Wikipedia:Reference desk/Archives/Science/2007 August 14

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 14

weather - humidity

what does relative humidity tell me that dew point does not tell me? what does dew point tell me that relative humidity does not tell me? Csnewman 00:26, 14 August 2007 (UTC)

I think you can calculate one from the other (given a known temperature and maybe pressure). So, they are just two different ways of presenting the same info. The two interesting values in each case are that you expect it to rain when it reaches 100% relative humidity (although this is at the clouds, not necessarily where you are) and expect dew when the temperature drops to the dew point. StuRat 07:50, 14 August 2007 (UTC)

Relative humidity is the ratio of the quantity of water vapor in a gas to the quantity that would saturate that gas at the same temperature. RH is expressed as a percentage of saturation at a certain temperature. It can also be defined as the ratio of the partial pressure of water vapor to the Saturated Vapor Pressure at the same temperature multiplied by 100. (to give %)

Dew point is the temperature at which condensation of water vapor in a gaseous atmosphere takes place.

--SpectrumAnalyser 20:15, 14 August 2007 (UTC)

Ice

Say you have a spherical container completely filled with water, completely closed, and you freeze it. If the material of the container was strong enough, would the ice compress when it expanded? How strong would a material have to be to withstand ice expanding without breaking? 68.231.151.161 00:38, 14 August 2007 (UTC)

If the material is strong enough, then the pressure within the container will rise as the ice 'tries' to expand. As you cool the container, the pressure will rise and the water will stay liquid at lower temperatures than normal. Eventually, the water will freeze into ice (at a lower temperature than normal), but it will be denser ice than would form at normal, atmospheric pressure. As to how strong the material would have to be, I'm afraid I don't have a definite answer, so you'll have to wait for someone else. It will have to be strong enough to resist the pressure difference between the inside and the outside of the container, and will depend on the volume of the spherical container, the thickness of the walls, etc. Skittle 00:50, 14 August 2007 (UTC)

You'd need to look at a Phase diagram for water. DMacks 00:51, 14 August 2007 (UTC)

Interesting question. This page says that what you are talking about is called "isochoric cooling" and can produce pressures up to 25 MPa (about 250 atmospheres) in a water pipe. But if the temperature continues to be lowered, the pressure will increase further, until at about -22°C (about -7°F) it reaches a maximum of almost 210 MPa (2,070 atmospheres)! If the temperature is lowered still more, one of the high-pressure phases of ice will then form. Obviously you would need a very strong container to withstand such pressures. This page (requires JavaScript enabled) will tell you how thick it would have to be, depending on the size, For example, for a design pressure of 40,000 psi (about 2,720 atm), a maximum stress of 60,000, and an inside radius of 2 inches, the pressure vessel would need walls 3.33 inches thick. Note: This is not professional advice! Do not consider obtaining a 3.33-inch-thick, 2-inch-inside-radius pressure vessel and conducting this experiment without consulting a suitably qualified professional engineer! --Anonymous, August 14, 2007, edited 04:20 (UTC).

That page seems to not mention what material the pipe needs to be made out of. Someguy1221 05:50, 14 August 2007 (UTC)

There, see what I mean? --Anon, 06:51 (UTC).

Some old story we had to read in school a century ago, about a guy who had a stock of cannonballs after the civil war he wanted to melt into scrap, but couldn't because of the residual gunpowder inside them which he couldn't clean out. Solution was to fill them completely with water and leave them outdoors in the winter until he ice broke them into pieces which could be thoroughly washed. Gzuckier 15:56, 14 August 2007 (UTC)

I never knew that cannonballs were hollow! --Dweller 16:45, 14 August 2007 (UTC)

They aren't - but seige mortar/bombard "shells" of the period probably would have been - which might be how the story originated. However, I'm doubtful that it would work because there wouldn't be an easy way to block up the hole you filled it through. You could possibly consider welding it shut - but if the things were full of gunpowder, that would definitely be ill-advised! Probably the easiest way would be to set them off and collect the shrapnel. SteveBaker 18:31, 14 August 2007 (UTC)

Remember, ice formes OVER water. So as ice forms, it will seal itself in.

Cannon can shoot bombs: see List of cannon projectiles. Rmhermen 04:17, 17 August 2007 (UTC)

No, actually. 2000 atm is quite enough to push the ice out of the way before busting through steel. 151.152.101.44 20:57, 14 August 2007 (UTC)

Yep. Those of us who used to get milk delivered in bottles in places with a real winter will remember that if it was delivered when nobody was home to move it inside, you would find the bottles with their caps elevated like this. --Anonymous, August 14, 2007, 22:13 (UTC).

about scientist of friction

hello i want to know about scientist of friction

Um, friction? If not, perhaps you need to be more specific. --jjron 09:07, 14 August 2007 (UTC)

I believe that article is about the science of friction, not about the scientists who study friction. As friction is generally well understood (with the exception of a few specific areas, like acoustic lubrication), it has largely moved out of the realm of science and into engineering, where civil, mechanical, automotive, aeronautical, and naval engineers spend a great deal of time evaluating the coefficient of static and dynamic friction of various material combinations, and study lubricants, fluid dynamics drag, etc. StuRat 10:14, 14 August 2007 (UTC)

A scientist of friction is usually called a tribologist, after tribology, the study of friction. Most these days are technically not tribologists but lubrication engineers (they come up with ways to use existing knowledge to produce new systems, rather than developing new knowledge). Laïka 11:12, 14 August 2007 (UTC)

On the other hand, maybe he means frictional scientists like Tom Swift or Buckaroo Banzai.

