How the hell did "prune juice" ever come to exist, since a prune is a dried plum and you can't get juice out of dried fruit? Do they mash them up and add water or something?
Isaac
There's a loophole.
Plums grown primarily to be dried are called "prunes", even before they're dried. They can also be eaten fresh, or juiced. Presto, a warrior's drink.
And now, some Bonus Botanical Trivia:
Many people believe nectarines to be a peach/plum hybrid. They're not. They're a smooth-skinned strain of peach, sharing an ancestor with the plum somewhere back in the history of stonefruit, but otherwise unrelated.
Somewhat fewer people believe nashi pears to be an apple/pear hybrid. They're not. They're a natural species, or, at any rate, as "natural" as the apples and pears that humans have been selectively breeding for thousands of years.
(I am greatly amused by Creationist publications that show a magnificent spread of delicious fruit and veg that God in His wisdom has provided for us; the Jehovah's Witnesses have a really nice version of this in one of their numerous happy-pictured books and pamphlets. I always have a hard time finding anything in those pictures that hasn't been gigantically changed from a near-inedible ancestor by human intervention. Possibly the coconut. Good luck opening that with your bare hands, Adam.)
Lemons, on the other hand, are a hybrid, though a pretty ancient one. Genetic analysis (PDF) has shown them to be a hybrid of the bitter orange and the citron.
If you always thought that grapefruit were hybrids too, you'll now be amazed to learn that you were right. The grapefruit only dates back to the 18th century.
Many people are also familiar with the factoid that, technically, the banana is a herb. Banana taxonomy has always been a nuisance, but this bit of pub-trivia information is not actually worth much.
In everyday grocery-shopping terms the banana is obviously a fruit, but in botanical terms it can defensibly be described as a berry, while the botanical "herb" is any non-woody flowering plant, most of which are inedible. (And, by the botanical definition, each individual kernel on an ear of corn is a separate "fruit". Don't get me started on cashews.)
All of these games with definitions and clashes between scientific and everyday terminology are pretty pointless. They make about as much sense as saying that because people who make coins for a living may refer to all of their input metals as "bullion", it is therefore sensible to invest in copper by the ounce.
Another one: In everyday usage, hardwood means wood that is hard. In scientific terms, though, it just means wood from non-flowering flowering [it wasinevitable I'd get one of these wrong, wasn't it?] plants, so balsa wood is technically a hardwood.
Finally, and perhaps most interestingly, it turns out that the tomato is technically an amphibian.
My mom's always been a strong proponent of vinegar as a miracle cleaner for almost anything, floors, windows, clothes, you name it. She recently discovered bicarbonate of soda, too, and has been using that for all sorts of stuff too, like in the dishwasher instead of the special powder.
When I visited the other day, she was washing the floors with a bucket that had hot water, bicarbonate of soda AND vinegar in it. Apparently the fizz when you add the vinegar gives you more "scrubbing bubbles". Except I vaguely remember from elementary school that an acid and a base cancel each other out, the example given having been exactly this, vinegar and washing soda.
So was my mom just washing the floor with salty water?
Bicarb is NaHCO3, acetic acid is CH3CO2H. Acetic is the acid in vinegar - cheap "white vinegar", which is rather more economical for cleaning things than 50-year-old balsamic, contains nothing but nice clean industrial acetic acid and water.
The reaction is:
NaHCO3 + CH3CO2H -> CH3COONa + H2O + CO2
Those products are sodium acetate, water and carbon dioxide. The CO2 is invisible but heavier than air, and can be poured out of the reaction container to extinguish a candle, said candle being the most dangerous thing that exists in boringly-safe-science demonstrations.
Sodium acetate is sometimes used as a flavouring, because it tastes like salt and vinegar all by itself. ("Salt and vinegar" snacks in the USA are apparently likely to be flavoured with sodium acetate; here in Australia I think that's illegal for some reason. I don't think it's toxicity; sodium acetate is pretty innocuous.)
If you mix sodium bicarbonate and hydrochloric acid, HCl, then the reaction is the same except instead of sodium acetate, you get sodium chloride, which is everyday table salt.
(For this reason, bicarb is a very effective antacid. A teaspoon full of bicarb can turn nasty acid-reflux indigestion into a series of hugely satisfying CO2 belches in seconds. You'll have a pretty darn high-sodium diet, though, if like me you end up eating several spoonfulls of the not-that-bad-tasting-when-you-get-used-to-it substance per day. In that case, hie thee to a doctor and get yourself a prescription for one or another acid-production-reducing drug.)
You'd want to be careful making salt from bicarb and hydrochloric acid, though, because if you don't get your stoichiometry right and not add balanced amounts of the reagents, then there'll be left-over bicarb or hydrochloric acid at the end. This is also what will happen if someone decides to make a cleaning product out of bicarb and vinegar; they probably won't titrate the mixture, and so will have a surplus of one substance or the other. Surplus bicarb, as a base, will clean greasy things by, essentially, turning the grease into soap. Surplus vinegar, as an acid, will clean things by dissolving various kinds of dirt, like mineral deposits ("scale"), or rust.
