Wikipedia:Reference desk/Archives/Science/2009 April 19

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April 19

Electric field therapy

Has any one every heard of it? roscoe_x (talk) 07:49, 19 April 2009 (UTC)

Mr. Google has. 88.112.62.225 (talk) 08:01, 19 April 2009 (UTC)

O yes, I've googled it first. But many of the links are selling products. I found this one [1].What I want to know is how common it is? And its affect to health. roscoe_x (talk) 08:10, 19 April 2009 (UTC)

It depends. It will affect your health in two and only two ways. First, it will consume as much of your available income as you are willing to give the peddlers. If you otherwise spend this on Hamburgers and cigarettes, it will improve your health. If the alternative is to pay for and go to a gym, it will usually be detrimental. For the second way it may affect you, see Placebo effect. In other words, it's complete bunk, but should be fairly harmless by itself. --Stephan Schulz (talk) 09:15, 19 April 2009 (UTC)

See also Electrotherapy but it sounds like a cranky idea to me. 89.242.147.172 (talk) 10:42, 19 April 2009 (UTC)

Have a look at this Quackology site,[2]. They don't rate it very highly. Richard Avery (talk) 11:42, 19 April 2009 (UTC)

There are a bunch of different type of electric field therapy and little evidence for effectiveness for any of them. It's not scientifically implausible that something like this could work, but there's hardly any actual positive evidence at this time. Looie496 (talk) 16:52, 19 April 2009 (UTC)

Of course, it's a scam. At best, it doesn't work, at worst it does some subtle and as-yet-not-understood harm. <sigh> SteveBaker (talk) 18:27, 19 April 2009 (UTC)

(I've got to stop looking at the websites for these things...it only makes me angry). [3] claims to explain how this works. Aside from the fact that this page is one large image - so you can't cut and paste the text (grrrr) and it's riddled with typo's - the actual "science" there is just a bunch of random jargon stringed end-to-end to look impressive. In the section "How the Electric Field Therapy works", we have:

• "EFT is a simple, non-invasive technique that uses static electricity...". OK - fact #1 - it uses static electricity.
• "The theory is that when an electric filed with high voltage AC is applied on a human body..." - well, firstly it's a "FIELD", not a "FILED" - but fact #1 said they use STATIC electricity. You can't have AC static electricity! Plus, if you apply high voltage AC to the human body (eg by sticking your fingers into an electrical outlet, it tends to DIE - not get better!
• "...cells metabolism is stimulated by supplementing ions and the Acid-Base Balance of Electrolyte in the blood results adjusted." ...that's not even a grammatically correct sentence. But what makes you think the pH of your blood is likely to be wrong? There is no science saying that any of the conditions it claims to fix is caused by an error in the pH of your blood - and even if it were - passing AC current at high voltages isn't going to change the pH. Do you really want to take a machine made by people who can't put together a single coherent sentence and have it try to electrocute you? I really don't think you do.
• "The H+ flow of ions in the organism can optimally be directed."...er what? Positively charged hydrogen atoms...like in water..."can optimally be directed"...Where? Why? What happens to all of the OH- ions that would be left behind? If you try to adjust the pH by moving the H+'s in one direction than you'll be adjusting the pH wherever the OH-'s would be in the opposite direction. But this is crazy! This is doing electrolysis on your blood! If it actually did this, it would kill you amazingly quickly!
• "This is important because beside others, the co-ordinates of enzyme, the Ca++ household, the proteins of transportation in the blood and the general acid-base-homeostasis depend on these flows of protons and ions." - OK - the only way to write a sentence with so little meaning is to open a dictionary of scientific terms and write them down at random. There is no way to even try to analyse what this sentence means!

It gets worse, the company claims to have "ISO9001" certification. I've actually been through the certification processes for those standards and they are pretty hard to get - they relate to how products are designed and documented and represent the result of frequent inspections of your processes, etc. However, when you click on the image of the "certificate" - it's not an ISO document - but something awarded by some unheard-of Chinese organization. It's certainly not a proper ISO-9001 certificate.

This is so very bad...it's beyond ridiculous. PLEASE - don't pay them a cent of your money.

SteveBaker (talk) 18:57, 19 April 2009 (UTC)

Thanks for all the information. My mom tried this. The one that made her belief there is something in this device is it made her feel electrical shock when the person touch her hand. roscoe_x (talk) 05:53, 20 April 2009 (UTC)

They probably connect you up to a Van de Graaff generator or something. That would make it feel like something is happening (because it is, you are becoming electrically charged, but that has no health benefits). --Tango (talk) 13:38, 20 April 2009 (UTC)

Viral infection

How is a viral infection detected and how is the responsible virus identified? —Preceding unsigned comment added by 173.49.18.189 (talk) 08:57, 19 April 2009 (UTC)

Do you mean by the body or in a laboratory? In the body it would be some kind of white blood cell which would then present a viral protein to another type of white blood cell which would then produce antibodies against it.

In the lab you can use lots of different ways - e.g. ELISA, polymerase chain reaction and antibody testing kits (the same technology as in pregnancy tests). I can't find a link to the last method. These all assume that you already know what the virus is. For unknown viruses it's a bit more difficult - I guess that genome sequencing would be used. Smartse (talk) 11:16, 19 April 2009 (UTC)

Thanks. The question is about how viral detection/identification can be performed in a lab context. What if the virus is unknown, or new? Would a clinical lab, say one at a hospital, be equipped to detect and confirm a new form of viral infection? Or is that only possible at a very well-equipped research lab? 173.49.18.189 (talk) 14:52, 19 April 2009 (UTC)

Hospital labs are equipped to detect known pathogens. Novel viruses would not routinely be screened for. The detection of novel viruses is a huge topic - there is no simple way to do it. Some viruses have been detected by looking for cytopathic effect in tissue culture. The hepatitis C virus was known to exist (as "non A, non B viral hepatitis") for more than a decade before Michael Houghton's group at Chiron discovered it by screening over a million cDNA clones in a phage library using serum from infected people to detect viral proteins. Today, techniques like random-primed PCR (with random-primed reverse transcription to detect RNA viruses) are available, but far from perfect. Of course once you've detected a virus, showing that it causes a specific disease is another challenge - we'll all loaded with viruses. --Scray (talk) 18:39, 19 April 2009 (UTC)

Photovoltaics and thermodynamics; ie, efficiency vs 'wasted' energy

Some considerable pondering hasn't cleared this up, and neither have I been able to find reasonable info in any of the articles I looked into, nor any of the archived reference desk questions some searches turned up.

Imagine a photo cell exposed to monochromatic photons are at the precise energy (ie, wavelength) which exactly matches the optimum energy for absorption in the cell; this is, I understand the band gap energy in traditional photo cells (eg, silicon). We will thus, be generating no phonons carrying 'excess' energy from the absorbed photons, there being an exact 'fit'. Phonon generation (energy therein will be wasted w/regard to electricity output) will be at least minimized. As nearly as I can make out, absorption and emission of photons being quantum mechanical events, there is no energy "leak" which eventually ends up in increased temperature of the photocell or attached structure.

It seems to me to be an exception to the usual energy conversion stricture which inherently includes some losses to the energy uber despot, entropy.

