Wikipedia:Reference desk/Archives/Science/2010 December 5

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December 5

Do other animals have blood types?

Well, do they? The Masked Booby (talk) 00:29, 5 December 2010 (UTC)

Yes, they do. You might want to read Blood type (non-human)- Nunh-huh 00:36, 5 December 2010 (UTC)

That article doesn't mention ABO blood types in other primates - see [1] for a quick overview. Wnt (talk) 03:25, 5 December 2010 (UTC)

Questions about Light: 1. Why 186k miles/sec? 2. What would a "photic boom" look like? 3. (see for yourself)

1. How come light "chooses" to go 186,000 miles/second? Why not 185k or 187k? What causes it to go no slower or faster than that seemingly arbitrary speed?

2. If we ever break the speed of light on a starship, whereas breaking the speed of sound on a plane caused a sonic boom, and we know what that sounds like, what would a photic boom look like?

3. In what language did God say "Let there be light?"

--Let Us Update Wikipedia: Dusty Articles 00:59, 5 December 2010 (UTC)

1. In the more sensible units used by most of the world the speed of light is 299,792,458 metres per second, i.e. very close to 300,000,000m/s, a lovely round figure. Maybe God was aiming for the latter figure and got it slightly wrong. Or maybe humans have the metre or the second slightly wrong. HiLo48 (talk) 01:30, 5 December 2010 (UTC)

2. For the time being, see Cherenkov radiation. --Wrongfilter (talk) 01:41, 5 December 2010 (UTC)

3. ...and who was he actually talking to? --Wrongfilter (talk) 01:42, 5 December 2010 (UTC)

The speed of light in a vacuum is determined by two physical constants, see Vacuum permeability#Significance in electromagnetism. And aparantly God did it this way so that it takes 500 seconds for light to go from the Sun to the Earth. Bubba73 ^{You talkin' to me?} 01:56, 5 December 2010 (UTC)

3: I've seen t-shirts printed with:

And God said...

$\nabla \cdot \mathbf {E} =0$

$\nabla \cdot \mathbf {B} =0$

$\nabla \times \mathbf {E} =-{\frac {\partial \mathbf {B} }{\partial t}}$

$\nabla \times \mathbf {B} =\ \ \mu _{0}\varepsilon _{0}{\frac {\partial \mathbf {E} }{\partial t}}$

...and there was Light.

(see Maxwell's equations;) WikiDao ☯ (talk) 02:16, 5 December 2010 (UTC)

I remember that. But a physics professor told me that you only need two of them for light (I forgot which two). Bubba73 ^{You talkin' to me?} 03:07, 5 December 2010 (UTC)

One of them Should be: $\nabla \cdot \mathbf {E} =4\pi \rho _{\mathrm {e} }$ . I think it would be the second last two that you need for light. Bubba73 ^{You talkin' to me?} 03:10, 5 December 2010 (UTC)

Here's the t-shirt design I've seen at the MIT museum (turns out it was designed in 1977 by an MIT Rabbi, no less!:). So, right, it looks like he's actually using the "Formulation in terms of free charge and current". WikiDao ☯ (talk) 03:29, 5 December 2010 (UTC)

Our article says:

"Maxwell's equations are a set of four partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These in turn underlie the present radio-, television-, phone-, and information-technologies."

But I will leave it for someone else to explain whether you need all four equations for there "to be light", or only the last two, and why... WikiDao ☯ (talk) 03:43, 5 December 2010 (UTC)

This is just my memory of a brief conversation > 30 years ago, so it could be wrong. But the second one basically says that there are no magnetic monopoles. The first one says that the electric flux through a bound surface depends on the electrical charge within it. (The first one would be similar to this if there were magnetic monopoles.) The prof said that you need only two for light, and if that is right it would be the last two (the interaction of electrical and magnetic fields). Bubba73 ^{You talkin' to me?} 05:07, 5 December 2010 (UTC)

Well, God would presumably say something more like A^μ,ν_,ν = J^μ. The four equations taught to undergraduates are an unnecessarily complicated pre-relativistic version of electrodynamics. Also, light in the real world is actually part of a more complicated U(1)×SU(2) gauge field, the rest of which is called the weak force. God apparently made the Standard Model gauge group first, then subsequently broke it to make light (and the W and Z bosons). I guess what I'm trying to say is that Genesis isn't very historically accurate.

The reason light "chooses" to go 186,000 miles/second is the same as the reason that one gram of hydrogen "chooses" to have about 6.022×10²³ atoms in it: we used unrelated standards of length, time and mass to define the units. -- BenRG (talk) 08:35, 5 December 2010 (UTC)

For 3, the Book of Genesis was written in Biblical Hebrew. Whether God actually spoke that language in the beginning is a question for religious philosophers. Buddy431 (talk) 03:56, 5 December 2010 (UTC)

Of course, Genesis was written by Moses ^(*) who lived after the Babylonian confusion ^(*), which caused a massive shift in languages ^(*). Presumably, God would have used the unified pre-babel language, if he used any human language at all. (*) ...if you buy the Biblical version. Moses was a mostly mythical figure who, if he existed at all, lived long before Genesis was written down. --Stephan Schulz (talk) 10:39, 5 December 2010 (UTC)

That would presumably be Enochian. 87.81.230.195 (talk) 22:15, 5 December 2010 (UTC)

Regarding the first question, it is meaningless as asked but there is an important question closely related to it. The number for the speed of light depends on the units that are used, and the units are essentially arbitrary. But there are several ways of combining physical constants that produce dimensionless values, that is, values that don't depend on arbitrary choices of units. One of the best known is the fine structure constant, formed by combining the speed of light, the Planck constant, the charge of an electron, and the Coulomb constant -- it has a reciprocal value of about 137.035999, and nobody knows why. Another, the value of which plays a huge role in determining the nature of our universe, is the gravitational coupling constant, the ratio of gravitational to electromagnetic forces expresses as a dimensionless value -- it is about 1.7518 x 10^-45. Dimensionless quantity#Dimensionless physical constants gives a couple more examples. Explaining why these constants have the values they do is one of the most important problems in physics -- one suggested answer is the so-called anthropic principle, which basically says they must have these values at least approximately or else we would not be able to exist to observe them. Looie496 (talk) 17:24, 5 December 2010 (UTC)

How DID I lose 3 pounds while I sleep?

I weighed 187 before bed, and 184 just some minutes ago. I don't understand. How would I lose weight while I sleep? Does carbon dioxide weigh heavier than oxygen as I breathe out? I didn't know I sweat out a lot at night. If I shed 3 lbs of skin, don't you think I would've noticed that on my bedsheets?

