Wikipedia:Reference desk/Archives/Science/2010 August 8

= August 8 =

North Pole?
How come there is no volcanoes in the North Pole? When was the last time that a continent sat on the north pole?--68.116.113.157 (talk) 00:41, 8 August 2010 (UTC)


 * There are lots of places with no volcanoes. There is nothing special about the North Pole in that respect, so I don't see why there needs to be a reason. This map from the late Jurassic period (so about 150 million years ago) shows land very close to, if not actually at, the pole (it's hard to tell from the image). That seems to be the most recent time there was any land near the pole. You can see the recent (last half a billion years or so) changes in the placements of continents in this animation: File:TectonicReconstructionGlobal.gif. --Tango (talk) 00:59, 8 August 2010 (UTC)
 * Volcanos (see article) occur where tectonic plates meet. There are no tectonic plate borders at the North Pole. Cuddlyable3 (talk) 14:19, 8 August 2010 (UTC)
 * Are you sure? I was going to say the same but then I came across File:Plates tect2 en.svg...a line goes off the top of the map and comes down the other side. Which suggests to me it passes pretty close to the North Pole. I realise flat maps can't properly illustrate a spherical(ish) globe, but... Vimescarrot (talk) 16:18, 8 August 2010 (UTC)
 * That map doesn't actually show the north pole. The north pole on that projection is a horizontal line infinitely far above the map, if I'm recognising the projection correctly. There is no way to know what that plate does north of the boundary of the map just by looking at that map. There is a whole chunk of the plate missing. --Tango (talk) 16:55, 8 August 2010 (UTC)
 * The source page says "Mollewide projections show the entire globe in one view, but are distorted significantly along the margins of the globe". Looie496 (talk) 17:50, 8 August 2010 (UTC)
 * You misunderstand. The map Vimes links to doesn't show the North Pole. The map I embedded certainly does, otherwise my answer wouldn't have made any sense. --Tango (talk) 17:54, 8 August 2010 (UTC)
 * On the other hand, there are plenty of Arctic volcanoes, especially in Iceland. ~ A H  1 (TCU) 18:31, 8 August 2010 (UTC)
 * Do we have any images of the major plates superimposed on a more modern projection, or something that preserves the curve a little better? (I'm also surprised by how weird it is to see a map like this centred over the Americas, but I don't think it would be reasonable to ask for one that matches my own expectations) Is Vimes's link a Mercator projection? The British Isles looks huge on it, and Africa very small. 82.24.248.137 (talk) 19:32, 8 August 2010 (UTC)
 * The boundary between the North American and Eurasian plates actually passes very close to the north pole, but it's difficult to find a picture that shows this properly. However I haven't seen anything that indicates volcanic activity at that part of the junction. Looie496 (talk) 20:03, 8 August 2010 (UTC)
 * You want the Gakkel Ridge, which passes pretty close to the pole and is an active (if extremely slow) spreading center - that article also has a useful image. Mikenorton (talk) 07:19, 10 August 2010 (UTC)

The Neurons or the synapses
Does it improve a person's memory or intelligence to have many neurons or is it in the synapses or the way that the neurons send the signals that is responsible for why some people have better memories than others? I believe Kim Peek (Rain man inspiration) was said to not have the corpus callosum, and it is in theory that some believe because of this his brain found a way of connecting around that at a superfast speed and that is what is responsible for his memory. Also, can the neurons or the way signals are sent be changed, I ask because I watched this documentary that had a guy from Enland who after an epilectic seizure when he was three he started to get this incredible memory and brilliance for numbers and on the same documentary there was a kid who got hit in the head with a baseball as a kid and ever since that he too had an afinnity to memorize dates and so on? —Preceding unsigned comment added by 70.136.158.173 (talk) 01:12, 8 August 2010 (UTC)

Also, could the size or shape of the brain have anything to do with it? (In the MRI taken of a woman who has memorized every day of her life from 1980 on and others like her there bains were slightly different shape and they were all left-handed.)