What have I missed, if anything? ww (talk) 10:07, 19 April 2009 (UTC)

One thing that you've missed is recombination. You might absorb every incident photon with no losses, but the resulting charge carrier pair might not make it to the external circuit. See Quantum efficiency of a solar cell. I don't know if this is the main method by which entropy gets its revenge, but it's certainly one of them. There is also the fact that "no real source of electromagnetic radiation is purely monochromatic" (according to Monochrome). --Heron (talk) 11:30, 19 April 2009 (UTC)

(ec) I am not sure that the second law of thermodynamics prevents 100% conversion of energy from one form to another. What it does do is prevent 100% conversion to useful work. This will certainly come into play once you start trying to extract current from your cell; if through nothing else the joule heating of the circuit. In any case 100% conversion of photons is not practically possible. Some of them will hit the crystal lattice and become phonons, some will be re-emitted before the electron can be made to do anything useful, all real crystals have flaws which will lose some of the photons etc etc. SpinningSpark 11:41, 19 April 2009 (UTC)

It does not violate thermodynamics because a pure plane wave of zero frequency width has zero radiation entropy, thus, there is no maximum possible conversion efficiency below 100%. The opposite extreme would be blackbody radiation at the same temperature as the cell itself, having maximum radiation entropy, none of the energy could be used to create work. Radiation obeys the laws of thermodynamics just as matter does. 129.2.43.42 (talk) 16:06, 19 April 2009 (UTC)Nightvid

My thanks all. It may be that the blinders of ignorance have loosened their hold slightly. My thought experiment was deliberately constructed to evade most practical issues and focus (ahemm..) on what seemed to be the key question. So the objection that there can't be a monochrome source is skew to the hypo; and likewise inefficiencies (ohmic and otherwise) during use of the electricity generated to something helpful (like running my automated beer fetching and opening robot, say). I was attempting to exclude such issues, mostly because they're as obiter dicta and so obscurantist. Recombination does indeed seem be something I'd not considered. Easy to miss, not being one at home in the labyrinthine complications of doped and dopey micro lattice structures, so I don't feel as terminally thick as I probably should. The anon contribution by 129.2... may be the very answer I was seeking, but alas I fell off the wagon as it made the turn into pure plane wave territory... In English this historian type can cope with, please...? ww (talk) 18:06, 19 April 2009 (UTC)

I don't know how helpful the analogy is, but in a volume of zero the mass of any continuous substance is zero, and if we have some mass in a zero volume the density is infinite. Similar considerations apply to the "spectral density" of light; if we have light with a continuous spectrum (that would give a continuous "rainbow"), the amount of light at exactly one frequency is zero. A finite amount of light at exactly one frequency (or in exactly one direction for that matter) has an infinite spectral density. This light can be thought of as coming from something "infinitely hot". Thermodynamics tells us that if the "hot part" of a heat engine is infinitely hot we can, in principle at least, convert heat to work with 100% efficiency. This of course is setting aside practical issues like reflection off the surface of the solar cell or carrier recombination. 69.140.12.180 (talk) 04:43, 21 April 2009 (UTC)Nightvid

Energy Saving Lightbulbs

What is the technology in energy saving lightbulbs? Are they simply minature versions of a flourescent tube, or are they something else? Do they flicker like flourescents? An example of what I mean is here http://www.amazon.co.uk/Philips-Energy-Saving-Lightbulb-Stick/dp/B001BYUIT2/ref=sr_1_3?ie=UTF8&s=kitchen&qid=1240136836&sr=1-3 I have only been able to find articles on either incandescent or flourescent lighting. 89.242.147.172 (talk) 10:32, 19 April 2009 (UTC)

It would be great if they really were "energy saving" but I think that would contradict the second law of thermodynamics. They are compact fluorescent lamps - I think this should help. Incidentally DYK that it's best to leave them on if you leave the room for less than 15 minutes because switching them on/off wears them out and could end up making them less efficient that incandescent light bulbs. Smartse (talk) 11:04, 19 April 2009 (UTC)

They are energy saving as you get the equivalent of 60w worth of light from an incandescent bulb for only 11w. The packaging of the 11w (230-240v) bulb I have says it gives 600 lumen, and is supposed to last 10000 hours. 89.242.147.172 (talk) 11:23, 19 April 2009 (UTC)

No, they don't flicker, but they are dimmer when first switched on and achieve maximum brightness after a minute or two. Richard Avery (talk) 11:35, 19 April 2009 (UTC)

I don't believe CFLs are likely to be less efficient then incandescent bulbs if turned off in less 15 minutes. They do wear out a lot that way however and so in overall energy consumption and pollution terms as well as cost to the consumer terms, leaving them on is sometimes better then turning them off. See [4] for example Nil Einne (talk) 13:22, 19 April 2009 (UTC)

It's important to be clear on this: CFL's have a MUCH longer lifespan than incandescents when used in "normal" ways. They are vastly more efficient because they generate much less heat - so the laws of thermodynamics are not violated! They are more expensive - but pay for themselves in a very short amount of time because of the energy savings and the increased lifespan. The only significant drawback is the mercury they contain - which is a nasty pollutant. However, they use a lot less mercury than old-fashioned 'striplight' flourescents - and right now, properly recycling the darned things - or cleaning up mercury pollution afterwards are a LOT easier than dealing with global warming caused by the wasted energy from incandescent lights. SteveBaker (talk) 18:24, 19 April 2009 (UTC)

Steve, we must disagree on what "normal" use is. To me, it's switch it on when you need it, then switch the damn thing off when you don't. I put a CFL in my bathroom and it was toast within about four months. The incandescent in the fixture beside it is still working two years later. It operates maybe 30 minutes a day, but gets cycled regularly (I work at home and drink a lot of water). Do the math on that and consider that it takes 9 cubic metres of landfill soil to neutralize the mercury emitted if you throw the bulb in the trash (which I don't, but many people do). Furthermore, during heating season, six months of the year, I save NO energy by having a more efficient light. All that happens is that the other energy source comes on. If that happens to be fuel oil in a furnace vs. hydroelectricity, I'm net-subtracting from combatting global warming. (Mercury emitted from new dam reservoirs is a wash vs. CFL bulbs).

If your definition of "normal" is leaving a light on all day in an air-conditioned house, unquestionably a CFL is better. Whereas I call that aberrant behaviour! :) Franamax (talk) 20:46, 19 April 2009 (UTC)

No matter ho much water you drink, that CFL should have been ok. You cannot generalize from a single fluke. Mine are warranted for 3 years minimum, and I have so far (3.5 years since I switched) had one dud after 3 weeks - and that was replaced without any discussion. As most electronics, CFL failure will tend to follow a bathtub curve, so a few early failures are to be expected. --Stephan Schulz (talk) 21:06, 19 April 2009 (UTC)

In contrast to the CFL sitting beside my couch, which has operated reliably for long durations for the last two years? I don't think it was a dud, although it's certainly possible that when you open a two-pack of bulbs one will fall into the bathtub :) I disagree, current switching is a known factor which shortens the life of CFLs compared to incandescents. My habit, ingrained in childhood, is to switch off a light when you don't need it. I find the suggestion that I should leave the light on, consuming energy, in order to save energy - paradoxical.

You're right in a way, I should have kept my receipt, gone back to the store, complained. They would have replaced the bulb and thrown the old one into their garbage. I've researched this a lot - switching and enclosed fixtures are failure modes for CFLs, or at least were. The various commentary available covers several generations of bulb, so I'm not sure. What I do know is that I use CFLs everywhere I find them appropriate. In the bathroom, where they are switched 10-15 times a day - no, I don't see the math that tells me I should spend more for no net energy/CO2 saving and use mercury in the process. The one in the living room, definitely. The bathroom, nope! Franamax (talk) 21:33, 19 April 2009 (UTC)

Well, our article claims (with sources) that a 5 minute on/off cycle reduces CFL lifetime to about that of an incandescent lamp, or alternatively, that one on/off cycle is equivalent to about 6 minutes of lifetime. With a rated lifetime of 6000h (the lower limit) they should be good for 60000 cycles, or about 10 years - half that if you also actually leave them burning 6 minutes (which also factors into their lifetime, of course). Even if the numbers are somewhat optimistic, it looks very much as if you hit the front side of the bath tub.... --Stephan Schulz (talk) 22:20, 19 April 2009 (UTC)

Not sure how to phrase this. :) Umm - we either need the bathroom light on for much less than five minutes or much more. Myself in particular. ;) If I need to operate the lamp for more than four times what I need the actual light for just for the privilege of paying way more to get the same lifetime as an incandescent, it is costing me and the entire planet. As I say, the living-room lamps are fine, they're on for hours at a time. No way I'm going to put the bathroom light on all day and night though. I can spend less money and emit less CO2 using incandescent lamps. So that's what I'll do. But yeah, I'll put another CFL in, track the lifetime and do the math. See you in four months! :) Franamax (talk) 23:43, 19 April 2009 (UTC)