How did this happen then? --70.179.178.5 (talk) 01:09, 5 December 2010 (UTC)

Most likely the scales aren't too precise, and have a margin of error of a few lbs either way. Or perhaps someone recalibrated them overnight, or they're not quite on a flat surface and so the weight wasn't distributed evenly. Are they digital or analogue? As for carbon dioxide, that's pretty irrelevant as, since it's the same density as air, it wouldn't register on the scales anyway. -mattbuck (Talk) 01:20, 5 December 2010 (UTC)

Are they digital or analogue? Digital. --70.179.178.5 (talk) 03:12, 5 December 2010 (UTC)

It's not at all unusual to lose 2-3 pounds during sleep through obligate water loss (about 700 cc of water lost in expiration, and the rest through transpiration (un-obvious sweating) through the skin. And more if you pee before weighing. - Nunh-huh 01:24, 5 December 2010 (UTC)

The oxygen you breathe in oxidizes sugars and fats in your body, turning them to carbon dioxide, which you breathe out. CO₂ is heavier than O₂. --T H F S W (T · C · E) 02:32, 5 December 2010 (UTC)

It's mainly water. Each breath has about .2g of water in it, which works out to 2.3lb overnight. Exhalation is about 6% water, and has a volume of about 5L, and a person breathes about 12 times per minute. Full calculation. Ariel. (talk) 07:57, 5 December 2010 (UTC)

True, and you also imperceptively perspire water vapour away from your skin all the time. 92.15.1.139 (talk) 14:00, 5 December 2010 (UTC)

Make sure the scales are on a level rigid surface and you stand in the same place every time. If they are on a carpet they will not be consistent.--Shantavira|^{feed me} 10:09, 5 December 2010 (UTC)

How do you know this for sure? Anyway, I'll take the digital scale to the kitchen and see if I weigh any different. --70.179.178.5 (talk) 18:55, 5 December 2010 (UTC)

You do lose some weight overnight due to water loss from breathing and sweating, but 3 lbs seems a bit high. Is it a balance scale ? That's the only type accurate enough for this type of measurement. A standard strain gauge or spring scale could be off by a pound or two just depending on how your weight is distributed on the scale. StuRat (talk) 16:36, 5 December 2010 (UTC)

CO₂ is heavier than O₂. According to the ideal gas law, one mole of a gas has a volume of 24.4 litres (at 25 °C). The molar-mass is that atomic mass in grammes. CO₂ weights 12 + 16 * 2 = 44 grammes; O₂ weights 16 * 2 = 32 grammes. Also, were you wearing the sames clothes? 3 lb / 1.5 kg is the weight of light clothing. Further; a set of spring scales are temperature-sensitive due to change in the elasticity of the springs. CS Miller (talk) 16:54, 5 December 2010 (UTC)

Boiling water

When one boils water there's a range of temperatures at which the water remains boiling and boils more violently when you turn up the heat. I've been having a dispute with my wife. My understanding is that the water temperature doesn't change but remains constant and the water roiling more is how the heat that can't go above the boiling point of the altitude you are cooking at, is dissipated. Anywyay, back to the fight. When she's cooking and is boiling something, she always keeps it the flame at full blast, and I always say she's wasting gas and running up our bill and the eggs/potatoes/whatever is not cooking any faster. She says I'm an idiot. Come what may, I am going to show my wife the responses to this post, so I hope someone can provide a scientific answer that settles this once and for all.--162.83.167.217 (talk) 02:43, 5 December 2010 (UTC)

You're right, but my advice is to keep it a secret. HiLo48 (talk) 03:18, 5 December 2010 (UTC)

A bigger flame will certainly heat water up to the boiling point faster, potentially saving gas. You're right that once the water's at a full boil, the temperature is constant no matter what the flame size (provided, of course, that the flame is big enough to maintain a full boil). But you boil eggs for what, a couple minutes? That amount of gas is going to be minuscule compared to what you're burning in your freezer all winter long (assuming you have a gas furnace). Buddy431 (talk) 03:50, 5 December 2010 (UTC)

You're both wrong, and you should stop fighting about it. Your wife is wrong in thinking that a higher flame will cook an egg faster, and you're wrong in thinking that her habit of using a high flame when a low one will do is going to make any appreciable difference to your pocketbook. - Nunh-huh 03:22, 5 December 2010 (UTC) To misquote the sixties: Make love, not potato salad. - Nunh-huh 03:41, 5 December 2010 (UTC)

You are partially correct, in that once the temperature of water reaches its boiling point, any additional applied heat turns water into steam faster, without raising the temperature of the water significantly. However, in the end your wife is more correct. Water that's boiling like crazy will consist of a larger fractional volume of steam bubbles than water that's boiling gently, and steam at 100° C and 1 Atm can transfer more heat to the food than liquid water at 100° C, because the steam can release its latent heat of vaporization. See the "Steam vs. Hot-Water Burns" section of this test answer page. The page pertains to causing burns on skin, but presumably cooking human skin is rather similar thermodynamically to cooking food. Red Act (talk) 04:21, 5 December 2010 (UTC)

Rereading this topic, I see that my statement "your wife is more correct" could be taken as insulting if it's taken to mean that I agreed with her position as described by your sentence "she thinks I'm an idiot". If you interpreted my statement that way, my apologies; that wasn't my intent. I just meant that her notion that vigorously boiling water cooks food quicker than gently boiling water is correct. I probably should have used better wording. Red Act (talk) 21:11, 6 December 2010 (UTC)

Perhaps my method of cooking the perfect boiled egg can save your marriage: 1. put eggs in pot, 2. cover with cold water, 3. heat over high until water boils, 4. turn off heat, 5. wait ten minutes, 6. eat the optimal egg. --Sean 19:21, 6 December 2010 (UTC)

Sean, doesn't your method vary with the size of the pan, the residual heat in the hob, and the room temperature? I follow your method with cold water, but then simmer for three and a half minutes for the "perfect egg" of average size. The size of the egg also makes a difference in the cold water method. Dbfirs 10:26, 9 December 2010 (UTC)

traction between the wheel and the tire

This one has always puzzled me. A concern in automotive engineering is often the loss of traction between the road and the contact patches of the tires on the driven wheels. What I have never heard of is concern over loss of traction between the metal wheel and the tire itself (never heard of a wheel spinning while the tire stays still). The coefficient of friction between rubber and road is much higher than rubber and the metal of a wheel and the area of the contact patch seems to be at least as large if not larger than the area of contact between the wheel and tire. The pressure on a contact patch includes about 1/4 of the vehicle weight while the air pressure forcing the tire to the wheel is about 30psi. (I suppose minus the external pressure forcing the tire away from the wheel.) So what am I missing here? (I do know that some specialized off road vehicles use bolt on bead locks to hold tires onto wheels, but this might be for different reasons, although they do probably run lower tire pressure.) --Leivick (talk) 03:09, 5 December 2010 (UTC)

As an off-roader, I can tell you for certain that we run lower tire pressures. The lower tire pressure puts a larger patch on the ground. It also makes the ride a bit smoother but that's just icing on the cake. The bead lock is to keep the tire on the wheel, it's not to keep the tire from spinning on the wheel. Dismas|^(talk) 03:18, 5 December 2010 (UTC)

I have heard of it, I've seen a hotrod spin the wheel inside the tire. Most of the traction problems people talk about is not straight ahead - there is rarely a problem with that, it's side to side slipping that is the problem. Ariel. (talk) 07:44, 5 December 2010 (UTC)

The bead lock article actually clears it up for me pretty well. I didn't realize that modern tires used special rubber compounds for the bead area. I was mostly thinking about how little traction tires have when going over metal plates (like those used in road construction). I could imagine a rubber material that could have a high static coefficient of friction when pressed against a metal tire. The rubber on the contact patch is probably quite different than the rubber on the bead surface seeing as the bead surface doesn't move and thus can be very soft without worries about wear. Is this right? --Leivick (talk) 08:42, 5 December 2010 (UTC)

