Wikipedia:Reference desk/Archives/Science/2008 May 29

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[edit] May 29

[edit] electrons and perpetual motion

To start off, I am a 17 year old high school student that has just completed his first year of high school-level physics. I'm pretty smart (at least I like to think so), but if this question can't be answered with vocabulary and ideas that someone of my edjucation level will be able to understand, that's ok. The answer to this question is probably in Wikipedia somewhere, but in my reading I've only seen equations and explanations that I don't think I'll ever understand until sometime through college.

Question: If there is no such thing as perpetual motion, how can electrons keep moving around the nucleus of an atom? My guess is that at the level we're talking about, there's not really anything like friction in the common sense, so there's nothing to act against the electrons after they are moving, but then where does the energy come from to move them in the first place?Hypershadow647 (talk) 00:03, 29 May 2008 (UTC)

The motion of electrons in an atom is perpetual motion in the ordinary English sense but not in the technical sense used in physics. A "perpetual motion machine" in physics is a system that violates the first law of thermodynamics (a "free energy machine" or "perpetual motion machine of the first kind") or the second law of thermodynamics (a "perpetual motion machine of the second kind"). The electrons in an atom don't gain or lose energy or entropy (at least not without interacting with the outside world), so they don't violate either of those laws. -- BenRG (talk) 00:39, 29 May 2008 (UTC)

You'll be happy to know that it's not a dumb question at all—in fact, it was a major objection to the original Rutherford atomic model, that the electrons seemed to whip around without losing energy, and if you imagine them behaving like any other gravitating object (like a planet) you'd expect that not to work out. Bohr's great insight, the insight which set off quantum mechanics (as distinct from the original quantum theory by Planck and Einstein), was that if you take for granted that the electrons can do that in specific orbitals, that they are not in "motion" in the same way that we understand it to be on a macroscopic level, then it all works out perfectly well mathematically and according to experiment, even if it doesn't make as much conceptual sense (this is the kernel of what would become the Copenhagen interpretation of Bohr and Heisenberg). Einstein thought this approach to physics was repulsive—to come up with answers that didn't make any intuitive sense, as he saw it—but Bohr's method won out in the end, and much of quantum mechanics is about taking for granted certain properties of the quantum world which are not shared by the macroscopic world. Why don't electrons behave like billiard balls, is essentially your question, and the answer is simply because the laws that govern billiard balls don't apply on that scale. --98.217.8.46 (talk) 01:10, 29 May 2008 (UTC)

Whoa, whoa, whoa. Bohr's atomic model did help inspire quantum theory, but it was completely wrong. Real electrons are nothing like the electrons in Bohr's model. They don't even have the angular momenta that Bohr thought they did. Bohr thought the angular momenta were 1, 2, 3, ..., but in fact they're 0, $\sqrt{2}$ , $\sqrt{6}$ , .... I think it was little more than dumb luck that his model gave the right values for the hydrogen spectrum (which could be measured to confirm it) despite having the wrong values for the angular momenta (which couldn't be measured). And quantum mechanics does explain the quantization of the orbits, it doesn't simply assume it the way Bohr's model did. Schroedinger's equation doesn't have any quantization built in; you have to solve it before you discover that the electron orbits are quantized.

There are two things that keep the electrons from dropping into a state of zero energy: the uncertainty principle and the exclusion principle. They're both hard to explain. The electron can't just fall into the nucleus because the nucleus is very small; that would give the electron a well defined position, which would mean that it would have to have a very uncertain momentum, which would quickly propel it out of the nucleus again. The "ground state" is a compromise between position uncertainty and momentum uncertainty. The exclusion principle comes into play when there's more than one electron in the atom. Without the exclusion principle they would all fall into the ground state, since systems "like" to have the lowest energy possible. With the exclusion principle you can only have one electron in each state, so some of them have to be in higher-energy states with larger angular momenta (meaning they're actually orbiting the atom, unlike the ground state electrons which have an angular momentum of zero and could be said to not be moving at all). -- BenRG (talk) 04:00, 29 May 2008 (UTC)

Bohr's approach was but one little way down the path of quantum mechanics and marked a distinct break with how things were done previously. That's the entire point I was making—not that the Bohr model ended up being exactly correct, but that the question is tried to answer was exactly the same one, and the methodological approaches used started QM down its rabbit hole. You don't get Copenhagen without the Bohr atom. You will note that I spoke specifically of Bohr's method. --98.217.8.46 (talk) 15:46, 4 June 2008 (UTC)

