Wikipedia:Reference desk/Archives/Science/2011 November 18

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November 18

Na₂CO₃

hello. I recently did an investigation with crystallization and am now in the process of writing my conclusion. I have been able to find the correct lattice system (monoclinic, cubic, etc.) for all of them except the above, anhydrous Na₂CO₃. Can someone help me out with regards to which of these it falls udner? Thanks. 24.92.85.35 (talk) 01:25, 18 November 2011 (UTC)

I don't know about anhydrous sodium carbonate, but the decahydrate crystalises in the monoclinic system. Plasmic Physics (talk) 01:56, 18 November 2011 (UTC)

For anhydrous sodium carbonate at normal conditions of temperature and pressure, it is monoclinic with space group C2/m, but above 760K it is hexagonal with space group P6₃/mmc.[1] Graeme Bartlett (talk) 12:42, 18 November 2011 (UTC)

"Infinite" resistance, lightning, and quantum tunnelling

I know that electricians speak of an open circuit as having "infinite resistance". Would it be accurate, though, to say that the existence of lightning proves that air has only finite resistance, and that quantum tunnelling implies the impossibility of truly infinite resistance even across a vacuum? NeonMerlin 05:52, 18 November 2011 (UTC)

An open circuit has a practically infinite resistance; the term is used because the multimeter is unable to detect it. Given enough voltage, there are no electrical insulators, however, merely less good conductors. Also, there's no need to invoke quantum tunnelling to explain the transmission of electricity across a vacuum. Classically, electric charge can be transmitted through a vacuum with no troubles. --Jayron32 06:11, 18 November 2011 (UTC)

While a vacuum does make a good insulator, it actually does not have resistance in an Ohm's law sense. Charges launched into a vacuum do not experience any resistance to their travel since there is nothing for them to collide with. Vacuum, it could be said, actually has zero resistance, not infinite resistance. However, vacuum will not usually conduct because it contains no charge carriers (it is a vacuum). To get it to conduct, charge carriers have to first be injected - see vacuum tube. SpinningSpark 12:31, 18 November 2011 (UTC)

Yes, but the force exerted by one charge on another charge works just fine in a vacuum. So you can "conduct" electricity across a vacuum. Actual electrons don't have to move across a region to count as electricity. Even in a wire, electrons don't "flow" down the wire like water in a pipe (despite the usefulness of the pipe analogy in other contexts). The better model of electricty in a conductor is the Newton's cradle: electrons crowded in a tight space and passing energy down the wire without themselves moving the length of the wire. In a basic sense, any movement of electric charge is electricity, and there's nothing inherent about a vacuum which stops this... Of course, there's the bit of problem that once you put some electrons inside a hard vacuum, it is no longer a hard vacuum... --Jayron32 14:46, 18 November 2011 (UTC)

I agree that conduction can take place in a vacuum, that is why I said that a vacuum has zero resistance. However, transmitting electricity across a vacuum without the use of electrons (or some other charge carrier) is not conduction, let alone ohmic conduction, it is launching an electromagnetic wave which is an entirely different phenomenon. The Newton's cradle model (or more accurately, the Drude model) is entirely unapplicable to conduction in a vacuum tube. The Newton's cradle model is poor anyway because most of the energy is not lost in at each collision of the balls whereas in a conductor electrons lose their forward momentum at each collision (hence the heating effect of an ohmic current). SpinningSpark 17:46, 18 November 2011 (UTC)

Our article on permittivity explains the concepts here, although it's a bit technical. Looie496 (talk) 16:19, 18 November 2011 (UTC)

Permittivity is a property of physical materials, not of the vacuum. The quantity vacuum permittivity is not the same thing, it is a dimensional constant dependent only on the system of units in use. SpinningSpark 17:59, 18 November 2011 (UTC)

Our articles on Cold cathode technology and our article on Vacuum tubes state that "cold cathode" tubes have some gas in them to allow conduction.Ionized gas molecules can certainly carry large currents. Cathode ray tubes and vacuum tubes certainly carry electron currents through a high vacuum from a heated cathode or filament. But a company called AFS says on their website that their high power tubes have cold cathodes and imply that they have vacuum rather than gas in them, and use a "strong electric field" to cause electrons to be emitted, allowing tens of thousands of amps. On page 7 they say "cold-cathode field-emission electron tube(s)" "just have to convince the electrons to propagate across a vacuum" as "plasma." How does this square with what Wikipedia says? Do the tubes really contain gas, or can "cold cathode" tubes conduct high currents of electron in a high vacuum? Edison (talk) 19:05, 18 November 2011 (UTC)

Vacuum has zero resistance, as long as you have free electrons in it to provide the current. The question is where are the electrons going to come from? The electrons can be provided by a cathode, but metals don't part from their electrons willingly. Some energy must be provided for that to happen. That energy can be provided by either heating up the cathode (hot-cathode) or by pumping the electrons out with a strong electric field (cold cathode). Dauto (talk) 19:53, 18 November 2011 (UTC)

Neutrino Circuit--real or a hoax?

http://www.youtube.com/watch?v=9uZ6elCRzNM This video. I googled and found nothing but copies of the same video and Wikipedia doesn't mention "neutrino circuit". Real or a hoax? Are you ready for IPv6? (talk) 10:26, 18 November 2011 (UTC)

There are about a zillion free energy hoaxes on the Internet. Wikipedia only mentions the few that have garnered significant media attention. (Blacklight Power, Stanley Meyer's water fuel cell, etc)

Just about any time anyone claims that the reason you haven't heard of something is because the "oil companies" are suppressing it, you may safely assume it's a hoax. The oil companies are very powerful, but there are a large number of equally powerful corporations that would love to have a free energy source.