Dream time vs. real time

How does perceived time in dreams correspond to real time? Often, I wake up 1 hour before class, go back to sleep, and wake up thinking that I've been dreaming for hours, when in fact only half an hour passed. One time, I heard a knock on my door in my dream that corresponded to an actual knock on my door in real life. I thought I woke up 4 minutes later, but the person who knocked said I woke up only 30 seconds later. So in my dreams, time seems to be significantly slowed. --140.180.3.134 (talk) 03:45, 5 December 2010 (UTC)

The answer to this is not known for certain, but I believe the most prevalent view is that dream time roughly matches real time in most cases, but with discontinuities -- there are often points in dreams where the situation abruptly changes, with a feeling that time has passed but no actual experience of the intervening events. It's very difficult, though, to study this with a fine level of precision. Looie496 (talk) 17:06, 5 December 2010 (UTC)

Those discontinuities are very interesting. If in reality you were driving your car and then instantly were in another location doing a different activity, you would be extremely confused and disoriented for a while. At the very least, you would definitely notice. The part of your brain that determines these things seems to be in a dormant state while dreaming though, presumably to keep you from awakening yourself frequently. Googlemeister (talk) 15:52, 6 December 2010 (UTC)

Actually time was speeded up if you dreamed 4 minutes in only 30 real seconds. --Sean 18:45, 6 December 2010 (UTC)

The perception of time is far more subjective than most people realize. consider the phenomena that the first time you drive down a new road it seems to be much lengthier than subsequent trips down it (I think everyone experiences this). It has to do, I think, with the amount of work the mind is engaged in - First trip down the road the mind is processing a lot of new information; tenth trip down the road is largely autopilot. Dreams are by definition novel, active mind-states, and so real world time passage (in those cases where the real world impinges on the dream) will likely seem much slower.

Actual passage of time in the dream itself is a different issue. dreamtime is not necessarily even approximately linear, since dream contexts can and do shift dramatically over the course of a dream. --Ludwigs2 19:54, 6 December 2010 (UTC)

Zincate

Hi. When Zn(OH)₂ is dissolved in an excess quantity of NaOH, what product(s) are formed? Zinc hydroxide has a high solubility/un-decomposes in strongly base (and acid) solutions, so that two different metal cations form around one oxide anion element. Does Na₄ZnO₃ exist, or is Na₂ZnO₂ always the only zincate compound synthesized? Thanks. ~AH1^(TCU) 03:47, 5 December 2010 (UTC)

Doesn't the linked article clearly state the formula for the stuff? "It is now generally accepted that the ionic species in alkali solutions of ZnO or Zn(OH)₂ contain Zn(OH)₄²⁻.^[7]" and previous statements giving the formula for the salt of that anion (with refs for various other effects). DMacks (talk) 17:36, 5 December 2010 (UTC)

A hot water will cool faster or a cold water will heatup faster.......?

let i have two glass one with hot water at 50C and other with cold water at 0C. ...and let assume the room temp. is 25C. I want to know if which glass water will reach at room temperature(25C) first...means which will transfer heat faster a cool one or hot one....? —Preceding unsigned comment added by Amit.meer (talk • contribs) 04:51, 5 December 2010 (UTC)

I don't have a direct answer, but this is likely to be somewhat complicated, as reflected in a somewhat related question from August 2008 and a related article, the Mpemba effect. I hope someone else can be more helpful. -- Scray (talk) 06:21, 5 December 2010 (UTC)

I would have thought that the warmer water will induce more vigorous convection currents in the surrounding air, and this will cause faster cooling, but the situation is complicated, and, in theory at least, neither reaches exactly 25C in finite time. Dbfirs 16:15, 5 December 2010 (UTC)

The glasses have equal heat conduction to the surrounding air, but the hot water surface loses heat by Evaporation faster than the cold weater surface gains heat by conduction. Therefore the hot water glass approaches 25 degrees faster. Cuddlyable3 (talk) 12:08, 6 December 2010 (UTC)

I would imagine that this depends on humidity. If you had air at 100% humidity at 25 degrees C then I think there would be a lot of condensation on the glass at 0C, and not so much evaporation from the 50C glass. I should add that this is just hypothesis, I haven't tried it or found citations. -- Q Chris (talk) 12:44, 6 December 2010 (UTC)

My theory is that since heat rises and the glass is open on the top, you are going to have easier heat transfer from hotter to cooler then v.v. Googlemeister (talk) 15:49, 6 December 2010 (UTC)

And, are radiative effects gojg to be significant? If the glasses are in sunlight by a window, will that change things as they are warmed by the sun? If it is very cold outside and they are by a window, will that change things as they radiate heat to the outside world and receive little back? 86.161.208.185 (talk) 23:41, 6 December 2010 (UTC)

Radiative warming or cooling will certainly depend on the temperature of the surroundings, and this includes the air outside a transparent window, and sunlight. Radiative cooling is proportional to the fourth power of absolute temperature (giving roughly double the heat loss from the hot water), but this means of heat loss is probably the least significant in many situations (?) because both glasses also gain heat by radiation from the surroundings. Adding a little black ink to the water would make a big difference if the glasses are in bright sun, even though the surrounding air in the room is kept at 25C. The situation is indeed complicated because there are at least five means of heat loss and gain. Conduction and convection (in the absence of turbulence, condensation and evaporation) are approximately proportional to the difference in temperature, but radiation is a "wild card" because it can depend on the temperature of the sun, 93 million miles away! In the absence of extremes like the window effect, high and low humidity etc. and with fan-assisted convection to give the same airflow to each, I suspect that there will be little difference between the two and that they will approach room temperature at similar rates, with the hotter glass approaching marginally faster, but it would be interesting to try an experiment. Can anyone find any research? Dbfirs 09:33, 9 December 2010 (UTC)

How can I design a gearbox (geartrain) to rotate a 2ton load using airgun.

hi.. i want to design a geartrain or gearbox to rotate a load of 2ton. and for power input to gearbox i want to use pneumatic power (air gun which is used to open and tight nut bolts...of tyre of car at car workshop)... i want to know that should i use gear train of 3 spur gear or should i use the worm and wheel..?.... as the speed (r.p.m)of air gun is generally too high, i thought worm and worm wheeel will be useful ....but i have no knowledge that how much torque is needed to rotate that 2 ton load....and so it is difficult to design the gear and gear parameters....the other problem is i don't knwo that these gears are available in market in what specifications...please help... thanks... —Preceding unsigned comment added by Amit.meer (talk • contribs) 05:01, 5 December 2010 (UTC)

You can rotate 2 tons using a feather (if there was no friction). The question is how fast you want to do it, and how much friction you need to overcome. How fast only you can answer, how much friction depends on how many gears you use (the more gears the more friction), and more importantly the size and quality of them. So, in short, there is no simple answer. Ariel. (talk) 07:40, 5 December 2010 (UTC)

What Ariel says, but if I were you I would first experiment with some sort of variable belt drive or bicycle gears, which will give you some idea of the torque required and the gear ratios to aim for.--Shantavira|^{feed me} 11:48, 5 December 2010 (UTC)

If I wanted to rotate a two ton mass, how I did it would depend on the purpose behind such a project. If it was for say: part of a flywheel energy storage system, then I would mount it vertically and use a simple oil film thrust bearing to reduce the friction to the lowest possible figure without resorting to more exotic solutions. An air-drill or gun should be able to drive that. However, for energy storage projects, a rotary vane pump of the type found in an air-guns is very inefficient. The other consideration to think about is containment. Depending on r.p.m., a two ton rotating mass could easily be endowed with sufficient kinetic energy to breach a brick wall and re-landscape half the street. --Aspro (talk) 13:30, 5 December 2010 (UTC)