Effects of gravitation from and on an object near light speed

Hello, I've got of the following question. Does an object (which is not a photon, thus has mass) which is traveling at relativistic speeds, have a higher effect on spacetime's curvature (since its mass is higher that it's rest mass). Also, if said object, was traveling at relativistic speeds, but towards a massive object, say a black hole, since it's accelerating and thus it's mass is increasing, at what I imagine some rate related to it's acceleration, can it also become a singularity before it passes said black hole's event horizon, since it's mass is increasing and its length is contracting ? 62.48.159.19 09:27, 14 August 2007 (UTC)

Yes, a moving object has more energy, and thus more gravitation. (It's not particularly useful to talk about it having more mass as if the motion caused mass increase; rather, the absurd amount of kinetic energy it must have to be moving at, say, 0.99c has inertia too which is simply added to its normal inertia.) However, I believe that the other relativistic effects (particularly time dilation) and the finite speed of gravity (c) would alter the dynamics around, say, a 1-centimeter-square rod with the mass of the Earth moving so as to have a measured length of 1cm so as to prevent it from acting like a black hole. After all, if we move ourselves fast enough it would appear that everything had become a black hole if that could happen, which would be completely inconsistent with still being able to see it all! --Tardis 15:19, 14 August 2007 (UTC)

An object wouldn't shrink from its speed when moving into a black hole. It would be shorter from the point of reference of the black hole than from the object, but the tidal forces would cause spaghettification and stretch the object and make it is significantly longer (and thinner) than normal. It would look like it's getting shorter from an outside observer due the light taking longer to get to them, and it would never appear to pass the event horizon. I don't think this is related to what you're talking about. — Daniel 02:15, 16 August 2007 (UTC)

Swirling Water

I've noticed that when I drain a tub of water, the water kinda spirals...why? I understand that there are lots of water molecules converging towards the 'drain hole', but then what happens?

Basically there will be non uniformities in the surface of the tub, and these make the water channel in a specific direction. There may also be currents set up in the tub from when you were using it, which would influence the water to drain in a particular direction. Be sure to understand that it is NOT due to the Coriolis effect (It has a tiny tiny influence compared to the others). Capuchin 10:02, 14 August 2007 (UTC)

Once a small preference to swirl in one direction starts, as the water approaches the drain, the conservation of angular momentum thing kicks in to make it swirl faster and faster. (The classic description of this is "Like when an ice-skater pulls his or her arms inward to spin faster.") Once the water is swirling down the drain in a particular direction, the viscosity of the water tends to make the stationary water further away want to spin in that direction too. So it only takes the tiniest of initial circular motion in the water to build up into that violent spiral vortex - and once the spiral forms, it becomes a self-sustaining thing. The story about the Coriolis effect making water spiral down in one direction in the Northern hemisphere and the opposite direction in the south is indeed an urban legend, although theoretically, if you did have a large body of water that was truly, utterly, stationary - with no initial preference to spin in either direction, it's possible that the Coriolis effect might have enough of an effect to make it tend to spiral one way or the other. However, in practice, it never does - even the slightest thing will cause motion in the water that carries on for hours afterwards and eventually determines which way the swirl will form. SteveBaker 18:17, 14 August 2007 (UTC)

I'd compare your swirling effect to any effect with many particles. If you throw a lot of marbles down a hill towards a drain they'll bounce around like crazy but then run in a motion around the drain due to gravity, surface anomolies, and to angular momentum. Consider also that a bathtub is designed to drain water when the plug is pulled, i.e. it's at an angle which will affect the water flow.

Car's rim

HI guys...I am a graduate in physics.I love physics for life...but I'm embarrased to say that I don't know some of the key concepts in it.....so please all those who read this ahead just give your idea...EVErybody has seen the automobile's rim rotating w.r.t tire RPM. When it slowly accelarates, the wheel's rim looks like its spinning in clockwise direction..With further increase in speed, the rim looks like its spinning in anti-clockwise direction..With varying RPM, the rim looks spinning in clock and anti clock direction rapidly...I hope you get the picture...What's the trick behind this?, that fools our brain?...Thanks in advance

It's more to do with the frequency of the lights illuminating the wheel or the camera taking the movie or the monitor playing the movie. You don't see this effect outside in the sun. It's a stroboscopic effect. When one frame is taken, the wheel might be in one position, then when the next frame is taken, the wheel may have rotated a whole 11/12ths of a rotation forwards, but our brain interprets this as a 1/12 rotation backwards, because that's a smaller change. This is how the illusion is made. With wheels which have a high rotational symmetry (due to spokes), which effect you see is highly dependant on the speed at which the wheels are turning, which is why you see the effect sometimes and not other times.

Also, please sign your posts using 4 tildes (~). Capuchin 11:24, 14 August 2007 (UTC)

Capuchin didn't monkey around, he spoke the truth. I would also add that a few cars come equipped with spinners, which do, indeed, rotate the rims at different rates and/or directions from the rest of the wheel. StuRat 16:51, 14 August 2007 (UTC)

But rarely backwards! (Unless reversing I think, I'm not hip enough to have some of my own). Yay for those little guys. They make my day. Capuchin 07:19, 15 August 2007 (UTC)

I thought I once saw a set spinning backwards, but perhaps I was mistaken, so I've crossed that part out in my response. StuRat 19:17, 18 August 2007 (UTC)