But isn't that precisely the point? The lifetime will obviously be negatively affected by the times you turn it on briefly (but the energy savings won't be by much), however since it doesn't have a continous 5 minute cycle the lifetime will almost definitely be better then a incandescent and as already mentioned, the every savings remains there and costwise they could easily work out better. Also as I already mentioned elsewhere the idea you should leave the lights on is bull, few people who actually know what they're talking about recommend, I'm someone who nearly always switches off my lights even if leaving my room for only several minutes and the lifecycle on my CFLs have been fine. Indeed those that do seem to have had short lifecycles have been ones that are usually left on for a long while! Indeed my biggest problem has been incandescent lightbulbs breaking when I turn off a light then realise I still needed it or whatever, turn it back on and 'poof' there goes the filament. Really the best thing to do is to stop worrying about 'keeping the light on' and just use the light like normal Nil Einne (talk) 01:39, 20 April 2009 (UTC)

Just to avoid a possible misunderstanding: Lifetime (for CFCs and incandescents) is usually giving in burning hours ("on time"), not in calendar time. --Stephan Schulz (talk) 11:44, 20 April 2009 (UTC)

I've yet to see any reliable data indicating that oft made heat claim has any merit and simple basic physics suggests it's probably bull so I'm going to call it that. Heat travels upwards. We all know that (or should if we're going to be making claims about using lightbulbs for heating). That's one of the reasons why we usually install heaters on the ground. If you've ever touched a incandescent you know they get very hot (so do flourescents of course but I digress) and so those the air around them (even more so if your bulb is in a fairly or completely fixture) and probably even the ceiling etc. So guess what? A (probably substanial) portion of the heat you generate with your lightbulbs is very likely not aiding you in heating your home in a useful way at all. You obviously do gain something, from the infrared and from the heat that does radiate or convect but it's very likely only small proportion of what you're generating and whatever the case is definitely not all the wasted energy even if you happen to have a fan installed on your lightbulbs to try and cycle all that heat arpimd (so your claim that you save no energy is surely wrong). It gets even worse if you're using something more efficient like a heatpump or a gas heater or whatever. The simple fact is if you want to generate heat, the best thing to do is to use something designed to generate heat (we call them heaters), not try and use something who's sole purpose is to generate light (which surprise, surprise, is what lightbulbs are for). Also do you really heat your home for 6 months of the year? P.S. If you really don't believe me that it's not simply generating the heat but actually having the heat radiate in a meaningful way, then there's this anecdotal story. I bought an oil column heater, these are common in NZ. This was a real cheapie. Several other people have similar oil column heaters of the same power (1000W). I've found the oil column heater is actually fairly useless at heating my room despite having the same power (well this is rated, I've never tested it to see if it is really what it claims) as these better brand oil column heaters. Clearly the better ones are much better at radiating heat. Obviously these would use more power overall in general since my heater is likely to get hot and switch off whereas the other heaters won't switch off as much but I still feel confident enough to say that these more expensive heater make more efficient use of the power they do use in most circumstances. P.P.S. If you have data indicating that using CFLs during heating season doesn't save energy (hopefully taking in to account any potential confounding factors) then I'm perfectly willing to eat my hat but I doubt it. Nil Einne (talk) 01:39, 20 April 2009 (UTC)

I have the feeling that they do flicker, even if I can only detect it subconciously. Although I use them they do not seem as nice as incandescent bulbs, but do feel like flourescent lighting which I do not like. I'd like it if someone could provide the evidence to prove me wrong. 89.242.147.172 (talk) 11:48, 19 April 2009 (UTC)

There are many different kinds of CFCs, with widely varying properties and prices. I use the Phillips Softtone, which after warming up looks much like an incandescent to me. It did cost me around EUR 6 or 7 per bulb, IIRC. Cheaper bulbs will quite possibly have worse performance, both in light colour and in flickering. --Stephan Schulz (talk) 12:39, 19 April 2009 (UTC)

Note it is important to consider what you mean by 'worse performance'. For example, everything else being equal, cool coloured fluorescents (cool daylight for example) generally have better colour rendition (see colour rendition index) then warm ones. (The phosphors uses makes a big difference however whatever the colour temperatures). However in Western cultures, warm coloured lights which are closer to incandescent lights are generally preferred particularly for living spaces and are sometimes considered more natural (even though cooler lights are far closer to the epitome of natural light, the sun and bright daylight). From sources I've read and anecdotal evidence these preferences don't hold for Asian cultures however where preferences are more varied and often tend to the cooler end of the spectrum. If you can only detect something 'subconsciously' it could easily be simply that you want flourescents to be bad so they are. Nil Einne (talk) 13:42, 19 April 2009 (UTC)

That's all very true - but since you can buy CFL's that match either incandescent light's color or natural daylight color - there is no way that 'color' is a good excuse for not buying them. The fact that you get the choice is a huge plus. SteveBaker (talk) 18:24, 19 April 2009 (UTC)

I agree and apologise if my response distracted from the main point. It's simply a pet peeve of mine that people commonly suggest warm lighting is better or more natural since it seems to be a cultural thing and the more 'natural' concept is IMHO flawed (as with most claims that something is more natural). An interesting thing of course is that flourescents (not necessarily CFLs) have already taken significant hold in a number of Asian countries, whether this is because of the difference in colour temperature preferences (since I believe most early flourescents had a fairly high colour temperature) or is partly the cause of the different preference of a combination of both I don't know. Nil Einne (talk) 01:08, 20 April 2009 (UTC)

I seem to get a peculiar feeling on my skin when in a room illuminated by flourescent light - as if it were something to do with static electricity perhaps. Perhaps its just my imagination. 89.242.147.172 (talk) 14:06, 19 April 2009 (UTC)

Well if you get a peculiar feeling on your skin, then that's easy to test in a double blind fashion. You leave the room and someone throws a coin, or uses a better random method to decide whether to put an incandescent or CFL in the room. The person then leaves the room. 5 minutes later (well a preset interval so there is no need for contact between the two of you), you come in to the room well blindfolded so you can't possible see what light is present. Repeat multiple times (I would say at least 20.) If you are able to detect reliably whether CFLs or incandescents are present, then we know you are somehow detecting CFLs, perhaps because of a peculiar feeling on your skin. Nil Einne (talk) 14:11, 19 April 2009 (UTC)

The trouble is that it may be the color rather than flicker or (the extremely unlikely) skin tingle that would clue the person in to which is which. CFL's exist that are a pretty good match for incandescent color - but they aren't 100% perfect. Controlling for color in your experiment is much harder. SteveBaker (talk) 18:24, 19 April 2009 (UTC)

Well 89 said that he/she believed there was a skin tingle. IMHO it would be useful for 89 to test this if it continues to influence his/her thinking. I agree it's rather difficult to control for CFLs vs incandescents if you can actually see the light whether that's because of colour or flicker or simply because you believe CFLs are bad (and you can easily tell by the colour and other factors) hence the reason I didn't suggest any double blind tests until the OP mentioned a skin tingle Nil Einne (talk) 01:08, 20 April 2009 (UTC)

One last comment here - I regard CFL's as an emergency measure. Incandescents are such a huge problem for global warming that we simply MUST get rid of them as quickly as we can. In many places around the world, laws are being passed to outlaw these ridiculously inefficient devices...and incandescent light bulb factories are gradually switching production to CFL's. They simply have to go - whether you like it or not. However, LED lighting is the true way of the future. LED lights have an amazingly long lifetime - they are yet more efficient than CFL's - you can dim them nicely (tricky with CFL's) and you can even get them where you can adjust the color they produce to whatever your heart desires (I have one of these). Because they last so long, whatever pollution they create when they eventualy DO die is relatively unimportant because we won't be throwing them away so often. The only limitation on them right now is that the really bright ones are pretty expensive. We're seeing them appear in things like car tail-lights and traffic signals where the consequences of failure and the cost of replacement are high. This year, I think there were more LED Xmas tree lights on sale than incandescents in local stores here in Texas. I have a really fancy tri-color LED "bulb" in my home-office area (you can actually turn it on and off and set the color remotely from your computer using X10 protocol over the power lines!) - it was really just a hideously expensive gimmick - but it does work. It's plenty bright enough - and you can adjust it to produce "warm" lighting or "cold" lighting depending on your mood (you can also make it flash in various colors in time with music on your PC if you're into 1970's disco lights!) But as soon as they get cheap enough (and they undoubtedly will), I plan on switching over my home to slightly saner LED lights completely. SteveBaker (talk) 18:24, 19 April 2009 (UTC)