 * Most neuroscientists believe that memory is stored by changing the strength of synapses, so the total number of synapses sets a limit on the amount of memory the brain can store. But this is only one factor: an effective memory depends on a whole range of systems for formatting, storing, and recalling memories. It's like in a computer memory is stored in little magnetic things, but if you just throw a pile of magnetic things on the floor, you don't get a usable memory system -- it all has to be organized properly.


 * It is very common for autistic savants to have extraordinary memory for details. Most don't have a missing corpus callosum, so I doubt that that has anything important to do with it. The stuff about signals connecting around at superfast speed is just nonsense. Basically we don't know where those extraordinary memories come from -- it clearly involves a higher level of brain organization than we yet understand. Changes in brain shape might be relevant, or they might not -- we just don't know yet. Looie496 (talk) 02:26, 8 August 2010 (UTC)


 * The hippocampus definitely does something uncertain to do with memory. Experienced taxi drivers (who can remember complicated maps) have enlarged ones, as the article mentions. 81.131.58.136 (talk) 02:56, 8 August 2010 (UTC)

Are the parts and even the strength of the synapses continually growing? Like for example our bones even though we stop growing and reach our full height our bones are still constantly rebuilding themselves and continue to go through a process? —Preceding unsigned comment added by 71.156.2.13 (talk) 03:52, 8 August 2010 (UTC)


 * There is quite a bit of evidence that synapses change continuously. A synapse is only a single signal channel, so they don't get more complicated over time, but they do get stronger or weaker.  Among other things, Giulio Tononi's theory of sleep proposes that synapses steadily strengthen over the course of each day, and then weaken when we are asleep -- there is substantial evidence to support this. Looie496 (talk) 04:52, 8 August 2010 (UTC)

Does chemical/hormones have anything to do with the strength or weakness of the synapse and or the neurons for that matter? —Preceding unsigned comment added by 71.137.245.165 (talk) 12:48, 8 August 2010 (UTC)


 * Absolutely. In fact, nearly every psychoactive drug exerts its effects by making some group of synapses either more or less effective.  There are a small number, such as caffeine, that act on neurons by non-synaptic mechanisms, but the great majority act on synapses.  There are also internal chemicals, such as norepinephrine, that act at least partly by modifying the strength of synapses in specific brain pathways. Looie496 (talk) 17:42, 8 August 2010 (UTC)


 * See Eidetic memory and neuroplasticity. ~ A H  1 (TCU) 18:27, 8 August 2010 (UTC)


 * The thing is that our brains definitely have a finite capacity - it's probably comparable to the size of the hard drive in your computer. But memories aren't (usually) stored with perfect fidelity.  I can recall my wedding day from 25 years ago - but I have almost zero memory of anything that anyone said to me on that day - other than a few snatches of the speech my father gave at the reception.  Because the brain has finite capacity - and we didn't evolve to live to be 100 years old - we must eventually be forgetting things at about the same rate that we learn them.


 * But we don't do that by totally forgetting one entire year of our lives for every year we live. Our more distant memories become fuzzier - and less important things get erased in order to keep a choice few of the most important.  It's as if we're continually summarizing less important (typically, older) information in order to make room for new, sharper memories.  I think it's clear that the more amazing the savant's abilities are, the more mentally crippled they seem in other regards.  Those who are able to memorize (by rote) unbelievable amounts of raw data - may well have damaged the part of the brain that is responsible for summarizing and discarding trivia.  The brain has a spectacular storage capacity - it's not at all surprising that one could memorize a million digits of PI or a thousand books - but perhaps these people are losing information that the rest of us would find essential to daily life?   It's hard to know because if such a person is incapable of (for example) managing interpersonal relationships or (maybe) holding down a job - we can't easily tell if that's because they are unable to remember some important class of data that relates to those more normal activities. SteveBaker (talk) 01:21, 10 August 2010 (UTC)


 * What tends to happen is that people with extraordinary event memory have difficulty generalizing -- they know all the facts but can't spot the patterns in them, can't see the forest for the trees if you will. Thus for most tasks they actually do less well than people with poorer memory.  One of the nicest descriptions of the phenomenon is in Alexander Luria's classic The Mind of a Mnemonist. Looie496 (talk) 23:02, 10 August 2010 (UTC)

