Wikipedia:Reference desk/Archives/Science/2007 December 10

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

December 10

Fan curve calculation

I'm looking for a fan curve (pressure-volume) for a particular fan - I've been told it's an "RB21 AVON" - and failing that, I've been told to find a "fan formula" so that it be calculated. Is there such a formula (with diameter, air density, and some other coefficients), or are these determined empirically? 150.101.23.102 (talk) 00:26, 10 December 2007 (UTC)

I've only ever seen this done empirically, with a device that measures volume/area/time of air coming out of the fan. Certainly far easier than solving some convoluted aerodynamics equation. Someguy1221 (talk) 00:39, 10 December 2007 (UTC)

Well, I've possibly found a more complicated formula - see here (measuring method, no anchor, about 1/3 down) - but that still looks like more of a "how to use a test rig to plot the curve" than a full CFD with fan parameters. 150.101.23.102 (talk) 01:02, 10 December 2007 (UTC)

Actually, assuming that I did have to perform a CFD analysis on it, what software would I be looking for? 150.101.23.102 (talk) 01:33, 10 December 2007 (UTC)

LED blinking with 555

How many LED's can I control with a 555 timer? It has a nominal output current of 200 mA, if a LED needs 20 mA — I believe this is a realistic value, correct me if it is not — I'll be able to supply 10 LED's. Am I right? Thanks in advance for your help. 217.129.241.169 (talk) 01:47, 10 December 2007 (UTC)

In theory, yes, but look at your datasheet for more accurate information. But I don't think it has any current limiting mechanism built-in so you will still need to have a resistor in series. --antilived^{T | C | G} 04:51, 10 December 2007 (UTC)

Yes, I'll obviously need to insert resistor in series with the LED's, my doubt was just related with the driving capacity of the 555. Meanwhile I saw a project similar to mine and it used a 555 to drive 20 LED's. Thanks! 217.129.241.169 (talk) 13:31, 10 December 2007 (UTC)

LEDs can be operated in series. I don't recall what the breakdown voltage is for the output transistor of the '555, but I'll assume that for the average '55, it's at least 12 volts (and probably 15). In that situation, you could easily operate 7 or 8 red LEDs in each series string connected to the '555's output and you've demonstrated that about ten series strings can be operated. If you need to go beyond that, use the output from the '555 to drive some garden-variety power transistor.

Atlant (talk) 18:06, 10 December 2007 (UTC)

Why do you need a resistor? Wont all the LEDs in series constitute enough resistance? I figure putting a resistor in there just reduces the number of LEDs you can light by wasting energy. --Seans Potato Business 21:36, 10 December 2007 (UTC)

The simple answer is, because LED's aren't resistors, so they don't have a "resistance"!

Think about an ideal diode: it passes current freely in one direction, and not at all in the other direction. So in one direction it acts (almost) like a 0Ω resistor, and in the other, ∞Ω.

Of course, you run a light-emitting diode in a forward-biased condition. But without a current-limiting resistor, it would draw (theoretically) your supply voltage ÷ 0Ω = an infinite amount of current. Not good.

(In fact, a forward-biased diode doesn't act exactly like a 0Ω resistor; there's a relatively constant voltage drop across it -- 0.6V for silicon-based diodes -- regardless of the current passing through it. Take that, Georg!) —Steve Summit (talk) 02:17, 11 December 2007 (UTC)

Erm, actually for a silicon diode under forward bias, its resistance is fairly accurately given in ohms by the expression 25/Ie where Ie is in milliamps. In a similar way, LEDs can also be said to have a non zero resistance altho Im not sure if the above formula applies to LEDs (but I think it may)--TreeSmiler (talk) 03:15, 11 December 2007 (UTC)

If there's a voltage drop with a resistor present, then there must be resistance from the LED - the energy it takes is what it uses to light up and that wouldn't happen unless it was forcing the electrons to do some work. Thus, you should be able to put no resistor and say, I don't know, 1000 LEDs and they will provide sufficient resistance to prevent too much current from flowing. If you insist on putting a resistor in the circuit, then surely you're just going to waste energy and voltage. The voltage drop over something is proportional to the resistance it provides so I don't see how you get a 0.6V drop over an LED without significant resistance. --Seans Potato Business 12:40, 11 December 2007 (UTC)

LEDs are commonly operated without any series resistance. Open up any of those cheap "LED flashlights" and you'll find that the circuit is nothing more than one or more LEDs paralleled across one or more batteries. By carefully choosing the battery size and the LED, you end up with enough series resistance to not blow up the LED. This isn't the way it's done in more "professional" circuits, of course. There, in low-drain appliactions, you'll find a resistor or an "RLED" (with a built-in resistor chip). In high-drain applications (like car tail lights), you'll find a specific IC dedicated to driving the LEDs.

Atlant (talk) 13:24, 11 December 2007 (UTC)

That's not true. Every LED needs a current to limit its current. A LED is, like a diode, formed by a PN junction. The current that flows through a PN junction is exponentially related with the voltage at its terminals, that means that applying a 9V voltage to a LED will make a huge current flow through it, burning it after some seconds. 193.136.173.43 (talk) 16:42, 11 December 2007 (UTC)

I assure you it is true and you can convince yourself of its truth by taking apart just about any cheap "LED flashlight". Your theory ignores the fact that there are lots of parasitic resistances strewn around the circuit: the LED has real resistance, the battery has a non-zero internal resistance, the wiring has resistance,and the switch has resistance. All of those resistances appear in series with the rather-small voltage that is left when you subtract the LED's "ideal diode" voltage drop from the battery's "open circuit" voltage. I also wondered about when I first saw it, but I've seen enough examples now to know that it's dead-standard practice in cheap LED devices.