The effect is properly called 'temporal aliasing' - but it can only happen when there is some kind of stroboscopic light or something. You see it under electric lights because those flicker on and off at the frequency of the power source (50 or 60 times per second depending on where you live). You also see it on the TV and in movies where the camera only takes a few dozen pictures per second. Under either of those situations, you are only seeing brief moments of the motion - it's not really a continuous motion but a bunch of still snapshots - which your brain tries to turn back into continuous motion for you. When something like a spoked wheel rotates, imagine what happens if it one spoke of the wheel were to be pointing (say) straight upwards in one snapshot - then in the intervening fraction of a second, the wheel were to turn exactly enough so that the next spoke would be pointing straight up. Since one spoke looks pretty much the same as the next, it would look as though the wheel hadn't turned at all during that fraction of a second. If the wheel were rotating just a bit too slowly for that to happen, then the next spoke would be just a little behind where the first spoke was in the first picture. Each new picture that we see has the top-most spoke be just a little more behind each time. The net result is that the wheel seems to be spinning slowly backwards. This effect happens with other things too - a picket fence for example may appear to be moving forwards at the same speed you are...anything with a repeating pattern built into it will do this. But it can't happen in normal daylight when you see the scene with your own eyes - although many people I've met will swear up and down that they've seen the effect - when you have them actually go outside and look, they have to admit that they aren't seeing it. But at night and on TV, it's a common phenomenon. You can easily destroy the effect by (for example) painting one of the spokes of a car wheel a different colour - then your eyes can latch on to the true motion and the illusion goes away. SteveBaker 18:07, 14 August 2007 (UTC)

Two small corrections. Incandescent lights don't flicker perceptibly, but that doesn't matter here because it's rare for them to be used for street lighting; most street lights are of types that do. And the flicker frequency is 100 or 120 per second, twice the power frequency. --Anon, August 14, 2007, 22:12 (UTC).

I always like to argue a bit. I don't think this is a purely stroboscopic effect, although that does happen. The human eye has a fixed refresh rate, I believe at about 10 times per second so it is perfectly possible to see this effect in constant sunlight, and I am sure I have done, although SteveBaker has put doubts in my head now! Cyta 07:24, 15 August 2007 (UTC)

NO! BIG MISCONCEPTION! The human eye does not have a 'fixed refresh rate' - completely the opposite in fact. If it did have a fixed refresh rate then maybe this strobe effect would work in daylight - but it doesn't so it doesn't. This is a very simple experiment - step outside - in daylight - and look at a car! SteveBaker 11:51, 15 August 2007 (UTC)

Agree with steve. Measurements of the "FPS" of the eye are done by flashing something on a screen for a few tens of ms and asking the subject if they noticed it. It isn't a measure of how the brain refreshes the image of the eye. The eye refreshes in a much more fluid way than the operating principles of monitors and other technology. And seriously? Have you ever watched somethign going at 10 fps. It's a very very discernable juddering effect. I know that some people can notice 60Hz monitors flickering - my dad does and makes me change it to 75Hz, so it's probably somewhere around that range. But the strobe effect won't work with just the eye because it doesnt "strobe" like that when it "refreshes". (I feel filthy trying to explain the eye in mechanical terms) Capuchin 13:21, 15 August 2007 (UTC)

I think the confusion comes about because each rod/cone photodetector in the eye has about a 1/10th second recovery time after it fires. That gives people the impression that all of the detectors simultaneously grab an image and send it off to the brain - and cannot take another one for 1/10th second. The truth is that each rod or cone fires and recovers independantly - out of sync with the others - so that while one individual detector has a 10Hz-ish response rate, the eye as a whole is seeing the world more or less as a continuum. What the eye sends to the brain isn't a 'picture' in the normal sense - it's a set of basic shape information like "a vertical line, half the height of the retina is moving left to right at 5 degrees per second". That information doesn't have a frame rate at all because it's described in terms of how fast things are moving and in which direction. That enables the brain to understand the scene presented to the eye in high level terms and cuts back on the amount of data that would otherwise need to be transmitted over the optic nerve.

The nature of that information flow also allows us to perform the important trick of interpolating the position of a moving object when it briefly vanishes from view. If you are primitive man, chasing a prey animal through trees, attempting to throw a rock at it so you can have it for supper, it's vitally important that you know where the animal is - even in the brief moments when it's hidden behind a tree. So you can imagine the in-between motion between the brief glimpses when you know where it truly is (this is called 'object constancy'). That same mechanism is what allows us to fuse together separate frames of a movie or TV show. But once again, people hear that we can use 'persistence of vision' (a poor term) to envisage a 10Hz flickery image as smooth motion - and go on to assume that the eye operates at a 10Hz rate - which is not the case. 1/10th second is just the right interval for the task we originally needed it for - imagine a rabbit running at 5mph - if it passes behind a 9" diameter tree trunk - then it'll be out of view for just about 1/10th second. SteveBaker 17:16, 15 August 2007 (UTC)

Sounds like I am wrong, strange I am sure I remember seeing a similar effect, ah well, the power of the memory to play tricks I guess. Is there some average focusing time though? Because as you move your head it isn't a smooth movement of vision, but a series of small focuses with jumps. Could this produce the effect under discussion? Cyta 07:13, 16 August 2007 (UTC)

You mean a saccade? Well, that does interrupt your vision - but they aren't regular, repeatable things. For a saccade to make a wheel appear to come to a complete stop, you'd have to be saccading away and back again at precisely the frequency that the wheel rotates (or some multiple/submultiple of it). Saccades just aren't that regular. As soon as you shift your mental attention (or even if you don't), the eyes will be saccading off to grab some more relevent information. I don't think this could explain it - but in any case, there is nothing to explain! The effect simply doesn't happen in sunlight (just as we'd predict) - and you can prove it to yourself in 10 seconds flat! Just go do it - look at a car in daylight. You don't see an effect - so there is nothing to explain away. Why are you trying so hard to explain an effect that doesn't exist - and which you can prove to yourself with quite possibly the simplest and most convincing experiment you could ever do?! SteveBaker 14:47, 17 August 2007 (UTC)

Human body drinking issue

HI! I have one doubt in my mind for a very long time...we use to piss whenever we feel that sensation.Either by drinking more water or by taking liquid food stuffs, we can keep our body under cool conditions...At those periods our urine color remians almost whitish..But when we reduce the water intake and control our urination for several hours, the color of the urine changes from normal whitish(close to that) to yellowish...And when our body gets more heat and if we subject to piss less often, then we use to feel pain(only males I think) at the tip of our organ..Why is that?...And also how much a normal human should drink water(in litres)?...

