Wikipedia:Reference desk/Archives/Science/2016 July 2

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July 2

Helium

It is said that,the block of element in periodic table can be known by the sub shell on which the last electron falls while writing the configuration. For eg. Sodium has configuration- 1s²2s²2p^6 3s¹.Here the last electron(s) fall(s) on S sub shell (here 3s¹) and we knew that Na lies in S block .

But what about Helium which has configuration (1s²) but falls on P-block? — Preceding unsigned comment added by Achyut Prashad Paudel (talk • contribs) 04:24, 2 July 2016 (UTC)

So perhaps you should put it above beryllium in your periodic table. The spectrum of helium does resemble alkaline earths. But in other chemical properties it more resembles neon. Graeme Bartlett (talk) 05:19, 2 July 2016 (UTC)

Anyway the reference is Periodic table#Placement of hydrogen and helium. Graeme Bartlett (talk) 12:15, 2 July 2016 (UTC)

If you really want to justify placing helium in group 2, it is true that helium's lack of a p-shell does make a bit of a difference compared to neon. Hypothetical neon compounds thus have even weaker electrostatic interactions and the orbitals involved in the bonding repel that much more strongly, to the point that the neon compounds fall over into instability. Helium on the other hand has some real hope of having marginal chemistry, such as the FHeO⁻ fluoroheliate anion. But while taking helium out of group 18 and floating it over the rest of the table with hydrogen (as Greenwood and Earnshaw indeed do) would indeed make it more obvious that neon is actually the most inert noble gas, not helium, I cannot think of a chemical argument to put helium in group 2 over beryllium. Neon is a much better fit.

Hydrogen and helium are problems in this way. If you look at the rest of the main-group elements, you think of them wanting to achieve a stable octet structure, with filled s and p orbitals. But period 1 has no s orbital, and while helium's 1s² situation, being a filled shell, is somewhat analogous to the filled ns²np⁶ situation of the other noble gases, hydrogen's 1s¹ situation really does not have a good analogue anywhere else. The periodic table has a kind of "early-installment weirdness" in the first row (more scientifically called the "first-row anomaly") that sorts itself out once we get to the typical elements from sodium onwards. Double sharp (talk) 15:13, 4 July 2016 (UTC)

P.S. I love this quote:

"It is customary to regard the bonding of the atoms from Li to Ne as normal and thus to consider the behavior of the heavy elements as 'abnormal.' Later on in this article we shall reach the conclusion...that it is rather the heavy elements which behave normally and not the more familiar elements of the first row." (W. Kutzelnigg, Chemical Bonding in Higher Main Group Elements, Angew. Chem. Int. Ed. Engl. 1984, 23, 272-295.)

This is of course carried up to eleven (or rather down to two) for H and He. Double sharp (talk) 15:21, 4 July 2016 (UTC)

(Much later): Well, what do you know: there are some really good reasons for helium over beryllium after all (see Wojciech Grochala's paper on the question). Double sharp (talk) 15:27, 10 March 2020 (UTC)

Not a fan anymore

I have this electric fan. It always worked. Now, when I plug it in, it just hums inside, but the blade doesn't move. It can spin the blade freely with no friction, as though it is not engaged, so I know it isn't seized up. Sometimes, when I plug it in, it works again just fine. The motor axle goes straight to the fan blade, so this is not about gears or anything. The hum is "electrical". What's going on here? Anna Frodesiak (talk) 04:55, 2 July 2016 (UTC)

What happens if you spin the blade while the fan is turned on and humming but is static? (Careful of your fingers!) Or perhaps that is what you have tried and nothing much happens. Thincat (talk) 09:29, 2 July 2016 (UTC)

Thanks for the reply, Thincat. Nothing happens. While turned on, it hums, and the blades can be spun either way freely. You can even 'give it a spin' and it will go round and round till it comes to a stop. No resistance. Odd. Anna Frodesiak (talk) 10:38, 2 July 2016 (UTC)