I've noticed (at least here in NZ) halogen bulbs that are compatible with ordinary sockets are now available. I believe these are being promoted because they are considered sufficiently energy saving that they will generally be allowed in places where incandescents will be banned. (Unfortunately this isn't going to be NZ since the new government abandoned that policy although given that nearly everyone else including China and our closest neighbour Australia are banning them I expect it's just a 'feel good' reversal since they'll probably still largely disappear.) They don't have the lifespan or anywhere near the energy savings of CFLs but if you absolutely refuse to use CFLs then IMHO you should use them. They are also dimmable although if used in dimmers they may not last as long (since they don't as hot as they should) nor save so much energy although it's my understanding they will always last at least as long as ordinary incandescents and will never use more energy. You're right about LEDs of course. They're also starting to get quite common in torches and other battery powered lights where the power savings (and longevity) is important and flourescents aren't possible. Nil Einne (talk) 01:08, 20 April 2009 (UTC)

I look forward to getting something similar to sunlight rather than the yellow gloom we are used to - although on the other hand it does signal the evening and hence probably helps you prepare to sleep. 78.146.75.232 (talk) 19:54, 19 April 2009 (UTC)

You may be interested in [5]. Also do note that cool daylight CFLs (and flourescents) with colour temparatures of around 6500K (lower are available if that's too high for you) are already vastly (IMHO and also based on colour temperatures) more similar to bright daylight then the yellow gloom and are available now (and in fact were the earliest flourescents I believe) Nil Einne (talk) 01:08, 20 April 2009 (UTC)

The great flaw of CFLs is that it costs about $1 (U.S) to recycle a CFL. An incandescent bulb can be purchased for less than 50 cents (US). A CFL can be purchased for about $3 or less with occasional rebates. Most consumers will not pay $1 to recycle a CFL, so they go into landfills and the mercury pollutes the environment. This is a neglected externality of the "bulb of the future." Edison (talk) 21:23, 19 April 2009 (UTC)

You're repeating some common, misleading arguments. There are organizations in many areas that recycle CFLs free of charge, including The Home Depot. You can also get mercury-free ones if that's for some reason not an option. If you get your power from a coal plant, the reduced power requirement saves more mercury from entering the environment than is actually contained in the light. Decent CFLs last longer than incandescents and obviously take less power, so your point on price is weak. -- Consumed Crustacean (talk) 21:30, 19 April 2009 (UTC)

Yes - that's true. I'd also argue against the "bulb of the future" moniker. CFL's are the bulb of today - they've been around for many years now. LED's (or perhaps, OLED's) are the bulb of the future. But at 10x to 30x the price of a CFL and 200x the price of an incandescent - it's hard to convince people to buy them until the price drops. But LED lights are good for 50,000 hours and use even less power than CFL's - so even at $50 to $100 a pop - they'll still pay for themselves over the very long term. 50,000 hours is a lot - if you put LED lights into a new house - the odds are good that most of them will still be in the house and working fine on the day it's demolished. So the issues of what toxic effects they might have in landfills is rather academic. You can flip them on and off as much as you want - they aren't as fragile as incandescents and CFL's so they work well in places like refrigerators and ceiling fan fittings and you can have them in any color you like - or even have RGB LED lights where you can adjust the color to fit your mood on any given day. Economies of scale will push the price of LED lamps down to more acceptable levels in not too many more years...so CFL's should be regarded as a stop-gap solution. SteveBaker (talk) 03:13, 20 April 2009 (UTC)

So, how would you solve a problem like lava lamps in the future? :) Mine goes through incandescent bulbs like shit though a goose. --Kurt Shaped Box (talk) 03:20, 20 April 2009 (UTC)

I have wondered the same thing. If incandescent bulbs become impossible to find we're going to have to come up with some sort of heating element/LED combo device to retrofit all of our lava lamps. (Also EZ Bake ovens if you're into that.) APL (talk) 03:54, 20 April 2009 (UTC)

I question your claim that "mercury-free" compact flourescent bulbs are available. Please provide a reliable source or a retraction. As for a merchant accepting old CFLs, the merchant still has to pay about $1 per bulb to get a recycler to accept them. It may be a public service on the part of the merchant, or a promotional method to increase traffic to the store, but it is still a neglected externality. If you Google 'recycle CFL you find that recyclers charge about 82 to accept 90 CFLs. A site that promotes use of CFLs says a consumer can buy a 30 bulb recycling container for $120, a higher cost per bulb than the initial purchase price. Is there presently a government audit system to verify that recyclers actually capture the mercury rather than charging a high price and then just putting them in landfill? Given the history of crooks in waste disposal in the U.S., it is just a matter of time until such a con is exposed. The bulbs still cost far more to recycle than incandescent bulbs cost to purchase, but the theoretical energy savings for the CFL would pay for the recycling. Has anyone determined how many CFLs get expensively recycled, compared to the number sold? I expect that the vast majority presently go into landfill and release mercury. Asking a merchant to provide "free" recycling will not work in the long run, because the recycling charge per CFL is about equal to the merchant's profit on the bulb. Edison (talk) 16:43, 21 April 2009 (UTC)

Okay, good thing you questioned me on that, as I'm no longer sure where I read that. As far as I can tell, they're still in the R&D phase. You've still ignored my point on coal plants and net mercury savings. On the rest of it: according to the compact fluorescent lamp article, price of CFLs at, say, The Home Depot, includes the price of recycling. This I can't verify due to the lack of source, but I haven't exactly expended much effort attempting to do so. Your entire point initially was that a consumer would not pay the extra dollar to recycle the bulb, which you've still failed to support; there was nothing to do with the costs and/or crookedness of the recycling process (which, uh, sources would be appreciated for). -- Consumed Crustacean (talk) 00:13, 22 April 2009 (UTC)

If CFLs are the same as flourescent lights, then why do CFLs take time to reach their peak brightness, but floursecent lamps get there more or less instantly? 89.240.60.225 (talk) 11:56, 20 April 2009 (UTC)

And then there are dimmers...

(Adding a subhead so as not to hijack the main thread.)

About 40% of the primary lighting in my house is on dimmers. I'm more than a little concerned about the "let's ban all the incandescents" argument, because I'm seeing no replacement for that capability. I tried a dimmable bulb last summer, and I tried another one that was supposedly some 'Next Generation dimmable' -- NEITHER OF WHICH LASTED THREE MINUTES.

So, what would the energy zealots have me do? I'm not about to replace all my switches with non-dimmable ones -- or is that what they would demand of me?

--DaHorsesMouth (talk) 01:06, 21 April 2009 (UTC)

Dimmers reduce the current to the incandescent lamp filament, causing it to emit less light (and heat). The CFLs need the rated current to keep the (usually mercury) gasses in motion, to create the light. Slower motion is not enough to create visible light.

I agree the the future is with LED lights...these all run at very specific voltages and amps, but are so small that they must be installed in banks. This makes if feasible to switch out some banks, to reduce the total light given. Of course your dimmer rheostats are not going to selectively switch out banks, so you will need to replace them anyway. If the power to the LEDs is wired to bypass the dimmer, then the dimmer could control a set of relays which switch various banks of the LEDs in or out, depending on the current through the dimmer. This conversion would probably cost a lot more than buying new switches. KoolerStill (talk) 07:56, 21 April 2009 (UTC)

The problem with most dimmers is that they are work as a simple potentiometer by reducing the voltage - and for incandescents, that's fine because brightness is somehow proportional to voltage. However, with CFL's there is a certain minimum voltage, below which they can't produce light at all - so they are generally not dimmable. Ditto with LED's. What's needed in that case is an alternative kind of dimmer. In the case of LED's, the solution is to switch them on and off hundreds of times per second with the ratio of on-time to off-time varying depending on how bright you want them to appear - that's called pulse width modulation or "PWM" for short. Some dimmers actually do work that way - the ones with a fancy electronic 'touch plate' design seem to do that. With some LED light fittings, the electronics inside (which are already converting 110v or 240v into the 5v or so that the LED's need) can detect changes in the supply voltage due to dimmer switches and 'translate' that into PWM automatically. I don't think that works with all LED fittings and I'm unsure about CFL's. Anyway - for DaHorsesMouth, I'd point out that humanity is in a state of crisis right now. If we don't take really drastic action to resolve this impending global warming catastrophy then an awful lot of people are going to die - many species of plants and animals are going to go extinct. Many more people will starve, lose their homes or find their cities going the way of New Orleans. You, personally, will be considerably more inconvenienced than by merely having no dimmers on your light fittings. So suck it up - get with the program and help to save the planet - nobody said it was going to be painless. SteveBaker (talk) 14:07, 21 April 2009 (UTC)