For these reasons, and also the fact that plain water plus elbow grease can clean a lot of things pretty effectively (the basis for the popularity of "laundry balls", which don't actually do anything), people may come to the conclusion that a vinegar-and-bicarb concoction is a super cleaner, when in fact they'd be better off using a smaller amount of only one of the ingredients.
(At least, in this case, mixing the compounds will do no harm. Mixing bleach and ammonia, on the other hand, may greatly reduce the amount of time you spend doing household chores, on account of how you may now be dead.)
Oh, and sodium bicarbonate is not "washing soda"; that's sodium carbonate, Na2CO3, which is commonly used to "soften" hard water, which contains dissolved minerals that prevent soap from working properly. Sodium bicarbonate is "baking soda", named for its use as a leavening agent; if you mix bicarb into batter that's slightly acidic, the fizzy-neutralisation reaction occurs and creates lots of little CO2 bubbles in the batter. "Baking powder" contains dry bicarb and acid powder (usually tartaric acid). Add water, and the components react and fizz.
Getting back to sodium acetate, a supersaturated solution of it is used in "phase change" heat packs...
...which "freeze", liberating heat, when disturbed with the little clicker device inside, or when otherwise slapped around. You put the pouch in boiling water to re-liquefy its contents; the things can be used over and over indefinitely, as long as they don't spring a leak.
You can do something similar to this with numerous other fluids, but sodium acetate's properties suit it very well to the purpose. Even if you don't actually need a hand-warmer, I strongly recommend you buy one as a toy, since you can get them on eBay for about $5 delivered.
All I really know about it is that it's technically called "cyanoacrylate", but the "cyano" part makes me nervous. The last episode of Mythbusters I saw had them sticking stuff to other stuff with superglue (which they called "super adhesive" for some reason) and they were wearing gas masks while doing it.
Am I endangering my health if I superglue a teacup together without lots of ventilation? My son's just now started building model airplanes and tends to stare so close at the model I'm expecting him to stick a propellor to his nose soon; is HE going to be poisoned too?!
Eva
At some point in the next few thousand words I may answer your question, Eva. You know how it is with me.
The magic acronym (or possibly initialism) to remember whenever you want to know how strongly a given substance desires to kill you is "MSDS", for Material Safety Data Sheet. You can find an MSDS for just about anything, provided you know the name of the substance in question. You usually don't need to know the exact chemical name, either; brand names, especially of pharmaceuticals, often work.
One popular substance can have a large number of MSDSes for it, sometimes with different data, because, for instance, a product sold under the same name by different companies may be made with different constituents. MSDSes may also differ even when they're talking about the exact same substance, because different manufacturers and importers and so on may have different testing regimes, or may just plain get stuff wrong. Generally speaking, though, you can trust MSDSes, even if you can't find one for the exact brand of, in this case, cyanoacrylate (which is known to the relevant chemists, and many hobbyists, as "CA") you're worried about.
When I say "just about anything" above, I mean it. Here's an MSDS (in PDF format, like most online MSDSes these days), for skim milk. Including rather excessive first aid procedures to employ in case the substance is ingested.
Here's one, and another, for olive oil. More over-enthusiastic warnings; apparently you're not meant to allow olive oil to make direct contact with the skin. MSDSes for innocuous substances are often like this, possibly for reasons having to do with the covering of arses, or perhaps because there was no "zero hazard" box for the MSDS-maker to tick.
OK, enough silliness. Search for MSDSes for cyanoacrylate, plus a common brand name or two like "Krazy Glue", and you'll get hits like this, this, this and this. Here's a whole page of MSDSes for Loctite products, including various other glues and threadlocks. There's a "safety" section in the Wikipedia article for CA, too, plus some MSDS links at the end.
What all of these agree on is that CA products of various kinds, from the water-thin stuff used to wick into gaps in plastic models through to various non-runny gel-type versions, are not nearly as poisonous as you'd think from their alarming "chemical" odour. The fumes are an eye and mucous-membrane irritant, and if you're sticking a whole room worth of furniture to the ceiling as they did on MythBusters then you'd be nuts not to wear some kind of breathing protection, but this stuff really isn't that bad. I don't think it even releases much in the way of horrifyingly deadly gases if you burn it, though again, this is not recommended.
(With regard to the title of this post, glues that people sniff to get high in a rather dangerous manner are generally based on some kind of solvent with psychoactive effects, though usually not effects that people living a life somewhere above rock bottom would consider worth the damage. Glues with no such solvent, like CA, PVA, hide glue or epoxy, often aren't particularly bad to inhale, which is just as well since they won't even get you high.)
Part of the reason why superglue isn't very poisonous is that its "set" state, a hard polymerised lump, isn't toxic. It's still listed as an "eye irritant" when hardened, but only in the way that sand is. And CA really wants to polymerise. All actual CA glue contains "inhibitor" chemicals in addition to the CA itself, to stop the stuff from instantly turning into a lump of plastic in the bottle. Several common compounds in the world, chief among them water, will "kick" CA into polymerising. And since your eyes and mucous membranes and so on are all rather damp, any CA vapour that hits them polymerises instantly.