Well, its humming so the motor is receiving power. A faulty capacitor might mean the motor wouldn't start without manual help so that seems unlikely. Things don't seem sticky. So, all I can suggest is that the motor has an intermittent fault and I know that really isn't very helpful. Someone here will know more about these things than I do. Don't leave the fan switched on and humming because it may burn out the motor. Thincat (talk) 11:56, 2 July 2016 (UTC)

Anna Frodesiak, let me guess, You can start it all time, remove the plug from the wall, set the switch to on, spin it with the fingers or something else, remove the fingers and put the plug to the wall while it is spinning. This problem is often caused by the phase shift capacitor. You can't start a bicycle with the pedal behind or down. But, moving the pedal in a position You can step on it to drive the wheel, You can start riding the bike. Motors, designed for three-phase electric power always start, but single phase requires other motor design like a shaded-pole motor or induction motor using a motor capacitor. It is also a brushless design, having less torque at low speed. Often the capacitor failed and needs to be replaced. Sometimes a coil has a mechanical failure, causing an interruption of the electrical circuit. Note: This motors operate on hazardous voltages. Only have a qualified person to repir it. The capctior shifts the phase of the AC from the power grid to a second coil of the motor. It can be seen similar to a further piston of a combution engine with separate timing or a third leg to ride de bicycle, no need to pull the pedal for start riding it, just use the foot with the pedal which is ahead to step on it. --Hans Haase (有问题吗) 17:32, 2 July 2016 (UTC)

Thank you for the thoughtful reply, Hans. So is the capacitor that black doohickey, a little black box with two wires coming out of it? If so, I can just put in a new one. Best, Anna Frodesiak (talk) 11:21, 3 July 2016 (UTC)

Two wires or terminals are typical for motor capacitors. The replacement needs same or higher AC voltage specification (V~) and same capacity in µF (farads). It needs so be specified and designed for this application and installed properly. A motor capacitor is usually below 20 µF, often 4 to 8 µF. An noise filter capacitor is far below 1 µF. Such capacitor should be avail for $2…5. If installed with thread, just screw in at the same place. When cabletied, the tie might be replaced. Check for position and reliable cabletie install. Too long or incorrect installed wires can cause safety problems. --Hans Haase (有问题吗) 11:39, 3 July 2016 (UTC)

Thanks, Hans. Actually, all these fans seem the same here, so I can just have someone pull one out of an old fan at the recycling lot for a penny. First, I will try one from another fan in the house to be sure that's the problem. I simply refuse to just go out and buy a new fan every time this sort of thing happens. Things are designed here in China to break. Planned obsolescence is a problem here and I hope they plan to make that plan obsolete soon. It is so wasteful and people get ripped off all the time. They throw it in the bin, shrug their shoulders, and then enjoy buying a new fan. Terrible. I even have someone here swap in new motors when they seize up (every two years). New fan = $40 USD, new motor = $5 USD. I have a box full of them, no kidding. :) Anna Frodesiak (talk) 22:51, 3 July 2016 (UTC)

One suggestion: Instead of using your fingers to start it, use an object like a ruler or chopsticks. It should hopefully fit through the screen, and, if it does get chewed up by the fan, it's no big deal. StuRat (talk) 11:44, 3 July 2016 (UTC)

Yes sure, the fingers must be already off before putting the plug into the wall. There's little time before the fan stops again form manual spinning it. --Hans Haase (有问题吗) 13:03, 3 July 2016 (UTC)

Thanks, StuRat and Hans. Actually, the screen is off and fingers work safely. The blades are plastic and could be stopped with a kitten. Anna Frodesiak (talk) 22:51, 3 July 2016 (UTC)

Fast, sharp plastic objects are still dangerous. Caution is advised. StuRat (talk) 20:21, 5 July 2016 (UTC)

Moving a large object

Many of us have heard a putatively Archimedean statement, which says basically "Give me a lever and a place to stand and I will move the world". Imagine a short and rigid lever: you push or pull one end, and instantly the rest moves. Imagine, now, a rigid lever far larger than the world: pushing or pulling the other end should cause the other end to move instantly. However, the result would be that movement would be "transmitted" from end to end instantly, faster than the speed of light. What's wrong with my scenario? I suppose it could be theoretical (e.g. the laws of relativity prevent movement from happening this fast), or practical (e.g. such a big lever could never be constructed rigidly; or the lever would compress or stretch, slowing down the movement so it's not instant), or a basic mistake by me (movement in the original lever just seems instant, but the time involved is measurable, and a longer lever would have a longer time between pushing/pulling and opposite end moving), but I don't know where I've gone wrong. Nyttend (talk) 05:16, 2 July 2016 (UTC)