I don't think an air gun would ever be able to get a 2 ton mass up to such destructive rotation speeds, on Earth, due to friction. StuRat (talk) 15:56, 5 December 2010 (UTC)

You would be surprised. Anti-rolling gyros' can weigh in from about half a ton, to many tons. The motive force required is quite small. Of course they can take an hour or more to spin up to speed, but once there, it takes just a few hundred watts to keep them spinning. Air-drills can manage that. Because of the weight, the thrust bearing are purposely made very simple and reliable. These units are designed by 'proper' engineers. Which is why I warn about large rotating masses going walkies when put together by the enthusiastic amateurs. I'm glad that the OP's who post here are not my neighbours!!!--Aspro (talk) 22:14, 5 December 2010 (UTC)

But one difference is that those gyros are precision balanced, while somebody pulling a car onto a platform will only have a very rough balancing, resulting in it wobbling and increasing friction dramatically as it picks up speed. Also, you can't expect a clean environment here, so pieces of dirt and dust will likely find their way into the bearings, also increasing friction. StuRat (talk) 02:30, 6 December 2010 (UTC)

We need to know a bit about the application to make recommendations. Since 2 tons is the approximate weight of a car and you are talking about using a car tool, I assume you want to rotate a car. If so, you'd want a circular horizontal platform with wheels underneath (like a large microwave tray) or maybe ball bearings. It could have gear teeth on the outside and the air gun could drive a smaller gear that meshes with this large gear. Note that this platform will add considerable mass and friction to the system, and parking the car so it's center of mass is above the center of mass of the platform would be critical. Also, you would want both gears to be recessed into the floor so you don't damage your tires (or toes) on the teeth. This would also aid lubrication, as oil would stay down in the recessed area.

Would this 2 gear system provide enough of gear ratio ? Let's do some math: If the large gear has a diameter of 250 inches and the small gear is 2.5 inches in diameter, that's 100 to 1 ratio. What's the rpm on the air gun ? If it's 1000 RPM, that would lower it to about 10 RPM, or one revolution in 6 seconds. That might be a bit fast. Perhaps one more gear to move that down to a 1000:1 ratio might be needed.

Also, see car turntable for examples of how others have done this. Note that one option is to rotate the turntable manually, with a long handle attached to the platform. As long as friction is low enough and you're not in a hurry and don't mind the exercise, this works fine. StuRat (talk) 16:02, 5 December 2010 (UTC)

One or two persons could rotate a steam engine, weighing far more than 2 tons, on a locomotive turntable, so the project you propose sounds feasible. It is a matter of keeping friction low in the support bearings: a central bearing to support it an keepit in place and ball bearing wheels near the edge to support it and keep it from wobbling. Edison (talk) 20:56, 5 December 2010 (UTC)

coffeciant of friction.......i really don't get it...practically...

I know it's the ratio of something..*****...... but i want to know what practially it means... when i say the coefficent of friction between the two surface is .75 then what does this means...?? —Preceding unsigned comment added by Amit.meer (talk • contribs) 05:09, 5 December 2010 (UTC)

You might find Coefficient of friction helpful. But think of it this way, say you have a body of mass M on a plane, with a coefficient of friction c. By Newton's laws, there is a force perpendicular to the plane (the Normal force), usually called N - if the plane is horizontal, then this is just equal to M * g where g is the force of gravity. The force of friction acts in opposition to any force attempting to move the body (eg gravity on an inclined plane, or someone pushing it), to a maximum force of N * c. Thus, to answer your question, the coefficient of friction is the maximum ratio of the resistance due to friction divided by the perpendicular force N.

For an example, consider a body of mass 1kg on a horizontal plane, with a coefficient of friction of 0.5. We'll take g = 10. Then N = Mg = 10N, and friction will counter any force trying to move the body along the plane with at most N * c = 10 * 0.5 = 5N of force. So we would need to apply more than 5N of force to move the object. -mattbuck (Talk) 05:45, 5 December 2010 (UTC)

1) A coefficient of friction of 0 means there's no friction at all, the object will slide on that surface forever. This is not possible in the real world.

2) A coefficient of friction of 1 means there's a great deal of friction, and you would need to push an object really hard to get it to move (the static coefficient of friction), and somewhat less to keep it moving (the dynamic coefficient of friction), and it will make a lot of heat as it does move. Note that a coefficient of 1, and even above 1, really is possible, although it requires some type of interlocking between the two objects, either mechanical (like small teeth) or chemical. StuRat (talk) 15:52, 5 December 2010 (UTC)

Formula One cars are capable of lateral acceleration over 5 g in corners which implies a coefficient of friction between tyres and tarmac much above 1. If the driver tries to go faster he may discover that the operative force is only Stiction. Cuddlyable3 (talk) 09:20, 6 December 2010 (UTC)

Formula One cars use a Spoiler (automotive), or more accurately, a Wing (automotive) to increase the normal force between the tires and the ground. Here's an interesting article on [2]. I've heard it said that an Indy Car generates about 5000 lbs of downforce at top speed. Given that the car weighs about 1500 pounds, the cornering isn't nearly as unbelievable (still impressive, though). It's often said that the downforce of such a car would be sufficient to allow it to drive upside down (again, at speed), though I've never seen anyone try it. Buddy431 (talk) 20:44, 6 December 2010 (UTC)

Why N2 is used in bladders for curing rubeer in tyre industry...?

In tyre industry the vulcanisation process is carried out in at a particular tempreature near about 180C..... in curing press....at the same time N2 is also supplied in bladder which at near bout 13kg/cm2 pressure......... i want to know if the purpose of supplying N2 is only to maintain pressure inside the tyre to maintain shape of the tyre or it has some other significance..... is N2 supplied to cool the hot vulcanised rubber to be solid...? —Preceding unsigned comment added by 220.225.96.217 (talk) 06:23, 5 December 2010 (UTC)

Nitrogen is used intead of ordanary air because the oxygen in the air degrades the rubber. Large expensive tyres on heavy plant, are also kept inflated with nitrogen for this reason. --Aspro (talk) 13:35, 5 December 2010 (UTC)

alcohol + water = less?

I heard someone say that if you put 1 l of alcohol in 1 l of water you end up with 1.8 l. That sounds like nonsense to me. Is it?
And while I'm at it; alcohol is lighter than water, so does it float on top of it? If so, that is something to consider when pouring a glass of whiskey. Should one shake (not stirr) first? Even beer?? :) DirkvdM (talk) 09:00, 5 December 2010 (UTC)

I don't know if your 1.8 number is accurate, but it is possible. Alcohol dissolves in water, this means that the molecules of alcohol fit themself inside the empty spaces between the water molecules. This also means that while it may be lighter, it's not necessary to mix alcoholic drinks. Ariel. (talk) 10:00, 5 December 2010 (UTC)

The number 1.8 is somewhat inaccurate. According to Ethanol#Physical properties, 1 L of water and 1 L of alcohol (more technically, ethanol) produce a mixture with a volume of 1.92 L.