Now, this is still not a good situation, since having a very thin layer of plastic accumulate inside your nose and on your eyeballs is not most people's idea of a good time, but the body can deal with tiny amounts of the stuff with no trouble. (This also means that all you probably need as the abovementioned "breathing protection" is a damp cloth tied around your face.)
You can take advantage of the effect water has on CA to accelerate its bonding, by for instance breathing heavily on the two pieces of something you're gluing before bringing them together, or even by spitting on the glue, in extremis. That won't give you a very good bond, but if you're in a hurry, it'll do. You can also sprinkle bicarbonate of soda on the glue, or dribble CA onto bicarb, to get an instantly set, hard but brittle filler material. (It's basically Bondo for plastic spaceships.)
There are also liquids, known as "CA accelerators" or "kickers", that give you an almost instant full-strength bond when they touch CA. You generally put glue on one piece, a spritz of accelerator on the other, then bring them together and zap, instant gluing of two parts that you didn't quite bring together straight, god damn it.
I'm not sure how much variation there is between the different accelerators; these days I just buy whatever's cheapest on eBay. Note that CA accelerator tends to be rather volatile and thus prone to liberate itself from the spray-bottle faster than many people can use the stuff. I recommend you keep the sprayer in a Ziploc bag.
The fact that there are substances that kick CA better than water does is the base for products like the one described in this MSDS, which is for a CA formulation used for fingerprint "fuming". You can do this neat little science trick with any CA, not just special expensive law-enforcement CA:
One thing hobbyists discover pretty quickly about CA, especially if they're using accelerator as well, is that the polymerisation process is exothermic. The glue gets warm as it polymerises, the increased temperature speeds up the polymerisation, and with enough glue and enough accelerator (or just CA by itself, if it's on something with a lot of surface area - cotton is particularly bad) the result is boiling polymerising CA. I don't trust any hobbyist who hasn't emptied five whole dollars worth of discount-store superglue into a very disposable container in the back garden, then added some generous squirts of accelerator, and stood well back.
This is another CA hazard. If you spill a lot of it on your cotton-denim jeans (or somehow just manage to deliberately use an unusually large amount), the profoundly crappy time you'd reasonably expect to have in your immediate future may be made significantly crappier by some nasty burns.
Anybody who's ever used superglue will have stuck the wrong things together, though with any luck just one finger to another, not a square foot of garment to singed flesh. If possible, a good way to remove CA is mechanically, with sandpaper or a file or, for many glue-on-skin situations, a disposable razor. (Or you can just wait; as the outer layer of your skin naturally flakes off, the glue will go with it.)
CA can also be dissolved with acetone, but the MSDSes for acetone are rathermorealarming than those for CA. There are less toxic glue debonders out there too; again, please accept my very personal recommendation of whatever's cheapest on eBay and isn't just acetone.
(CA is also not just kicked into polymerisation by water, but also slightly soluble in it. So a long hot bath or shower may help you out, provided you have enough un-stuck limbs to be able to operate the taps.)
While I'm giving unrequested buying advice, as far as CA itself goes, I just buy it from discount shops. Given CA's irritating propensity to go hard in the bottle, I like the few-dollar cardboard oblongs with multiple little separately-bubble-packed tubes, the more and the smaller the better. Unless you've got an ongoing meaningful relationship with a local hobby shop - which I recommend; it's worth paying a bit extra for stuff if wise counsel on various subjects, or just hours of entertaining chat, is available in return - I see no reason to buy fancy brand-name CA for almost any job.
Getting back to that alarming cyano group which is indeed hanging off the few different, but effectively almost identical, kinds of CA molecule, it is in this case not much to worry about, but certainly is if it's hanging off something less complex, like a potassium or hydrogen atom. I find the lethality of various cyanide compounds almost amusing, since it's yet another sign of the absence of "intelligent design" of even this one planet, let alone the whole universe.
I mean, what's the element that's the basis of all life on this planet? Carbon. What makes up 78% of the planet's atmosphere? Nitrogen. (Don't miss this sample!) What do you get when the two of them get together? Cyanide, a deadly poison. It's sort of the opposite of the sodium-plus-chlorine thing.
And while I'm rabbiting on, I was also amused by MythBusters' and/or Discovery Channel's determination to call the glue they were using "super adhesive", a term that doesn't really exist in nature, to the point where a couple of slip-ups when someone said "superglue" anyway made it to air. This is in line with MythBusters' general self-censorship policy, in which no brands not integral to the myth are blurred or taped over or covered with new labels reminiscent of Repo Man.
Sometimes this policy seems to make little sense, though. In a recent special episode, MythBusters shot a .50 AE round from a Desert Eagle into watermelons, and they called the gun a Desert Eagle, even though there are various other firearms that chamber that round. But in the episode a while back where they demonstrated what a bad idea it is to wrap your hand around the cylinder of a .50 Smith & Wesson revolver when firing it, not one mention was there of the brand of that gun, though anybody familiar with the preposterous hand-cannon arms race of recent years could have mistaken a S&W Model 500 for anything else.