In both small and large levers, the act of manipulating the lever propagates from one end to the other at approximately the speed of sound in the material. For real world rigid objects, the speed of sound is typically a few km/s. This is of course much slower than the speed of light (300,000 km/s). So for a 2-m lever, the impulse to move might take 1 millisecond or so to move from one end to the other. Such times are measurable with the right equipment, but wouldn't be observable to an unaided human using normal-sized levers. Dragons flight (talk) 05:35, 2 July 2016 (UTC)

A StackExchange thread calculates that the "place to stand on" would have to be about 20 million light years away, so Archimedes would be dead long before the earth moved... AndrewWTaylor (talk) 12:21, 2 July 2016 (UTC)

There's no such thing as perfect rigidity, which is what you'd need for the movement to be transmitted "instantly". As with the spherical cow, the rigidity of a level is sometimes assumed to be perfect (or simply ignored) because it is true (or irrelevant) at a certain given level of accuracy. Matt Deres (talk) 13:03, 3 July 2016 (UTC)

This nice video answers your question. https://www.youtube.com/watch?v=JTvcpdfGUtQ Joepnl (talk) 02:15, 6 July 2016 (UTC)

Brain development

I have many students in institution , they all seems to be more interested on the topics those I taught , but everyone (mostly) say that they are not able to memorise the matter and always ask me about those ways of making their mind more sharper. Here , I want to give them non - medical ways for that . Probably those ways that are easier, doesn't requires any cost,not any medicine and are very efficient .So, What Can I suggest them for? — Preceding unsigned comment added by Achyut Prashad Paudel (talk • contribs) 05:17, 2 July 2016 (UTC)

You might find our article on Memory improvement of interest.--Shantavira|^{feed me} 06:46, 2 July 2016 (UTC)

Flash cards are cheap and effective. They can make their own. Mnemonics are also useful, like "Many Equestrian People Buy Pretty Handsome Horses Of Noble Descent" to remember the methane series: (Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane, Octane, Nonane, Decane).

However, with smart phones that can access the internet whenever we need to look something up, memorizing large volumes of facts is now only likely to earn you money on games shows like Jeopardy. Some rather basic facts, like alphabetical order and the times tables, on the other hand, do still come in handy. StuRat (talk) 11:51, 3 July 2016 (UTC)

Iron Balls

In a paint spray can why there is always a heavy ball which clearly rattles like a bell ?124.253.244.80 (talk) 05:48, 2 July 2016 (UTC)

According to [1]: "Some cans, such as spray-paint cans, have a ball bearing inside. If you shake the can, the rattling ball bearing helps to mix up the propellant and the product, so the product is pushed out in a fine mist." Dragons flight (talk) 06:09, 2 July 2016 (UTC)

Horses living underground

Recently, I visited the Wieliczka Salt Mine, which was a lot of fun. During the tour, they showed how horses had been used for labour in the mine. We were shown stables in the mine, and various horse-drawn equipment, etc. According to the tour guide, young adult horses would be lowered into the mine and would live nearly their entire life in the mine, only being taken out again when they were no longer able to be productive. The guide said that the horses were well-fed and well-cared for. Even assuming that is true, I would tend to imagine that living decades underground with historically poor lighting and being expected to regularly engage in difficult labor would be far from an ideal life for a horse.

However, the guide went on to make a factual claim that struck me as rather suspicious. She said that the horses living in the mine often lived longer than their counterparts on the surface. She then attributed the increased longevity of the horses in the mine to the "good air". She compared this to people with asthma and allergies, so I think she was talking about the lack of pollen and other allergens, though I could imagine that "good air" might also mean the relative absence of horse pathogens. Suggesting the horses lived longer in the cramped and very unnatural environment of a salt mine seems very weird to me, and smells a bit like propaganda to make people feel less bad about the fact the horses were there. (The last horse didn't leave the mine until relatively recently - the early 2000s, if i recall correctly.)