If the electron were a ball bearing circling inside a hemispherical bowl, or a weight tied to a string circling around the point of suspension, friction and air resistance would cause it to drop gradually to lower and lower levels. The reason electrons do not show such a gradual decay of energy level, as it was explained to me back when, is that even taking it as a sort of standing wave, it might be expected to bleed off energy like an electromagnetic wave circling on a loop of conductor. The quantum effect is the theory or observation that it cannot drop gradually to imperceptibly lower energy energy levels as the circling weights could in the mechanical analogies. Only a quantum of energy can be absorbed or emitted, changing it to a higher or lower energy level, thus no slight gradual decay of energy level is possible.Edison (talk) 02:20, 29 May 2008 (UTC)

It's true that only a quantum of energy (a photon) can be absorbed or emitted, but the energy of a photon can be arbitrarily small, so in effect the energy that can be emitted isn't quantized at all. So this can't explain why the electron orbit doesn't decay. -- BenRG (talk) 04:00, 29 May 2008 (UTC)

However, the stable energy states around a nucleus are quantized as well, and this imposes the appropriate restriction on the possible photons that can be emitted. Confusing Manifestation(Say hi!) 04:12, 29 May 2008 (UTC)

I don't know about our 17 year old OP, but reading through this thread I'd feel confused (and not just in manifestation). So can we explain it in understandable terms without sacrificing enough to run into my favorite Feynman quote?--71.236.23.111 (talk) 16:57, 29 May 2008 (UTC)

It may be helpful to the OP that a "perpetual motion machine" is essentially something that can perform work indefinitely without an external source of power. Electrons moving around a nucleus, or moons orbiting a planet orbiting a sun, are perpetually in motion, although there is no energy expenditure to maintain that motion. ~Amatulić (talk) 18:34, 30 May 2008 (UTC)

[edit] antioxidants to delay ageing and damage of cells

what combinations work and do they¿ i know you cant give medical advice, but what does science say so far about the effectivity of them¨¿thnx guys —Preceding unsigned comment added by 75.69.26.8 (talk) 03:20, 29 May 2008 (UTC)

The empirical evidence to date has been quite disappointing; the benefits are largely theoretical rather than demonstrated. - Nunh-huh 05:07, 29 May 2008 (UTC)

While antioxidants are great at scavenging damaging free radicals generated via metabolism, which can denature proteins and injury other macromolecules such as nucleic acids and lipids, there is no conclusive evidence that they prevent general cellular senescence. See telomeres. Wisdom89 _{(_T / ^C)} 06:45, 29 May 2008 (UTC)

[edit] Career

What jobs and career options are there in relation with animation? —Preceding unsigned comment added by 117.194.225.94 (talk) 03:55, 29 May 2008 (UTC)

If I had to surmise, I'd say a viable option would be 3D video game development. Wisdom89 _{(_T / ^C)} 06:42, 29 May 2008 (UTC)

There are tons of video animation companies for everything from bad car commercials to popular children's movies. -- kainaw ™ 12:57, 29 May 2008 (UTC)

There are quite a few unique ones, too. If you like kids you can run animation workshops at museums or fair events, for one. What job you can find and end up in might not be the same as the "standard" job description. And even that can differ from one place to another. --71.236.23.111 (talk) 16:35, 29 May 2008 (UTC)

[edit] Thermo Pond Epoxy

Hi. I saw a '3rd Rock From the Sun' episode where mary albright puts this chemical on dick solomon's desk, because earlier they had been playing impractical jokes on one another. Dick comes in and sits at his desk. as soon as he puts his hands on the desk, they are instantly stuck. Then he uses too much force to get his hands off and he accidently puts his head on the desk and then is stuck to this position. My question is: Can this chemical really do that sort of thing, paste objects together that quickly?Jwking (talk) 05:32, 29 May 2008 (UTC)

Yes, some types of superglue can. There are loads of documented cases of people putting it on toilet seats, glasses etc. Not a very sociable think to do. If you put it on a non-porous surface skin will bond almost instantly. There have also been eyeball to eyelid and similar in accidents. I'd stick to clingfilm for jokes... --BozMo talk 06:22, 29 May 2008 (UTC)

Since detaching the stuck human can require medical assistance this way exceeds funny. The doctors and lawyers will laugh all the way to the bank, though. (Neither legal nor medical advice)--71.236.23.111 (talk) 16:21, 29 May 2008 (UTC)