(In fact, with enough energy, petroleum can be synthesized from waste! You can't tell me that ExxonMobile wouldn't like to get their hands on that before BP does!) APL (talk) 10:38, 18 November 2011 (UTC)

There's also the recent Energy Catalyzer although the merits of that article are still debated by some Nil Einne (talk) 14:43, 18 November 2011 (UTC)

I'm not sure what the gag is here, but it occurs to me that it'd be pretty easy to hide some batteries in the two thick "feet" on the base of the device. There's probably wires running through that otherwise unnecessary horizontal brace running along the center of the device. The whole device is held together with screws, so it can probably be opened to replace the batteries. APL (talk) 10:56, 18 November 2011 (UTC)

A stack of 3 volt button cells inside the rods would also get about the required voltage to make this work. I am not a physicist, but I would bet my house that there is no way this could possibly be done powered with solar neutrino flux. The device simply does not have a large enough cross-section to capture enough power even if there were a practical way of doing it. The article says the flux is 10¹⁴ (SI units) and solar neutrino says the energy of solar neutrinos is up to 18 MeV. The average energy is way less than this, but even giving the benefit of the higher figure and assuming 100% capture efficiency this only returns a power of 16 μW per square metre and the device has nowhere near that large a cross-section. Even with 1 m² that is barely enough to cause a crackle on your radio, let alone light a 25 W lamp. In any case 100% capture efficiency is entirely unrealistic for such a small device. The vast majority of neutrinos pass straight through the entire earth as if it wasn't there. Filling the entire distance from here to the sun with lead would only succeed in stopping half of them. In short, yes it is a hoax. SpinningSpark 12:17, 18 November 2011 (UTC)

One rod is made of 73 different elements, one rod is made of 74 different elements. That's a large chunk of the periodic table but okay.... Nil Einne (talk) 12:45, 18 November 2011 (UTC)

Does the sun really only produce that little energy in neutrinos? Thats less than 1 in 60,000,000 compared to photon energy. Surely the nuclear reactions involved disappate more than that fraction of energy in neutrinos. Can any one confirm that from the 4H→He +2 neutrinos reaction? I have no doubt that this machine does not work as promoted either. Graeme Bartlett (talk) 12:53, 18 November 2011 (UTC)

Before someone else points it out, I made an error in the power calculation above, I forgot to multiply by 18x10⁶. But there is still no way it is going to work. SpinningSpark 12:55, 18 November 2011 (UTC)

While we're here, the Blacklight articles says "that produces electricity for 1 cent per KW". This doesn't make much sense as it's a unit of power, not energy. Unless they make that mistake? Grandiose (me, talk, contribs) 13:29, 18 November 2011 (UTC)

Just because you piqued my curiosity, could you clarify: you're saying now that the Sun radiates about 1/3 the energy in neutrinos that it radiates by light? Can we nail down that ratio precisely - is it the result of some kind of equipartition theorem in the fusion reaction...? Wnt (talk) 14:55, 18 November 2011 (UTC)

Are you addressing that to me? I don't know any more than there is in the Wikipedia articles from where I got those figures. SpinningSpark 17:22, 18 November 2011 (UTC)

The Sun radiates only about 3% of its energy as neutrinos, see Proton-proton chain reaction. Icek (talk) 19:34, 18 November 2011 (UTC)

Neutrinos can also be produced by the CNO cycle and the Urca process, but those are not very important in the sun. Dauto (talk) 22:59, 18 November 2011 (UTC)

Ordinary little Christmas tree lights only use a couple of volts each and could easily be powered by batteries supplying a few volts to the rods. But the large light bulb is a puzzle. If it were running on 120 volts, it seems like the guy would get a shock when he touches both of the rods, which he appears to do several times. However, they make 12 volt 25 watt bulbs with the same shape and base as regular 120 volt bulbs. So if I were called on to build a device which would perform like the one in the video (not implying that this is how his device works), I would hollow out the large pieces of wood which make up the base, and are clearly not needed to support the claimed small weight of the device, and put in 8 C-size alkaline batteries, or D-size if they could fit, to equal 12 volts, then wire several of the little Christmas tree lights in series to use the 12 volts, or put several 12 volt Christmas lights in parallel, and buy a 12 volt, 25 watt A19 bulb for the big finale. D or C batteries could easily supply the 2.1 amps required by the bulb. Maybe even AA could do it for the short time required. The big light bulb could also be of the sort used in the "human powered light bulb trick," where a real looking bulb actually has LEDs or a small flashlight bulb and a battery inside, and is lit by shorting the two parts of the base with a piece of metal. See "Brain powered light bulb." The thermal inertia evident in the time it took the big bulb to go dark is consistent with an incandescent 12 volt bulb rather than a little flashlight or led bulb inside the globe. When he said "73 elements in one rod and 74 in the other," that might mean "73 units of some type" rather than 73 different elements of the periodic table. Here is a 1921 article using the term "battery elements" to refer to the lead plates in a storage battery. This use of "element" has a long history in electrical terminology. 1872: "On testing the battery of 80 elements..." The rods look a lot like the carbon rods once used in the big "ignition" dry cell batteries. Edison (talk) 16:59, 18 November 2011 (UTC)