So, is there any reliable evidence that horses living in Wieliczka or other salt mines actually did live longer than average? (Either compared to horses in general, or perhaps just to horses subjected to similar workloads.) If there is some verifiable truth to this claim, then is there supporting evidence to show what factors allowed the underground horses to survive longer? Dragons flight (talk) 06:06, 2 July 2016 (UTC)

Sounds unlikely. Pit ponies were worked hard. Our article on pit pony states that "One 1911 writer estimated that the average working life of coal mining mules was only 3½ years, where 20-year working lives were common on the surface" and gives a citation. I guess the air quality might have been a bit better in a salt mine, but even so....--Shantavira|^{feed me} 07:09, 2 July 2016 (UTC)

It depends a lot on the shape of the mine. For mines with adit access, it was easy to walk ponies in and out of the mine and so they tended to be pastured above ground from time to time. For deeper mines where a cage lift was needed, it's difficult to get them in and out, so they often did it just once. Some countries did have laws though where ponies were required to spend some time above ground at least once a year, or (much later on) to be retired to pasture above ground. Horses aren't stupid though and some were known to realise what was going on and decide that, "they weren't going back down there again".

Much depends on the mine too. Salt mines are generally healthy, as they avoid damp, dust and toxic air, the big problems for coal and other mines. They also have larger working passages than coal mines. There's also much anecdotal evidence that pit ponies (in well funded mines) were well cared for as the miners also appreciated having a big of the surface down with them. Working horses on the surface in Victorian and Edwardian times didn't have a particularly good time of it, so "living longer" could be as much about the short lives of those above as long lives for those below. Andy Dingley (talk) 10:25, 2 July 2016 (UTC)

And for a reference:- Dear Old England: A Description of Our Fatherland by Jane Anne Winscom, 1867 (p. 101) which says of a salt mine near Northwich in Cheshire: "Horses sometimes live in salt-mines, like the ponies in coal-pits, but the horses thrive better than the ponies, the salt being very wholesome". Alansplodge (talk) 19:53, 2 July 2016 (UTC)

Domestic turkeys and chickens are often kept at low light levels, e.g. 10 lux. Long photoperiods combined with low light intensity can result in blindness from buphthalmia (distortions of the eye morphology) or retinal detachment. I would be surprised if the horse came out with perfect vision. DrChrissy ^(talk) 20:10, 2 July 2016 (UTC)

Their life expectancy may have been longer due to them not getting cold stress in the winter months. Yet, the bigger question is, did they really spend their entire lives under ground? A horse is a mammal like you and I, that requires sunlight to form vitamin D. Without it we die eventually. Maybe these horses and pit-ponies had a vacation on the surface from time to time. After all, just like modern haulage vehicle needs their oil changed regularly in-order that these expensive mechanical contrivances continue to work – so the pit ponies must have been so serviced. vitamin-d-for-horsesJust that this may have not been recorded – history is littered by such omissions. Chickens (and pigs which are also raised in the dark) are a very unfair comparison as they NEED vitamin supplements in a modern poultry & pig farms (and both enjoy (?) very short lives)... Mine never did, as they were free-range (ensuring they and their eggs were very tasty, unlike what now is on offer at the local supermarket).Vitamin D3 Deficiency in poultry--Aspro (talk) 21:35, 2 July 2016 (UTC)

Of Welsh pit ponies: "Horses that worked shaft mines stayed underground for life". Alansplodge (talk) 14:54, 4 July 2016 (UTC)