Urine is clear when we drink a lot of water because it is less concentrated (there's more water per unit of urea). Like when you put a bit of orange squash in a glass, as you put more water in the color of the orange squash becomes clearer and clearer. The pain (pain?) to pee you are refering to is probably just the sensation that your bladder is full and you need to empty it, the sensation gets stronger and stronger until you do pee. If it is very painful you should see your doctor. Healthy adults should drink somewhere around 2-3 liters of water over the course of a day, but i'm sure most people don't manage that much, it depends on the excercise and electrolyte intake you have, too.

I hope I managed to answer most of your questions, but It was hard to work out what some of them were so please reply if you have further questions. Also, please sign your posts with 4 tildes (~). Capuchin 11:32, 14 August 2007 (UTC)

Qualification: Healthy adults should take in 2-3 litres of water over the course of a day; this can be from food as well as drink. If urine is very dark in colour, there is a good chance of dehydration. If someone is experiencing pain after a certain action, they should probably avoid the action if possible and definitely see a doctor. Skittle 22:30, 15 August 2007 (UTC)

The pain is at "the tip of the organ". I'd see a doctor about that, it could be a sign of a disease which is aggravated by hot conditions, such as herpes. StuRat 16:41, 14 August 2007 (UTC)

It could just be pain from holding up urine in your bladder for one er... "big release" which comes out quicker that your anatomy can handle? SGGH ^speak! 22:18, 14 August 2007 (UTC)

Speaking of bladders, can the bladder get tired from holding in a large quantity of urine for too long, even if very little is still trickling in? I'm afraid to do the obvious home experiment...Someguy1221 22:20, 14 August 2007 (UTC)

I believe that "holding it in" when your body is telling you to pee will increase the risk of urinary tract infection. In fact it's mentionned in that article as something that "seems sensible but has not been tested" to avoid UTI. No medical advice and all that, but don't do it ;) Capuchin 13:14, 15 August 2007 (UTC)

Moldy bread

I've noticed that recently the incidence of bread mould seems to have increased drastically; formerly bread kept in our kitchen only went mouldy in exceptional conditions (if it was kept for weeks in damp conditions for example), but recently, virtually every loaf of bread, produced in a number of different factories, bought from several different shops, and in a number of different varieties, becomes spoilt by mould before reaching the end of the loaf; sometimes, even bread sealed in unopened plastic packets becomes mouldy, days or even weeks before "best before". Bread consumption rate has not changed, nor have the conditions in our kitchen to the best of my knowledge. Is there any reason for the rapid increase in bread mould; someone suggested maybe removal of a banned preservative or change in recipe is causing this; have any anti-mould agents been banned recently? Laïka

You might want to try buying two identical loaves at the same shop at the same time, then giving one to a friend to monitor in his kitchen. That should reveal if it's the bread or your kitchen. 147.197.230.174 12:02, 14 August 2007 (UTC)

You should simply buy freshly baked bread, and eat it within a 48hour period. Factory made bread not only tastes foul, it also contains all sorts of crappy preservatives to make it last longer. "Days" or "weeks" before the "best before" date suggest to me that you are indeed referring to this poor excuse for bread. Still, it should not go mouldy that fast. Make sure the temperature isn't too high, that no sunlight can get to the "bread" and the cupboard isn't, well, mouldy. If none of that is the case, then there's something seriously amiss with the shop you're getting the bread from. Maybe they're tampering with the best before dates, maybe they're printed in error, or maybe they have indeed done something to the recipe.81.83.82.123 12:09, 14 August 2007 (UTC)

I have had great tasting as well as lousy tasting factory made bread, home-made bread, and "artisan" made bread. Maybe your "crappy" tasting "factory made bread" got too close to your soap box. Why buy a loaf of bread every 2 days if your household cannot consume a pound of bread in 48 hours? Edison 15:05, 14 August 2007 (UTC)

Buy a smaller loaf? Or freeze half? Seriously, it's not rocket science. You don't buy 100lbs of meat and then spend the rest of the month eating it without putting it in a freezer, do you?

Which jurisdiction do you live in? If we know that, we can perhaps see if the laws on preservatives have changed. It might also be good to see the type of mold. Perhaps you can e-mail or call the bakery company and ask. -Arch dude

A small increase in temperature or humidity will encourage the growth of (most types of) mould, sometimes dramatically--going from the winter heating season with its tendency towards low humidity to spring and summer may be responsible. Note as well that mould is the result of spores that settle on the bread and then grow; if your kitchen has accumulated a large load of spores from previous contaminated loaves, then subsequent loaves are likely to pick up spores more rapidly and mould more quickly. TenOfAllTrades(talk) 14:56, 14 August 2007 (UTC)

I suppose we could add a disclaimer about "Ask a baker" or "Ask a mycologist" for bread mold advice. I claim no expertise in this area and only cite info found on the net or from general reading, including a baking textbook, and from baking various types of bread in the home. The surface of fresh-baked bread is generally free of microorganisms because of the heat of baking, but ropy mold may survive in the interior of bread. The surface can become contaminated while the bread is cooling or being wrapped, while it is sliced, or while it is stored. [1]. Bread can be attacked by various types of mold. There are over 600 varieties of bread mold. Some of them are listed at [2]. Note that the same mold may grow on bread and other substances, and some of these are toxic to humans. In general, some spores are just part of nature. But if mold from bread or other sources has left a high concentration of spores in an area such as your kitchen or your breadbox, it stands to reason that a loaf of bread placed there subsequently might get moldy quicker than if the mold spores were not drifting around. See Black bread mold, [3] at Madsci.org. Edison 14:59, 14 August 2007 (UTC)