The rods in that video look exactly like graphite rods in 6V lamp batteries. I've used them for electrolysis, and the batteries are pretty easy to take apart. --140.180.3.244 (talk) 18:50, 18 November 2011 (UTC)

The 6 inch tall "Number 6" ignition cells I remember put out 1.5 volts. They don't seem to make gigantic carbon zinc cells like that any more. Now they have a look-alike with 2 smaller "F size" alkaline cells inside, wired in parallel: [2]. Edison (talk) 04:50, 19 November 2011 (UTC)

Why would 73 units of some type and 74 units of the same type result in the charges differing? In any case, the specific comment was "73 different elements", I don't see why you would use different if you intended to imply they were 73 units of one type. (I'm not saying having one extra element would definitely make one positive and one negative but it makes slightly more sense.) Nil Einne (talk) 23:31, 18 November 2011 (UTC)

He did point out that the voltage stayed the same as he moved the light up and down the rod, possibly intended to show it was not a Voltaic pile with, say, 73 "elements" of copper, zinc and paper soaked in saline in one side and 74 such elements in the other side. I agree the phrasing is puzzling. I would expect only a small voltage difference between a stack of 73 and a neighboring stack of identical "elements," if each was the same sort of neutrino catching doo-dad. Did he ever imply the "elements" were stacked, or concentric cylinders, or a mixture, or some other configuration? Edison (talk) 04:50, 19 November 2011 (UTC)

In the video he said

Now it's compressed. The material in this, is made of; one rod is made 73 different elements, the other side is made of 74 elements. There's one additional element, and that's why one is negative and one is positive. One builds up the electrons. The neutrinos that strike the earth 24 hours a day and other things striking the earth, the combination of them causes electrons to build up in one of these rods and it depletes them in the other side.

I don't see any way this can be read as to mean anything other then there being 73 different elements in one side and 74 different elements in the other side. Even if he didn't mean chemical elements (and I would suggest if he didn't I would suggest element was a rather poor choice of terminology in the context), he clearly meant different stuff, not just 73 units of the same thing. Also he's clearly saying the additional element means a potential difference to arise between the two rods (because of neutrinos and whatever else). I don't know and didn't pay much attention to what he talked about later, listening to the intro was enough to convince me he was talking nonsense, which was my main point from the beginning.

BTW there's some suggestion the actual intented meaning was element 73 and element 74 [3] [4]. In the Youtube link it's mentioned the clip comes from a movie "Undeveloped Tech". It's easy to find that movie shows stuff invented by DL and his company BWT although I'm not sure which one, if any, he is in the specific clip. Per the earlier refs, DL appears to have a fairly poor reputation even among free energy people (also look for him at http://peswiki.com). According to [5] at one stage he couldn't even spell neutrino on his website (can't find the wording anymore although there's still this very low quality video [6] which uses that spelling and I think [7] is related to the DL guy somehow).

Mind you, as said it is unclear if DL has any real involvement, the main 'inventor/s' (perhaps the person demonstrating the device in the video?) and companies/people involved seem to be not directly connected to DL. In the earlier refs and [8] it's also suggested the element thing and possibly even neutrino was just a smoke screen by the inventor to cover how the device really worked and how simple it is. Funny enough for something so simple they appear to be having lots of problems making the device.

P.S. I censored the name but the links do name him directly which was fairly unavoidable. If people feel this is a problem per BLP, feel free to remove them.

Nil Einne (talk) 12:06, 19 November 2011 (UTC)

Perhaps the "73 elements" terminology comes from half-understood 19th century electrical books or articles, jumbled together to sound impressive. Interpreting the phrases in the video could be like trying to interpret David Copperfield's magical patter as descriptions of scientific phenomena. Edison (talk) 03:27, 20 November 2011 (UTC)

hydrogen bonding

Why phosphoric acid is a syrupy liquid? — Preceding unsigned comment added by Jyotiprakash hati (talk • contribs) 18:35, 18 November 2011 (UTC)

The title you chose for this section is the answer to your own question. 67.169.177.176 (talk) 22:45, 18 November 2011 (UTC)

exercise not improving fitness

For the last three weeks I've been running a route three times a week that is 4.6 miles (7.4 km). I've been very interested to see how my performance is improving over this period, especially as I have entered a December race. Unfortunately, my watch says I'm getting slower and my body tells me it's working harder. I'm generally curious about the effect of regular exercise on fitness and have just read the exercise article. Now I'm stressed because of this: "where most people will see a moderate increase in endurance from aerobic exercise, some individuals will as much as double their oxygen uptake". How do I know if I'm one of the unlucky few? Judging by the last three weeks of running, it seems I might be! 86.7.42.12 (talk) 18:45, 18 November 2011 (UTC)

Have you considered that you might just be tired?