When is a heat sink necessary for a 7805 voltage regulator?
Hello! I'm a (newbie) hobby electrician. I'm planning on using an AC adapter that outputs 9V 210mA connected to a 7805 voltage regulator for my project. How do I know if a significant amount of energy will be wasted as heat? The circuit will be in operation mostly 24/7. When is a heatsink connected to the regulator required? The circuit will be working at room temperature. I think part of the difficulty I'm having understanding how the voltage regulator will work is because I've only worked with DC from batteries, and learned that when 9V is connected with a 1KΩ resistor in series to an ammeter, the reading will be 9mA, but I don't know why the AC adapter's specs show 9V at 210 mA. I would think (probably wrongly) that 9V would output 9A because of the ammeter example I gave. I'd also appreciate any help on understanding this now that I'm moving on from working with AA batteries. Thank you!--el Aprel (facta-facienda) 02:21, 8 August 2010 (UTC)
 * The regulator will reduce the voltage by varying its resistance. The current flowing through times the voltage drop will turn into heat.  The AC adapter rating is just the maximum, so you have to know how much current your circuit will consume,  If it only takes 10mA then no heat sink will be needed, but the full 200mA dropped by several volts, will need one. Graeme Bartlett (talk) 03:15, 8 August 2010 (UTC)
 * At risk of straying into wikibooks/wikiversity territory, you need the output voltage of the bridge rectifier. This will be the RMS of the step-down transformer you are using. Then take (Vin - Vout) / Iout. This will give the power drop across the voltage regulator. The voltage regulator's datasheet will give a maximum operating temperature. Say it is 70&deg;C, this is fairly typical. Given a room temperature of around 30&deg;C (a bit on the high side), the voltage regulator can not be more than 40&deg;C above ambient. Say the transformer is at 12V AC. RMS is 17V DC. The drop across a 9V regulator is 8V (check the maximum input voltage, but they are normally around 50V). 8V * 210mA is 1.68W. That is 0.04W/&deg;C or 25&deg;C/W, assuming the heatsink is in free air. In enclosed cases you need a bigger heatsink (higher value for W/&deg;C, smaller for &deg;C/W), if you are using fan-cooling then a smaller heat sink is needed. CS Miller (talk) 11:04, 8 August 2010 (UTC)


 * You haven't mentioned the current draw of the load for the 7805. Once you know the load current, the rest is quite straightforward. As Csmiller hinted above, small unregulated wall-wart power supplies have very poor load regulation, that is, their output voltage at no-load can be more than twice their rated output voltage (i.e., example, 20V at 0 mA, 15V at 20 mA, but 9V at 210 mA). In this example, if the load draws 20mA, the 7805 will sink (15V - 5V)*20mA = 200mW, (9V-5V)*210mA = 840mW, etc. Alternatively, you can just use an off-the-shelf regulated wall-wart. If it's output is not clean enough, add passive RC filters.East of Borschov 18:12, 9 August 2010 (UTC)


 * I think you mean "If its output has too much ripple,...", and adding a passive RC filter will reduce the voltage to below the nominal regulated value i.e. Vout = Vreg - R Iout. Cuddlyable3 (talk) 19:48, 12 August 2010 (UTC)

Maximum knowledge
Omniscience is impossible. For instance, the uncertainty principle states that we can not simultaneously know an elementary particle's position and momentum. So what is the maximum level of knowledge that can be attained, and is there any realistic measure of attaining it?--220.253.219.83 (talk) 04:24, 8 August 2010 (UTC)
 * The maximum knowledge that can be obtained is the amount of knowledge that our brains are capable of holding. That's an infinitesimal fraction of the information in the universe, so it doesn't really make sense to talk about obtaining it.  The information in a teaspoon of water is vastly beyond our ability to comprehend. Looie496 (talk) 05:04, 8 August 2010 (UTC)