(If you haven't been watching the nutty progression of ever-more-wrist-smashingly-powerful handgun cartridges and the you've-gotta-be-kidding-me guns that shoot them, compared to which the action-movie-staple .50 AE Desert Eagle's .44-Magnum-ish bullet energy looks positively feeble, then you could be forgiven for thinking a short-barreled Model 500 was some kind of flare gun. I wonder if even this has been surpassed by now?)
The "super adhesive" thing is particularly nutty, though, since they could have just called it cyanoacrylate.
I was watching "Industrial Revelations" on Discovery, and I noticed a lot of Industrial Revolution factories running from one power source, a steam engine or waterwheel, with power distributed via a load of parallel overhead shafts, which brief Googling tells me are called line shafts. A belt runs from each shaft to each working machine, often with a free-turning wheel next to the one that drives the machine so the belt can be moved over onto the free wheel to "turn the machine off".
What I can't figure out is, what kept the belts on the wheels? They don't have ridges on the edges to contain the belt, they're not V- or U-profile with a matching belt shape, they're just flat metal as far as I can see, yet the belts don't fall off.
Traditional flat leather drive belts were a pretty good piece of technology. They weren't even as much of a death-trap as you might think just looking at them, since they often had enough slack that getting some piece of yourself or your clothing caught between belt and pulley wouldn't necessarily whip you into the air or smash your face into the machine. Getting your hand caught in the moving parts of the steam engine or waterwheel gearing on the other end of the lineshafting system was bad, bad news, but if only a belt had grabbed you, you had at least a fighting chance of yanking yourself free. There usually wasn't even enough pressure between belt and wheel to instantly crush your hand.
But this arrangement looks even more insultingly physically impossible than lineshaft setups. That dang belt should fall off the engine right away, shouldn't it?
Occasionally, there's a flat belt that runs on a spool-like pulley with raised flanges on the edges, like the small receiving pulley in the above picture, or this one:
On all but the biggest of the Towers-of-Hanoi stepped sections of that pulley, the belt can only fall off on one side. But where's the power for the stepped pulley coming from? Another dang flat pulley, that's where!
Free-spinning idler wheels weren't the only way of stopping a machine, either; the middle belt in this piece of lineshafting...
...has been taken off the wheel to stop it driving. That's "taken off", though, very probably not "fallen off". Left to its own devices, it'd stay where it was meant to.
The secret is that the "flat" pulleys on which the belts are running are not, actually, flat. If you look closely, at for instance the stepped pulley picture above...
...you can just about see that the pulley surface profile is slightly convex, or "crowned". The profile of the pulley is sort of like that of a wooden barrel, except less pronounced.
Wherever a flat belt is on a crowned pulley, it will tend to move towards the centre. This effect is reliable enough that some of the pulleys in a flat-belt power-transfer arrangement actually can be completely flat, as long as every belt runs over one or more crowned pulleys somewhere else.
For practical purposes, you can stop here. Slight convex profile to pulley equals flat belt staying in the middle of the pulley. Provided all other pulleys are well enough aligned, at least; if the pulleys aren't lined up very well then even if all of them are crowned, the belt may still "walk" off one of them. But basically, crowned pulleys equals centred belts.
If you want to know why crowned pulleys work as they do, things get a little more confusing. Confusing enough, actually, that the question can be presented as a puzzle, or even as a "paradox".
(Crowned pulleys are much more confusing than tax brackets, but I think less confusing than wind-powered vehicles that travel faster than the wind.)
The edge of a flat belt that is closest to the middle of a crowned pulley will be stretched a little more than the other edge of the belt, because the crowned pulley has a greater diameter in the middle. This gives the belt-edge toward the middle of the pulley higher tension and thus more traction than the other edge. So wherever the more tense, higher-traction portion of the belt wants to go, the whole belt will tend to go.
Any given point on the portion of the belt in contact with a pulley will, by definition, contact a point on the pulley. But when the pulley is crowned and the belt is not in the middle of it, the slight bend in the belt means a point on the tenser side of the belt, closer to the middle of the pulley, will be unable to stay in contact with the same point on the pulley as it rotates. The slight bend in the belt created by the crown profile points the belt away from the middle of the crown profile. All parts of the belt in contact with a pulley "want" to stay in contact with that same part of the pulley - that's sort of the whole point of friction belts on pulleys. But because the tenser edge of the belt, closer to the middle of the pulley, has more grip than the other edge, the whole belt tends to climb to the middle of the pulley.
This illustration from The Elements of Mechanism, which I found on this page explaining the aforementioned "paradox", may help you visualise this. It certainly helped me. The point on the pulley (in this case two truncated cones, not a smoothly curved crown) which is under point "a" on the belt will end up at point "b" as the pulley rotates. The belt tries to stay frictionally stuck to the same part of the pulley, so it climbs to the middle.
(A "perfect" crowned pulley with a smooth curve is a bit of a nuisance to make, so some crowned pulleys have a flat centre and curved, or even conical, ends, and some are as shown in the above picture, just two truncated cones stuck together base-to-base. These designs don't work as well - a belt will wander on the flat part in the middle of the first type, and the ridge in the middle of the second type reduces grip and wears the belt - but they work well enough for many purposes.)
