)

You have anologies in other languages, too: human <=> human being; Mensch <=> menschliches Wesen; humain <=> un être humain. As above "pre-scientific" notation. 213.169.162.159 (talk) 09:16, 26 February 2012 (UTC)

...and 'ser humano' in Spanish. Richard Avery (talk) 14:08, 26 February 2012 (UTC)

Being. ~AH1 ^(discuss!) 18:53, 26 February 2012 (UTC)

And "istota ludzka" in Polish. Equivalent terms exist in practically all European languages. They are all calques for Latin philosophical terms from Aquinas that are in turn calques for Greek terms from Aristotle. Dominus Vobisdu (talk) 18:59, 26 February 2012 (UTC)

Actually, isn't "being" a verb? Maybe we should take this to the Language desk. ;) Wnt (talk) 19:42, 26 February 2012 (UTC)

No. It's a noun derived from a verb. Specifically, a Gerund. Dominus Vobisdu (talk) 19:45, 26 February 2012 (UTC)

It just sounds better, wouldn't you say? Vranak (talk) 03:11, 1 March 2012 (UTC)

Oxosulphuric acid

What is the name for the analog of sulphuric acid with oxygen in place of sulphur (H₂O₅)? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 20:15, 25 February 2012 (UTC)

Sulphuric acid is H2SO4.--92.29.192.13 (talk) 21:19, 25 February 2012 (UTC)

Super oxygenated water is all I can find. Apparently, it is not very stable.--92.29.192.13 (talk) 21:23, 25 February 2012 (UTC)

Oxygen doesn't form that sort of structure ("O with four bonds to atoms"). H₂O₅ is all linearly attached "hydrogen pentaoxide", not a "sulfate-like core". DMacks (talk) 21:32, 25 February 2012 (UTC)

That acid can't exist, so I assume it is a hypothetical acid, in which case you'll have to invent a new name, possibly peroxygenic acid. Plasmic Physics (talk) 21:36, 25 February 2012 (UTC)