[edit] Brain + Sex

What part of the brain becomes more active during sexual arousal? Or at least, what part controls that. --68.90.143.120 (talk) 06:30, 29 May 2008 (UTC)

The Raphe nucleus and serotonin are thought to be involved. Wisdom89 _{(_T / ^C)} 06:35, 29 May 2008 (UTC)

Well, speaking as a guy, it's pretty clear your entire brain goes on vacation. Gzuckier (talk) 20:47, 29 May 2008 (UTC)

[edit] Under a burning night sky

I'm curious why the sky is orange during nighttime. First of all it was 10 pm so I discount the sun. It is relatively cloudy and rainy during the night and the clouds are actually the ones that is a dull orange. I live in Manila so the air pollution is strong but I can't account fo r the orange hue as we use fluorescent lamps here rather than oil or fires.--Lenticel ^(talk) 06:58, 29 May 2008 (UTC)

Sodium street lamps? (See the section on light pollution) 81.174.226.229 (talk) 08:41, 29 May 2008 (UTC)

Also the article on Light pollution Jdrewitt (talk) 12:00, 29 May 2008 (UTC)

hmm... I think you're right. It is Skyglow. No wonder this never happens when I'm in the provinces. --Lenticel ^(talk) 12:38, 29 May 2008 (UTC)

Hi. This happens here too, where I live in Southern Ontario. The streetlights cause the clouds to turn orange, and this can cause it to reflect into my bedroom window, where there are no nearby streetlights, so much so that I can see penumbral shadows of objects and even read printed text in the dark (but just barely and only with night vision adaptation caused by pupil expansion). When there are no clouds, or when the clouds are high, the light isn't strong enough to be reflected. However, if there is say a snowstorm, the light will illuminate the nearby snowflakes the way the sun illuminates dandelion seeds flying on a cool, windy, clear spring day. I've also noticed that the far away cloud bases sometimes appear darker than the nearby clouds, perhaps escaping the light pollution. Hope this helps. Thanks. ~A H 1^(TCU) 17:16, 31 May 2008 (UTC)

[edit] Power Generators

I was wondering how efficient generators are at converting mechanical energy from the rotating magnet to electrical energy? 61.69.132.119 (talk) 07:32, 29 May 2008 (UTC)

I think it's about 80-85%, which is from actual, but half-remembered, research. If someone doesn't give the definitive answer, I'll try and get back to you. It's definitely quite high, because the overall efficiency is about 40%, with most of the losses at an early stage, I think in the production of steam. 130.95.106.128 (talk) 11:25, 29 May 2008 (UTC)

I second the 80-85%, but can't quote a good source either. I know the figure from adding up losses for overall systems in which case it would be stated as a 15-20% loss. --71.236.23.111 (talk) 17:05, 29 May 2008 (UTC)

Surely it is higher than 80 to 85%, given that in 1879 Thomas Edison's "Long legged Mary Ann" generator achieved 82% mechanical to electrical efficiency, a sizeable increase from the 40% of earlier generators, with his reasearch team over the next decade achieving 90% efficiency [1] . If present day 1000 megawatt utility baseload generator were 15% to 80 20 % inefficient, it would get heated up by 150 to 200 megawatts, which seems unlikely. [2] cites 93% to 97% efficiency for small hydro generators. [3] says a wind generator has achieved 98% efficiency. [4] cites an 800 megawatt generator's efficiency being increased from 98.93% to 99.01% efficiency via stator rewinding and other improvements. I wonder if the lower efficiency deducts the auxiliary equipment such as coal handling, plant lighting, pumps etc? Edison (talk) 19:12, 29 May 2008 (UTC)

I guess not all your losses are heat. You lose quite a bit to vibration. I've seen the 80-85% ballpark used in wind turbine and cogeneration setups. (And I've usually seen those come out on the low end rather than overshoot the target.) It should be straightforward enough Joules in Watt out, but there's a lot of leeway in who measures what where as usual. If you look at steam temperature/wind energy to power output you get a different number than if you look at torque to output. 71.236.23.111 (talk) 22:43, 29 May 2008 (UTC)