See Dimmer. I have seen theatrical lighting dimmers from the 1920's which were giant rheostats which introduced resistance in series with the light to dim it, and I have seen large Variacs (variable autotransformers) from the 1950's which dimmed lights by reducing the voltage more efficiently, but modern light dimmers (for the last 25 years at least) use electronic switching to vary the portion of the AC cycle during which the current goes to the bulb. A rheostat type dimmer for controlling several hundred watts of lighting would dissipate a huge amount of heat and could not be contained in a plastic casing in a light switch, as is common. CFLs are readily available which work well with these dimmers. I have had some in use for a couple of years with no problem. They get gradually dimmer, but the color temperature stays about the same, while incandescent bulbs get gradually redder as the dim. Then at a minimum very low level of brightness, they go out altogether, while incandescent bulbs just keep getting redder and dimmer down to just perceptible. Dimmable CFLs cost 2 or 3 times as much as non-dimmable ones. Non-dimmable CFLs fail when operated with a dimmer. Edison (talk) 16:27, 21 April 2009 (UTC)

Rheostats are still an effective switching for DC voltages, which is what I had in the back of my mind when writing the above. The biggest advantage of LEDs, from the power saving point of view, is that they run on extremely low voltages, 3 to 5 volts, and run well on DC. This makes them very suitable to run from local installations of batteries, or from solar panels. They can give quality lighting on home-generated power, in other words, and on very small amounts of it at that. As solar panels produce DC, this also saves inverter costs, and the total needed is much less than running any conventional 110 or 240 lighting systems.KoolerStill (talk) 16:39, 21 April 2009 (UTC)

Editors skilled in electronics have reminded me in previous similar questions that LEDs are commonly controlled by electronic circuits which can regulate the current, or which can operate them on a duty cycle, and not directly from a battery. The brightness could be adjusted by reducing the portion of the cycle they were turned on. That is more efficient than introducing a series resistance which would waste energy in I²R losses. Switching is higher efficiency than a rheostat. Edison (talk) 22:58, 21 April 2009 (UTC)

How long do American electric kettles take to boil?

I'm used to 220volts, but in the US you only have 110volts. Does that mean that American electric kettles take an extremely long time to boil water? And are electric room heaters unknown in the US? I was reading the BS 1363 article and its discussion. Edit: I've just tried boiling one litre of tap water in a 240v electric kettle, and it took 2m 45s to boil, and exactly three minutes before it turned itself off automatically. 89.242.147.172 (talk) 11:30, 19 April 2009 (UTC)

In principle, you can get the same amount of energy independent of the voltage - you just draw a higher current. I would expect US equipment to be made to the relevant specifications. In other words, a simple heating device designed for 230 volts will work in the US, but will be much less effective. A device designed for 110 volts will likely fail, possibly in spectacular and dangerous ways, if connected to 230 volts. More complex devices will run into trouble either way, although many modern electronic devices are built to accept 100-240 volts, 50-60 Hertz, to be usable off nearly any mains supply in the world. --Stephan Schulz (talk) 11:58, 19 April 2009 (UTC)

Electronics - yes, but I have never seen a universal supply kettle or anything else that takes considerable power. SpinningSpark 12:16, 19 April 2009 (UTC)

American plugs are rated at 15 amps so the maximum power that can be extracted is 1650 watts. Poking around on US eBay their kettles seem to be generally in the 1000 to 1500 watt range, although I did find one that claimed 1750 watts and can boil seven cups in 5 minutes. By contrast, British plugs are rated at 13 amp and kettles are usually in the 2000 to 3000 watt range which is considerably more. Before anyone points it out, I know that 220 volts x 13 amps is less than 3kW but the British supply is not really 220 volts. It used ot be 240 volts, when we signed up to European harmonization at 230 volts (edited) the only thing that changed was the specified allowed variation in voltage! Don't believe everything you read on labels. So in summary, American kettles will boil slower, but not extremely slower as you state. SpinningSpark 12:16, 19 April 2009 (UTC)

In NZ, the maximum current allowed by our normal plugs is 10 amps. Looking at my kettle which is the cordfree kind, it's rated at 2200-2300W. Nil Einne (talk) 13:18, 19 April 2009 (UTC)

I'd also point out that electric kettles are relatively rare in the US. When we wanted to buy one, we had quite a hard time of it. They aren't things that are ubiquitous in every household...and indeed the one we have has a low capacity and is incredibly slow compared to the ones we've owned in the UK. Most of the time, if we want boiling water, we use either the microwave or a pan on the stove - the former is faster for small quantities - the latter is faster for large quantities because the kettle is so small. There are similar issues with many other appliances. Mains-electric lawnmowers, for example, are relatively pathetic affairs compared to the ones we had in the UK - so relatively few people have them. Whether 110v or 240v was the better choice is a tough one. 110v is undoubtedly much safer around the home than 240v - but the hassle of having to have special multi-phase 220v outlets for running things like washing machines and hot-tubs is a real pain. I've recently been looking to buy an air compressor for running pneumatic tools in my garage - the 110v ones are crap but having to wire a special 220v outlet just to run the darned things is a real pain. I suspect that when electric cars and plug-in hybrids become common - the UK will use three-phase 480v charging outlets - and here in the US, we'll be stuck with 220v...which is going to be painfully slow. SteveBaker (talk) 18:01, 19 April 2009 (UTC)

That explains (partly) why they dont drink tea in the US - they cannot boil the water. They must just have coffee percolators. Makes me wonder how they "invented" instant coffee. Using an electric stove for cooking must be difficult too. 78.146.75.232 (talk) 19:47, 19 April 2009 (UTC)

I doubt that 100V is much safer than 240V (or 230V these days, +/- enough that it makes no difference). It's probably a bit safer, but is it really "much safer"? Especially since UK appliances will generally draw less current than their US equivalents (most appliances don't use anywhere near the maximum power available, so the UK and US ones will presumably use the same power) - current can be just as important as voltage when determining risk from electrocution. As for electric cars - UK charge points in public car parks, etc., might use 3-phase, but ones in private garages at people's homes won't - individual houses aren't generally connected to a 3-phase supply. It would probably be easier to increase the current than the voltage - the 13A limit is determined by the wiring inside the house, a separate high-current connection to the mains supply could be made (I don't know what the maximum current that can be drawn by an entire house is, but it is clearly much greater than 13A, although perhaps less than 13A times the number of sockets in the house). --Tango (talk) 19:05, 19 April 2009 (UTC)

Certainly 120 (or 100) volts is much safer than 240 (or 230). About half the current will flow, reducing the likelihood that the shock victim will be unable to let go of a bare wire, and lessening the chances of stopping the heart action.See page 160 of Electric wiring, residential by Mullin:[6]. Twice the current could make all the difference. The painfulness of an electric shock also increases with higher voltage at a higher than linear rate. Edison (talk) 19:51, 20 April 2009 (UTC)

First, the correct nominal North American voltage is 120, not 110 or 100. As to safety, 110-120 V is slightly better than 220-240 V in terms of shock hazard but slightly greater in terms of fire hazard since higher currents are required. However, a factor of about 2 is only going to matter if a shock or overheating incident is right on the boundary between serious and not-serious.

To match the original poster's original research, I just measured a liter of water into a Canadian 120 V electric kettle and boiled it. This kettle is old enough that it doesn't have an automatic shutoff and if it ever had a wattage rating displayed it's illegible now. But it took between 5 and 6 minutes to reach a full boil, and I think you'll find that'd be typical today. Although the circuits are rated for 15 A, nobody makes appliances that draw a full 15 A because one other thing on the circuit would trip the breaker. The tap water going in would have been between 5° and 10°C, so, taking the mid-range of those numbers, the current draw would be 92.5/(120×5.5) kilocalories/volt-minute = just about 10 A. I'd be surprised if you can get a significantly more powerful one in North America. --Anonymous, 19:40 UTC, edited 19:45, April 19, 2009.