Poland is a very religious and superstitious country. Apparently it's the "negative ions" in the salt mine, or some other "hand wavy" explanation. Something the tour does not tell you is that you can pay a lot of money to stay in the salt mine, there's a section reserved as an exclusive "health spa" which has many "healing miracles" claimed to it, and the thing is that the majority of people involved would believe this 100%, there's no "scam" going on. There are many places in poland where "miracles" happened and people do not question it critically and you will be very very unpopular if you do. I suspect the "horse story" is just an anecdote which "sounded good" and "fit the narrative" so is taken uncritically to be true. I'm Polish by the way (I left as a child) and I deeply love and miss Poland, (I've been to wieliczka twice and would go back again) but this is one "hokey" aspect of Polish culture which is not my favorite. Vespine (talk) 00:21, 4 July 2016 (UTC)

It's impossible to weigh up what might be behind such a claim, even if it were undoubtedly true. Better fed? Better looked after? Less likely to have an accident (ironically)? Less likely to end up being eaten? Less exposed to illnesses? Who knows? --Dweller (talk) Become old fashioned! 09:39, 6 July 2016 (UTC)

Copper battery contacts

Copper reacts with atmospheric oxygen to form a thin layer of copper oxide on its surface. Does this mean that unplated copper (or brass for that matter) is unsuitable for making battery contacts? That such parts much always be plated with another metal? Or does the thin layer of copper oxide not affect conductivity? Can'tTrustHillary (talk) 07:19, 2 July 2016 (UTC)

Unplated copper can be used for battery contacts in cheaper devices, but oxidation is indeed a problem. Higher-quality contacts are tinned with solder, nickel-plated, or even gold-plated - the battery contacts in your mobile phone will almost certainly be gold-plated, for example. Brass doesn't suffer from oxidation, but its electrical resistance is rather high: it's commonly used for mains contacts, where the voltage is much higher than in a battery-powered system. This document from Energizer goes into some more technical details. If you have a device with bare copper battery contacts, it's a good idea to clean them with emery paper or something similar when changing the battery. Tevildo (talk) 08:47, 2 July 2016 (UTC)

So do these cheaper devices fail over time? Or does the extra oxide layer not cause complete failure but just increased resistance and thus wear down the battery faster? Can'tTrustHillary (talk) 12:42, 2 July 2016 (UTC)

What usually happens is that the voltage supplied to the device falls off as the contact oxidises and the battery discharges, so that the device stops working. Cleaning the contacts without replacing the battery can often revive the device for some more hours of operation. When it gets very bad, you may find that the device doesn't work even with a new battery. Tevildo (talk) 13:44, 2 July 2016 (UTC)

Why would you want to make battery contacts? If it's a one-off for a short demonstration, then just make them out of anything. If you're becoming a battery box factory, then get better advice than us and you can afford a more robust production process. Mostly I never make them (and I use a lot of battery boxes) - I either buy boxes, or I make boxes (wooden laser cut) by using the ready-made contacts from other commercial battery boxes.

Copper isn't too good because it also has poor behaviour as a spring. You can make a spring contact from brass or (if plated) steel. Copper though has a low yield strength, so it would get pressed flat and lose contact. Must battery box contacts now are plated steel, in the form of a wire spiral volute spring.

As to corrosion, then the oxide is one problem, but so are other compounds, formed from leaking batteries. Although we're no longer using paper-wrapped zinc carbon batteries which leaked as soon as you looked at them, it is still a near inevitability that any holder for primary cells will see leakage during its working life. Copper oxide is a thin passivation layer which will give adequate contact for some years. Crystals of copper salts though can almost immediately form a non conductor.

Brass is also no more expensive than copper, and often cheaper. Copper is a nuisance to form on a punch press, brass is benignly well-behaved. Brass is stronger, thus can be made thinner and cheaper. Andy Dingley (talk) 12:13, 2 July 2016 (UTC)

I'm not making anything. I just noticed that quite a few electronics around the house use copper or brass battery contacts and that that they noticeably darkened over time. I was just wondering why that's all. Can'tTrustHillary (talk) 12:42, 2 July 2016 (UTC)

Just re-read my OP and realized why you assumed I'm making battery contacts. It must've been the "unsuitable for making battery contacts" part. My bad. I meant that in the general sense of "why is A made out of B". Can'tTrustHillary (talk) 12:46, 2 July 2016 (UTC)