I (130.95) will probably have to take back what I wrote. My notes, based on the rather old (1970s) book Man, Energy, Society by Earl Cook, tell me that the steam generator (in combination, called a furnace, boiler and superheater) is about 85% efficient, not at all what I thought, and the generator is 46% efficient. The figure of 46% refers to the steam energy going in (187 G Cal per hour) and the electrical energy coming out (86 G Cal per hour), so 86/187 is about 0.46. This means I know virtually nothing about the efficiency of the generator magnet itself, which is a small part of the whole contraption, so the other guesses are more reliable, and more relevant. In short, the answer I gave, "quite high," seems to stand up well :). 203.221.127.63 (talk) 17:09, 30 May 2008 (UTC)

[edit] Mortality rates expressed as hazard rates

Does anyone know where I could find mortality rates calculated as hazard rates? I'm trying to answer questions like, if a Canadian man makes it to the age of 93, what are his odds of surviving to 94? If an American baby reaches the age of 6 months, what are the odds of her surviving the remainder of her childhood? Thanks. moink (talk) 12:36, 29 May 2008 (UTC)

I once did a similar analysis on South African mortality rates and I used Statistics South Africa for the data so I would presume that the United States Census Bureau would be able to help you for US data. Do you know how to calculate the probabilities you need from actuarial tables? Also, see force of mortality which is the hazard rate in survival analysis with respect to mortality. Zain Ebrahim (talk) 13:20, 29 May 2008 (UTC)

Thank you! I think I just needed the term to search for. moink (talk) 16:45, 29 May 2008 (UTC)

You're welcome. All the best with your work. Zain Ebrahim (talk) 18:08, 29 May 2008 (UTC)

[edit] Horrible plastic smell!

I bought some plimsolls online from River Island, but they stink of plastic like they're from some crappy discount shoe warehouse.. I've put them outside to air but they still smell awful. I thought I'd ask you clever lot for some advice. Whack on the Febreze? Air tight container with some Bicarbonate of soda? Thanks in advance! 79.78.46.84 (talk) 12:57, 29 May 2008 (UTC)

You bought some "lines painted on the hull of a ship to help determine displacement and draft"? Where are you planning on using them? There has got to be another definition for that word.... -SandyJax (talk) 15:01, 29 May 2008 (UTC)

Um yes? plimsolls Nil Einne (talk) 15:05, 29 May 2008 (UTC)

(after edit conflict) Plimsoll shoes have a very pungent smell of rubber, I've always found, but not plastic which doesn't really smell much. Theres not much you can do about it. It fades with with time, I find. Fribbler (talk) 15:07, 29 May 2008 (UTC)

Also see wiktionary:plimsoll Nil Einne (talk) 15:08, 29 May 2008 (UTC)

My guess would be that this is the culprit Thiourea. Vulcanization makes rubber usable, but sulfur is known to combine into truly stinky substances. The thing is that you want to avoid the rubber drying out (which would ruin it.) So the baking soda will eat the smell, but in an airtight container will also desiccate the rubber which will make it brittle. Fragrance oil might cover the smell, and you could use some activated charcoal insoles. 71.236.23.111 (talk) 16:10, 29 May 2008 (UTC)

Maybe your ~~posh~~ expensive plimsolls from River island were made in a sweatshop just like the ones from a "crappy discount shoe warehouse"? Astronaut (talk) 16:20, 30 May 2008 (UTC)

[edit] A doubt

How does one cite matter from a chapter in a book that has contributors that are different from the editors mentioned on the cover and elsewhere? Which all authors are to be included in the citation?

Kindly help me with this doubt.

Thanks in advance.

Regards.

PS: Have posted the same query on the talk page of citations, but am not expecting a quick reply. Thanks.

—KetanPanchal^taLK 13:30, 29 May 2008 (UTC)

I would err on the side of more information, listing both the chapter's editors as well as the editors of the entire book. Roughly working from the MLA guidelines, "Works with Multiple Authors" should list the individual author first, followed by the group:

Heller, Steven, ed. The Education of an E-Designer.
Heller, Steven and Karen Pomeroy. Design Literacy: Understanding Graphic Design.

Hopefully this example helps. Nimur (talk) 14:23, 29 May 2008 (UTC)

Incidentally, that's not how you cite chapter authors. Chapters are cited separately and then in the same citation you cite the main work and note the editors. E.g. Paul, Diane B. (2003), "Darwin, social Darwinism and eugenics", in Hodge, Jonathan and Radick, Gregory, eds., The Cambridge Companion to Darwin, Cambridge University Press, 214–239--98.217.8.46 (talk) 15:23, 29 May 2008 (UTC)

Thanks Nimur, for your reply. But, my doubt was more about the technical aspect since I'd never seen the {{cite book}} in its expanded form, had only seen its abbreviated form at WP:Citation templates. Surprisingly, I got my doubt cleared at the talk page of citations itself! Bye. Take care. —KetanPanchal^taLK 14:54, 29 May 2008 (UTC)

[edit] Optimal nap/sleep time

I've read somewhere that if you're not having a full night's sleep, or if you're just taking a nap, certain sleep durations are better than others because you'll not be waking up during the "wrong" part of the sleep cycle. I think two recommended durations were given in the article I read, one for short naps and the other for longer sleep.