Anyway, shot in the dark, let's see if this comes up blue: overtraining. --Trovatore (talk) 18:48, 18 November 2011 (UTC)

I get plenty of sleep and while I'll occasionally be more tired on some days than on others, what I've been noticing is a gradual but steady decline in my performance. The distance I run is split in half; 2.3 miles at the beginning of the day and the same again in the evening. So as far as exercise goes, it's nothing! Almost certainly not enough to consider it overtraining. I've also been making sure I get enough protein in my diet and enough water. 86.7.42.12 (talk) 18:58, 18 November 2011 (UTC)

Well, I can't tell you. But I am curious about your "unlucky few" remark. You do know that increasing your oxygen uptake is good, not bad, right? --Trovatore (talk) 19:41, 18 November 2011 (UTC)

Oh. Well, I figured it meant that you needed twice as much oxygen to do the same amount? Embarrassed that I've completely misunderstood that then... 86.7.42.12 (talk) 19:50, 18 November 2011 (UTC)

Well, needing more oxygen when resting could be bad, although if you have a larger body now due to more muscle mass, even that might be OK. StuRat (talk) 19:54, 18 November 2011 (UTC)

So you understand that high oxygen uptake is different from breathing hard? Breathing hard — hyperventilation — may mean a lot of oxygen is going in, but then just going straight out again. Oxygen uptake is how much actually goes into your blood, and the only reason it goes into your blood is because you're using it, for aerobic metabolism. --Trovatore (talk) 20:18, 18 November 2011 (UTC)

Right, and a high oxygen intake when resting probably means a high basal metabolic rate. That could be good, in a way, in that you would tend to burn more calories and avoid obesity, but, ultimately, having a lower basal metabolic rate is probably better, as long as you can reduced your calorie intake accordingly, as this will slow the oxidation in the cells (yes, antioxidants also help, but not 100%). StuRat (talk) 20:47, 18 November 2011 (UTC)

I understood the passage to mean higher oxygen uptake when exercising. --Trovatore (talk) 21:24, 18 November 2011 (UTC)

If you want to be diagnosed to see if you have some medical condition, this is not the place. If you want an idea of how people react to suddenly starting a strong exercise regimen, look at boot camp. Marines show up and immediately start a hard exercise regimen. It usually takes about a month before they start getting used to it. After three months, they are capable of performing the regimen without a real problem. -- kainaw™ 19:46, 18 November 2011 (UTC)

Something else to watch is your diet. You are presumably hungrier now that you are exercising, and how you satisfy that hunger could make a difference. If you reward yourself after each run with a dozen donuts, then you might do more harm than the exercise did good. StuRat (talk) 19:57, 18 November 2011 (UTC)

All I can suggest is to take a weeks brake. Then run another 4.6 miles. If you're back to a shorter time, then I have a clue, but I can't break the Wikipedia policy on medical advice to tell you here. Different people take different times to get rid of lactic acid build up, sometimes 48 hours or more. You will just have to work this out for yourself. --Aspro (talk) 20:25, 18 November 2011 (UTC)

Or perhaps a week's break. A weak brake belongs on the car of your least-favorite politician. :-) StuRat (talk) 04:39, 19 November 2011 (UTC)

You've not said how long the race you're training for is; from that regime I'll guess it's a 10k. Some observations:

• To be on the safe side, regardless of your age, get a check up from your doctor. There are any number of things that affect your performance at long-duration aerobic exercise. Better to discover if you have any of them now than during the race. I ran a race in September and passed several people (who looked younger and fitter than me) being helped by marshals, including one who was getting CPR.
• I wouldn't expect to notice any appreciable gain in 3 weeks. Unless you're in your late teens or early 20s, like Kainaw's marines, I'd expect you'd see only very modest improvement in 3 months. And those guys suffer.
• Ideally you'd train with some more experienced runners; see if there is a local running club. If you really can't do that, at least get a decent running book (Paula Radcliffe's how to run is pretty accessible).
• This morning+evening thing you're doing is very strange. It takes several miles to get warmed up and ticking over nicely; you're stopping just as the unpleasantness is beginning to pass.
• Every programme I've seen has you running significantly longer on one day (usually Sunday) than the others.
• If you've not run regularly before, 4.6 miles is quite a lot.
• Crazy people, who've heard that "exercise needs willpower" show up underprepared for a race and think they can will their way to the end. Maybe they can, but maybe they'll die. The willpower part comes in for all those long runs in the dark and the snow months before the race.

-- Finlay McWalterჷTalk 20:58, 18 November 2011 (UTC)

Thank you for such comprehensive advice! The race is a 5k and held once a month, so I intend to run it regularly and am focusing on improving my time. When I feel up to it I'll start working on 10k.

• I'm 27 and have had check-ups a few times in the past ten years. No problems and I have perfectly average blood pressure (assuming that blood pressure is an important thing to consider for exercise).
• I'm okay with suffering! I wasn't necessarily thinking I'd be seeing a lot of improvement in 3 weeks, but definitely not expecting for it to get harder.
• The book is a great idea, and I've been meaning to go along to a local club for a while. I'll make a point of doing that now.
• The morning then evening thing is because I'm running to and from work. I'm also running with a small backpack, so I'm carrying some extra weight in addition to my 9 stone (though it really isn't much).
• I really should be running the full distance for the race for proper practice, yes. I'll do that.
• I used to run fairly regularly (as in about 6 months ago) and my time for a 5k race in July was 27 minutes, so I'm not a complete beginner.
• Definitely not thinking of going into it without proper practice! But thank you for the warning.