Atlant (talk) 17:51, 11 December 2007 (UTC)

It doesn't make much sense to say everything "needs a current to limit its current." 193.136.173.43 may have meant every LED needs a resistor to limit its current. It is true that with the proper combination of forward biased voltage drop in the LED and proper battery voltage, the circuit can be operated without a dropping resistor (and with higher efficiency due to that absence of power wasted heating up the resistor rather than producing light). If you had two LEDs with a 0.75 volt forward voltage drop across a 1.5 volt battery, there would be no point in adding a sereies resistor. If you used a 9 volt battery, you would need to drop the voltage to 1.5 volts by means of a resistor which dropped the voltage by 7.5 volts at the current consumed by the LEDs, selected by application of Ohm's Law. Or you could use 12 of the same LEDs in series across the 9 volt battery with no resistor and get more light with the same battery life. A refinement in the cuircuit design would be to make sure the circuit operates not only ith a new battery, but down to some specified "dead battery" voltage. Edison (talk) 17:11, 11 December 2007 (UTC)

(Just FYI: No visible-light LED drops 0.75 volts; they all drop about the same voltage as the energy (in electron volts) of the photons that they emit, so a red LED drops about 1.6 volts, green 2.3V, blue 3.0+V, and so on. See Light-emitting diode#Considerations in use.)

Atlant (talk) 17:51, 11 December 2007 (UTC)

Then make it "2 volts" for the LED forward voltage drop at normal operating current place 6 of them across a 12 volt battery, or 1.5 volts and place one across a 1.5 volt battery. Still no resistor required. Edison (talk) 18:49, 12 December 2007 (UTC)

Also keep in mind that if you light the LEDs briefly in sequence, so they're flashing so rapidly you can't see it, you can probably light quite a few more LEDs with your limited power before the flicker becomes noticeable. Have fun! -- HiEv 23:01, 11 December 2007 (UTC)

So ,Wikibrains, if there were 5 red leds connected in series each dropping a voltage of 1.9V @ 10 mA, and you connected this string across a 12V battery with zero internal resistance, would the leds (or one of them) be destroyed? —Preceding unsigned comment added by TreeSmiler (talk • contribs) 01:50, 12 December 2007 (UTC)

What sort of a battery has zero internal resistance? What sort of an LED has zero internal resistance? What sort of wire? Ultimately, Ohm's law still rules, although it is certainly possible to drive LEDs beyond their I_f maximum rating.

Atlant (talk) 13:35, 12 December 2007 (UTC)

A regulated power supply with the terminals delivering 12 volts would be the equivalent of a 12 volt battery with zero internal resistance, up the the point where its current limit was reached. Edison (talk) 18:29, 12 December 2007 (UTC)

And that, of course, isn't a battery but is closer to an ideal voltage source.

Atlant (talk) 01:12, 13 December 2007 (UTC)

If you have a 12V battery that is only allowed to provide 10 mA of current, then you need 1k2 ohms of resistance or 6 190 ohm LEDs. If LEDs have a lower resistance than this, then I don't see how you could achieve 1.9 V with 10 mA. --Seans Potato Business 14:51, 12 December 2007 (UTC)

A forward biased diode is not well described as having a certain ohmic resistance, but rather by having a certain voltage drop. Even that will increase slightly as the current increases. It is an active device, not a resistor. Edison (talk) 18:46, 12 December 2007 (UTC)

No no. A semiconductor diode is a semiconductor diode. It obeys the diode equation. The incremental resistance at any point is defined as the ratio of the voltage across it to the current through it (Ohm's law)--TreeSmiler (talk) 02:04, 13 December 2007 (UTC)

The real reason for driving an LED from a voltage source through a resistor, rather than directly from a hypothetical voltage source of the exact same voltage as the LED's forward voltage drop, is that the resistor-plus-voltage-source approximates a constant current source. LEDs like to be driven with a constant current. If you tried to drive a LED from a low-impedance voltage source, the output of the LED would be insanely sensitive to the source voltage. Every tiny fluctuation in the source voltage, and every tiny change in the LED's temperature, would cause dramatic changes in the LED's brightness. A small rise in voltage would be enough to destroy the LED. With a series resistor acting as a current source, small changes in the LED's temperature and the source voltage do not noticeably affect the LED's output because they do not affect the current much. Yes, this wastes a bit of power, but not as much as you waste when your LED melts. --Heron (talk) 21:27, 12 December 2007 (UTC)

Surely you waste zero power when your led melts. If it fails o/c there is no current; if it fails s/c there is no voltage. In either case, no power is wasted.

However I think you have hit the nail on the head with the above explanation of why you need resistors in series with leds: to stabilize the current through them. But its ok for a string of leds to be drive by a current source, I think everyone would agree. In this case, power is necessarily dissipated in the current source.--TreeSmiler (talk) 01:58, 13 December 2007 (UTC)

Err if you fail s/c in the real world you draw maximum current and maximum power (not no power). --Dacium (talk) 02:55, 13 December 2007 (UTC)

Sorry but power is volts times current. So a s/c device can dissipate no power however much current flows.--TreeSmiler (talk) 02:59, 13 December 2007 (UTC)

Euler's astronomy

Hello, I was wondering if anyone would be able to point me toward some information on Euler's contributions to Astronomy? The section of the article concerning it doesn't say much, and Google results have been unfortunately sparse. I'm particularly interested in his work on comets, and the moon - anything I've been able to find hasn't offered more than a few sentences. Thank you, 81.102.34.92 (talk) 13:54, 10 December 2007 (UTC)

Corn, an artifact?