I can't find the article and don't remember what the recommended durations are. Can someone point me to some good articles/resources on the subject? —Preceding unsigned comment added by 71.162.242.15 (talk) 16:53, 29 May 2008 (UTC)

Power nap doesn't state a duration, but you might get that in the original NIMH study cited. --71.236.23.111 (talk) 17:11, 29 May 2008 (UTC)

[edit] Shindo earthquake scale

When measuring earthquakes there are different sclaes, like Mercalli intensity scale and Richter magnitude scale. Why did the Japanese invent their own scale "shindo" (Japan Meteorological Agency seismic intensity scale) (In Japanese called 震度)? Moberg (talk) 16:57, 29 May 2008 (UTC)

As the article's lede notes, the Japanese scale serves a function distinct from that of the Richter scale (and, for that matter, it predates the Richter). The Mercalli scale serves a similar purpose, but again postdates the Japanese scale (1902 vs 1884). — Lomn 17:21, 29 May 2008 (UTC)

("lede", what does it mean?) Ah okey, predates, thanks! But why is the Japanese the ones who made their own scale and not China or Australia or some other country? Moberg (talk) 18:55, 29 May 2008 (UTC)

(lede is an alternative spelling of lead and refers to the beginning of an article) --Tango (talk) 19:03, 29 May 2008 (UTC)

(ok! :)) Follow-up-question: I think that Taiwan uses this scale or a similar one. Is there any other other countries that uses the shindo- (or a from the shindo-scale derived) scale? And if so, why? Moberg (talk) 19:24, 29 May 2008 (UTC)

[edit] Original rotation of solar system.

Under the Nebular Hypothesis, the rotation of the solar system is caused by the contraction of the original cloud and, as it condensed, the rotation went from perhaps once every million years to that which it is today. The principle of the conservation of angular momentum is applied. The question is, however, what caused the original rotation? Any generally accepted reasons for the cause under this hypothesis, or are we still guessing?Boblaw1 (talk) 17:52, 29 May 2008 (UTC)

I don't think you need anything to cause it - chances are, when you add together the angular momenta of all the particles in the cloud, you're going to get something non-zero, the chance of all the random movements cancelling out exactly is vanishingly small. That tiny, but non-zero, value is then increased dramatically when the cloud collapses. --Tango (talk) 18:16, 29 May 2008 (UTC)

It's in the nature of gravitational capture for things to orbit around each other. Even if the whole cloud were to magically stop spinning, the next thing to wander close enough to start orbiting it would make the system be rotating again. --Sean 13:40, 30 May 2008 (UTC)

I'd say a slight rotation was imparted on the solar system by the rotation of the galaxy. So, the next question will be where that came from. There must have been some initial rotation right after the Big Bang (or perhaps even before), just like the dispersal of matter couldn't be exactly equal or we never would have gotten galaxies and solar system forming in the first place. StuRat (talk) 00:17, 1 June 2008 (UTC)

[edit] Nuclear reactors and fissile materials

What distinguishes the following technologies?

Uranium enrichment
Plutonium reprocessing
Graphite reactor
Light water reactor
Heavy water reactor

--141.161.98.180 (talk) 18:12, 29 May 2008 (UTC)

Might they all be distinguished by being the subject of homework questions? Edison (talk) 19:13, 29 May 2008 (UTC)

We have Wikipedia articles on each of those. Uranium enrichment, nuclear reprocessing, graphite moderated reactor, light water reactor, heavy water reactor. The different reactor types use different neutron moderators (which has different implications for what type of fuel you put in them and what types of products you get out in the end), uranium enrichment is about increasing the percentage of U-235 in a given sample of uranium, reprocessing is about taking spent fuel and getting out certain materials created by the fission reactions (such as plutonium). For more details, read the articles. --Captain Ref Desk (talk) 19:34, 29 May 2008 (UTC)

[edit] Diodes

what exactly will happen when we short two terminals of a common type of diode ,does there be any charge movement and a resulting effect on depletion region202.125.143.75 (talk) 19:25, 29 May 2008 (UTC)