I'd go with humidity. It has been exceptionally humid where I live recently (Detroit), and, as previously mentioned, that has a dramatic effect. I've seen bread last indefinitely without getting moldy in dry climates, where it just mummifies instead. StuRat 16:08, 14 August 2007 (UTC)

My preferred bread gets mouldy very easily, in a matter of days. Other breads can stay good for up to a week. However, when I buy the same bread (at the same supermarket chain) at my mother's it stays good much longer. So both those factors also play a role. But there's also the possibility that bakers used to add all sorts of preservatives that are forbidden now. Especially in the fifties and early sixties, when there was this notion that anything synthetic is better than nature. Nature was even considered 'dirty'. And still is by some people (and to some extent by almost everyone). But I digress. Anyway, mouldy bread is pretty 'dirty' (if you eat it, that is). DirkvdM 09:41, 18 August 2007 (UTC)

Benzodiazepine

please list all possibilities for testing positive ie;..urinalysis, for Benzodiazepine. please list all known drug and chemical relatives associated with this subject.

Please list all chemical associations and or any and all reasons to test positive for Benzodiazepine. all known narcotics...etc. all biological refrances. —Preceding unsigned comment added by Jamdmp1850d (talk • contribs)

Wikipedia:Medical disclaimer: The medical information provided on Wikipedia is, at best, of a general nature and cannot substitute for the advice of a medical professional (for instance, a qualified doctor/physician, nurse, pharmacist/chemist, and so on). Wikipedia is not a doctor. -- JSBillings 13:48, 14 August 2007 (UTC)

Judging by the wording, a "do your own homework" is more in order. 147.197.230.174 13:53, 14 August 2007 (UTC)

I didn't want to be too snarky, but I had also considered adding, "... or consult your parole officer." -- JSBillings 14:17, 14 August 2007 (UTC)

The Tour de France is over, you are not going to get that medal back... Lanfear's Bane

In all seriousness, have you looked at Benzodiazepine? -- JSBillings 18:12, 14 August 2007 (UTC)

Detection Periods can be found here. Why would one test positive for benzos? Almost certainly because they had taken one. Mrdeath5493 04:05, 15 August 2007 (UTC)

Electromagnetic Waves

Can someone please define clearly, how electromagnetic waves are related to magnets.

Does magnetism explain this ? StuRat 15:04, 14 August 2007 (UTC)

Sorry, not really. That just tells me that all magnets are electromagnets which Im aware of. I was thinking more of how electromagnetic waves are related to magnets. And do all magnetic fields generate electromagnetic waves?86.139.54.68 15:08, 14 August 2007 (UTC)

The simple but not very enlightening answer is the Poynting vector: if integrating that vector's flux through very large surfaces surrounding your system gives a non-0 answer, you have produced EM waves. Since it involves ${\vec {E}}$, any purely magnetic field will not radiate. However, since changing magnetic fields produce electric fields, in most cases a changing magnetic field (e.g., a spinning magnet) will radiate. We don't seem to have magnetic dipole antenna, but dipole has some discussion of them radiating. --Tardis 15:30, 14 August 2007 (UTC)

(I may have to award another platinum plated spork for that answer! I think the OP wants a SIMPLE answer!

Simply: A magnet produces a magnetic field - you know that. If you put a compass onto the table and spin a magnet around or wave it about in some way - then the compass needle will also move around. This shows that the magnetic field is changing. When the magnetic field oscillates, then a magnetic wave is produced - just like if you move the cone on a loudspeaker you get changes in air pressure - or sound waves. Magnetic waves are what light, radio, X-rays, etc are. Unfortunately, you'd have to spin your magnet around very fast before you'd get any light out of it - 10,000,000,000,000,000 times per second would just about do it - but a mere million times a second would be enough to generate radio waves that an AM radio could pick up. We talk about electromagnetic waves rather than just magnetic waves because electrical fields and magnetic fields are actually different aspects of the same underlying thing. You can see that is the case if you spin a magnet around inside a coil of wire, then you'll generate some electrical current in the wire - and if you apply some voltage to the wire, you can make the magnet move. (That's how electrical generators and motors work.) Since it's a lot easier to change the voltage in a wire than it is to spin a magnet around, we make radio transmitters with rapidly varying voltages instead of spinning magnets - but the end result is the same. SteveBaker 19:25, 14 August 2007 (UTC)

This is my first attempt at helping out on WP so apologies in advance for any errors or over-simplification... SteveBaker may be correct but in my more simplistic explanation I understand that magnetism is a magnetic force exerted on materials (most notably on metals) while electromagnetism is a combination of the electric field and the magnetic field and is a fundamental force. Maybe this will help in conjunction with Steve's answer above:

Radio waves can cause nuclei of some atoms to change their magnetic orientation. This is known as nuclear magnetic resonance (nmr) spectroscopy and is the basis of the new medical application magnetic resonance imaging (MRI). Atomic nuclei, e.g. hydrogen nuclei or proton, behave like little magnets and can align themselves with or against an applied magnetic field. It is possible for the nuclei to "flip" between the "with" and "against" states; the energy diffrence between the two states is quite small and corresponds to radio frequency waves and 60 or 90 MHz radio waves are commonly used to effect the "flip". The precise radio frequency required depends on the chemical environment of the atomic nucleus as the magnetic field felt by a nucleus is influenced by neighbouring atoms which of course are potential magnets. NMR spectroscopy can be used to deduce the chemical environment of atoms and hence the structure of molecules. The nmr spectrum of bromoethane CH3CH2Br has two signals corresponding to (i) the methyl CH3 protons and (ii) the methylene CH2 protons.