Thanks everyone again for the help and advice. I really want this running thing to work! Sorry about the confusion with oxygen uptake, the sentence felt like it should read 'negatively' and I didn't think about it enough! 86.7.42.12 (talk) 08:58, 19 November 2011 (UTC)

Let me mention food again as an issue. I go onto a cross-trainer twice a week. This has the advantage of measuring energy expended and heart rate (and has a big-screen TV where I can get my weekly fix of either Star Trek or The Borgias). It makes a huge difference if I go up without a snack (typically 6 hours after lunch), or if I have a banana or a small sandwich about an hour before training). --Stephan Schulz (talk) 09:11, 19 November 2011 (UTC)

It seems to me that you are not training intensely enough. You should exercise for 20 to 30 minutes, at a heart rate around roughly 150 bpm (the optimal value will vary from person to person, for someone below 30 it will more likely be a bit higher than 150 bpm than lower). You may find out that you can only run fast for say, 12 minutes. In that case then you should execise for 12 minutes at that high intensity for a while. You should do this 4 to 5 times per week. This sort of exercise will lead to significant improvements in fitness. You should then gradually increase the time you exercise to 30 minutes or even longer.

Also you should exercise for running the distance you want to run, but you do that at a slower pace, simply to get used to running that distance, not per se to improve fitness. Now, because you aim to run only 5 km, you may not need to do this, you'll likely be able to get to that 5 km simply by increasing the amount of time you can run fast. To run 5 km, you only need to run 23 minutes at a speed of just 13 km per hour, which is a reasonable target for someone aged 50, let alone for someone aged 27.

With proper training almost everyone around 30 years of age should eventually be able to run the 5 km in less than 18 minutes, perhaps close to 16 minutes, which is still way off the World Record of 12 minutes and 37 seconds. Count Iblis (talk) 21:02, 20 November 2011 (UTC)

Solar sail max speed

If we assume a solar sail based object in vacuum with the following criteria:

• Constant source of radiation pressure along the traveling distance,
• No other effects of forces such as gravity.

Then what would be the maximum theoretical speed (with respect to a stationery observer where the solar sail started)? I couldn't figure if acceleration will tend to zero with time or stay constant while the relativistic mass increases.--109.74.32.237 (talk) 18:59, 18 November 2011 (UTC)

The maximum speed is (unsurprisingly) the speed of light (Assuming the radiation is constant). The acceleration (as seen by the "stationary" observer) tend to zero as the relativistic mass increases. Dauto (talk) 19:35, 18 November 2011 (UTC)

But, of course, you'll never get near that because:

1) You will move away from the star providing the radiation pressure.

2) There is no pure vacuum in space, and at high enough speeds the drag from space dust would be enough to counter the radiation pressure. And the solar sail will also be destroyed by that dust at some point. Even if stationary, micrometeors would still eventually destroy the sail. StuRat (talk) 20:02, 18 November 2011 (UTC)

1) The OP didn't say anything about stars. He mentioned a constant pressure

2) He also mentioned he wanted us to ignore all other effects

-- Dauto (talk) 20:47, 18 November 2011 (UTC)

Forces analysis

In the solar sail article (Mathematical survey section) it was written: "The light force can be separated into the normal force (away from the light source) and the tangential force as a function of the angle A of the sail face to the light. The Normal Force per area = 8/9 $cos^{2}A$ + 1/9 $cosA$. The Tangential Force per area = 4/9 $sin2A$."

May I know how this was calculated or at least a reference for that? I've tried to used conventional calculations for Lightness number from the Wikipedia article highlight and found it to be around <math>1.46*10^{-3} * \frac{mass}{area}</math> $1.46*10^{-3}*{\frac {area}{mass}}$ (The article I am writing is in an Arabic questions and answers site here). --89.189.69.209 (talk) 14:42, 20 November 2011 (UTC)

There is something missing in this equation. The units don't match. Dauto (talk) 15:57, 20 November 2011 (UTC)

If you were referring to the last equation, it is derived from the light force acceleration to gravity ratio and the constant $1.46*10^{-3}$ should have units of $kg/m^{2}$. There may be some mistakes (I've corrected there above) in the equation I derived but what concerns me now is the Wikipedia previous phrase analyzing force to its normal and tangential components.--Almuhammedi (talk) 19:30, 21 November 2011 (UTC)

Specific Gravity of various substances

My son needs a reference point for the specific gravity of Styrofoam, Pine Wood and Pine Bark. Its for a 7th grade science project, and he and I have both surfed, but can't find a reference that looks like they really know what they're talking about. He's doing an experiment to show the bouyancy of objects in water. The teacher picked the objects. If you can just point me in the right direction I would appreciate it immensely. Tanstaafl37 (talk) 20:45, 18 November 2011 (UTC)

Have you considered measuring it, instead of taking it from table? It's not too hard. Dauto (talk) 20:48, 18 November 2011 (UTC)

The experiment should provide the answers (which he is to perform) ... or am I missing something?--Aspro (talk) 20:53, 18 November 2011 (UTC)

Aw-shucks, I think I see a problem with our article on Specific gravity, is far too complicated for this grade. Just find out how the teacher wanted the experiment to be done, and just do it.--Aspro (talk) 21:06, 18 November 2011 (UTC)

Yes, it's not written for 7th grade, is it? There are numerous variations on a simple experiment to measure specific gravity of irregular buoyant objects. They all use Archimedes' Principle (a shorter article more suited to your son's level of study). Is your son expected to design his own experiment, or has he been shown a standard method? Dbfirs 23:46, 18 November 2011 (UTC)

At the 7th grade level, "specific gravity" is essentially equivalent to "density." There are a few nuances - if you want to be technical - but if you just want to measure specific gravity, you do so in the same way you measure mass-density: you weigh the object; and you measure its volume (usually by measuring the displaced volume of water when you submerge the object). If you want to get into the finer nuances, you can discuss the difference between mass and weight; you can discuss the different effects that temperature and pressure have on volume of water and volume of the test object; and you can discuss sensitivity analysis and experimental error. You can also discuss why we measure specific gravity; and you can discuss the benefits of normalization of the measurement to a common substance (e.g., water). Nimur (talk) 01:04, 19 November 2011 (UTC)