Presuming that the diode was not connected to a circuit then probably nothing since unlike say a capacitor, I'm pretty sure a diode doesn't keep a charge. If the diode is connected to a circuit, then I believe shorting it will basically remove any potential affect the diode may have Nil Einne (talk) 19:51, 29 May 2008 (UTC)

A reverse-biased diode does effectively act like a small capacitor, storing a tiny amount of charge. I don't suppose most diodes have enough capacitance for it to have any measurable effect in practice, but apparently some do: see varicap. —Ilmari Karonen (talk) 21:48, 29 May 2008 (UTC)

I am guessing that the questioner is thinking that shorting the ends of a diode together will have some effect of connecting the P-type side of the diode to the N-type side. However, they are already connected in the P-N junction inside the diode. Connecting them again will have no effect. By bringing up the depletion region, the questioner is implying that the diode is in a circuit and has current passing through it. Shorting out the terminals in that case will mean that you have a parallel circuit. The short has a resistance of nearly 0 ohms. The diode is commonly treated as having 0 ohm resistance, but that is not true. The good ones are 300 ohms (if my memory from circuit design 15 years ago is correct). So, until the current surpasses the capability of the short, there will be basically no current in the diode. The depletion region will quickly dissipate. If you take a good electronics course, this is the type of problem you'll see on a final: At 0.02 seconds after shorting out the diode, what is the potential difference between the terminals? -- kainaw ™ 21:49, 29 May 2008 (UTC)

[edit] Electronics home project

Well I would like to build a switch which is switched on and off by clapping your hands twice. What do I need, and how do I do it? Bastard Soap (talk) 20:29, 29 May 2008 (UTC)

You can order a kit to build a sound-activated switch (which seems to cost exactly the same as The Clapper). This page also has a link to a PDF version of the assembly manual which includes a theory of operation section. --LarryMac | Talk 20:41, 29 May 2008 (UTC)

Or order everything separately [5] (This doesn't have to be cheaper in the end. I've had a couple of DIY projects that added up to more in parts than the thing off the shelf. Less fun buying it, though.) Be sure to follow local code in all projects involving home installations! Have a licensed electrician check any circuit before connecting to your house electricity. (No advice, but if you burn down the house your insurance won't pay and your spouse will never shut up about it.)--71.236.23.111 (talk) 22:55, 29 May 2008 (UTC)

[edit] Nuclear Power Plant on "automatic mode"

In the event of a hypothetical doomsday scenario how long would a modern nuclear power plant be able to supply electricity before it shuts down? I know an that nuclear power plant's have some kind of dead man's handle mechanism as a failsafe but let's assume that it was turned off. How long could a modern nuclear power plant work on it's own? For hours, days, weeks or years? Mieciu K (talk) 21:57, 29 May 2008 (UTC)

Hours would be easy, and days would be likely. Weeks would be a stretch. This assumes that the system has some sort of automatic load following, or that the load on the generator remains constant. so it just has to do steady-state operation. If "doomsday" meant that the load dropped abruptly or that it dropped gradually and there was no load following, the generator would trip followed by the shutdown of the reactor. Edison (talk) 23:34, 29 May 2008 (UTC)

(Caveat: I'm only an expert on US Navy nuclear propulsion systems) Think of your car's engine. Things happen inside the engine too fast for a human to contol, so we automate things like lubrication, fuel and air intake, and valve timing. Whether controlled by a carburetor or an electronic fuel injection system, there is an automated control system that looks at system outputs like speed, exhaust temperature, and exhaust chemistry, and adjusts system inputs like fuel and air to maintain whatever it was preset to. Completely separate from that, there is a system that looks at lube oil pressure, and adjusts oil flow to keep that constant (that system is pretty simple; it's just a dump valve attached to a spring, so if oil pressure rises the valve opens more, dumping more oil back to the sump, etc). If you walk away from your car after you start the engine, what happens? It stays running until something goes wrong beyond the capacity of the control system. The engine will shut down when the fuel tank runs empty and fuel pressure drops to the point where the injectors or carburetors can't compensate. The engine will shut down if the car is in an enclosed space like a garage, and the oxygen concentration goes down to the point that it won't support combustion any more. The engine will shut down if a part breaks. The engine will shut down badly if it runs out of oil. Etc, etc, etc.