Electric space heaters in the U.S. designed to operate on 120 volts only draw about 12.5 amps. so they only put out about 1500 watts of heat, enough for a small room. 240 volt heaters are also available, which can heat a larger room. 120 volt electric kettles in the U.S are sadly underpowered and do an inadequate job of boiling water compared to those running on 220 or 240 volts in the rest of the world. The inadequacy of 120 volts is the result of being the pioneer country in having central station electricity for heating, lighting and power. Poor insulation on the available wiring in the 1880's and a concern for safety from electrocution resulted in a distribution voltage lower than desirable in the light of 2020 hindsight. Edison (talk) 21:18, 19 April 2009 (UTC)

Here in Canada, (where many of us call electric utility poles "hydro poles") I can roughly confirm Anonymous experience with an electric kettle. I can boil 8 ounces of Canadian water and make a cup of coffee in roughly one Canadian TV commercial break (that's two minutes), so the power output is just fine for me.

The difference between a 120V vs. 240V shock is about three feet farther across the room in my experience. :) Don't try this at home kids. Luckily there was lots of space around the water heater.

And in Canada at least, getting the higher voltage is pretty straightforward. The bulk of the streel-level distribution system runs at 240V, but the step-down transformer has a centre-tap, so it's a three-wire system. In your circuit-breaker box, you tap one or other of the 120V legs for most uses. For the big stuff, you just tie two breakers together by hooking up a different coloured wire to one of them. Voila, double the voltage, no problem at all! Of course, you need to use properly rated wire when you do it. Really big stuff like industrial machinery is a whole different story. Franamax (talk) 00:27, 20 April 2009 (UTC)

Unless your appliance is the kind that's always on when it's plugged in (as my kettle is), it also needs to be double-insulated if you're going to power it that way. Most devices in both Britain and North America are designed so the power switch is on the "hot" wire and the rest of the internal wiring is connected to the "cold", but when you take 240 V from a 240/120 circuit, both sides are "hot". (This is a safety issue, not a function issue.) --Anonymous, 00:45 UTC, April 20/09.

@SteveBaker: Here in the greater Toronto area, house current (117v @ 60Hz) electric lawnmowers are the norm; gasoline-powered ones are the conspicuous exception. And practically everyone has an electric teakettle in the kitchen. That situation is opposite to my experience growing up in the States, where electric lawnmowers were scarce and electric teakettles practically unknown. Many of the teakettles are garbage, and I have serious qualms about boiling water in a plastic vessel imported from a country well known for falsifying statements of compliance with material specifications and counterfeiting safety approval and certification labels. I therefore tend to boil water in a pot on the stove, or in glass in the µwave oven. But yes, 117v electric teakettles tend to be significantly slower by subjective perception than the 220-240v items with which I've had experience outside North America. —Scheinwerfermann ^T·_C00:54, 20 April 2009 (UTC)

There aren't many plastics that melt below 100C and I'm pretty sure that they'd have thought of that when selecting which plastic to use! Most - if not all - electric kettles in the UK are plastic 'jug-style'. After all, you don't want a material that's a good conductor of heat OR electricity when you're designing an electric kettle. The ones you find in the UK are pretty highly refined gadgets - they are fast, even when there is a lot of water in there. They have auto-shutoff and a fill level indicator. They also have cords that are curled up like a telephone handset cord so they don't dangle off the edge of the countertop where kids or pets might grab them and pull a quart of boiling water onto themselves. By comparison, the ones I've found in the US date back to the stone age. SteveBaker (talk) 02:56, 20 April 2009 (UTC)

Re Americans not drinking tea. We do, just not hot and in liters. We make about a gallon of ice tea in one batch. which is then put in the fridge for later. So there's no rush as to how long it takes to brew it. For professionals there's stuff like this [7] 16.3 gallons / hour doesn't sound that slow. 76.97.245.5 (talk) 10:07, 20 April 2009 (UTC)

I thought the reason people didn't buy electric lawnmowers was because they didn't like being tethered? It seems like every one I've ever seen has had the cord repaired in at least one place. APL (talk) 12:47, 20 April 2009 (UTC)

Repaired by whom? I would replace a lawnmower cord, rather than repair it... --Tango (talk) 18:37, 20 April 2009 (UTC)

This thread is turning into a forum discussion of transatlantic life styles. Ahem. Electric kettles work by Joule heating for which the governing equation is P = I²R = V² / R where P = heating power in watts, I = current in amps, R = resistance in ohms and V = voltage. Therefore if V is doubled then P increases 4 times. That is why a US kettle for 120 V would work spectacularly (and dangerously, so don't do it) in Europe on 240 V. Conversely a European kettle would take 4x as long to boil in the US, other things being equal. In practice similar powered kettles are made with resistive heating elements of 4x higher resistance in Europe than those in US. The quoted voltages (which can vary by 10% or more) are actually the r.m.s values of alternating supplies, as are the I values in the equations above. As the the article explains, the r.m.s. value gives the average heating power. A couple of trivial consequences of the differing supply voltages not involving kettles are

• machinery outdoors, such as lawnmowers, on long extension cables may perform weakly due to voltage loss in the cable resistance in the US while this is rarely experienced in Europe
• using an extension cable while it is tightly coiled is never advisable but it is more liable to overheat in the US than Europe. Cuddlyable3 (talk) 16:59, 20 April 2009 (UTC)

All of the above is negligible compared to the effect of watching or ignoring the kettle. ike9898 (talk) 20:04, 20 April 2009 (UTC)

Just as a side note, not all kettles are electric. There exist the metal kettles you can but on the stove, which boils over 10L of water in about 15 minutes or so, but that's just an estimate (120V). ~AH1^(TCU) 00:38, 21 April 2009 (UTC)

Since you say 120 V, I assume you are talking about an electric stove. In that case, that estimated time sounds awfully low. Even if we assume a starting temperature of 25°C, the current draw at 120 V would be 750/(120×15) kilocalories/volt-minute, which is 29 A. Stoves do have heavier wiring than normal outlets, but no way would a single stovetop element have that much power. My stove is on a 120/240 V circuit and is breakered at 40 A on each hot, and that has to supply the oven as well as the four elements ("burners"). I remember that on a previous stove where I had occasion to look at the wiring, each burner was on a 15 A fuse and got 120 V.

I just tried pouring a liter of water into a saucepan and bringing it to the boil. It took 7 minutes -- longer than the electric kettle. The heating element is probably comparable to the one in the kettle, but the saucepan does not fully cover it. I also put half a liter of water in a smaller saucepan. It had so much less contact area with the element that it took longer to boil, about 8 minutes. --Anonymous, 08:34 UTC, April 21, 2009.

HIV

I found out recently that in many countries, men who have sex with men are nto allowed to donate blood because they are in a higher risk group for HIV. But I don't see what the problem is; surely as long as you're tested and you're negative then you're fine to donate blood. What else is there to it? Thanks 92.7.20.157 (talk) 11:50, 19 April 2009 (UTC)

As they are high risk group, the can be infected while donating, and testing sample with conventional methods gives false negative results, while they are truly infected but in a phase called window period where HIV infection is not detected by conventional methods, so by rejecting these donations, we decrease risk of blood borne transmitted HIV Maen. K. A. (talk) 12:03, 19 April 2009 (UTC)

The problem is simply that the tests aren't 100% reliable (especially in the early stages of infection). There is a risk with any donor, but the risk with men that have had sex with men is greater and someone has determined that the former risk is acceptable and the latter isn't (probably because you can exclude men that have had sex with men without significantly reducing blood stocks, so there is very little cost and a significant benefit). --Tango (talk) 13:23, 19 April 2009 (UTC)

We have an article MSM blood donor controversy. As has been mentioned no test is 100% effective. In particular many of the older tests had large windows where a HIV would not be detected, however the new tests common in developed countries are better and have a shorter window. The bans vary from country to country but as our article notes, for a number of reasons the bans are sometimes controversal, particularly when it's a lifetime ban even more so when other high risk groups such as sex workers and intravenous drug users don't receive a similar ban. There are also questions raised about whether all MSM should be considered in the high risk group or whether some may be lower risk then people who are currently allowed to donate. [8] [9] [10] Nil Einne (talk) 14:06, 19 April 2009 (UTC)