 * I think you may be misunderstanding the uncertainty principle. We often say things like "we can not simultaneously know", but that is misleading.  The correct understanding is that particles do not ever have both a fixed position and momentum.  It's not a statement about our lack of knowledge.  It is an expression that trying to assign momentum and position to everything is inherently futile because those properties do not exist.  A hypothetical omniscient deity could know everything there is to know, and still wouldn't know both position and momentum, because the universe simply doesn't allow particles to ever have well-defined values for both.  Personally, I don't see any objections in quantum mechanics to a deity having omniscient knowledge of the present and past.  However, the principle of wavefunction collapse (as presently understood within our limited knowledge) would seem to imply that even a deity would never be able to uniquely predict what would happen in the future, since quantum mechanics inherently involves randomness.  Dragons flight (talk) 05:26, 8 August 2010 (UTC)


 * Unless the deity is the source of the randomness. You are assuming a deity that lives in the universe, but the deity could be the universe. i.e. the deity is to the universe as you are to a thought in your mind. Meaning each and every particle and interaction in the universe is specifically "animated" by the thoughts of the deity. Ariel. (talk) 07:59, 8 August 2010 (UTC)


 * No. All of those are the same as saying quantum mechanics is wrong, because the events would no longer be truly random.  You can either say the universe isn't random (because the deity guides it, etc), or you can say the universe is random and future outcomes aren't known till they occur, but you can't have it both ways.  Of course, a deity could certainly make a predestined universe appear random to beyond our ability to ever tell the difference, but that's not the same as saying the universe actually is random.  Dragons flight (talk) 08:19, 8 August 2010 (UTC)


 * What's the difference? From a physics point of view I mean, not a philosophical one. If it's random beyond our ability to tell the difference, if there is no physical test or ability to tell otherwise, then it is random. The Equivalence principle works exactly the same way: Since there is no way to tell the two forces apart, they are the same. Ariel. (talk) 08:30, 8 August 2010 (UTC)


 * Collapseless interpretations of QM don't present this problem --Atemperman (talk) 05:36, 10 August 2010 (UTC)


 * Information (not exactly the same as knowledge, but close enough) is strongly tied to energy. It takes energy to manipulate and store information. See Limits to computation and google for "Ultimate physical limits to computation" for more exact numbers. Another limitation is the speed of light - you can not know about anything not within your light cone. Additionally as you store more information the size of your "brain" gets larger - eventually it gets large enough that it takes a long time (because of the speed of light) to retrieve information from the other "side" of it. This greatly limits the speed at which you can think. (So you can know a lot, but not be able to think fast, or know less and think faster.) There is no direct answer to your question since you did not specify what is doing the "knowing", but maybe what I wrote was interesting anyway. Ariel. (talk) 07:59, 8 August 2010 (UTC)


 * Infinite knowledge may or may not be impossible, as infinite knowledge of information would be impossible, but knowledge is distinct from information. There are also ways that ordinary people use to increase knowledge to much higher levels (ie. mystical self-hypnotic mind-expanding introspective spiritual experience), but not in a way that true omniscience could be achieved at any given point in time. The electron conundrum is likely connected to the quantum zeno effect. Knowledge cannot be quantified, and therefore may have little to do with the possible interconnections between neurons and synapses (~∞). As for the deity as universe argument, this is called pantheism. Also, it would be possible for an omnipotent being to create a door he cannot open. All the being has to do is make the door so that it is impossible to open at a specific time, but make it able to be opened when he decides it can. ~ A H  1 (TCU) 18:14, 8 August 2010 (UTC)


 * Even forgetting quantum theory and uncertainty and things like the speed of light limitations on information transfer - this is a tricky problem. Ultimately, you need N bits of storage to store N bits of information.  If the smallest imaginable item of information (let's say the spin on a particular fundamental particle) could be stored in the most compact way imaginable (which would probably be using the spin of a remarkably similar fundamental particle!) then the very best you could possibly do would be to store all of the information about half of the universe using the other half of the universe to store it!