Alcohol not only dissolves in water, the two substances are miscible. That means that alcohol will not float on top, regardless of what ratio of the two you use. Red Act (talk) 10:46, 5 December 2010 (UTC)

Note that, while a solution of ethanol in water (like whiskey or vodka) won't spontaneously separate into two phases under normal conditions, it is certainly possible to float one atop the other and have them remain reasonably separate for the length of time required to serve and consume a drink. Layered drinks rely on differences in density caused by variations in alcohol content and dissolved sugar, as well as the reduced mixing and diffusion that occur at lower temperatures in narrower glassware. (Our article has a four-layer demonstration shot illustrating the highest-alcohol, lowest-sugar ingredient on top, two increasingly-sweet middle layers with lower alcohol content, and working down to a non-alcoholic high-density bottom layer.) TenOfAllTrades(talk) 18:40, 5 December 2010 (UTC)

(ec) Regarding the first question, see miscibility.

Regarding the second question see the diagram on the right, taken from the article ethanol. If I interpret the diagram correctly, and haven't made a calculation error, a mixture of 1 liter of ethanol and one liter of water should occupy about 1.922 liters. The volume is reduced because the mixture of the two molecule types packs slightly better than each molecule type on its own.

calculation
mL g/mL g g/mol mol Ethanol 1000 0.789 789 46.07 17.13 Water 1000 1.0 1000 18.02 55.49 Mole fraction of ethanol = 17.13/(17.13+55.49) = 0.236 From the diagram, at mole fraction 0.236, the volume is reduced by about 1.07 mL/mole. (17.13+55.49)moles × 1.07 mL = 77,7 mL 2000 mL - 78 mL = 1922 mL.

--NorwegianBlue ^talk 11:23, 5 December 2010 (UTC)

Thanks, that answers my questions more than adequately. DirkvdM (talk) 15:00, 5 December 2010 (UTC)

The general situation is an example of the volume of mixing. DMacks (talk) 17:32, 5 December 2010 (UTC)

Light pipe into fibre optic cable

Light pipes, as seen in the diagram in the article, have a wide diameter. Is it possible to squeeze the light into a fibre optic cable and expand it again at the other end? Would just having a large lens to focus the light on the end of the fibre optic cable be enough, and similarly at the other end? Thanks 92.15.1.139 (talk) 14:15, 5 December 2010 (UTC)

Yes, but with some limitations:

1) Light pipes are to move a broad spectrum of light a short distance. Fiber optics are to move one particular frequency of light a long distance. So, if you sent regular light down a fiber optic line, you'd expect more of the light to change into heat than with a single frequency.

2) The lasers used to generate the light ray for fiber optics send it straight down the fiber, while the light from the light pipe will be headed in all different directions, and therefore bounce off the sides. This will also reduce the length the light can be transported.

So, yes, you could do it, but it may not get you much benefit. StuRat (talk) 15:37, 5 December 2010 (UTC)

Yes, the light pipe article you linked discusses commercially available ones using optical fiber. 75.41.110.200 (talk) 19:31, 5 December 2010 (UTC)

Thanks, but I'm not sure about 1) as FOCs are often used to look through in medical or other situations. Regarding 2), unless the cable is in a dead straight line it will encounter bends and light will be bouncing off the sides. 92.15.18.168 (talk) 22:42, 6 December 2010 (UTC)

1) Those medical fiber optics only transport the light a short distance, so can work with normal light.

2) If light strikes the inside of the fiber optic wire at a shallow angle, it will reflect more efficiently and need fewer reflections, allowing it to travel further. Thus, both avoiding sharp kinks in the cable and sending the light beam parallel to the wire are critical for the most efficiency and longest range.

3) (New item) There will also be losses at both conversion steps, from the light tube to fiber optics and back again. StuRat (talk) 23:28, 6 December 2010 (UTC)

So have you got a PhD in fibre-opticology or is the above just guessing? 92.29.113.166 (talk) 00:01, 9 December 2010 (UTC)

There seems to be an assumption in that Q that nobody with anything less than a PhD in fiber-optics could possibly answer your question correctly. If that was the case, you likely wouldn't get an answer at all, since we probably don't have anyone here with that particular qualification. StuRat (talk) 08:11, 9 December 2010 (UTC)

dreams predicting the future

Sometimes i get dreams and they happen again in real life sometimes its something that affects me other times its not and sometimes I can dream of disasters like Katrina, 2004 tsunami, 2009 chile and haitian quake, hurricane Gustav, Ike and Henna, or typhoon megi before they happen most of the time they happen a month or a few years later so far so I end up remembering the dreams when they happpen im just wondering what exactly causes these and if I end up dreaming really destructive happening should I tell someone? —Preceding unsigned comment added by 213.94.238.86 (talk) 14:20, 5 December 2010 (UTC)

Start by recording the dreams (I do this). One likely problem is confirmation bias. Let's just take one example. You said you predicted Hurricane Katrina. Did you predict the city ? The date ? The name Katrina ? That no organized effort would be made to evacuate people ahead of time ? That the levies would break ? That the Bush administration would be slow to respond ? That the New Orleans police would go nuts and start shooting people at random ? That the stadium would fill with refugees ? Or did you just have a dream about a generic big storm ? If so, big storms happen all the time, so that really doesn't have any predictive value.

I'd bet if you had the actual prediction written down at the time you dreamt it, you will find it's not as "on the nose" as you remember it, after the fact. But, if it still seems like a very accurate prediction, then I suggest you have your recordings of future dreams notarized before the event, so you can later prove you dreamt it ahead of time. Once you have a proven track record of success, then people may take heed of your future warnings. StuRat (talk) 15:27, 5 December 2010 (UTC)

Well I dream about what i am doing on that day not actually dreaming tha time im in it and hearing it on the news in the dream. --83.71.80.201 (talk) 17:38, 5 December 2010 (UTC)

"The easiest way to predict the future is to invent it." -- Alan Kay. Ginger Conspiracy (talk) 04:06, 7 December 2010 (UTC)

The problem with chocolate milk

The powder or liquid you add to milk (or powder you add to water to make cocoa) don't seem to be adequately soluble, leaving clumps of chocolate and sugar at the bottom of the cup, no matter how hard I stir. I assume that heating the milk or water helps, but it's still not very satisfactory, and I don't always want hot chocolate milk. My questions:

1) Which component(s) have a low solubility in water and fat (in the case of milk). Is it the sugar (or possibly corn syrup in the case of syrup), cocoa, or both ?