Because radio waves represent small amounts of energy they represent little hazard. Radio waves from radio transmitters are passing through our body all the time. When analog radio waves are picked up by an antenna they result in the formation of a tiny electrical signal which is amplified and converted into sound and/or vision and is the basis for radio and television reception and other forms of telecommunication.

Sorry for potentially hijacking this thread but then how are photos created if it's just a spinning magnet? I thought photos were more than merely particle models for light. --antilived^{T | C | G} 06:08, 15 August 2007 (UTC)

The electromagnetic wave carries energy. When that energy hits something - there may be some change to the thing it hits as the energy is absorbed. In the case of photographic film, there is a chemical change that is caused by light (an electromagnetic wave) of an appropriate frequency hitting it. That chemical change can then be exploited to trigger more chemical reactions when you develop the film to change the colour of a pigment which ultimately forms the photograph. The light energy could also hit a carefully designed piece of electronics in a digital camera - ultimately resulting in a signal being sent to a computer and a picture being stored. SteveBaker 13:09, 15 August 2007 (UTC)

Why does a hand in warm water while asleep cause bed wetting?

Is there a scientific reason for the above? Everything I google comes up with rubbish reasons along the lines of "it just happens" there must be a biological reason. SGGH ^speak! 18:06, 14 August 2007 (UTC)

I guess the first question is: Is it even true? That sounds like an urban legend. SteveBaker 22:21, 14 August 2007 (UTC)

I have only anecdotal evidence, but I believe this to work. Not only did this work on me (sadly), but it has worked on a couple of friends too. I have no clue what the cause is. Mrdeath5493 04:01, 15 August 2007 (UTC)

The Straight Dope's Cecil Adams addressed this on 28 June, 1991:

Insofar as it works at all, the pan trick depends on the power of suggestion--simply thinking about water, or in this case dreaming about it, makes you want to go to the bathroom. The effectiveness of the stunt is a matter of debate. Some urologists scoff at the idea. But other medical types have been known to tell patients having a tough time urinating after rectal surgery to put their hands in warm water. Merely letting the water run in a nearby sink sometimes works, too. I tried it once without success, perhaps because my richly deserving would-be victim was dead drunk. But I've gotten too many testimonials from satisfied perpetrators to think the whole thing's a fraud. The suggestion need not be tactile. Recently I heard a talk by an architect who was trying to deal with the problem of men unable to perform in public restrooms. His solution: mount pictures of waterfalls over the urinals. Audio stimuli work too. Rockpocket 05:20, 15 August 2007 (UTC)

Is the warm really necessary for this 'trick' to work other the fact if you put your hand in cold water particularly freezing cold water you may not fall asleep? Also, um BTW, is there any particular reason you and your friends wanted to urinate in bed? BTW were you all males? Nil Einne 16:11, 15 August 2007 (UTC)

In my experience, this trick does not work. DuncanHill 16:13, 15 August 2007 (UTC)

Lanthionine

I need a small amount of a chemical called lanthionine. Although it is not expecially exotic, I can't find it in any catalog I have or on by searching Google. Any help? Or can you think of some good reason this chemical wouldn't be available? ike9898 19:16, 14 August 2007 (UTC)

TCI has it. And Acros used to, maybe call 'em and ask what happened to their supplier? DMacks 19:47, 14 August 2007 (UTC)

Thanks, you saved the day. ike9898 19:55, 14 August 2007 (UTC)

Adoption

How do I go about adopting my wifes 36 year old daughter

That's not really a question for the science desk! But in any case, it's a legal question and we aren't allowed to answer those. SteveBaker 19:34, 14 August 2007 (UTC)

You would need to consult a lawyer, as the process will be different depending on your constituency. If you are the from the USA, http://www.adoption.com/ might be a good place to start. Rockpocket 19:42, 14 August 2007 (UTC)

There are several websites that give general information about adult adoption like here http://adoption.about.com/od/typesofadoption/a/adultadopt.htm, but that does not take into account that you are already the person's step father. As above, information off the internet can not be used as legal defence, you will need to talk to a lawyer. Vespine

Perfectograph

i have a perfectograph made in new york city dated 1925 what is it and what did it do?

HMM! Does it look like this [4]?--SpectrumAnalyser 20:44, 14 August 2007 (UTC)

Cool! A gizmo for training morse code operators by the look of it. SteveBaker 22:20, 14 August 2007 (UTC)

Plato on third region beyond true and false, fuzzy logic precursor

Hello all, in a wikipedia article about founder of fuzzy logic, Lofti Zadeh, there is a following claim: "Plato laid the foundation of what is now known as fuzzy logic indicating that there was a third region beyond true and false." Does anybody know more specifically about Plato's opinion on this? Thnx, Mel

Maybe that's a reference to Plato's analogy of the divided line? Not sure. ←Ben^B4 20:37, 14 August 2007 (UTC)

No, its not got to do with it, although its kind of intriguing. mel

Well, if it was Plato, it wasn't exactly a deep insight. We're all pretty aware that there is "True", "False" and "Er...kinda". SteveBaker 22:59, 14 August 2007 (UTC)

yes, you´re right, but i must say that internet is sparkling with the attribution that i brought out above, and none quote the source. it seems to me a little bit like this case: http://www.geniebusters.org/915/04e_ex01C.html

I've taken the claim out of Lofti Zadeh based on this discussion. Of course if a reliable source turns up, it can go back in. --Allen 23:39, 14 August 2007 (UTC)

You should ask on the humanities desk - the experts there are more likely to know87.102.4.73 08:23, 15 August 2007 (UTC)

Physics?