I read somewhere that "an ear of corn is an artifact: corn can survive only if man removes the kernels from the cob and plants them." If it is an artifact, how has it come into being in the first place? Thanks. --217.11.17.252 (talk) 14:19, 10 December 2007 (UTC)

I think you find that animals can remove the kernels too.They cache them, forget a few caches & voila, the corn sprouts. BUt domestic corn, I think is a tetraploid of the native maize so the germination rates tend to be lower. —Preceding unsigned comment added by 24.226.90.6 (talk) 15:09, 10 December 2007 (UTC)

I assume you read it here? I honestly wouldn't take that as a source of real philosophical insight (there is nothing so dull and misleading as a dedicated Marxists' view of the world), but the guy's point is that domesticated corn has trouble growing without human intervention. Whatever wild species corn came from (there are many theories, see maize) obviously had no trouble reproducing itself but was quite different from what we eat today. The point of the article there is that corn has been domesticated for over 500 years, and that the domesticators were native American peoples. --24.147.86.187 (talk) 15:18, 10 December 2007 (UTC)

• Basically all domesticated crops and animals are "artifacts" in the sense that they have been altered far beyond their pre-human form, and would either go extinct or be greatly endangered without human intervention. But there's nothing wrong with that (well, at least if you ignore developments in the last few years like genetic modification, which is still controversial). Similar interactions between plants and insects led to the rise of flowers and edible fruits. It's just that we humans have taken the process to an extreme by allowing crops and animals to focus very little energy on defending themselves and more of their energy on being tasty. --M@rēino 20:34, 10 December 2007 (UTC)

You might want to consider though that classical plant breeding is a lot more complicated then people think. Over the last decades in particular, it doesn't just involve people selecting the best traits that arise naturally. Mutants are induced by radiation [1] and chemical means for example. Unlike with genetic modification, no testing is required of the resulting product. Also I disagree with your view that there's nothing wrong with that in the sense that there's no reason to presume that even tradiational crop breeding methods are somehow 'super safe'. There is easily the chance that we will select traits which are harmful to humans or to other organisms (beyond as desired for pest control). If you compare any one of our crops with its ancenstor you can see we have made very major changes. Consider for example the extremely bizzare forms of Brassica oleracea we have like cauliflower, broccoli, cabbage, brussel sprouts and how 'fucked up' these plants are. Bringing it back to the original question take a look at [2] (see section "Corn or Maize") to get an idea how much we have changed our current crop plants. My point is not that I think any of this is wrong but simply that we have and continue to very severely change the crop plants we eat. Genetic modification is significantly faster (so there is a somewhat higher chance we may do something bad which we will notice but it may come a bit late) and does carry some additional risk but a lot of the risks are overplayed (for example when people talk about minor chances of increasing cancer when there is no reason to presume that we haven't already or won't do that via other means but just don't know because the only way you detect such things is by carefully controlled scientific tests). The ironic thing is that given the opposition to genetic modification, many breeders are using GM as a 'proof of concept' and then using tradiational means (i.e. radiation, chemical mutagens etc) to get what they want even though it means that a lot more has changed in the process (most of which the breeder won't know the effect of). This also means the cost is in fact a lot higher and more out of reach of poor farmers. Note that the issue of the introduction roundup resistantance or protection of crop varieties (which existed long before GM) or things such as terminator genes are seperate issues from whether GM itself is somehow a potentially harmful or dangerous technique. Nil Einne (talk) 12:03, 11 December 2007 (UTC)

Thanks for all comments. Omidinist (talk) 08:05, 11 December 2007 (UTC)

medical abbreviation

VDRL (veneral deseas research laboratory) in context with RCPF . What means the abbreviation RCPF? —Preceding unsigned comment added by 217.186.79.16 (talk) 14:31, 10 December 2007 (UTC)

In the context of laboratory testing it may be Rate-Controlled Programmed Freezing. This is to do with the procedure of freezing specimens in a non-damaging way. Richard Avery (talk) 16:01, 10 December 2007 (UTC)

The VDRL and Reiter protein complement-fixation are both tests for syphilis. 152.16.16.75 (talk) 10:34, 11 December 2007 (UTC)

Problems with Blazars

On the article, blazar, it says the accretion disk is quite small. Only 10^-3 parsecs in size. While the article on Q0906+6930 states that the event horizon is on the order of 1000 times that of our solar system, making the accretion disk many times larger than that. Possible error here? 64.236.121.129 (talk) 14:44, 10 December 2007 (UTC)

I agree that the Blazar article is confusing in this respect. I think the answer is that in a blazar, the observed emission does not come from the accretion disc (which would give a broad emission line spectrum, as in a quasar), but instead comes from the relativistic jet. The "tip" of the jet, as you can see in the illustrations, is much smaller than the accretion disc, and could be smaller than the SMBH event horizon. -- Coneslayer (talk) 15:13, 10 December 2007 (UTC)

Q0906+6930 says event horizon volume not size, so presumably they mean a radius 10 times that of the solar system. If one canonically defines the solar system as 40 AU (orbit of Pluto), then that would be 400 AU or 2*10^-3 parsec. That number isn't so different from what blazar says. I think the problem is the fuzzy way in which the terms are being defined, and not that they are actually radically different sizes. Dragons flight (talk) 15:21, 10 December 2007 (UTC)

The problem is, we are talking about the accretion disk, which should be much much larger than the event horizon. Sure enough, most scientists agree, if you look at a black hole, you probably won't even see the event horizon because it will be dwarfed by the much larger accretion disk. So while the event horizon should be 2*10^-3 parsec (if your calcuations are correct), then the accretion disk should be much larger than that. 64.236.121.129 (talk) 15:50, 10 December 2007 (UTC)

Well supermassive black hole suggest 10^5 to 10^10 solar masses. 400 AU is equivalent to 2*10^10 solar masses, i.e. at the extreme upper limit. However, if supermassive black holes really vary over 5 orders of magnitude, then presumably there must be a large range of variation in accretion disks too. Dragons flight (talk) 15:58, 10 December 2007 (UTC)