I'm rather surprised if you have many things around the house which use copper, rather than brass. Everything I can see here which is modern is plated steel. I even have some brand new knife switches rated at 32A [sic] which are made of copper plated steel (dreadfully badly made things, I wouldn't trust them over 50V). Older stuff (1950 to 2000) is almost all brass. I've found one 1920s wooden-bodied torch which uses copper, but those are flat plates and the spring was supplied by brass strip springs on the battery itself. Andy Dingley (talk) 17:20, 2 July 2016 (UTC)

In my experience (in places in Canada where I've lived), household wiring is invariably copper. Typically the bare ends of the wires are gripped in either screw terminals (hopefully, cleaner ones than those seen on this old switch) or else, where two wires come together, in wire nuts. If bare copper gets coated in oxide, you would not think this would be safe for household power circuits. --69.159.60.163 (talk) 23:17, 3 July 2016 (UTC)

Battery connectors, as raised by the OP, are a slightly different situation.

There is a concept in connector design, that of a "gas tight" contact. The idea is that this is a metal-to-metal contact so tight that atmospheric gases, including water vapour, cannot penetrate between them - thus avoiding the corrosion problem. This is mostly seen as a deliberate feature in IDC (Insulation Displacement Connector) contacts, such as punch-down blocks. If a connector is inevitably going to be wet long-term (any outdoor telecomms) the tendency is to use jelly-filled connectors, which are filled with inert petroleum jelly, in order to exclude moisture.

Your switch contacts would be termed "pillar" contacts in the UK, rather than screw. The bared cable is wrapped around them in a loop or partial loop. It's considered a mechanically better restraint than the screw terminal with the end of a screw thread acting to compress a cable within a hole or loop. However it also has lower contact pressure, so is more susceptible to not being gas-tight and seeing more corrosion. That's one reason they're not favoured for low voltage telecoms work. Andy Dingley (talk) 00:16, 4 July 2016 (UTC)

Decrement table used to calculate pregnancy rate

According to Wikipedia,a decrement table is used in birth control studies to calculate the pregnancy for each month. How do you calculate the pregnancy rate for each month by using a decrement table? — Preceding unsigned comment added by Uncle dan is home (talk • contribs) 14:33, 2 July 2016 (UTC)

Assuming you mean the Decrement table article, it would be nice if the article explained what it actually is. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:38, 2 July 2016 (UTC)

Moving a small object

Suppose Archimedes and the Earth trade places at opposite ends of the lever, and suppose Earth is replaced with a tiny pebble. Archimedes, very close to the fulcrum, quickly presses the lever down a few inches—the pebble, far far away at the other end of the lever, moves much faster, maybe exceeding the speed of light? I'm assuming a rigid lever with a fast speed of internal transmission. If the answer is that particle physics prevents a sufficiently great speed of transmission, then what if we lived in a world with solid particles the size of a lever, in which relativity still holds? Loraof (talk) 14:57, 2 July 2016 (UTC)

A key finding from relativity is that there are no solid objects, not really. See Ehrenfest paradox. There are a lot of ways to make a whip, but none that flick faster than light. The closer to lightspeed you make the tip go the heavier it is, so if nothing else it would outweigh the Earth at some point. Wnt (talk) 15:24, 2 July 2016 (UTC)

(ec) You answer the first part yourself - you cannot get a bar sufficiently rigid to enable a push at one end to result in light-speed at the other end. Presuming such a lever existed, then presumably it would get v.heavy at the end tending to light speed, requiring massive & presumably infinite force to be exterted at the other end. So, no dice. --Tagishsimon (talk) 15:25, 2 July 2016 (UTC)

Thanks! Loraof (talk) 15:37, 2 July 2016 (UTC)

The recent sound answer by Dragons flight that "In both small and large levers, the act of manipulating the lever propagates from one end to the other at approximately the speed of sound in the material." should curtail ideas of a rigid lever with a fast speed of internal transmission. AllBestFaith (talk) 16:25, 2 July 2016 (UTC)

One could put a powerful rocket on the far end of the lever with a speed-of-light control signal from the near end and thus create a system that acts like a lever with faster-than-sound propagation. Of course the result wouldn't actually be a lever, and in fact would work better with the lever removed... --Guy Macon (talk) 18:03, 2 July 2016 (UTC)