The following is my suggestion for the simplest way to measure specific gravity: equipment needed -- kitchen scale with a zero function, dish of water, spike (or finger). method: 1) weigh the object. 2) place a dish of water on the scale and zero the scale. 3) push the object into the water until it is just submerged (a spike instead of a finger will give better accuracy). 4) read the weight. 5) divide the first weight reading by the second. I leave it to your son to explain why this gives the specific gravity of the object. Dbfirs 12:59, 20 November 2011 (UTC)

Essentially, specific gravity is the density of an object compared to the density of water (potentially calculatable by weight in water vs. weight in air, IIRC). The article says the experimenter needs to maintain a standard "control" for the experiment in terms of temperature and pressure. That should not be too difficult, but increases in difficulty when trying to do it with accuracy: do it in the same day.

I remember creating a method of making the measurements without needing to use weight scales of any kind, needing no measuring equipment except a ruler. This is quite inaccurate for most objects (I determined the SG of a metal nail to be 8) but for rectangular masses it may be no problem. All that is needed is to compare the water displacement due to volume in the water (required to submerge the entire object in water without adding your hand, or simply measure the dimensions of the object to find the volume), versus the displacement due to mass of the object. To find the mass displacement, put the entire object in a non-permeable container such as a plastic one, and measure the volume of the water displaced by the container holding the object, averaging it out from the wave motion in the larger container that holds water. Divide the mass displacement by the volume displacement, and this is your SG; however I have not experimented using very large objects, nor those with SG>1.0. The error created by the container itself is also uncertain. Can somebody expand on my method? ~AH1 ^(discuss!) 21:52, 20 November 2011 (UTC)

MO theory and the shapes of molecules

can one determine the shape of molecules using only MO theory?(i.e. without using hybridization model?)--Irrational number (talk) 20:55, 18 November 2011 (UTC)

Yes. 67.169.177.176 (talk) 22:48, 18 November 2011 (UTC)

um.... A bit more explanation?like how?(i'm the OP)--81.31.191.221 (talk) 05:27, 19 November 2011 (UTC)

Actually, for covalent molecules you don't even really need either the hybridization model or the details of the MO theory. All you gotta do is figure out the Lewis structure, and that will give you the shape of the molecule. Of course, when you start talking about transition metal coordination complexes (like for instance nickel carbonyl), then you have to figure out the d-orbital splitting to determine what basic shape the complex will assume (e.g. octahedral, square planar, tetrahedral, etc.) and whether there will be any Jahn-Teller distortion. However, often you can determine the basic shape just by seeing how many ligands are bound to the central atom, e.g. if there are six ligands then you can bet your life that the complex will be octahedral. For complexes with four ligands, it's more complicated because they can be either tetrahedral or square planar, which you can often figure out from the number of d-electrons on the central atom. Likewise, for complexes with five ligands there are two different geometries (square pyramidal and trigonal bipyramidal), but it doesn't really matter because they'll be in equilibrium anyway. In any case, once you're done with this, you'll have to determine the d-orbital splitting in order to see if there's distortion, and that's where the MO theory comes in -- you have to figure out the symmetries on all the orbitals, then mix and match the orbitals with the right symmetry, then place the electrons according to the aufbau principle, and that will tell you if there's any Jahn-Teller distortion. 67.169.177.176 (talk) 07:44, 19 November 2011 (UTC)

to put it another way, is it possible to verify the shapes (derived by other methods) using MOT?--Irrational number (talk) 19:13, 19 November 2011 (UTC)

Yes, it's very much possible to verify the shape of a molecule by matching the symmetries of the electron orbitals. 67.169.177.176 (talk) 21:08, 19 November 2011 (UTC)

Yes, there are a number of methods. One of the more common ways to do it is, I believe, the Hartree–Fock method. Needless to say, you need a computer to do any calculations on anything but the very simplest of molecules. Basically, you have a way to calculate the (approximate) energy for any given configuration of atoms in space, and you minimize that energy. Buddy431 (talk) 18:33, 20 November 2011 (UTC)

Yikes, memories of porting Hartree-Fock analysis (in FORTRAN) from a mainframe to a PDP-11 back in college, must be nearly 35 years ago, now. PЄTЄRS J V ►TALK 18:56, 20 November 2011 (UTC)

Note that this whole discussion is about determining what the molecule is predicted to be based on various theories. The molecule "just is" and has its shape as a fact of nature. All we can do is rationalize why using theories verify the theories (studying them to find which ones agree with fact). And then use the theories to predict what other molecules might probably look like in the domains where the theories are known to be consistent with nature, rather than using the theory to prove the fact of the structure. DMacks (talk) 19:35, 20 November 2011 (UTC)

Yes, that's what I was gonna say -- all these methods could be used to predict the shape of the molecule, but you still have to verify it experimentally. 67.169.177.176 (talk) 21:34, 20 November 2011 (UTC)

Extension cord AMP ratings

I need to buy 3 extension cords. The store seems to have them in ratings of 10A, 13A, 15A, 20A, and 30A. So, what's the minimum required for:

1) A 1500 watt, 120 volt space heater.