There is a large variance in sizes. Maybe the article should be changed to relect that? It's a little misleading as it is. I should point out that Q0906+6930 is larger than your upper limit, at 16 billion solar masses. 64.236.121.129 (talk) 16:33, 10 December 2007 (UTC)

Solar Cell Production Cost

Related to a previous question... I'm trying to collect as many peer-reviewed studies on the cost, resource impact, and environmental impact of producing solar cells. This is not the consumer cost as the previous question apparently asked. This is the production cost. Obviously, solar cells don't fall out of gumdrop trees in the butterfly forest. That is why I want to get a well-rounded view of the production cost. -- kainaw™ 19:07, 10 December 2007 (UTC)

I've seen $8 per watt quoted for home/roof installations...with 30 cents per kilowatt hour as the 'lifetime' cost (which is two or three times more than electricity bought from the grid). For large scale power-station stuff $3.10 per watt for currently-available cells - perhaps half that when some of the newer technologies come on-line. "First Solar" claim[3] to have gotten the price down to $1.19 per watt. But even at $1 per watt, it's still borderline whether it's worthwhile - depending on the number of cloudless days you get per year, how close to the equator you are and how long your panels last (which is always an unknown with the latest and greatest solar panels until they've been in service for a few years).

I'm not 100% sure if this is still true - but at one stage, there was a premium on square solar cells because where cost is no object (eg in a spacecraft) one can pack them tightly in a grid. Since silicon wafers are circular, this meant that there were a bunch of curved sections of the panel that were cropped off the edges. These were vastly cheaper than the square variety - but you couldn't get so much energy efficiency because they couldn't be packed together very closely. If this is still the case, then you may need to be more specific about the application. SteveBaker (talk) 20:24, 10 December 2007 (UTC)

• The US Dept of Energy has some publications that may be of interest to you. --M@rēino 20:38, 10 December 2007 (UTC)

Why not use hexagonal solar cells? They'd pack just as well, but wouldn't require cutting as much off. By the way, I remember hearing that solar cells only give off a little more energy over their life than is required to produce them, is that correct? — Daniel 01:00, 11 December 2007 (UTC)

Well, taking the numbers I got for domestic rooftop panels (cost $8 per watt to buy, produce electricity averaging 30c per kwatt.hour) - means that they have to be good for 24,000 hours - It's hard to imagine it would take 24 kwatt.hours to make a single 1 watt solar panel...that's a lot of energy. I don't know why they don't make hexagonal panels - but there is no wastage with the round ones - they just have to sell the offcuts for less than the square bits in the middle. SteveBaker (talk) 01:42, 11 December 2007 (UTC)

I'm sorry that I wasn't clear enough that I'm looking for production cost, not installation or operation. I have found that current production creates 1.5 tons of carbon dioxide per ton of silicon processed into half a ton of functional solar cells (assuming the silicon is nearly 100% pure - purification of silicon has cost/waste also). The monetary cost is similar to computer chips since the "wafer" process is very similar. I've found valid research into reducing the cost and environmental impact of solar cell production - which is what I was looking for. -- kainaw™ 14:13, 11 December 2007 (UTC)

It's utterly ridiculous to claim that the CO2 produced in the manufacture of a solar panel is anything other than utterly negligable compared to the amount it saves us over it's lifetime. Let's crunch the numbers to show you how utterly insane that idea is! One ton of lignite coal (which is about 75% carbon) will burn to produce about a ton of CO2. In the process it produces 28 MJ/kg of energy - so if coal-fired power stations were 100% efficient (which they most certainly aren't) then in round figures, it would produce about 7 MWhrs per ton of CO2 emitted. So the question is will 0.33 of a ton of solar panels (which produced 1 ton of CO2 during manufacture) be able to produce 7MWhrs of electricity during their lifetime? Well, the calculation I did above suggests that a domestic solar panel (which is a lot less efficient than a commercial setup) can produce power for an average of 24,000 hours before it craps out on you. So one ton of solar panels only has to be able to produce 300 Watts per hour to save more CO2 than it produced during manufacture. Since one $600 rooftop panel (with probably less than a kilo of cell inside) produces about 150W - then so long as it didn't require more than half a ton of silicon wafers - it's a net CO2 win. I strongly suspect that a 150W panel contains about a kilo of silicon wafers - so solar panels produce about 1/500th the amount of CO2 compared to a coal fired power station...and that's assuming that you didn't produce any CO2 to build the power station (which you did) and that it's 100% efficient (which it isn't) and that digging a ton of coal out of the ground and getting it to the power station didn't create any CO2 (which it did). I also picked a rooftop domestic solar panel for my calculations - and for sure commercial setups are more efficient. No - the claim that solar panels produce more CO2 during manufacture than they save is a statement that is utterly without merit. SteveBaker (talk) 14:54, 11 December 2007 (UTC)

Photovoltaic#Greenhouse_gases says: Life cycle greenhouse gas emissions [...for photovoltaics...] are now in the range of 25-32 g/kWh and this could decrease to 15 g/kWh in the future.[65] For comparison, a combined cycle gas-fired power plant emits some 400 g/kWh and a coal-fired power plant 915 g/kWh and with carbon capture and storage some 200 g/kWh. Nuclear power emits 25 g/kWh on average; only wind power is better with a mere 11 g/kWh....QED. SteveBaker (talk) 15:00, 11 December 2007 (UTC)

Put a different way, it takes 1-4 years for a solar panel to recoup the energy used in its manufacture. Whatever it can produce during the years of operation beyond that might then be considered carbon free. Dragons flight (talk) 15:06, 11 December 2007 (UTC)