Just wondering, but would it be possible to make a copy of the keystrokes + cpu instructions when you're 100m away from the said computer? Not with current technology, but wondering if this is possible. 206.77.151.206 20:38, 14 August 2007 (UTC)

From EM radiation? I don't think so, it isn't distinct enough to recover the system state (if it was then WiFi could be unlimited speed.) But see TEMPEST. ←Ben^B4 20:49, 14 August 2007 (UTC)

Maybe better placed on computing desk.--SpectrumAnalyser 20:50, 14 August 2007 (UTC)

Eavesdropping a video signal remotely has been possible for some time - see van Eck phreaking. With a suitable directional detector I imagine monitoring a USB cable and retrieving keystrokes would be a similar task. Doing so from a distance like 100m will require a more sophisticated detector with a highly directional waveguide and a sensitive detector, perhaps (sf authors would have us dream) a SQUID). I would be surprised if major intelligence agencies didn't already have this capability, although perhaps not at that range. Doing the same for the CPU is a whole other matter - you're not detecting the emanations from one wire or two or four, but tens or hundreds of thousands, all right beside one another and signalling at insanely high frequencies. -- Finlay McWalter | Talk 20:51, 14 August 2007 (UTC)

The OP says "Not with current technology" - so we're mostly interested in the physical limits of what's possible - not what we could actually built tomorrow. I think that some technique involving not one van Eck detector but a huge array of them surrounding the building might get enough spatial/holographic radio detection data to extract useful information from the PC. Personally - I'd bet on being able to hack into the PC via an unused wireless port - or perhaps to send a nanotechological flying robot into the building and to have it sneak into the PC's case and latch itself onto some interesting data lines.

Got a follow up - Would it be possible to read contents of a cd/casette through the paper casing? 206.77.151.206 20:53, 14 August 2007 (UTC)

That seems almost impossible. Unlike the phreaking above, where the video/usb signals are stretched out and time and space (and are comparatively large, because they're actively emanating) a cd or magnetic tape would be very hard to examine. The data is stored in very small locations, they're entirely passive, and they're physically smushed together making picking one from all the others almost impossible. -- Finlay McWalter | Talk 21:12, 14 August 2007 (UTC)

I would think that reading a CD through a paper envelope might be possible - paper will transmit some amount of laser light - with the right lenses...maybe. Also, the data on a CD has levels of error protection that would mean that even if you could only read a portion of the disk, it might be possible to reconstruct the data. SteveBaker 22:14, 14 August 2007 (UTC)

Well, you could always beef up the laser output to flash burn the paper away...you know...if you were really didn't feel like take the thing out and doing a nice resealing job on the sleeve once you were done. Someguy1221 22:17, 14 August 2007 (UTC)

I think the paper will diffuse the light far too much (you're aiming for pits 100nm deep and only a few times that apart). Similarly I thought about using ultrasound, but that would have to be of an extremely high frequency to get the wavelength down low enough to detect a 100nm pit. Perhaps a scanning x-ray probe would work? -- Finlay McWalter | Talk 22:24, 14 August 2007 (UTC)

I think reading cassettes with paper in between would work without major problems though... --antilived^{T | C | G} 06:05, 15 August 2007 (UTC)

Even a simple twisted-pair would preclude radiation (to a sufficient level) that it isn't likely to eavesdrop on a signal inside of it. I think the hard part would be discerning which detected emissions corresponded to which signals. Inside a modern computer, thousands of independent wires, solid-state connections, and data bus cables all have unique signals on them simultaneously. Even if each were perfectly and noiselessly radiating towards your ideal eavesdropping antenna, the interference would prevent any sort of meaningful interpretation. Crosstalk is enough of a problem even inside the microcomputer, since signal levels are being pushed to ever lower voltages and power levels. Furthermore, you would need a very nice "ideal antenna" that can pick up radiation across a huge bandwidth - from kilohertz for some of the video and audio signals, up to gigahertz for the CPU clock and the RAM access. Nimur 15:05, 15 August 2007 (UTC)

Not strictly related to the question at hand, but close, and still fascinating : http://applied-math.org/optical_tempest.pdf 69.95.50.15 15:23, 15 August 2007 (UTC)

This one blows my mind [5] it's a camera that works by figuring out how all of the light in the visible scene got where it finally ended up. With some clever math, the camera can take photos around corners. Their math effectively allows them to interchange the position of the camera with the position of the light source. In their example, they show a picture of a hand of cards - taken from the back side of the cards, calculating what's on the front of the cards by inferring how they scattered light onto the wall behind!! If you can do that - it's hard to believe that you couldn't do similarly amazing things with an array of RF detectors. Think about how CAT scanners image the interior of solid objects. These kinds of technique are what convince me that you could decypher a CD-ROM through a paper sleeve. SteveBaker 17:53, 15 August 2007 (UTC)

SteveBaker might want to amend his quote to: "With some clever math, a large number of images, and total control over the scene lighting conditions, the camera can take photos around corners." It's still amazing; there's a very nice video of the technique which is probably comprehensible in a non-technical way. Nimur 23:50, 15 August 2007 (UTC)

Well - that's perhaps overstating it. You do need some special lighting - lighting that 'scans' the scene - and you need to take a lot of photos, but that's not difficult. The presentation at SigGraph a few years ago included a comment from the author that it is possible that the light from a TV screen or computer monitor might suffice to perform this trick - since the raster scan produces light that moves relative to the scene (albeit very quickly). In a dimly lit room with a single CRT-type display showing a nice, bright image - it might be possible to reconstruct the scene without having additional control over the light source - given enough time to integrate the images. It definitely wouldn't be easy though. SteveBaker 11:58, 16 August 2007 (UTC)

Not a question!

A WikiCookie for you!