2) Premixed chocolate milk doesn't seem to want to separate. Why is that ? Is it an additive (carrageenan ?) ? If so, why can't this additive be provided in either chocolate powder or syrup for those who mix them up at home ? StuRat (talk) 15:16, 5 December 2010 (UTC)

Sound like it is a phenomenon of wetting (or rather lack of) you are describing. When all else fails, read the preparation instructions on the side of the tin. It might say: Add powder first. Then pour a little milk in the cup and mix into a smooth past. Slowly add more milk whilst stirring. Problem solved. Works with custard powder too. I don't understand the bit about pre-mixed not separating . Are you adding it cold to hot liquid or what. What's it say on the instructions?--Aspro (talk) 15:53, 5 December 2010 (UTC)

By "premixed" I mean chocolate milk you buy in a store, already mixed together in the final drinkable form. StuRat (talk) 16:05, 5 December 2010 (UTC)

Yes, I think that's it. Even cocoa powder alone, when put into cold (soya, in my case) milk will form little clumps, of which I think only the outside is wet. Even vigorous stirring with a spoon doesn't break these up. I use an immersion blender (which I assume generates localised pockets of pressure strong enough to smash the little cocoa clumps) which produces a genuinely mixed chocolate solution. -- Finlay McWalter ☻ Talk 16:00, 5 December 2010 (UTC)

Thanks, but what's the issue with chocolate syrup then ? It's already wet, but still doesn't want to mix with the milk, it wants to stick to the bottom of the glass. StuRat (talk) 16:05, 5 December 2010 (UTC)

The chocolate syrup probably has a gelling agent added. These gels are engineered to 'be gels' between certain temperature ranges. The molecule are cross linked with their neighbours so resist dispertion. They be made to thicken up as they get warmer or the other way around. Food science can get quite complicated but it enables companies to use cheap ingredient in products which they can then sell with a higher profit margin. So if the syrup is primarily designed for pouring over say cold ice cream it will also resist dispersing in very cold milk. It might however mix with warm milk faster. Oh dear. This is making me feel hungry again. Reminds me: another excellent wetting agent for cocoa powder is a good shot of whiskey. --Aspro (talk) 16:35, 5 December 2010 (UTC)

Let me just note that milk, including chocolate milk, is actually a suspension rather than a solution -- in other words, the solids are not really dissolved, merely swirling in the liquid like dust swirls in air. That makes the physics quite different. Looie496 (talk) 17:00, 5 December 2010 (UTC)

And why do they use a suspension ? Sugar is water-soluble, correct ? Is cocoa only fat-soluble ? If so, then the best we could do is to have fat and cocoa droplets suspended in the water, I suppose. StuRat (talk) 17:04, 5 December 2010 (UTC)

"milk [...] is actually a suspension". "why do they use a suspension ?" Who's "they"? The cows? And why the French punctuation? I was sure you didn't use to put a space before your question marks. 86.164.31.131 (talk) 17:29, 5 December 2010 (UTC)

I was talking about cocoa in hot water when asking about "they", but I suppose it also applies to milk. That is, why can't the chocolate and sugar be dissolved directly in the water portion of milk rather than in the suspended fat clumps ? And I've always used French punctuation, I just prefer it. StuRat (talk) 18:27, 5 December 2010 (UTC)

The issue is that Chocolate isn't one thing; cocoa is a mixture of chocolate solids, which includes both water soluble and fat soluble substances. Its why milk, which is a mixture of fats and water, is an ideal thing to dissolve it in. See cocoa butter, which is the fat soluble part of chocolate. It won't actually dissolve in the water part of the milk; the fatty part of the milk helps keep the chocolate in suspension. When you buy the premade bottled chocolate milk, the milk contains emulsifiers which help keep the chocolate in suspension. The reason that the chocolate isn't dissolved in the water portion of the milk is that, by and large, it doesn't. The laws of physics are a mean bitch... --Jayron32 18:38, 5 December 2010 (UTC)

So then, there's no way to make a better cocoa powder (or syrup), which would mix easily with water or milk using just a spoon to stir it ? I must either dirty a blender or else end up with half of it stuck to the bottom of the cup ? StuRat (talk) 19:18, 5 December 2010 (UTC)

I have had no issues with mixing Hersey's Chocolate syrup with milk. The trick, though, is to add the milk to the glass first. This "cushions" the syrup, and prevents it from contacting the sides of the glass and sticking. The syrup itself has no problems dissolving in (cold) milk - it's just that if it sticks to the sides of the glass, it never gets a chance to dissolve. You can also get do a passable job of mixing powdered cocoa in cold liquid, if you use the opposite proceedure: Add all the dry powder to the glass, and then add a small amount of liquid. Stir until it becomes a thick paste. Here you're using the viscosity of the solution to your advantage to help wet the powder. Keep adding a small amount of liquid at a time, gradually thinning out the mix, until you get a full mug. Warming the liquids in either case will help, as not only will the warm liquid be better at dissolving things, it also has a lower surface tension, so it will be better at wetting powders. -- 174.31.212.34 (talk) 19:51, 5 December 2010 (UTC)

I have this problem when adding Hershey's syrup to a gallon of milk, it goes straight to the bottom, without mixing noticeably. I wonder what you are doing differently. StuRat (talk) 21:21, 5 December 2010 (UTC)

The syrup does go straight to the bottom (as would be expected from its higher density), but in my experience the surface of the syrup (and the surface of the glass) is coated with a thin film of liquid from the passage through the milk, inhibiting adherence to the surface of the glass. Vigorous stirring can then mix the syrup with the rest of the liquid. If you leave it for a while (perhaps because you're adding a gallon's-worth of syrup through a cup's-worth dosing nozzle) the coating may dissolve/dissipate, leading to surface sticking. When dealing with large volumes, I'd recommend adding a portion, immediately mixing, then another portion, etc. I'm also not sure if there is a glass/plastic wetting difference which may affect the wetting/sticking properties at the syrup/container interface. -- 174.31.212.34 (talk) 03:06, 6 December 2010 (UTC)

This really works. I'd also recommend starting with a small amount of boiling water poured on the cocoa powder and sugar, to dissolve it more easily and completely, forming a thick paste, then slowly make it up with the milk, stirring after each addition. Think about adding slightly less liquid than is already in the cup each time (so maybe 1:0.8 ratio of contents:new milk each time, then stir). The boiling water might be less convenient if you live in the US, where the desire for tea hasn't led to a kettle that's always being boiled through the day. I cannot speak to syrup. 86.164.31.131 (talk) 21:26, 5 December 2010 (UTC)

The small-amount-of hot-water-to dissolve-the-crystals-then-add-the cold-milk method is exactly what Ovaltine used to recommend for a cold version of the drink when I was a youngster. Deor (talk) 02:09, 6 December 2010 (UTC)

Does using Dutch process chocolate make a difference in mixability? Ariel. (talk) 20:31, 5 December 2010 (UTC)

Thanks for your replies, everyone. It looks like I'm stuck using all those workarounds because nobody has come up with cocoa which readily dissolves in water and/or suspends in milk. StuRat (talk) 05:05, 8 December 2010 (UTC)

Rainbows around light

What causes rainbows to appear around bright lights? Is there some sort of interference effect caused by the eyes lens? 74.15.138.27 (talk) 16:56, 5 December 2010 (UTC)

For street lights, it's usually water droplets in the air. StuRat (talk) 17:01, 5 December 2010 (UTC)

Thanks for the answer. Would there still be water droplets in the air if we're in winter? 74.15.138.27 (talk) 08:22, 6 December 2010 (UTC)

Yes, it's called Fog, see Fog#Types for the supercooled variety. Cuddlyable3 (talk) 09:07, 6 December 2010 (UTC)

I would have noticed fog. And, correct me if I'm wrong, only water droplets can form rainbows, not ice crystals. 74.15.138.27 (talk) 16:27, 6 December 2010 (UTC)

Ice crystals can form sun dogs, which can be rainbowy. --Sean 18:51, 6 December 2010 (UTC)

A low level of fog may not be visible, especially at night. Let's say visibility is down to 1/4 mile, due to fog. If you can only see a small portion of that distance, due to the darkness, you may not notice the fog. And fog is common in winter, so long as the temperature is above freezing. StuRat (talk) 23:19, 6 December 2010 (UTC)

Well, it's a few degrees below freezing here, but not much. Anyhow, if this is an effect due to raindrops, would it be analogous to to rainbows? Because I thought rainbows always formed on the opposite side of the sun. 74.15.138.27 (talk) 03:33, 7 December 2010 (UTC)

Lens flare diagram.