Now, with that in mind, let's look at a reactor plant. Again, nuclear reactions happen too fast for a human to control, so there are completely automated systems that make minor adjustments to keep the plant running at whatever settings the operators decide. Of course, there are very stringent limits on what the operators are allowed to decide. Some limits are set by the physicists who designed the reactor, some are set by the engineers who designed the steam plant the reactor is there to drive, some limits are set by government safety analysts who want to prevent accidents, and some limits are set by the Navy's operational needs.

However, if started up within those limits, a reactor plant will run on it's own until something happens that the control system can't handle. To the best of my knowledge, there is no "Dead man switch" that would make the plant shut down - and cause a regional blackout - just because there's a flu bug going around and all the operators had to run to the bathroom at the same time. That's the whole point of the automated systems - the humans look for trends, while the machinery runs itself.

Running out of fuel isn't a problem for several years, perhaps decades, depending upon fuel level. Fuel poison buildup shouldn't be a problem for a plant designed to run at full capacity for years at a time, as an electric generator plant would be; that would be factored into the design and the control system would compensate. Leakage shouldn't be a problem, either, as the nuclear side is designed to not leak, and minor leaks would be compensated for by an automated makeup system.

Any reason for the plant to shut down if the operators "just walked away" would come from the steam plant and electric generator side. Again, an electric generator plant would be designed to automatically compensate for changes in load, because they happen too fast for humans to follow. And, if we assume that nothing breaks, and no turbines run out of lube oil, and the river keeps providing cooling water, the reactor plant will keep providing electricity until the electric load on the grid drops to below minimum power. Another point. These plants generally have more than one of everything. More than one core, more than one turbine, more than one oil pump for each turbine, etc, simply because we are aware that things break, and we don't want to have a blackout just because an oil pump broke.

We can figure that the coal-fired and oil-fired plants will fail first, because no one refilled the coal hoppers and fuel tanks. Still, the hydro-electric plants and nuclear plants will have no problem with the remaining load, because there's no people left anymore, so there's not much using electricity. Major system loads will be automated lights for cities and highways and other things like that that turn themselves on. And, if a plant that is fueled for, say, five years at full power finds itself only running at ten percent power, well, then, it has enough fuel to run for 50 years at THAT power level.

Eventually, though, some random storm will down some power lines, and the transformer farm will isolate itself from the grid to protect itself, and _then_ the power plant will find itself running with no load. The steam plant can run at idle, just waiting for someone to shut the breakers again. but the reactor can't. Those fission by-product poisons we dismissed earlier DO have an effect. It's minor compared to the rest of the reactor running under full load, but when running at idle they have an effect that adds up. They will continue to contribute heat, and the control system will throttle back the fission process, until it is forced to shut the plant down. Safely, automatically, with no people involved. At least, that's the way US Navy plants are designed. When will that happen? Who knows? A day, a month, 5 years? Can't tell. But the nuclear and hydro plants will stay running until some outside factor makes them shutdown. -SandyJax (talk) 14:53, 30 May 2008 (UTC)

There was a straight dope "staff report" on this subject as well. [6] APL (talk) 16:27, 30 May 2008 (UTC)

Ok, thanks for all the answers. Mieciu K (talk) 07:22, 2 June 2008 (UTC)

[edit] List of Physics equations

We have a page which lists lots of trigonmetric equations. Is there a similar page for physics equations. In particular "v = u + at", "s = ut + 0.5at^2", "F = ma", etc. Maybe also for electrical equations "V = IR" "Q = VC" etc. If it does exist you can't find it by searching for "v=u+at". -- SGBailey (talk) 22:24, 29 May 2008 (UTC)

We have list of equations (not all Physics), list of relativistic equations and list of equations in classical mechanics (this one has the constant acceleration equations, but I think the TeX stops search working). Algebraist 22:42, 29 May 2008 (UTC)

[edit] Logic gates

I was mulling over how to make demonstration logic gates using many different technologies. Obviously a mechanical AND gate can be made with two input levers which can be up or down pushing on opposite ends of a floating pivoted beam where the central pivot holds a flagpole. If both are down, the pole is down, if only one is up the the beam pivots and the pole is down, if both are up then the beam itself is raised and hence the pole is up. What other technologies are there and how might they be demonstrated? Hydraulic (liquid pressure?); Pneumatic (flow (or not)); electromechanical (relays); electronic (transistors and voltages); Pure optical (no idea how); Magnetic (how?), etc etc. Suggestions and advice on a postcard please... -- SGBailey (talk) 22:35, 29 May 2008 (UTC)