2) A 500 watt, 120 volt space heater.

3) An average-sized 120 volt refrigerator with top freezer compartment. StuRat (talk) 21:01, 18 November 2011 (UTC)

Copper is going up in price. Seize this opportunity to grab some extensions that you can use in the future for higher wattage appliances. Spend now save latter! --Aspro (talk) 21:15, 18 November 2011 (UTC)

Fire marshals will tell you never to use an extension cord with any of those appliances. That said, for permanent circuits, 14-ga cable will carry 15A/1800W, 12-ga will carry 20A/2400W. While short cords can be de-rated, as can intermittent loads, if you're hooking up resistance heat loads or major appliances with motor starting requirements, it's best to go with a cord that matches the permanent wiring requirements. A refrigerator requires a 20A circuit, and the space heater at a full 1500W will start to heat its cord, so I'd go with 20A cord for that and keep each cord to the shortest length that will suffice. You should never coil or bunch excess cord - I know someone who who had to throw a smoking extension cord out the window after it was hooked to a space heater while coiled. The 500W heater can probably live with the 14-ga/15A cord. That's the lowest I would go for loads like that. Anything smaller would be for intermittent use or light-duty lighting. Acroterion (talk) 22:44, 18 November 2011 (UTC)

Thanks. I was thinking the 500 W heater would only need 1/3 the amp rating of the 1500 W. Why isn't that the case ? As far as avoiding coiling the cord, how should you take up the excess ? StuRat (talk) 22:58, 18 November 2011 (UTC)

The 500 W heater would be OK with the 10A extension, but it is always better to err on the safe side, as advised by Acroterion. I too have seen coiled extensions melted by a load within the rating because of the induction in the coil. Best advice is to uncoil the full length and avoid any repeated loops. Random looping will be fine as long as there is no bunching. Even better would be to find a route that allows the full length to be stretched out, but this is not strictly necessary as long as loops are large and not repeated in the same direction. Dbfirs 23:35, 18 November 2011 (UTC)

I doubt there is much heating due to inductance, compared to the resistive heating in the cord. Coiling it up decreases its ability to dissipate heat. Anyone care to calculate the inductance of an extension cord? Edison (talk) 04:31, 19 November 2011 (UTC)

How would "induction in the coil" be affected by having equal and opposite currents in the two closely spaced conductors of the extension cord, inherent in AC (or DC, for that matter) power supplied to a device? Edison (talk) 03:18, 20 November 2011 (UTC)

You could get by with a relatively small cord on the 500W heater (say, 16-ga - 18-ga is lamp cord and not at all satisfactory), but with long-term heavy loads it's best to be conservative. At small wire gauges there's really very little copper, and a little damage can severely impair current-carrying capacity. Also, voltage drop increases as wire gets smaller or longer, which is a reason to keep cords short: the drop in voltage is the result of resistance, which produces heat. Vacuum cleaner cords tend to have very small wire gauges and heavy loads, and they can get quite hot, but vacuums are rarely on for more than a few minutes at a time. Using a cord only as long as it has to be lets you avoid coils, too. Acroterion (talk) 03:04, 19 November 2011 (UTC)

How about if you coil half the excess in one direction, then the other half in the opposite direction, attached to the first coil ? And as for "being on the safe side", if I need 15A to be safe with 500W, why don't I need 45A to be safe with 1500W ? StuRat (talk) 00:04, 19 November 2011 (UTC)

Two half-coils in opposite directions would be much marginally better than a single coil because you would cancel out most of the inductance, but I would feel safer with looser random looping. You are correct that watts are proportional to amps, in fact 500 watts is less than 4.2 amps so a 5 amp extension lead (available in the UK) would technically be adequate. Thin cable is easily damaged, hence the advice not to run it near the limit. 1500 watts is only 12.5 amps at 120 volts, so in fact the 15 amp extension would serve the purpose (but I would prefer to use 20 A to give a safety margin). If the space heater has a fan, this will draw extra current, especially as it starts up. Dbfirs 00:36, 19 November 2011 (UTC)

Nope, no fan. A pile of random coils would present too much of a trip hazard, so I'll try reversing the direction and monitoring the temp. StuRat (talk) 01:55, 19 November 2011 (UTC)

You could get by with a relatively small cord on the 500W heater (say, 16-ga - 18-ga is lamp cord and not at all satisfactory), but with long-term heavy loads it's best to be conservative. At small wire gauges there's really very little copper, and a little damage can severely impair current-carrying capacity. Also, voltage drop increases as wire gets smaller or longer, which is a reason to keep cords short: the drop in voltage is the result of resistance, which produces heat. Vacuum cleaner cords tend to have very small wire gauges and heavy loads, and they can get quite hot, but vacuums are rarely on for more than a few minutes at a time. Using a cord only as long as it has to be lets you avoid coils, too. If it's a loose coil you're probably OK. Acroterion (talk) 03:04, 19 November 2011 (UTC)

Why aren't extension cords on reel [9] a problem for inductance ? StuRat (talk) 01:59, 19 November 2011 (UTC)

Why do you think they aren't? Any decent extension cord on a reel for consumers always says to fully extend/unwind it before use (or at least for high loads), the same as do vacuum cleaners and such. (Our article also notes the requirement.) Nil Einne (talk) 03:03, 19 November 2011 (UTC)

Most cords on reels are used intermittently, so heating and inductance don't present such a problem. Acroterion (talk) 03:04, 19 November 2011 (UTC)