Sumo wrestling with Glare.

I think it may also be due to Glare (vision), where your eye is refracting the light in much the same way as in a Lens flare. WikiDao ☯ (talk) 04:39, 7 December 2010 (UTC)

It could also be due to the ingestion of certain substances in some cases. :) WikiDao ☯ (talk) 04:43, 7 December 2010 (UTC)

(Hypothetical) Gravitation in the Center of the Earth

For this question, let's pretend that the Earth's center doesn't contain magma or other extremely high temperature material that would inhibit travel. If someone were to begin to dig a hole from one pole (straight through the center of the core) to the other, it would be logical to assume that for the first half of the journey, they would be digging down, while for the last half, they would be forced to dig up - against gravity. But at the very center of the core, where would gravity be pulling from? Would the digging human become crushed under the force of gravity coming from all sides, or would he float, suspended by the equilibrium? Or would none of these situations occur? Any answers would be much appreciated! Stripey the crab (talk) 19:36, 5 December 2010 (UTC)

Well, gravity would pull him in all directions at once EXCEPT directly back along the shaft he'd just dug. He'd probably float slightly towards the Himilayas or something due to variations in mass, but nothing significant. If the earth was just a hollow sphere, gravity would be 0 everywhere inside the sphere. -mattbuck (Talk) 19:44, 5 December 2010 (UTC)

(edit conflict)Assuming that the Earth is of uniform mass, (or of concentric spheres, each of uniform mass), then gravity increases linearly as you ascend from the centre of the earth; in the centre of the Earth there is no gravity, and you would float. The relevant formula is $g_{local}{=}{\dfrac {g_{surface}\cdot r_{local}}{r_{surface}}}$ CS Miller (talk) 19:50, 5 December 2010 (UTC)

(edit conflict) x2 See Shell theorem. The earth is very approximately a sphere (yes, it is an oblate spheroid shape, that is it bulges a bit in the middle, but the deviations from a true sphere are small enough to be ignored on this scale). Any sized hole at the exact center of the earth turns the earth into a "shell"; there is zero gravity inside that hole regardless of your location inside the hole. Thus, as long as you are part of a symmetrical open space, that contains the center of the earth at the center of the open space, you experience no net gravitational pull at any point inside of that space. The Shell theorem article goes through the proof of that. --Jayron32 19:51, 5 December 2010 (UTC)

BTW, don't confuse being inside a hole in the center, with being inside a tunnel in the center. If you are in a hole then you float with no force in any direction, but if you are in a tunnel then gravity is pulling you to the exact gravitational center of the earth, but not very hard - the closer to the center the less the gravitational force. Ariel. (talk) 20:24, 5 December 2010 (UTC)

It troubles me that the above analysis give the impression the person would float around randomly in the central void. If one were in a hollow spherical void in the center of the Earth, analysis above says there is no gravity, but would a person lying on the edge of the void along the equator experience a slight centrifugal force tending to hold him to the wall? What if the void left only a thin shell of the Earth near the surface (assuming the Earth did not fly apart from the loss of gravitational attraction of all the now-missing internal mass). Wouldn't the centrifugal force press the person strongly outward? Edison (talk) 20:51, 5 December 2010 (UTC)

The centrifugal force is pretty small on the surface of the Earth, and does cause a slight difference in the apparent force of gravity at the equator versus poles. It's large enough to be measured, but not enough to notice. StuRat (talk) 21:17, 5 December 2010 (UTC)

Specifically the centrifugal force corresponds to the centripetal acceleration, whose magnitude is v²/r. In round numbers the Earth's circumference is 40,000 km and its radius 6,400 km. So at the equator the acceleration is (40,000 km/day)²/6,400 km = 250,000 km/day² = 0.033+ m/s² or just about 1/300 of the normal acceleration due to gravity. And that's at the surface of the Earth. At the surface of a smaller void near the center, r would be much smaller and v would be smaller in the same proportion, so the force would be reduced in proportion to r. --Anonymous, 22:08 UTC, December 5, 2010.

So, according to the first equation above, there is a linear decrease in the weight you feel yourself to be as you approach the center of the Earth. So at one tenth the radius of the Earth (6,371 kilometres (3,959 mi) → still 637.1 kilometres (395.9 mi)) away, you'd feel one tenth as heavy as on the surface (180 pounds (82 kg) → only 18 pounds (8.2 kg)). At still 6.37 kilometres (3.96 mi) away from the center you'd weigh only 0.18 pounds (0.082 kg), which is pretty close to weightless. And then you'd be completely weightless at the exact center, and then become gradually heavier again as you went up the other side. WikiDao ☯ (talk) 17:33, 7 December 2010 (UTC)

Health effects of reflective heater

infrared radiant heater

What are the health effects of a reflective heater (seeing as there is no article)?Smallman12q (talk) 19:40, 5 December 2010 (UTC)

Seeing as there is no article, can you tell us what a reflective heater is please? -mattbuck (Talk) 19:47, 5 December 2010 (UTC)

Think this is sales and marketing Über speak for what everyone else calls radiant heating, and as always Wikipedia has an article about it. They use a shiny concave reflector.--Aspro (talk) 20:03, 5 December 2010 (UTC)

Health effects: Well, the sun warms you in the same manor, but radiant heaters don't have the UV component to their electromagnetic spectrum, so one might start suffering from rickets if you completely substituted this form of heating for sunshine. If one sits too close, blistering of the skin may result. Sticking one's fingers through the grill may result in burns and with electric versions – an electric shock. RHU's may cause burns (both conventional and radiological). Cutting them open to see what's inside may also provide one's doctor with an interesting case of stupidity triumphing over commons sense. --Aspro (talk) 20:34, 5 December 2010 (UTC)

Dropping it in the bath might be very bad for health, causing electrocution. An electric heater typically heavily loads the electric circuit, and if other heavy load is on it, or if an undersized extension cord is used, the overheating could cause a fire, bad for health. If it is too near something flammable, ditto on fire and health effects. It causes less pollution at the point of use than a heating system using fossil fuel. It allows heat to be produced in the room it is needed, improving health in a building with uneven or inadequate central heating. There is not the problem of carbon monoxide poisoning from a blocked flue or leaky heat exchanger on a furnace burning fuel, and obviously no health problems from gas leaks, gas explosions, or boiler explosions. Edison (talk) 20:42, 5 December 2010 (UTC)

If its electric, there is the obvious danger of tripping over the electric cord and braking one's neck etc. A cord of a wall mounted unit may also provide a strangulation hazard. These heaters are beginning to sound really dangerous _ they ought to be banned.--Aspro (talk) 21:16, 5 December 2010 (UTC)

I've never actually used one of these myself, but I suspect you might have a problem similar to the one you have with a car without seat warmers. That is, the side towards the heater gets too hot and the side away from the heater gets too cold. This is uncomfortable, and can result in sickness if you get a mix of sweat and cold (or could mimic/mask sickness). As in the car, this problem should dissipate as the heat disperses evenly throughout the environment. But, unlike a car, you may not have a coat on to protect you from the cold until it does. Also, moving out of the target area of the radiant heater will chill you, too.