Where did you get the statement: "solar panels produce more CO2 during manufacture than they save"? I didn't write that. I didn't imply that. I didn't reference anyone else who stated that. Where did this rant come from? -- kainaw™ 15:14, 11 December 2007 (UTC)

That was in User:User:DanielLC's comment above in response to your question. Rmhermen (talk) 16:04, 11 December 2007 (UTC)

Thanks - I didn't relate the response to the intended statement. -- kainaw™ 17:04, 11 December 2007 (UTC)

Sorry - yes, Wikipedia indentation style makes it tough to target specific responses yet stay in the right order of the flow of the conversation. I was using the "1.5 tons of CO2 per ton of solar panels" number that you provided to respond to Daniel's claim - so I couldn't put my reply immediately after his without making it confusing as to where that number had come from. Sorry if it sounded like I was accusing you of saying something daft. SteveBaker (talk) 19:02, 11 December 2007 (UTC)

Measuring Snowfall vs Rainfall

I've been trying to find a precise answer to this question, but I can't seem to dig it out, so I thought I'd post my first question to the Wiki Ref Desk. When meteorologists predict that there will be 1 to 2 inches of snowfall, what are they exactly saying? Will there be 1 to 2 inches of loose, unpacked, powdery snow, or will there be 1 to 2 inches of water left after the snow has melted? If it is the former, what is the ratio of water within the snowfall? I'm mostly interested in the use of this in the U.S., but if someone in another country has any information about it, I'd be interested in that, too. Thanks. --Saukkomies 19:24, 10 December 2007 (UTC)

This link should help. -- kainaw™ 19:30, 10 December 2007 (UTC)

Yeah, thanks for the link, Kainaw. It did answer part of my question. But there still remains unanswered the question of what exactly do the forecasters mean when they say that we should expect 2 to 4 inches of snowfall? Our Canadian colleague below indicates that when they state "snow accumulation", it means the depth of the freshly fallen snow. But often all they say is "2 to 4 inches" and leave it at that. Does this mean 2 to 4 inches of the snow depth of fresh snowfall, or 2 to 4 inches of precipitated moisture - which would be much deeper than the freshly fallen snowfall depth. I'm still confused... --Saukkomies 20:36, 10 December 2007 (UTC)

What I get from that link is that 2 to 4 inches is an average accumulation over an area. The example from the article is that an area with 6 inches in one spot and no significant cover in another should be marked as 3 inches of accumulation. It also points out that water content is very much a matter of how the person processes it. It explains that snow floating in the wind will have difficulty making it into the precipitation container, reducing the amount of water detected. Another option is to take a plug of snow and melt it, but that depends on how well packed the snow is and where you took it from. Now, another way to ensure that you know 2 to 4 inches is accumulation is to consider what the accumulation would be if it was a measurement of water. Consider the high density level in our article of 15%. In other words, in an inch of snow, 15% of it is water. Using simple algebra, if 2 to 4 inches of water fell as snow, it would be 13 to 27 inches of snow accumulation. Obviously, they are not calling for 1 to 2 feet of accumulation, so they mean 2 to 4 inches of accumulation. -- kainaw™ 13:14, 11 December 2007 (UTC)

They're forecasting the depth of snow, not the equivalent depth of rain. My understanding is that it's approximately a foot of snow to an inch of rain, though this varies wildly based on the sort of snow. Our article notes that newly fallen snow has a density of 5%-15% (which is in the ballpark of the foot-to-inch conversion) but that density increases as packing and settling occur. — Lomn 19:30, 10 December 2007 (UTC)

I was under the impression that it might be even more subtle than that. Here in Texas, it snows about once every couple of years - and initially, the ground surface is generally above freezing. Hence, most of the initial snowfall just goes into chilling the ground so that subsequent snow can settle. So when they make an estimate here, there ought to be much more to it than simply one-foot-per-inch-of-water. SteveBaker (talk) 20:00, 10 December 2007 (UTC)

I would expect that it is more subtle than that. However, it seems that forecasts tend to be things like "6 to 10 inches" or "up to 2 inches", which have quite a lot of room to cover subtleties. — Lomn 21:36, 10 December 2007 (UTC)

Canadian forecasts sometimes refer to "snow accumulation", which makes it clear that they are indeed talking about the depth: the initial snow that (as we say here) "doesn't stick" doesn't count. In after-the-fact reports where they show "precipitation", that's the melted amount, as explained here. --Anonymous, 22:01 UTC, December 10.

Henry Margusity at AccuWeather just put up a blog post on snow ratios. -- Coneslayer (talk) 17:57, 12 December 2007 (UTC)

Why no lightning during a snow storm?

I thought lightning was caused by static electricity from clouds rubbing against each other. Doesn't this happen during snow storms as well? So why no lightning during them? 64.236.121.129 (talk) 20:06, 10 December 2007 (UTC)

See thundersnow.148.177.1.211 (talk) 20:07, 10 December 2007 (UTC)

So the next question is, what's different about cold-weather storms that makes lightning less likely to form? The article describes it as rare, which fits my experience in Toronto -- summer thunderstorms, maybe twice a month if you count small ones; winter thunderstorms, maybe once every 10-15 years. But it doesn't say why. So why? --Anonymous, 22:11 UTC, December 10.