The crowned-pulley effect isn't very strong unless the crown shape is very pronounced, which would make the belts wear out quickly; this is why it can't compensate for more than slight misalignment of the pulleys. Pulleys with raised edges of one kind or another - including V-profile belts and pulleys and their relatives - can tolerate much more misalignment.
(An exaggerated crown shape does make the crown effect much easier to see, though. Famous Web-woodworker Matthias Wandel has a page about the crown effect too, that includes an exaggerated pulley.)
Although the era of lineshafting has long passed in the Western world, flat belts and crowned pulleys survive as conveyor belts, and in the strangest other places - the paper-handling machinery in photocopiers, for instance!
You can also set up a model steam engine to run a whole model machine shop via tiny line shafts. Most such setups, however...
Why do nuclear power stations (and other power stations, for that matter) have cooling towers in that weird half-hourglass shape?
I presume the guys who built them knew what they were doing, but what did they know that I don't?
Ian
I pledge to eventually answer your question, Ian, but first I'm going to rabbit on interminably about power stations.
The cooling tower has become emblematic of nuclear power stations, and the white "smoke" drifting from the top of them is a source of vague nervousness for a lot of people.
But, as you say, other kinds of power stations have cooling towers too. I live less than an hour's drive from Lithgow and the Mount Piper and Wallerawang Power Stations, able to produce 3.4 gigawatts of coal-fired electricity between them; Mount Piper has two cooling towers, Wallerawang has one. The "smoke" that comes out of these towers is actually just clouds of tiny water droplets.
(Once again, if you can see it, it's not "water vapour". Clouds, and the visible "steam" squirting out of a kettle or a steam locomotive, are liquid water droplets with a ceiling temperature of 100°C at sea-level air pressure. It's possible for actual invisible-vapour steam to be swirled in with condensed droplets as it mixes more or less chaotically with the outside air, but "pure" steam is invisible, and has no ceiling temperature. Put your hand in the visible portion of the steam coming out of the side of a locomotive and you may get scalded, but putting your hand in the invisible jet close to where it's exiting may flense the flesh from your bones.)
Power stations need cooling towers, or some other heat-sink like water from a convenient river, because they are heat engines. Heat engines, as I've written before, become more and more effective as the temperature difference between their "hot end" and their "cold end" increases.
A heat-engine that makes this fact obvious is the now-quite-standardised sort of "coffee cup" Stirling engine...
...which stands on a wide circular displacer-piston cylinder and can run on the heat from a cup of coffee or tea, or backwards on a cup of ice-water. I've got one that runs like this, but really low-friction versions of the design can run on the heat from a human hand, if the ambient temperature is cool.
(You can pay quite a lot of money for a jewel-like Stirling engine {or, more interestingly, a kit to build one}, but this eBay dealer, in addition to being called "Stirlingeezer" which ought to be a reason to buy from him all by itself, sells quite beautiful engines and kits that are guaranteed to run from hand-heat. If enough people buy stuff via the above affiliate link to Stirlingeezer, I shall soon be able to afford one of his engines!)
(Oh, and if you're short of money, you can get a Stirling kit for $US30 delivered, or conceivably less if you get lucky with your bids, from this guy in China.)
Conventional power stations, whether fired by coal, combustible gas of one kind or another, or a nuclear reactor, make their electricity by turning a turbine connected to a generator. Gas-fired stations can do this directly with a gas turbine, which is essentially a jet engine tuned for shaft-turning power, rather than thrust. Coal and nuclear stations make electricity less directly, by using the heat of combustion or nuclear fission to boil water and run a steam turbine.
(I think there are also gas power stations that use steam turbines. There are definitely gas power stations that burn the gas in one turbine, and then run another, different turbine from the hot exhaust of the first one.)
Anyway, that's the hot end. A well-designed heat engine will try to get its cold end as distant in temperature from the hot end as is practically possible. The ratio between the two temperatures, expressed in Kelvin (or any other temperature scale, as long as it starts at absolute zero), determines the maximum possible efficiency of a heat engine.
Sometimes "the cold end" is synonymous with "the exhaust temperature"; that's how it works for internal-combustion piston vehicle engines, and steam engines too. A classic example of the latter is the triple-expansion compound steam engine. This has one small piston for the fresh, hot, high-pressure steam right out of the boiler. The medium-heat, medium-pressure exhaust from this first piston powers a medium-sized piston, and the low-heat, low-pressure exhaust from that piston in turn runs one or more even bigger pistons. (This can theoretically be extended to even more stages, but in practice quadruple-expansion was about as far as anyone could get before the gain in efficiency wasn't worth the extra complexity and friction.)
Steam-turbine power stations, on the other hand, may emit exhaust gases from the burning of fossil fuels, but the system that makes the actual electricity is a closed, Rankine-cycle steam/water circuit. The burning fuel or fissioning atoms heat cool water to steam, the steam turns a turbine or three, and the turbine exhaust then goes to some sort of cooling device, generally a heat exchanger, that dumps the final unusable portion of the water's heat somewhere.