This isn't a question, it's a cooKie of appreciation for Sci Ref Desk people. I've gotten some great help here recently; keep it up! ike9898 21:08, 14 August 2007 (UTC)

Well, thank you so much...but we've got to share it? There's at least 23 regular contributors and the average mass of a chocolate chip cookie is 30g which at about 8kcal per gram means we only get...oh well, perhaps if we weighted the cookie share according to the number of bytes of response normalised and tracked as a moving average over the period...oh...sorry. THANK YOU! IT WAS DELICIOUS! :-) SteveBaker 21:58, 14 August 2007 (UTC)

Quick! Eat it before the seagulls get to it! — Scientizzle 22:49, 14 August 2007 (UTC)

Don't see Kurt around here... bibliomaniac15 00:46, 15 August 2007 (UTC)

Awww... it's great to be thought of! --Mdwyer 01:26, 15 August 2007 (UTC)

What about those who don't provide great help? -- Kainaw^(what?) 01:42, 15 August 2007 (UTC)

Bran cookies with raisins. SteveBaker 01:45, 15 August 2007 (UTC)

Since this isn't a question, I propose that tasty looking cookie should be removed from the desk. But, y'know, so not to clutter up the talk page, I'll volunteer my page for it... Rockpocket 01:45, 15 August 2007 (UTC)

The reference desk does not provide gustatorial advice or services. Please contact me in real-life to resolve this cookie. DMacks 03:18, 15 August 2007 (UTC)

Alas, I've turned off cookie support in my browser (black helicopters and tinfoil hats, doncha know), so I can't enjoy it at all. :-( --Steve Summit (talk) 20:47, 15 August 2007 (UTC)

Do you still have granola bar support enabled? I'll trade you for your share. Confusing Manifestation 02:21, 16 August 2007 (UTC)

Laser Cutter in Woodshop

For those of you familiar with laser cutters...

I want to cut some 1" thick hard wood boards on the school laser cutter. The policy says that we can not cut over .5". I am wondering if this is a technical limitation or not. After reading the article on laser cutters it sounds like running the cutter slower would allow you to scale up the thickness of the material. Or Running the entire cut pattern twice.

Anyone want to chime in on this?

Thanks,

E

Have you considered asking whoever is in charge of that fantastical piece of destruction, or consulting its manual? Your teachers probably won't appreciate your setting the building on fire ;-) But why not use a table saw? Someguy1221 22:03, 14 August 2007 (UTC)

Laser cutters can cut all sorts of interesting shapes that a table saw can't touch. But seriously - there is a reason why they said the limit was 0.5" - if it could actually cut 1" safely/effectively then the limit would be 1".

There may be a physical reason -- the optics might not be able to focus the laser any farther than .5" from the cutting head. --Mdwyer 01:24, 15 August 2007 (UTC)

I did ask, the .5" limit was quoted as "we have never tried anything thicker". Thats why I am looking for technical reasons- to see if I should pursue the issue. The optics may play a roll and I hadn't considered that.

My reason for using the laser cutter is as stated, you can design some really funky shapes, I am making a bookcase with interlocking sections of mahogany and maple and the joints I designed would be hell to cut on a table saw.

Thanks for the feedback

Well, you could – with permission – try cutting a thicker piece. I wouldn't be surprised if the limit was due to the maximum working distance of the optics. Note that if you set up your patterns very carefully, you could (in principle) do a thicker piece by cutting a 0.5" from one site, turning the piece over, and cutting another half inch from the other side.

Your best bet, however, might be to jot down the manufacturer and model number and look up the specs online. I suspect that any company that's selling laser cutters these days will be hip and trendy enough to have a web site. Even if the specific information (product manuals, specifications sheet, etc.) isn't on the site, with any luck they'll have an email address or other contact. A politely-worded email can work wonders.

By the way, how the heck did you get a laser cutter in your wood shop? Back when I was your age, we had to use saws! TenOfAllTrades(talk) 01:56, 15 August 2007 (UTC)

Saws? You were lucky to have saws, we had to use our teeth! DuncanHill 11:01, 15 August 2007 (UTC)

Ah-hah! Then get permission to try something thicker. Bring a fire extinguisher! SteveBaker 01:57, 15 August 2007 (UTC)

Band-saw and/or router might work (depending on the pattern) assuming they are available to you; if not, keyhole hand-saw, chisel, and drill-press can suffice. stack the two pieces together when cutting so the results will interlock even if your hand slips or whatever. DMacks 02:06, 15

August 2007 (UTC)

Heh, we have a laser cutter in the shop because I am in college. We have a couple of them actually, and a water jet cutter, (and a rapid plastic prototyper, which is particularly cool!). Anyways, Ill do the spec thing and see what I can dig up. I have been trying to use all of this high-tech equipment for one thing or another. I realize that I can make a book shelf using a variety of manufacturing techniques, I specifically designed this one to be laser cut, there is something attractive about going from CAD to glue-able pieces in one step!

(Im sold on the fire extinguisher thing though..)

Ahh, now I see:) Well put the precision to the test...cut the 1-inch piece it as if it were .5 inch, turn the piece over in reality and in CAD, cut it again, and see how well the two cuts align. DMacks 02:16, 15 August 2007 (UTC)

The laser cutter at my school has software where you can set the thickness and material of what it's supposed to be cutting, and adjusts the speed, etc automatically. Have you tried playing around with the options to see if yours can do that? (Also, I'd advise checking with a scrap piece of wood before you try the real thing - the edges can end up a bit burnt) 147.197.230.174 09:02, 15 August 2007 (UTC)

Kids these days. "If you can't do it with electromagnetic radiation, then it ain't worth doing." Capuchin 09:38, 15 August 2007 (UTC)

When you say school, are your referring to secondary school/high school or university/tertiary level college (higher education)? Nil Einne 16:37, 15 August 2007 (UTC)

If you mean me(147.197.230.174), secondary school. 80.169.64.22 19:10, 15 August 2007 (UTC)