At the bottom of the thundersnow article is a link to a straightdope article explaining the likely reasons why thunder-snowstorms do not often develop. In short, the conditions conducive to lightning-generating storms are not produced frequently in winter weather conditions. Link reproduced here: Straight Dope staff report: Why don't snowstorms produce lightning? User:Sifaka 01:37, 11 December 2007 (UTC) (currently IE-xiled) —Preceding unsigned comment added by 128.196.149.20 (talk)

I saw lightning during a snow storm only once, and it was quite pretty. Edison (talk) 02:58, 11 December 2007 (UTC)

Yes, I too have seen lightning during snowstorms - but on several occasions. One was during the night of my 40th birthday party in Chicago! --Saukkomies 23:35, 10 December 2007 (UTC)

Tongue twister

What makes it hard to say toung twisters?--Sivad4991 (talk) 20:14, 10 December 2007 (UTC)

Most of them require you to align mouth, tongue, vocal cords, lips into some configuration - then require you to switch to a radically different configuration more quickly than is easily possible. Try saying "She sells sea shells on the sea shore" very slowly and pay attention to how your tongue and lips have to move around before each of the S's and you'll feel exactly what I'm saying - each S requires a totally different configuration of mouth parts. Now try the non-tongue-twister "She shears sheep in the shade" (Which I just invented!)- and because all of the S's are "Shhh" sounds, you can feel that it's a lot easier to say. Hmmm - we need a word for these kinds of sentence. "Tongue-straighteners" perhaps! SteveBaker (talk) 20:30, 10 December 2007 (UTC)

She sold sea-shells on the sea shore. DuncanHill (talk) 22:48, 10 December 2007 (UTC)

According to tongue twister, "The sixth sick sheikh's sixth sheep's sick." is the hardest tongue twister. Ow! I think I just pulled a muscle! SteveBaker (talk) 04:39, 11 December 2007 (UTC)

Also, see the bottom of this thread from the ref desk archive which has really good answers to the same question [4]. Azi Like a Fox (talk) 06:29, 11 December 2007 (UTC)

Yes - I was thinking about that myself. The odd thing is that "The sixth sick sheikh's sixth sheep's sick." is hard to read even without speaking or subvocalising it - just looking at the words is tough. But "She sells sea shells on the sea shore" is easy to read in my head but harder to say. So perhaps there is something for the theory that it's a cognitive thing as well as a mechanical musculature issue. SteveBaker (talk) 14:29, 11 December 2007 (UTC)

I would hazard to guess that the similarity between each of the words in "the sixth sick sheikh's sheep's sick" makes saying the rhyme a cognitively-demanding task, as well as physically challenging. This would similar to the phenomenon of semantic overload, when saying the same word over and over reduces the meaning of that word to nil (try it). See also mantra. Vranak (talk) 20:49, 11 December 2007 (UTC)

These are just things you dont say in everyday life (probably)-- thats why they are hard. A few hours practice and you can say any tongue twister-- Its just like learning to play a difficult tune. Practice on this easy one first:

Moses supposes his toses are roses,

Moses supposes his toses to be.

Moses supposes his toses are roses.

Moses supposes erroneously!

--TreeSmiler (talk) 02:14, 12 December 2007 (UTC)

Lightning striking a lake

One time while I was camping there was a giant thunder storm. Before the storm I was walking next to a lake near by and saw some ducks on the lake. During the storm I saw a lightning bolt hit the lake which was amazing I mite add. When the lightning bolt hit it made me think about the ducks, what hapens to ducks if a lightning bolt hits the lake while there are on the lake do they just turn into rost duck (mmm sounds good)? Amd same with the other little lake cretures do they just get fried to?--Sivad4991 (talk) 20:31, 10 December 2007 (UTC)

Lightning can certainly kill people on the water's surface, so the same is true of ducks, but lightning also dissipates fairly quickly when striking water. Additionally, in most lake settings, terrain dictates that actual water strikes will be comparatively rare. — Lomn 21:33, 10 December 2007 (UTC)

The electrons that make up the bolt come from the whole earth, basically, but the intensity of the current in the volume near the place the bolt "hits" will diminish with distance from the strike. It's sort of like the reverse of the way sound gets weaker as the spherical wave that makes it up increases in area. Ducks too near the strike will be zapped, as will the aquatic creatures. Those farther away (how far depends on the strength of that particular bolt) will survive. This does not take into account the direct effect of the field around the bolt, which could be expected to act on nearby fauna as well. --Milkbreath (talk) 22:19, 10 December 2007 (UTC)

Being smaller makes you less vulnerable to lightning striking close by. The voltage difference per meter gets less and less as you get further from the point of impact - and the closer together your points of contact to the ground/water, the less potential difference there is across your body. (One piece of advice they give you when out in the open during a storm is to keep your feet close together). A duck has a considerably smaller voltage across it than a human for that reason. Water conducts electricity better than the ground does - so one would expect the lightning to dissipate outwards faster than on land - and also the resistance of the water and the bodies of the animals bobbing around in it isn't so different - so less of the current will flow through the animal than on land. You're still going to get animals killed - but there are good reasons to assume that it would be less severe than on land. —Preceding unsigned comment added by SteveBaker (talk • contribs) 21:59, 10 December 2007 (UTC)

Unknown arachnid species

I came across this arachnid, which lived at the entrance of these caves in southern Brazil. I assume they're some species of the amblypygi order. Does anyone know exactly what species it might be. I would love to put it in a wikipedia article. -- MacAddct  1984 ^{(talk • contribs)} 20:34, 10 December 2007 (UTC)

I count 7... There is one out of focus on the right of the photo (I think that is a leg anyway) Shniken1 (talk) 04:20, 11 December 2007 (UTC)

There's another out-of-focus one coming straight towards the lens, just by its left mandible. I think this one actually had part of that limb injured, so it's shorter than it should be. -- MacAddct  1984 ^{(talk • contribs)} 04:34, 11 December 2007 (UTC)

Oh that was probably in re: to me when I said I only saw 6 legs, but then I saw the others so I removed my post. I also took the <smalls> off the below comment because it is funny. Like The Ring. Saudade7 23:30, 11 December 2007 (UTC)