Maen. K. A. (Madhero88), the problem lies with false negative results, not false positives. Axl ¤ [Talk] 16:31, 19 April 2009 (UTC)

Oh... So sorry my mistake, your so right Axl Thank you for correcting me :-) Maen. K. A. (talk) 16:38, 19 April 2009 (UTC)

The posters above are certainly right about higher-risk groups being excluded for medical and test-accuracy reasons. Another factor to consider is the experience of blood "suppliers" in the last few decades. The onset of HIV was not well handled (to say the least) and resulted in many infections among blood recipients and criminal charges for officials in at least Japan and France. A similar occurence happened with hepatitis C in Canada. If you were the director of a blood-donor clinic and knew you could go to jail if you did the wrong thing, would you go the safe route and bar entire classes of people, or would you say "nah, it'll probably be OK"? In Canada, last time I checked, you are asked if you spent any significant time in the UK in a certain time period and barred if you did, there is some risk that you might be carrying a TSE. You can generally still donate blood for research purposes in any case - but it will not be used in humans. Franamax (talk) 00:52, 20 April 2009 (UTC)

Ah, but it's nowhere near that simple, for this assumes male potential donors will answer honestly when asked the screening questions. There are those who feel a ban on gay male blood donors is not justified and/or not justifiable, and so will lie. This is not just a public health issue that can be resolved with a simple risk/benefit calculation; it has significant sociopolitical aspects and implications as well. —Scheinwerfermann ^T·_C00:59, 20 April 2009 (UTC)

Clearly, there are at least these two goals: (i) make the blood supply safe, and (ii) legal protection for those collecting blood. As Franamax so aptly pointed out, the latter is a major driving force, and is served by asking the question of every donor. Lying on the part of the donors does not significantly increase the legal exposure for those who run the blood collection. I don't deny what you're saying about this being a complex web of issues, but the (somewhat simple-minded) thinking behind the screening questions is pretty clear. --Scray (talk) 02:19, 20 April 2009 (UTC)

The iceberg that sunk the Titanic

Am I likely to have any of the water molecules from this iceberg in me now? How many? In the same way that my next breath is said to contain molecules breathed by Julius Caeser, the dinosaurs, and so on. I've been wondering what volume six billion water molecules woould have, if everyone in the world has one molecule from the iceberg in them. I wantched a tv programme last night (Channel 4, UK) that said the iceberg was ten times the size of the Titanic. Incidently, some of the details given in the programme were different from those given in the Wikipedia articles. 89.242.147.172 (talk) 13:55, 19 April 2009 (UTC)

• If one assumes that the Titanic iceberg was only 10 metres on a side (and it was certainly larger than this), it contains 10³ metric tonnes of water whereas the total water in the oceans is 1.3 x 10¹⁸ tonnes. Assuming this is all now thoroughly mixed we can expect this proportion of Titanic iceberg water to be in an average (say 100kg) human. Dividing by the molecular weight of water (34 u) gets about 10¹² (one trillion) molecules per person. SpinningSpark 14:59, 19 April 2009 (UTC)

So that comes out as one molecule in every 100 people, or thereabouts? Or am I misinterpreting your numbers? --Tango (talk) 15:12, 19 April 2009 (UTC)

I don't know whether or not you are misinterpreting my numbers, but you are certainly misinterpreting my interpretation of my numbers;

${\frac {10^{3}}{1.3\times 10^{18}}}\cdot {\frac {100}{1.67\times 10^{-27}\times 34}}\approx 10^{12}$

SpinningSpark 15:49, 19 April 2009 (UTC)

So there were some molecules, or atoms, from the iceberg in the cup of tea I've just drunk. Wow! 78.147.243.82 (talk) 16:42, 19 April 2009 (UTC)

Actually, there are even more than that, Stephan Schulz has pointed out that the molecular weight of water is 18, not 34, but was kind enough not to write my mistake on the page. SpinningSpark 17:21, 19 April 2009 (UTC)

Ok, I did misinterpret you. I thought the 10¹² was the number of water molecules in a typical person, rather than the number of water molecules from the iceberg in a typical person. (Had I thought about it, I would have realised that was far too small a number...) --Tango (talk) 16:39, 19 April 2009 (UTC)

• (ec)Well, 1 g of hydrogen has 6E23 atoms, or 6E14 (600000000000000) per person. For H2O, or water, 18g holds as many molecules. Titanic displaced about 50000 tons. Depending on what "10 times means", the iceberg should have between 500000 tons and 50000000 tons. You do the math ;-). The other way round, 6 billion (US reading, i.e. 6E9 ) water molecules is about 1E-17 mol, or 0.000000000000000018 g (give or take a few zeros if I miscounted). --Stephan Schulz (talk) 15:03, 19 April 2009 (UTC)

I'm not sure this is a meaningful question - in a body of liquid water the molecules of H₂O and constantly dissociating into H⁺ and OH^- ions and recombining (but not necessarily in the same order), so the same molecules don't exist for long. I'm not sure how long a typical water molecule lasts for, but it's probably much less than the time since the Titanic was hit - although actually, you need to measure it from when the iceberg melted, which could have been at least a few years later. --Tango (talk) 15:12, 19 April 2009 (UTC)

You are a spoilsport ;-). But what about George Washington's axe? Especially since most atoms are indistinguishable... --Stephan Schulz (talk) 15:23, 19 April 2009 (UTC)

You can just rephrase the Q as "do I have any of the hydrogen or oxygen atoms in me from the Titanic's iceberg ?". StuRat (talk) 15:52, 19 April 2009 (UTC)

Yes, you could, but that is a different question and could have a different answer. It's also a harder one to answer - I'm not sure how to determine how much oxygen or hydrogen there is in circulation - lots of it is locked up in the crust, mantle and core of the Earth, but the upper layers of the crust are involved in the mixing going on, so it becomes very difficult to work out where to draw the line. --Tango (talk) 16:39, 19 April 2009 (UTC)

Well, actually, the answer is easy, it's "yes". The difficult question to answer is "how many", that there is at least 1 is easy enough to show by taking extreme estimates. --Tango (talk) 16:52, 19 April 2009 (UTC)

The really difficult question here (aside from all of the annoying nit-picking) is whether sufficient time has elapsed for the atoms from the iceberg to have mixed sufficiently with those in the rest of the planet to make it reasonable that one or more would have made it into your body. The more recent the event, the less likely that is. So when you pick the more common choices like water that Julius Caesar peed - then the odds are much higher than for someone who was around in the 1990's. The Titanic iceberg only melted something like 100 years ago - so I doubt that it has been completely mixed into the rest of the water on the planet to the degree that Julius Caesars excretions have. But the efficiency of that mixing is undoubtedly the key here - and I'm not aware of any way to measure or even estimate the degree of that. If all the water in the world were not being swirled around by tides and such - I suppose you could use the Maxwell–Boltzmann distribution and the Random walk math to make some kind of measure of the rate at which the molecules would propagate outwards...but that would neglect other water transport mechanisms like tides and clouds being pushed around by the wind...it's a tough thing to attack convincingly. SteveBaker (talk) 17:45, 19 April 2009 (UTC)

Well, we can make a rough guess. [11] lists the self-diffusion constant of water as 2.2x10^-9 m²/s. Using Fick's_law#Example_solution_in_one_dimension:_diffusion_length, with the concentration at position x=0 of 55.5 mol/L (the concentration of pure water), we find that at a range of 20,000 km (half the circumference of the earth) and at 97 years the concentration would be 55.5 mol/L * erfc(3853500). Now erfc(10) is about 2x10^-45 (by [12]), and a larger parameter only makes the value less. (Note that extending the time to 2 thousand or 65 million years doesn't appreciably change things. Even at the age of the universe at 14 billion years we're still only at erfc(320).) Working in the other direction, in 97 years diffusion results in a concentration of 1 molecule/L at a distance of 38.9 meters (65 million years is 32 km). - I'd say that diffusion would play only a minor role, and other transport mechanism are much more important. -- 128.104.112.117 (talk) 23:28, 19 April 2009 (UTC)

Darn! I was hoping that the distance due to this kind of diffusion would be so big that we could neglect other sources. Oh well - so it's basically down to figuring out how much the waves, tides, winds, etc stir things up...that's really tough to figure out. SteveBaker (talk) 02:16, 20 April 2009 (UTC)

Since a rough calculation shows that the evaporation from the ocean over 100 years couldn't amount to more than 100 meters total, and the average ocean depth is several kilometers, there's a reduction factor of somewhere between 10 and 100 right there. Also, a lot depends on whether the ocean water in the region where the iceberg melted stayed on the surface or sank into the abyss, where it would have stayed for many years.