In both small and large levers, the act of manipulating the lever propagates from one end to the other at approximately the speed of sound in the material. I challenge that, in practice i believe the propagation is actually much closer to the speed of sound than the speed of light, even in very "rigid" materials. I did read this somewhere, I'll see if i can dig it up. Vespine (talk) 00:02, 4 July 2016 (UTC)

You appear to be challenging the assertion propagates ... at approximately the speed of sound because you recall reading that propagation is actually much closer to the speed of sound. Okay. Possibly a job that doesn't need doing, but on you go. --Tagishsimon (talk) 00:10, 4 July 2016 (UTC)

That was my pre-coffee dyslexia, even though I copied and pasted and read that sentance 3 times, i swear in the morning it said "speed of light", in my head anyway.. please ignore me. Vespine (talk) 04:13, 4 July 2016 (UTC)

In recompense, I offer Faster-than-light which I think is quite a good article! Especially the sections about things which may appear as violations, which are actually not, specifically phase velocities above c and group velocities above c. For example, if you shine a laser pointer at one end of a galaxy and move it to the other side very quickly, the laser "point" can be made to move in excess of he speed of light, but there's no "thing" actually moving faster than light and no information can be transmitted faster than light using this "effect". Vespine (talk) 05:47, 4 July 2016 (UTC)

Temperature indicating coolant - by color

I am looking for a liquid coolant that indicates its temperature by color change. Ideally it is cheap, non-toxic, non-corrosive, has low viscosity and shows changes in the range 20 to 90 deg C. AllBestFaith (talk) 17:57, 2 July 2016 (UTC)

See Thermochromism for our article. Cobalt chloride solution is the traditional substance for demonstrations of the phenomenon - see, for example, this video - but I'm not sure about "non-toxic". Tevildo (talk) 18:40, 2 July 2016 (UTC)

Most of the substance there would be toxic, but leuco dye may be the least, being used on teeshirts and other consumer products. Graeme Bartlett (talk) 23:14, 2 July 2016 (UTC)

This site has an interesting suggestion - measure the temperature by more conventional means (such as a thermocouple) and illuminate the coolant with different colours of LED depending on the temperature. If the requirement is purely decorative, this might work. Tevildo (talk) 09:15, 3 July 2016 (UTC)

Or use separate cooling and thermochromism chemicals, exposing the small quantity of the later to the temperature of the former, so it always indicates the temperature, without decreasing the cooling efficiency significantly. StuRat (talk) 11:57, 3 July 2016 (UTC)

Temperature indicating coolant - by magnetic property

I am looking for a liquid coolant that indicates its temperature by change in magnetic property such that it can be detected by an inductive sensor outside aluminium pipework. Ideally it is cheap, non-toxic, non-corrosive, miscible with water and can indicate changes in the range 60 to 100 deg C (or to 120 deg C in a pressurized cooling system). AllBestFaith (talk) 18:04, 2 July 2016 (UTC)

I do not know if there is a commercial product or even a patent for a system. Perhaps you can take advantage of superparamagnetism or ferrofluid. Inductance may change slightly as temperature changes. Filling your coolant with magnetic nanoparticles to make a ferrofluid may cause unwanted blockage or coating though. Dissolved substances in a water based coolant that could affect your magnetic field may include ammonium iron(III) sulfate or manganese based Tutton's salts. However using conductance changes, or measuring the speed of sound will likely result in less noise. And putting a thermometer on the outside or the aluminium pipework would be almost as good. Graeme Bartlett (talk) 23:04, 2 July 2016 (UTC)

It's quite hard to usefully measure temperature by measuring flowing coolant. The problem is that coolant is there as a heat transfer medium, so the coolant's temperature isn't usually the thing you're primarily interested in. Coolant changes its temperature around the circuit, so will its temperature at any one location reflect the temperature you need to know? If it's an approximation and you're measuring a system average temperature or looking for changes away from the regular operating temperature, then (as Graeme notes) a simple external temperature probe will do fine. If you need a better defined temperature than this (such as the temperature of the hot thing itself), then you're probably going to need to attach a sensor directly to that, not just measure that of some coolant downstream in the circuit. Andy Dingley (talk) 08:45, 3 July 2016 (UTC)