That's a Macis deadus. Once you've touched it you die in a year :-P Nil Einne (talk) 11:40, 11 December 2007 (UTC)

I think it is some kind of this (?) Opiliones based on pictures on the web. But maybe you will have to send the picture to an entomologist at a university. Maybe it doesn't even have a name yet. So many living things in South America are disappearing before we even know they exist. Thanks Weyerhaeuser! Saudade7 23:45, 11 December 2007 (UTC)

I would say it's definitely some form of harvestman (Opiliones), as the spiked pedipalps are downward facing rather than sideways as in amblypygids, although it's not a species I have ever seen before.

splenocyte irradiation

Why might someone irradiate splenocytes that they're using to stimulate CD4+ T cells for an in vitro proliferation assay? Is it to stop them from proliferating or alternatively kill any among them that are proliferating, leaving those that aren't? --Seans Potato Business 21:24, 10 December 2007 (UTC)

To prevent the splenocytes, most of which will be lymphocytes, from proliferating as a response to the major histocompatibility antigens of the CD4+ T cells. By irradiating the splenocytes, you measure only the proliferation of the CD4+ T cells, and not the proliferation of the splenocytes. --NorwegianBlue ^talk 15:51, 11 December 2007 (UTC)

Aspirin and Coughing

Not a medical question

Coughing says coughing is a side affect of aspirin?

Is this true it's not mentioned in Aspirin? A quick google finds the opposite might be true.

If aspirin does causing coughing, through what action? Caffm8 (talk) 22:32, 10 December 2007 (UTC)

The British National Formulary No. 29 (March 1995) lists bronchospasm as a possible side-effect in hypersensitive patients. DuncanHill (talk) 22:37, 10 December 2007 (UTC)

As does the current edition (No. 54). DuncanHill (talk) 22:41, 10 December 2007 (UTC)

"Side-effects generally mild and infrequent but high incidence of gastro-intestinal irritation with slight asymptomatic blood loss, increased bleeding time, bronchospasm and skin reactions in hypersensitive patients.". DuncanHill (talk) 22:42, 10 December 2007 (UTC)

A rare side effect then? Most googling shows that aspirin is used to suppress coughs. But I suppose that's not odd, aspirin lists head-ache as a side effect.

Maybe it shouldn't be listed as a cause in Coughing. Perhaps because a lot of articles mention ACE Inhibitors (which do cause cough) given with aspirin (which suppress it) and the article mentions both. Caffm8 (talk) 22:54, 10 December 2007 (UTC)

I am not aware of aspirin being used to suppress coughing - BNF suggests its use as an analgesic, an anti-inflammatory, and an antipyretic, in mild to moderate pain and pyrexia, and in inflammatory joint disease, as well as its use as an anti-platelet. Interestingly, our aspirin article does not seem (at a cursory reading) to mention the special hazard of its interaction with warfarin. I am reluctant to edit the article extensively, but do find this odd. DuncanHill (talk) 23:08, 10 December 2007 (UTC)

Well this is the first google hit.

Yes, the aspirin article is misleading, it states "Taking aspirin with alcohol or warfarin increases the chance of gastrointestinal hemorrhage." and since most people have probably take aspirin with alcohol and suffered no adverse affects they might think the same of warfarin

It's why you shouldn't take medical advice from wikipedia  :). While (i believe) most things are probably correct through the power of many eyes, you never know what's missing and that's often more important Caffm8 (talk) 23:41, 10 December 2007 (UTC)

I always look up any medicines I am given on the BNF (its website allows access to anybody for free, so long as you register). This follows an embarressing incident at the chemists when a doctor had issued an illegal prescription for a controlled drug for a child I was looking after. DuncanHill (talk) 23:45, 10 December 2007 (UTC)

Sweet biscuits (= cookies) and savoury crackers

I've been told by a professional chef that if you put unwrapped sweet biscuits into the same airtight container as unwrapped savoury crackers, they'll all go soggy fairly quickly; whereas if you put them in different containers, each will retain their freshness for a lot longer. Is this true, and if so, what makes it so? -- JackofOz (talk) 22:57, 10 December 2007 (UTC)

Well, crackers tend to be very dry while cookies are moist. So putting them together, the crackers would absorb some of the water given off by the cookies, making the crackers get soggy. I don't see how it would affect cookies much, though. -- MacAddct  1984 ^{(talk &#149; contribs)} 23:21, 10 December 2007 (UTC)

If anything I would presume they would dry out Nil Einne (talk) 11:38, 11 December 2007 (UTC)

OK, so the crackers may go soggy and the biscuits may dry out. In time, they'd approach a sort of equilibrium, I guess. That makes more sense than what I was told (or thought I was told, to be charitable). Thanks for the enlightenment. -- JackofOz (talk) 13:59, 11 December 2007 (UTC)

Being salty - it makes sense that the crackers would absorb water if there is some floating around in the container because they'd be rather strongly hygroscopic. But I wonder if the somewhat oily nature of the crackers is what is messing up the biscuits? We should perhaps be a little more precise with our language here: In the USA, the things that are called 'cookies' are often made deliberately soft - the things that the Brits and Australians call 'biscuits' are only the harder, crunchier types (think 'Oreos') - so they are also pretty dry. The things Americans call 'biscuits' are a totally different thing altogether (more like bread). I find it hard to believe that there is enough water in a sweet biscuit to make anything soggy. An American-style soft choc-chip cookie though - that could easily be moist enough to do it...but then it's SUPPOSED to be soft - so this complaint wouldn't apply. I could believe that mixing soft cookies with crackers would make the crackers soggy and the cookies go excessively hard though. Humectant is an interesting article in this context. SteveBaker (talk) 14:22, 11 December 2007 (UTC)

related (sort of) Cake/biscuit tax conundrum. DuncanHill (talk) 14:25, 11 December 2007 (UTC)

A complex microbiological summary of this chemical process is to be found on this page --Cookatoo.ergo.ZooM (talk) 10:08, 14 December 2007 (UTC)

CIDP -- referrence to the Minnesota Quality Pork Processors

I looked at the three references regarding this interesting subject, one as recent as 12-8-2007. I do not see any reference that any of the employees were recovering. The articles only referred to those who were suffering from CIDP.