This "somewhere" can be a separate water supply, either a river, large lake or sea, or it can be evaporating water in a cooling tower. Once the heat exchanger has cooled the closed system's water in whichever way, that water is pumped into the boiler again, and the cycle continues.
You might wonder why you need to dump heat from the turbine exhaust, when you're only going to heat the water up again in the boiler. There are two practical reasons for this.
The first reason is that the exhaust from a power turbine is almost all still water vapour, because, in brief, turbines made to run on a flow of hot gas do not like it if the gas condenses to liquid inside them.
The second reason is that the pump that returns the water to the boiler has the opposite preference; it only works with liquid water. It would be possible to use a gas pump instead and make a system in which the working fluid is always vapour, but the energy needed to run a gas pump against pressure from the boiler is high, while the energy needed to run a water pump is trivial (by power-station standards), on account of the incompressibility of the water.
The upshot of all this is that standard 20th-century power stations are pretty miserably inefficient. Today, there's much more effort being made to reduce the heat wasted, by for instance transferring some of the heat of the turbine exhaust to the water feed between the pump and the boiler, or by using some of the waste heat to keep nearby buildings warm ("cogeneration"). These sorts of measures can only go so far, though, so cooling towers of one shape or another will continue to be built.
Which, finally, brings us back to the classic cooling-tower shape.
Cooling towers actually come in all shapes and sizes; large air conditioners, for instance, often have evaporative coolers for their chillers, but those coolers don't look anything like a power-station cooling tower.
Power-station coolers have to have very large capacity, so they inescapably have to be very large. Power-station coolers also have to provide a decent convective "stack effect", also known as "draught" (or "draft", in the less-demented American spelling). But, importantly, power-station coolers don't really need to be able to hold up much more than their own weight, plus any remotely plausible wind loads or shifts of their foundations.
The classic curvy cooling-tower shape fits all of these requirements. In engineering terms, because cooling towers don't need to hold up an interior full of offices, they can be built as a "thin-shell structure". You could build a cooling tower out of giant Great-Pyramid stone blocks if you wanted to, but a surprisingly thin reinforced-concrete shell, built in layers from bottom to top (not unlike the way 3D printers work), is the usual solution. And the builders almost never balls it up.
Objects of this shape are called "hyperboloid structures"; they're strong for their weight and so have been used for all sorts of masts and towers and, sometimes, ordinary buildings too, and they're particularly suited for use as cooling towers. The large area at the bottom of the hyperboloid gives lots of room for evaporation, the "waist" accelerates the gas mixture (I think because of the venturi effect), and then the widening opening at the top encourages turbulent mixing with the ambient air. (Air gets into the tower in the first place via an open latticework section around the base.)
The final question that occurs to me in this area is why cooling towers are hyperboloids, but factory chimneys are cylindrical (or close to it - they often taper a bit toward the top).
This is because the cooling tower wants to move a vast amount of low-pressure air. The evaporating warm water at the bottom of the tower produces a steam/air/water mixture that isn't much warmer, and thus less dense, than the ambient air, so it has little buoyancy compared with the ambient air, won't move terribly fast, and so has to pass through a really wide pipe. Factory chimneys, on the other hand, are moving a much smaller volume of much warmer gas, usually combustion-product "flue gas". This is usually quite a lot hotter than ambient, so it rather wants to go up a chimney and doesn't need a wide one; you just need a nice long chimney, both to get a strong stack effect and to discharge the gas as high up as possible, to spread the pollution by dilution, as it were.
(Incidentally, The Secret Life of Machines addresses the stack effect in episode five, on central heating. And while I'm on the subject, the extraordinary documentary Fred Dibnah, Steeplejack features the titular working-class hero climbing hundreds of feet up a brick chimney and then perching on scaffolding that looks as if it were assembled by blind drunkards and knocking the chimney down by bashing bricks, one by one, into the flue. It has to be seen to be believed.)
Something has been bugging me for a while, but I didn't want to ask anyone in case it sounded racist, which it isn't, because some of my best friends are members of the inferior races which Asians like me will soon enslave.
Internet anonymity lets me ask YOU, though:
Why do old black people so often not look as old as identically old white people?
I'm asking now because I've seen a few excellent examples in just the last few days.
I was watching Joe Morton in Eureka, and he looks EXACTLY THE SAME as he did in Terminator 2, 20 years earlier.
And look at this guy! He just died at age 75, but in the picture of him performing a year ago he could be 50, or 40 even.
And then I watched a recent Daily Show where George Clinton did a walk-on, he's 70 but looked 50, tops.
(OK, there was some latex work there too. But you can't see that clearly even in HD, and he still looks old.)
I know black people don't actually LIVE any longer, quite the opposite here in the States, but looking young your whole life has to be some consolation. How/why does it happen?!
Z
I have heard this phenomenon described as "black don't crack", but I, like you, don't know whether it's safe for non-black people to call it that in company.
In the case of people in TV and movies this phenomenon is, of course, at least partially the result of makeup, lighting and plastic surgery. But you're right when you say that it happens in "real life" too.
The reason is actually quite simple.