But in another sense all this is sort of irrelevant. The principles of quantum mechanics say that when water molecules are packed tightly enough so that their wave functions overlap, they do not have individual identities. There is actually no such thing as "the specific water molecules that made up the iceberg." Looie496 (talk) 02:53, 20 April 2009 (UTC)

History of Motion Detection

I want to know the History of Motion Detection Technique. somebody plz help. —Preceding unsigned comment added by 210.212.183.113 (talk) 14:48, 19 April 2009 (UTC)

I don't have an answer, but it might help other volunteers if you indicate whether the subject you have in mind is security or psychophysics. SpinningSpark 16:01, 19 April 2009 (UTC)

Optical flow Cuddlyable3 (talk) 11:52, 20 April 2009 (UTC)

Cartographic projections & Symmetry

Is there a cartographic projection such that if you cut it in half along the equator, rotated one half 180 degrees, and overlaid it on the other half, all points would be overlaying those directly opposite them on the globe? I've got a kid here who wants to dig holes through the earth and would appreciate this sort of visual reference.  ;-)

Thanks! -- 208.120.228.214 (talk) 15:08, 19 April 2009 (UTC)

I am no expert on this subject but I very much doubt that there is a named projection for this. I certainly cannot find anything even remotely like that in my textbook (Steers) on map projections, nor does our article map projection. Of course, such a projection is possible, just about any surface is possible to project, it just hasn't been done. Can I suggest you download Google Earth which may help your youngster to visualise this by manipulating the globe? SpinningSpark 15:39, 19 April 2009 (UTC)

No, certainly not without some bizarre discontinuity or pathology. This is forbidden on account of parity within the surface. A pure rotation keeps left turns left turns and right turns right turns. Yet if you travel on the surface of the Earth and turn right, a person remaining opposite you will make a left turn. 129.2.43.42 (talk) 15:58, 19 April 2009 (UTC)Nightvid

Is the map in the Antipodes article what you are looking for? Rmhermen (talk) 17:03, 19 April 2009 (UTC)

Ah, just beaten to it! I don't think there's a specific projection name for a map that shows antipodes. It's not really a projection per se, but a way of overlaying other projections. --98.217.14.211 (talk) 17:07, 19 April 2009 (UTC)

After looking on the article about antipodes, I found it amusing that there is almost no overlap of land on land in those superposed maps. Dauto (talk) 22:19, 19 April 2009 (UTC)

In that case, you'd probably be amused by the Land hemisphere (unless you already clicked through to it). Incidentally, the first external link in the Antipodes article is an entertaining mindscrew to get your head around :) 80.41.73.141 (talk) 23:44, 19 April 2009 (UTC)

I am willing to offer my garden for digging experiments.Cuddlyable3 (talk) 11:46, 20 April 2009 (UTC)

Decay chain

Hi, what are the equations used to calculate how much of each substance there will be at a certain point in time in a decay chain? Thanks, 96.255.93.227 (talk) 19:49, 19 April 2009 (UTC)

For radioactive ion beams. Axl ¤ [Talk] 20:48, 19 April 2009 (UTC)

More generally, I think you want the Bateman equations. Axl ¤ [Talk] 07:06, 20 April 2009 (UTC)

Why does the universe seem to be older than it is?

If I look in one direction using a powerful telescope I can see objects that are 13 billion years old. Now if I look in the opposite direction, I can also see 13 billion years old objects, which means that these objects are separated by 26 billion light years. However the age of the universe is only 13.7 billion years old, so how come the distance between these objects appears to be bigger than the universe itself? Laurent (talk) 21:54, 19 April 2009 (UTC)

If an explosion travels at lightspeed, its radius will be (lightspeed x time). Its diameter (which is what you're measuring) will be twice that. Vimescarrot (talk) 21:56, 19 April 2009 (UTC)

That doesn't answer your question. Never mind. It's something to do with relativity and the different perceptions of time and...and...and someone else will come along and correct my mistakes and explain it better, so I'd be better off not trying. Vimescarrot (talk) 21:57, 19 April 2009 (UTC)

The real reason is the metric expansion of space. Things cannot move faster than the (always constant) vacuum speed of light through space. But it's space itself that is expanding. Consider, as an example, a rubber balloon that doubles in diameter every second. Even if ants living on it are constrained to 5 cm/second, the distance between two given points on the surface will eventually increase much faster, and two ants at these points will move apart with the same speed even if they don't move with respect to the surface at all. For concrete numbers, assume the balloon has a diameter of 10 cm, and the ants are 1 cm (measured along the surface) apart. After 1 second, the balloon will have a diameter of 20 cm and the ants will be 2 cm apart. After 3 seconds, the balloon will have 80 cm, and the ants will be 8 cm apart. After 4 seconds, the balloon will have 160 cm, and the ants will be 16 cm apart - oops! The distance between the ants now has increased 8 cm in one second (and even weirder, they think they have not moved at all).--Stephan Schulz (talk) 22:34, 19 April 2009 (UTC)

As the last poster has pointed out the distance between objacts in the universe is expanding almost exponentially, so that distant galaxy that emitted some light 13.7 billion years ago that you can see now has been moving away from us all that time at ever increasing rates and is much further away now than 13.7 lightyears. (more like 40 lightyears if memory serves), and twice that distance if you compare galaxies in oposite sides of the sky. Dauto (talk) 22:42, 19 April 2009 (UTC)

I'm presuming you mean billion light years. SpinningSpark 07:12, 20 April 2009 (UTC)

Yes, I meant billion lightyears. Sorry about that. Dauto (talk) 20:32, 20 April 2009 (UTC)

Our article on the observable universe clarifies some of the misconceptions around the connection between the size of the observable universe and its age. Gandalf61 (talk) 10:59, 20 April 2009 (UTC)

Thanks for your answers. I still don't quite get it but I'm going to go through the articles you've suggested and post back here. Laurent (talk) 11:35, 20 April 2009 (UTC)

The universe isn't expanding exponentially yet. For most of the time since the big bang the expansion has been slowing down (roughly following a t^2/3 curve), then a few billion years ago it flattened out and now it seems to be speeding up slightly. The simplest model that fits the current data is ΛCDM, which predicts exponential expansion (e^Ht) in the future, but it's too early to be confident that it's right. It's possible that the universe might end in a Big Crunch or Big Rip in as little as ~10 billion years. It depends on the nature of the dark energy. -- BenRG (talk) 12:20, 20 April 2009 (UTC)

Naively, a Hubble constant of 70.1 ± 1.3 (km/s)/Mpc does automatically result in an exponential growth. The growth is proportional to the size, which essentially is the definition of an exponential process, unless I'm very wrong (which is not impossible ;-). Of course, constants aren't and I think this one is particularly not... --Stephan Schulz (talk) 13:01, 20 April 2009 (UTC)

You are correct... the Hubble constant, H₀, is the value of the Hubble parameter H(t) at the present day (t₀). If we practice cosmology long enough, we may have to denote H₀ with an epoch, as we do with stellar coordinates. -- Coneslayer (talk) 12:54, 21 April 2009 (UTC)

And you're correct that it's constant when the expansion is exponential. In ΛCDM it approaches a final value of about 60 km/s/Mpc. That's the H in the e^Ht from my last post. It was much larger in the past. At the decoupling time (when the cosmic microwave background was emitted) the Hubble parameter was around 10⁶ km/s/Mpc. During inflation it was... oh I don't know, but something very large. A random forum discussion I googled says 10⁵² km/s/Mpc, which seems believable enough. -- BenRG (talk) 18:48, 21 April 2009 (UTC)