As I am a physician and have treated people with CIDP (etiology not yet known but I don't they involved brain matter because that's a pet hobby with me), I wanted to see any documentation of these employees' recovery. I did not find one.

Whoever added that little "now you know the rest", please tell me your source. This is a very important subject for me. —Preceding unsigned comment added by Borderbumble (talk • contribs) 23:10, 10 December 2007 (UTC)

The chances that you would reach the editor who made that change via the Reference Desk are rather slim. In searching the CIDP article's history page, it appears that User:Saudade7 first added the information on 4 December. You can ask him about it using his talk page. --LarryMac | Talk 23:30, 10 December 2007 (UTC)

By magical chance I am here! Yes, I listed those articles. At the time of my listing them one of them said that all but two of the people had recovered. I got one or two of the articles from Yahoo News which I have found in the past to edit reports without acknowledging the change. In general this has been due to grievous grammatical issues with the article. By all means, if you find another article that claims that no one is recovering, and if the articles I provide no longer claim that people have recovered, change / edit the story! As a matter of fact, I think stories are more interesting the more apocalyptic they sound! (although I am not religious, waiting for an actual apocalypse, or wanting people to be sick). But that's what the stories said at the time I accessed them and I tried to be NPOV. Best, Saudade7 03:56, 11 December 2007 (UTC)

Update: I left a message for Borderbumble on his/her talk page. Saudade7 04:09, 11 December 2007 (UTC)

Licky fingers & placky bags

Why or how (or why and how) does licking the tips of one's fingers make it easier to seperate and open plastic bags? DuncanHill (talk) 23:31, 10 December 2007 (UTC)

It works for book pages too. Maybe because by licking your fingers you are making them slightly sticky with your saliva, which gives you friction and allows you to grip the edges of the bag or book page. 70.171.229.76 (talk) 23:57, 10 December 2007 (UTC)

I have never needed to do it for books (I must say, if I found anyone using spittle-covered fingers to turn the pages of my books, I'd tear their arms off!). I've also only needed to do it with plastic bags comparitively recently - say in the last year or so. DuncanHill (talk) 01:44, 11 December 2007 (UTC)

If your finger tips are very dry they don't have good grip. Some people will have drier finger tips than others and won't need to lick them.Shniken1 (talk) 01:48, 11 December 2007 (UTC)

At post offices in Australia, the workers are supplied with a damp sponge to use, presumably as it's more hygienic. Confusing Manifestation(Say hi!) 02:57, 11 December 2007 (UTC)

OT but saliva does contain antibacterial agents, so might perhaps be more hygenic than a damp sponge that's had various people's fingers in it??-Shantavira|^{feed me} 10:01, 11 December 2007 (UTC)

Well that depends. Presuming you have good hygenie and don't go putting your unwashed fingers into your mouth or onto your face then probably not. Besides that, if you do it right you barely touch the sponge. The point is to wet the stamp or the envelope flap directly not to use your fingers. In NZ, very often they have wet rollers as opposed to sponges so it's easier to wet envelope flaps. Edit: um sorry didn't read this properly. I thought we were talking about wetting stamps or envelopes flaps hence the post office comment. However I still think a sponge will usually be more hygenic for the first reason Nil Einne (talk) 11:34, 11 December 2007 (UTC)

But consider the murder method in The Name of the Rose / The Name of the Rose (film)...

Atlant (talk) 13:28, 11 December 2007 (UTC)

With new plastic bags in which merchandise is placed at a store, the manufacturing process leaves the bags with the opening tightly sealed . Apparently the cutting apart of the bags at the factory seals the plastic together, so that if you try to separate the edges with dry fingers, the adhesion of plastic to plastic is far greater than that of the fingers to the plastic. When the fingertips are moistened, the friction seems to increase allowing the bag to be opened. The sticking together of the plastic may be in part because the manufacturing process actually seals the plastic together (unintentionally) due to the compression, or perhaps to heat from friction. Another factor may be atmospheric pressure, with an absence of air between the tightly adjacent plastic. Bags which have been between rollers to give them a textured surface adhere together less. A third factor may be static electric attraction between the plastic surfaces, for whatever reason. Physics handbooks like the CRC handbook give coefficients of friction for many combinations of materials (diamond on ice has very low friction). Is there a reference anywhere for the friction of human finger on plastic bag material, dry and moist? Licking the fingers is highly inadvisable. A sponge is effective and sanitary. Edison (talk) 17:02, 11 December 2007 (UTC)

A sponge used by hundreds of other shoppers? Some of whom will have personal hygiene problems? Really? DuncanHill (talk) 17:06, 11 December 2007 (UTC)

I did not make clear whose use of the sponge I was advocating. A "personal sponge" is advisable for any store personnel who must open bags, like clerks/baggers/cashiers. I would not recommend sharing. If a stickypad (equivalent to a loop of masking tape) were provided for shoppers the bags would be easy to open. Edison (talk) 18:25, 12 December 2007 (UTC)

This was discussed on the ABC Online Forum. It seems to be due to a "liquid bridge", which is due to the adhesion and cohesion of water, which are due to the polar nature of the water molecule. Presumably this force, though useful for opening plastic bags, is not as strong as the friction between rubber and pavement, which is why water makes roads more slippery. --Heron (talk) 20:56, 12 December 2007 (UTC)