When you get older, your skin loses elasticity and you get more wrinkly. The principal factor in the visibility of wrinkles is light, or more precisely shadow. Wrinkle-hills cast shadows in wrinkle-valleys, and those shadows play a big part in making a face look old.
If you've got pale skin, wrinkle-shadows show up very clearly. But the darker your skin is, the closer to the shadow shade it all is naturally, and the less obvious are the wrinkle-shadows, and the less old you look. That's really all there is to it.
Rub your face with lampblack and, no matter what colour your skin was before, it'll now be so dark that wrinkle-shadows will be almost invisible. Do the same thing with titanium dioxide powder and every tiny line will stand out clearly, unless you're only illuminated by a light right next to the viewer.
(This is why the built-in flash of a compact camera tends to make everybody's face look flat and weird - but not wrinkly! A photographer may use a "beauty dish" to add a controlled amount of this effect to a portrait.)
This same phenomenon can be seen in some peculiar places. Take the moon, for instance.
A full moon is much more than twice as bright as a half moon, because of what's called the "opposition effect". The effect is partly caused by the retroreflective qualities of lunar regolith - it tends to reflect light back the way it came. There may be some quantum weirdness involved too. But the opposition effect occurs mainly because the lunar surface is very uneven, thanks to meteorite impacts and no erosive forces. So there are lots and lots of shadows when the moon is illuminated from the side from our point of view, making it half-full, but there are almost no shadows at all when it's full, and illuminated by the sun looking over the earth's shoulder, as it were.
(The albedo of the moon is surprisingly low - it's about as dark as an asphalt road. It seems so brilliant in the night sky because it's illuminated by direct sunlight, not because it's actually the pale grey it seems to be when compared with the surrounding dark sky.)
The "black don't crack" phenomenon is one small part of numerous more-or-less-racist theories that explain one or another kind of physical advantage that dark people are supposed to have over pale people.
One of the more popular of these theories is that black slaves were literally bred to be stronger and healthier, since there wasn't much of a market for longsighted asthmatic cotton-pickers. Whether the claim is that this breeding was forced by slave-owners, or was just a result of brutal natural selection that caused weak slaves to often die before reproducing, though, it's pretty clear from genetics and genealogy that it actually didn't happen.
There is evidence for something like this in some situations. It's hardly surprising, for instance, that a number of successful very-long-distance runners have come from cultures where, for centuries or even millennia, being good at cursorial or persistence hunting has been a way to get more wives and offspring.
Even in these situations, though, there are many confounding factors. Running is something almost anybody can do, almost anywhere. It requires no expensive equipment or special facilities. So poor countries, regardless of culture, produce more runners than they do, say, golf or polo players. (And every now and then along comes a little white guy who's accustomed to spending days on end rounding up sheep, on foot.) For the same reason, you don't see many bobsled teams from countries where it doesn't snow.
(While I'm digressing, here's a note even less relevant to the original question: Because I'm in Australia, thedailyshow.com doesn't want to show me that George Clinton video. I just get a "Sorry, this video is unavailable from your location" error. If you have the same problem, you can solve it with the Modify Headers Firefox extension, which lets your browser say it's asking for the page on behalf of a US IP address. Find instructions on how to do this here.)
I recently saw a news article that linked to this government page: http://ga.water.usgs.gov/edu/earthhowmuch.html
...which says if all Earth's water (liquid, ice, freshwater, saline) was put into a sphere it would be about 860 miles in diameter.
Now I understand an 860-mile sphere is massive, so even though that sounded small I could accept it, until they state the estimated volume of water on earth at 332.5 million cubic miles.
So how do you cram 332,500,000 cubic miles into a 860 mile sphere?
Matthew
Quite easily, actually!
The volume of a sphere is four-thirds pi times the radius squared cubed [Sorry I left that error there for so long, commenters!]. So if the radius is 1 unit, the volume is 4.19 cubic units.
The radius of an 860-mile sphere is 430 miles. 430 cubed is 79,507,000. Four-thirds pi is about 4.1888. Multiply that by 79,507,000 and you get about 333,038,143, a number less than 0.2% larger than 332,500,000. The difference is accounted for by variations in precision in working out the number, since this is really only a ballpark figure and taking it to nine significant digits is silly.
To "sanity check" this if, like me, you always feel mildly nervous about the order of operations for a calculation like 4/3Πr^3, consider the volume of a cube 860 miles on a side.
The volume of a cube is of course just its edge-length cubed, and an edge length of 860 miles gives a volume of 636,056,000, a nice sane-sounding 1.91 times the volume of the sphere that'd neatly fit in that cube.
My own second-favourite way-to-visualise-the-quantity-of-something is that all the gold in the world (not including gold we have yet to dig up or somehow extract from seawater) would make a cube only 20 to 22 metres on a side, depending on who you ask. To help visualise the size of the cube, 21-ish-metres is about the length of two city buses parked nose to tail.
Because gold weighs 19.3 grams per cubic centimetre, though (11.16 ounces, or 10.16 troy ounces, per cubic inch), a 21-metre-on-a-side cube of gold would weigh 178,737 tonnes. So I suppose you wouldn't have to worry too much about someone stealing it.