I beg to differ. As an illustrative anecdote, some (OK, about 30) years ago one of my bookshop colleagues started to vacuum our shop's carpet at the end of the day, as was routine. However, she neglected to first unwind the extension cord reel as was also usual, and after only a minute (very roughly) the reel started smoking, and on inspection significant amounts of the cord's plastic insulation and the plastic body of the reel were found to have melted. {The poster formerly known as 87.81.230.195} 90.197.66.55 (talk) 13:14, 19 November 2011 (UTC)

Much depends on the machine and the level of economy the manufacturer applied to the cord; I have two vacuums: one cord gets hot, the other doesn't. Per the note below, I think in most cases the problem is heat dissipation rather than inductance, as it's my understanding that the hot and neutral currents cancel inductance in a coiled AC cord, but it can become a significant issue where the hot alone is a coil, or where the hot and neutral are separately routed, as in knob and tube wiring. Acroterion (talk) 14:58, 19 November 2011 (UTC)

Does anyone here have a reference (or even a strong physics argument) suggesting that a coiled extension cord heats excessively due to inductance, instead of due to the concentration of heat source and lack of dissipation, as stated in our article? -- 203.82.66.198 (talk) 13:49, 19 November 2011 (UTC)

Acroterion is correct. The two wires inside of the cord carry opposing currents so they won't cause any induced currents in a neighboring coil. The overheating is such situations is really caused by bunching up too many warm wires together which prevents efficient cooling. Dauto (talk) 17:37, 19 November 2011 (UTC)

(edit conflict)::I wasn't thinking very clearly when I implied that inductance was a significant effect. In fact, at any instant, the current is flowing both ways in the coil, and only a systematic difference in the layout of live and neutral would cause a significant inductive effect. The main heating effect must be from the resistance in the wire and lack of heat dissipation in the coil. My advice for very loose loops is therefore much better than the suggestion of reversing the direction of half the coil. The safe answer, of course, is to use an extension of appropriate length and avoid any close aggregation of cable so that air cooling is optimised. I've modified my reply above. Dbfirs 17:44, 19 November 2011 (UTC)

Really, putting resistance heaters (in particular) on extension cords is a very bad idea. There is never a perfect connection between the heater plug and extension cord, leading to heat buildup at the connector. If you have that few outlets that you need extension cords (implying an older house with fewer outlets and likely all just 15 amp circuits), all the more reason, you'll also get heat build-up at the wall outlet connection. As for the fridge, not a hot (no pun intended) idea either, there's a reason that compressor-driven appliance (A/C, fridge, freezer,..) instructions state you should not use an extension cord. (And most electrical code requires a dedicated circuit for the refrigerator, that is—moreover—nothing else plugged in, anywhere, on that circuit.) In the meantime, if you are going to use an extension cord temporarily for any reason, it should introduce minimal resistance, so: copper, as short as possible, and as high an amperage rating as you have available. (Higher amp connectors will also dissipate heat better.) PЄTЄRS J V ►TALK 19:18, 20 November 2011 (UTC)

Extension cord heating

Follow-up Q: A longer cord means more energy is dissipated as heat. What about a thicker cord ? Does that mean more, less, or the same amount would dissipate as heat, assuming an identical length and load ? StuRat (talk) 15:06, 19 November 2011 (UTC)

Less. For DC, anyway, the power that a wire turns into heat is $P={\frac {I^{2}\rho \ell }{A}}$, where $I$ is the current through the wire, $\rho$ is the resistivity of the material (usually copper) that the wire is made out of, $\ell$ is the length of the wire, and $A$ is the wire's cross sectional area. So with everything else constant, a wire with a larger cross sectional area will generate less heat. The equation above is a combination of equations found at Electric power#Direct current and Electrical resistance#DC resistance. The power turned into heat is really somewhat more than in the equation above due to it being an AC current instead of DC, but I think it's close enough for this discussion. Red Act (talk) 16:48, 19 November 2011 (UTC)

Yes, much safer to use thicker conductors. Double the cross-section of the wire and you halve the total heat to be dissipated in the coil (approximately, ignoring the skin effect). AC calculations are exactly the same if you use RMS values (which nearly everyone does). Dbfirs 12:48, 20 November 2011 (UTC)

Flexible electrical insulation which is thermally conductive

Next follow-up Q: Is there any material which could be used for wire insulation which would conduct the heat away from the wires rather than holding it in ? Ceramics seems to be good electrical insulators and at least some conduct heat fairly well, but they aren't flexible enough. Perhaps some composite material containing ceramics ? StuRat (talk) 18:08, 19 November 2011 (UTC)

Small ceramic beads and manganese oxide copper sheath cables, are already used in some applications were silicon rubber or PTFE insulation is not heat resistant enough. Conducting heat away is the wrong approach. As I suggested at the start, get the highest amperage extension that you can afford – and educate everyone within the household, about the dangers of leaving them coiled up whilst in use (the fire brigade in the UK have a neat way of getting this across to new recruits – they show them photographs of the chard remains of those who who left their extensions coiled up).--Aspro (talk) 18:33, 19 November 2011 (UTC)

How about if you put the coils in a pail of water (and hope that the insulation is intact) ? :-) StuRat (talk) 18:46, 19 November 2011 (UTC)

Already invented. its called an electric immersion heater. Are your life and fire insurance polices up to date? :-)--Aspro (talk) 19:14, 19 November 2011 (UTC)