As for more serious risks, electrocution is always possible, especially when used near water, and fire is a risk, if it's placed where it can be accidentally covered by blankets or papers. Some models may have overheat sensors, but I wouldn't want to bet my life on them working. StuRat (talk) 21:06, 5 December 2010 (UTC)

Modern designs mitigate some of these dangers such as exposed live elements, though there are still some risks. As a supplement to background heating, I find electrical radiant halogen heaters to be excellent, but even the modern versions can be uncomfortable when used as the only source of heating because of the directionality mentioned above. Using two (one on each side) helps to provide a more comfortable all-round warming. Dbfirs 22:06, 5 December 2010 (UTC)

Electric radiant heaters usually have a red hot element that burns any dust that falls on it. This will produce an unpleasant smokey smell particularly when the heater is first used after a summer break in a room that is vacuumed regularly. Despite claims to the contrary, domestic Vacuum cleaners puff out fine dust, itself a health problem. Cuddlyable3 (talk) 08:55, 6 December 2010 (UTC)

And every once in a while, an insect or spider will die in there, filling the room with the horrible stench of flaming insect. I prefer convection heaters, used on low, as they don't do this and are much safer, as far as burns and fires are concerned. StuRat (talk) 23:13, 6 December 2010 (UTC)

I chose instead many years ago a plugin room-sized electric heater which looked like a baseboard radiator, and which was filled with water. I chose that rather than an oil filled one because water does not burn, if things went very wrong with the unit. I cannot set the draperies on fire, and a child could not stick something metal into it and get electrocuted by touching an energized heating element. The only downsides were that it is slower to warm up a room, and you still hear a little click as it cycles on and off. The plus side it it has worked well for many years and provides an even room temperature, and does not put out a red glow to disturb sleep. Edison (talk) 03:23, 8 December 2010 (UTC)

Help with electricity (mAh calculations and whatnot)

I have a unit here, it takes DC 9V. I don't know its drain, but on the wall-wart it says 850 mA. So let's presume that this unit drains 800 milliamps.

Now I'm making sort of an outboard battery. To get the 9 volts (nearly), I chained six NiMH AA batteries together.

Each of the AA is rated 2350mAh. So six would be 14100 mAh? Is that right?

Consider that's right, and the unit drains 800 milliamps, the maximum theoretic runtime would be 14100/800 = nearly 18 hours of play? Are my calculations right or is something completely off? --96.21.156.211 (talk) 20:35, 5 December 2010 (UTC)

Unfortunately no, that's not right. If you hook up the batteries in series, then the voltage goes up, but the amps stays the same. If you hook them up in parallel then the amps goes up and the voltage stays the same. In your case you are increasing the voltage, so the amps stays at 2350mAh giving you a runtime of 2.9 hours. Ariel. (talk) 20:50, 5 December 2010 (UTC)

No offense, but that's completely wrong, although your final answer is right. The voltage goes up in series, yes, but "the amps stays the same" doesn't make sense. First of all, for any given resistive circuit V=IR, so increasing the voltage would increase the current. Second of all, we KNOW from the wall wart that the unit, when supplied 9 volts, drains 800 mA. 6 NiMH AA batteries, when connected in series, would provide 9 V and therefore drain at 800 mA.

Second point: if you hook them in parallel, "the amps" doesn't go up. Hooking them in parallel doesn't change the voltage across the terminals, so the current doesn't change. The current through each battery actually decreases because there are now more batteries for the current to come from.

Third, 2350 mAh isn't a measure of current. It's a measure of charge; that's why the unit if mAH (milli-ampere-hour) and not mA. 6 batteries would indeed give you six times the amount of charge. The reason it doesn't give you six times the runtime is because, from each battery's perspective, you have a chemical reaction at the positive terminal accepting electrons, and another reaction at the negative terminal giving off electrons. If you have 600 mA running through the circuit, regardless of how many batteries you have, each battery's chemical reactions have to accept electrons at 600 mA and give them off at 600 mA. The battery's dead when all of its chemicals have reacted, so how quickly it runs down depends only on how much current flows through it. --140.180.128.247 (talk) 05:40, 6 December 2010 (UTC)

Feeling pedantic? You need to understand that when answering a question you need to answer in terms the questioner will understand. One way to do that is to use their terminology in your answer. Additionally it's not possible to explain every detail immediately - you will overwhelm them, and often it doesn't help them because that's not what they want to know. Ariel. (talk) 08:17, 6 December 2010 (UTC)

@140.180.128.247, no offense either, but Ariel's comment "the amps stays the same" makes sense when you understand Ariel is correcting the OP's expectation that a higher (milli-) amp rating is achieved at the higher voltage by chaining (i.e. connecting in series) the AA batteries. It is not relevant to introduce Ohm's Law V=IR because the unspecified unit is unlikely to represent a constant resistance R. I don't know what the unit is, but if it is a consumer device with transistors it is likely to draw a fairly constant current. Cheap "wall warts" are often simple unregulated DC supplies with uncertain specifications, and the unit may work happily below 9V as the batteries age. Cuddlyable3 (talk) 08:44, 6 December 2010 (UTC)

I'm not feeling pedantic. "amps" clearly refers to a current; in no situation have I ever heard it refer to a charge. The OP never once referred to the mAh rating as "the amps"; the only time his post included "the amp" was in 800 mA, which is a current, so it's not the terminology the OP used in his question. I was certainly confused by Ariel's post for quite a while, so I don't know how I'm being pedantic by clarifying it after I finally understood it. --128.112.38.41 (talk) 16:39, 6 December 2010 (UTC)

Alright then. Thanks. —Preceding unsigned comment added by 96.21.156.211 (talk) 20:58, 5 December 2010 (UTC)

And also consider that you can seldom use all of the charge in a battery. Typically, when they get low, the voltage drops to a point where it becomes unusable for the given device, long before all the charge is actually used up. StuRat (talk) 20:59, 5 December 2010 (UTC)

You also need to know that NiMH cells produce only 1.2 to 1.3 volts for most of the time (though possibly 1.4v when newly charged), so you might need to have seven cells in series if your equipment needs at least 9 volts. To gain capacity, you could use two sets of series-seven connected in parallel, but it might be more economical to buy a high-capacity 9v rechargeable battery. Dbfirs 21:53, 5 December 2010 (UTC)

When cells or batteries are connected in parallel, the weakest one, or the one with the lowest unloaded voltage, will tend to drain current from the strongest one. The mAh which could be provided would be the sum of the mAh ratings of all the parallel connected batteries only if they were electrochemically identical. Certainly I would not connect a partially used one in parallel with several brand new ones. It might be best to remove the batteries when the device is not in use to avoid internal discharge. That said, I have seen a professionally manufactured device (ground megger, used to measure low resistances) which indeed used several D cells in parallel to get more output amperes than one cell could provide, and they were left in parallel when the device was not in use. Leaving batteries in parallel was avoided with more expensive batteries such as large lead-acid cells. Edison (talk) 20:09, 6 December 2010 (UTC)

Yes, I agree that there can be problems when cells are connected in parallel, especially with lead-acid. I was thinking of identical NiMH cells, charged at the same time, where problems should be minimal unless one cell fails. It is safer to buy higher capacity cells, but these are often not available at comparable cost. Dbfirs 10:46, 9 December 2010 (UTC)