(Unless you are very wealthy, you probably can't buy a large enough lump of gold - especially at today's outrageousprices - to really appreciate its density. At current prices, one kilogram of gold would cost you more than $US51,000. Tungsten, however, is 99.7% as dense as gold - I'm sure counterfeiters have gilded tungsten for profit many times - and it's much more affordable, though still expensive. The good people of RGB Research {here on eBay US, here on eBay UK, here on eBay Australia} have their one-kilo tungsten cylinders on sale again for a mere $US220 plus rather pricey delivery. If you can afford one, and have the slightest interest in science toys, I urge you to buy one; my own tungsten cylinder is one of my most treasured possessions. And one of the most durable, too; if the house burns down the tungsten cylinder, like my Bathsheba GrossmanMetatrino, will be sitting intact in the ashes.)
My most-favourite way-to-visualise-the-quantity-of-something is that if you breathe on an ordinary marble, the thickness of the layer of condensation from your breath on the marble is approximately to scale with the thickness of the atmosphere on the earth.
(And another one, that doesn't really make anything much easier to understand but is prime stoned-party-talk, is that a human is about as much bigger than an atom as a galaxy is bigger than a human.)
How do the little rectangular anti-theft tags work?
I get how the big anti-theft stickers work. They've got an obvious square spiral antenna that I presume collects enough microwatts from an incoming signal to run a little transmitter that sends another signal out.
But the little tags don't have any circuitry inside. I cut one open, and there are just some tabs of springy metal in there - two pieces next to each other, and a smaller piece separated from the other two by a clear plastic membrane.
The metal sticks to a magnet, but that's the end of my ability to figure out what it does.
Is there invisible nanotechnology in these things, or something? Hey, maybe they're a placebo!
Kim
If they're a placebo, the alarm systems in shops seem to really believe that it works.
What you're looking at there (here's a more elegant cutaway picture on Wikipedia) is called a magneto-acoustic, or acousto-magnetic, tag. Which is one of those things that doesn't really sound as if it ought to work, but does.
The first two of the three tabs inside are, I think, a couple of pieces of amorphous metal - which is quite an exotic material to be stuck to commonplace consumer items just to stop people stealing them. Amorphous metal is, in a way, the opposite of nanotechnology; it's metallic glass, special because it lacks the microscopic crystal structure of normal metals.
The third tab is a piece of less exotic, medium-coercivity metal. When that third piece is magnetised, the two other strips, which are sitting loose in their little plastic coffin, become quite easily moved by external magnetic fields. (They're amorphous metal because that's already unusually easy for external fields to move.)
The security gateways as you leave the store emit a pulsed magnetic field up in the tens of kilohertz, at the resonant frequency of the amorphous-metal strips. When next to their mildly-magnetised buddy, this quite tiny field causes the amorphous-metal tags to buzz, and to continue to buzz for a very brief moment after each pulse of the external field. This very brief "ringing" period causes a tiny change in the magnetic field of the third strip, which an antenna in the security gateway, very implausibly, detects. And off go the sirens.
The thingy at the checkout that deactivates the tags is a degaussing coil. It more-or-less demagnetises the third strip, which both reduces the magnetic sensitivity of the other two strips, and removes the field which the other two strips modulate. So now the sirens don't go off.
I am entirely unable to think about any security system without immediately trying to figure out ways to defeat it. (I try to avoid airports nowadays. They make me feel like Jackie Chan in a deckchair factory.)
One obvious but impractical way to defeat magneto-acoustic tags would be to degauss them yourself; I don't know how strong the degausser needs to be to achieve this, though. You might be able to pinch stuff if you just smuggled a CRT-screen degaussing wand into the shop, and found somewhere to plug it in.
Swiping your own rare-earthmagnet across the tag would, if anything, probably make it work better (by more strongly magnetising the third strip), but I wonder if leaving a magnet or three stuck to the tag, in a Halbach array if you're fancy, might silence it. Just chopping it bodily off with a potato peeler would probably do the job too, of course, but where's the fun in that?
(If you can magnetise tags yourself with a ten-cent eBay magnet, then you could pry them off things you've bought, reactivate them, and attach them inconspicuously to things which other people may innocently carry into shops. You could, is all I'm saying.)
The square-antenna type of tag, by the way, is also pretty simple. It doesn't actually have anything fairly describable as a transmitter in it, but is rather a tuned circuit that resonates somewhere in the low megahertz. This makes it detectable, if a nearby transmitter/receiver combo rapidly sweeps its output through the relevant frequency range and looks to see if something is managing to suck up some energy at the appropriate frequency.
This kind of tag is deactivated by, essentially, blowing out the capacitor essential to their resonance with a higher-powered signal. I think a shoplifter could probably defeat these tags by just dragging a knife across them a couple of times, though, breaking the circuit. I haven't actually tried this, though, because it'd mean missing out on all of the fun of a good old-fashioned armed robbery.
Perhaps someone who's worked in retail since fancy security tags came into vogue will enlighten us in the comments.
I would also like to hear from anybody who's successfully used the "just lob the item high over the security gate and into the hands of your partner in crime" technique.