Our school used to have a cool board with chutes, marbles and switches that switched LEDs on/off. --71.236.23.111 (talk) 23:02, 29 May 2008 (UTC)

Here's an idea for an optical OR gate. Light shines down both tubes, but each one may be blocked by a piece of cardboard. Where the tubes combine is some sort of prism/two-way mirror set-up that combines the two beams together and sends it down the bottom tube. If one tube or the other is open, or if both are, then there will be light coming out the bottom.

     |    |         |    |
     |    |         |    |
    -+----+-       -+----+-
     |    |         |    |
     |    |_________|    |
      \     *     *     /
       \    *     *    /
        \   *     *   /
         \  *     *  /
          \ *     * /
           \*     */
            |     |
            |     |

I think that perhaps to make a XOR gate you could consider how to make a NOT XNOR gate first - pick a medium that involves something flowing (like water, or sand), and let each switch divert the flow in opposite directions somehow. So having neither, or both on, lets the flow continue as it was, while having just one diverts it (into a secondary channel, say). You can then turn that all around, and make that secondary channel the primary one, and you've got a XOR. Confusing Manifestation(Say hi!) 23:38, 29 May 2008 (UTC)

Hmm. Yes. Remember that all logic operations can be built from NAND or NOR if you are really desperate. --Prestidigitator (talk) 01:04, 30 May 2008 (UTC)

Please be sure to see fluidics. Also, Tinkertoy computer [7] [8].

Atlant (talk) 15:27, 30 May 2008 (UTC)

[edit] How to calculate minimum wind speed to cause white caps on ocean

Wiklearn (talk) 23:42, 29 May 2008 (UTC)How is wind speed related to the height of ocean waves? At what wind speed would white caps appear? Wiklearn (talk) 23:42, 29 May 2008 (UTC)

You might try looking at Beaufort scale—in particular, the table that relates Beaufort numbers to wind speeds and to surface conditions on the open ocean. Deor (talk) 13:08, 30 May 2008 (UTC)

[edit] Definition for trompe on wikipedia please

Trompes are simple devices that use the power of falling water to compress air. Trompes were used widely in the 19th century. They were used to provide the air for mines, blast furnaces pneumatic tools for alpine tunnel making, etc. The town of Cobalt in Ontario, Canada claims to have the only hydraulic air compressor (Trompe) left in the world http://www.cobalt.ca/ragged_chutes.htm Researchers in England have tried to use low pressure trompes (attached to an enclosed wind turbine and electric generator) to extract energy from the water running over weirs in a cost effective way. (Pdf is available if required). I believe that a dedfinition of trompe would help show people in poor countrys that energy is available from low grade (low head) hydro sources without the huge investment in machinery that is currently a barrier to using these (often high volume) sources of energy. —Preceding unsigned comment added by Gaiatechnician (talk • contribs) 23:46, 29 May 2008 (UTC) Gaiatechnician (talk) 23:47, 29 May 2008 (UTC)

Not sure what you're driving at, but have you looked at "Trompe"? --Milkbreath (talk) 01:25, 30 May 2008 (UTC) Thank you but at the top it says "This article does not cite any references or sources. (November 2006)

Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed". I actually wrote it and my poor effort has been sitting there with that proviso (and doubt) over it for a long time. Just hoping to get it fixed. My sources are long ago photocopys from engineering manuals. I have made some so I know they exist. Gaiatechnician (talk) 15:04, 30 May 2008 (UTC)

Ah. Now I see. That message about sources is just Wikipedia houskeeping. It serves two purposes, basically, the way I understand it: to warn the reader that there is no proof of the accuracy of the information, and to flag the article for possible deletion if it sits like that too long. Your concern is jusatified, I think. This is the Reference Desk, I'm sure you know, and it is not the right place for announcements of this type (though I think you will have gotten the attention of people who have the knowledge to fix the article). You should try the talk pages of whatever WikiProjects are concerned with things like trompes, for instance, Wikipedia_talk:WikiProject_Engineering and Wikipedia_talk:WikiProject_Technology. --Milkbreath (talk) 15:53, 30 May 2008 (UTC)

Such devices have existed. Within the last year there was an article in New Scientist showing methods of using low-head flow to compress air. The water flowed in the shape of a U taking air bubbles down to the submerged air reservoir in the middle. The water leaves at a lower level.Polypipe Wrangler (talk) 10:23, 30 May 2008 (UTC)