An application could be to measure thermal gradients throughout an engine cooling system with less inconvenience and error sources than attaching a plethora of temperature probes. AllBestFaith (talk) 16:16, 4 July 2016 (UTC)

Circuits and electric fields

Hello, I am slightly confused on the physics of direct electric circuits. Here is what I have been taught:

• Batteries (sources of emf) provide a constant potential difference between its terminals;
• Electrons (charge carriers) gain a potential difference by being forced by the emf to go from the positive terminal to the negative terminal;
• If the battery is attached to a closed circuit, the battery having a potential difference by definition means an electric field exists in the wires of the circuit;
• Electrons from the negative terminal of the battery enter the wire, but the electrons already in the wire itself also move due to the presence of this electric field. The circuit is electrically neutral, however, because the same number of electrons entering the circuit are also exiting the circuit into the positive terminal.

If the following pieces of information are true, something is bothering me; I have seen voltage graphs where, the voltage through the battery increases, then in the wire the voltage is constant, then decreases when passing through a load. But this seems odd to me; if there is an electric field in the wire, then there would be equipotential lines, which indicate to me that the potential of the electron would change. Wouldn't the potential energy of the electron increase as it passes through the wire if this is the case? Textbooks I've read tend to indicate that the electrons only gain their energy when they pass through the battery, but not inside the actual circuit itself (until it is lost through a load). This would mean there is no field in the wire... but if there were no field in the wire, a current would not exist. Can someone shed some light on this? Thanks. 74.15.5.167 (talk) 21:27, 2 July 2016 (UTC)

To keep thinks simple, think of the wire as a perfect conductor. When connected there is no field in the wire, but there is a current. The battery adds energy to the electrons, the load removes it. The wire normally adds no energy. Can you believe there can be a current with no voltage? (If you can't you will have to understand what happens when the circuit is first connected and the current starts to flow. This however is not DC and needs understanding of how the energy of a current or charge relates to the space around it.) However if the wire has resistance, then the wire will remove energy and there will be a small voltage across the wire. Graeme Bartlett (talk) 22:03, 2 July 2016 (UTC)

The battery itself will not cause more than eviqvalent to its voltage to use a static charge which is able to pull small items. High voltage wires keep the dust on it. Rubbing a balloon on plastic cloth, it is able to pull hair. Toner is pulled by static charge. Altering fields (radio), caused by oscillating or bursts when beginning to pull current from the battery, electrical circuits behave like an antenna. Pulses fade out. Longer wires, higher the impedance and higher powered fields increase the inducted voltage on the wires of the circuit. Electric and magnetic fields can not always be shielded the same way. There are lots of design variants to kill interference. Shields keep it off, wires in twisted pair receive the same wave 180° shifted which causes to add and subtract the same signal. --Hans Haase (有问题吗) 20:43, 3 July 2016 (UTC)

Electricity was originally understood to be a kind of fluid, hence the name "current". The Hydraulic analogy (see article) is still useful in understanding DC electric circuits. One can think of a conductor as a pipe and a potential difference as equivalent to a difference in pressure between two points. AllBestFaith (talk) 20:44, 3 July 2016 (UTC)

The potential on the electron is easy to figure out - it's what's measured in volts. (Well, the potential energy per se is in electron volts, which is to say how much energy a difference of so many volts actually takes in an electron, but the volts are the electrical potential.) Ohm's law tells you how the volts decrease with resistance. Now how do the electrons manage to move in the wire, when there is such a weak electric field? Because it has really low resistance. Remember, the amount of current flowing through a resistor and a plain wire has to be EQUAL in order that you're not making charge out of thin air, so that means the field is much weaker in the wire. (I'm ignoring more complicated effects like inductance and capacitance for now, which kind of let you have minor temporary inequalities in the amount of charge at some point) Wnt (talk) 03:01, 4 July 2016 (UTC)