 * That is a crazy upper-limit - gathering and maintaining the information would require machinery comprising some very large fraction of the universe! Also, history is a part of knowledge.  If you used 90% of the stuff of the universe to store information about the other 10% then you'd only be able to store that information for 9 instants in time...which is a pretty crappy kind of history!  It is a little tricky though - you might argue that you could store half of the information about the universe onto the other half - then you'd now know everything about 100% of the universe because the two halves are now identical.  You might also argue (classically) that if you know everything possible about some chunk of universe at some particular instant in time - then you could extrapolate forwards and backwards in time to know the state at some arbitary time in the past or future.  Aside from the quantum and chaos theory issues with that - there is now another problem.  The half of the universe that you're storing the information on is affecting the half that it's trying to store...that means that the progress of your extrapolation calculations will affect the future in such a manner as to make your extrapolations incorrect.


 * We could also get into annoying philosophical arguments about what it actually means to "know" something. For example, we know the equation for calculating PI to an arbitary number of digits.  Does that mean that we "know" what all the digits of PI are?   Well, maybe...I don't know the 30th digit of PI - but I could probably find it out soon enough to be useful - so it's not much of a stretch to say that I "know" it.  But suppose I wanted the googol'th digit?  That might take my computer a billion years to calculate it...I don't think anyone would claim that they know what that digit is.  If (as in the hypothetical case of using one half of the universe to store the information about the other half) my computer cannot retrieve the information faster than I can go out and measure it again - is there even any point in storing it.  If I want to know the albedo of the 3rd moon of the 4th planet orbiting alpha centauri - but that information happens to be stored in a part of my computer that's 100 light years away - then it's easier to send out a probe a few light years and measure the albedo than it is to ask the computer to fetch the information for me.  Do we "know" something if the cost to retrieve that knowledge takes longer than collecting the knowledge itself?


 * This is an interesting question to think about - but you're definitely not going to get a single good answer!


 * SteveBaker (talk) 00:44, 10 August 2010 (UTC)

Tetra Fish Fins
Do the fins of a tetra fish grow back? My wife and I have a 150 gallon aquarium filled with all kinds of tetra fish,and the other day we noticed that one of the smaller fishes front fins were nibbled on and the fish is flapping constantly.We seperated the fish from the pack and we were wondering if the fishes fins will      ever grow back? If so how long? —Preceding unsigned comment added by 173.105.146.216 (talk) 10:04, 8 August 2010 (UTC)


 * Added new section header Rojomoke (talk) 10:27, 8 August 2010 (UTC)


 * Generally the consensus seems to be yes they will grow back. 87.102.23.179 (talk) 11:48, 8 August 2010 (UTC)


 * Yes, they grow back; they could not reach the same size as the original fins. 82.59.69.48 (talk) 13:13, 8 August 2010 (UTC)

SOx cooling atmosphere
This recent article at Ars Technica mentions (from a report it covered) : ..result of a stronger cooling effect from sulfate aerosols.. (I'm not particularily interested in the article or conclusions/methodology)

I was wondering if any figures were available on the cooling effect of 'sulphate aerosols' - ie what magnitude of effect has/will have Flue-gas desulfurization had/will have on the worlds temperature, particularily the isolated effect of not adding sulphur to the atmosphere - not the overall balance taking into account burning coal. Thanks.87.102.23.179 (talk) 11:46, 8 August 2010 (UTC)
 * Are you wondering if it might be better to leave in the sulfur in order to cause global cooling? It's not worth it. The acid rain is far worse, To get rid of CO2 we need those plants, and acid rain kills them. Ariel. (talk) 05:18, 9 August 2010 (UTC)
 * A few weeks ago the paper-edition New Scientist had an article claiming that the last few decades' efforts to reduce the UK's sulphate emissions to lessen the acid rain impact on Scandinavia had (i) had little impact on the acidity, because most of it (82%?) comes from Scandinavia itself, the North Sea or other parts of continental Europe, and (ii) had resulted in a measurable increase (0.8C?) in the UK's average temperature due to the lessening of the aerosols' cooling effect. Unfortunately the article seems not to be on the NS website. (87.81 posting from . . .) 87.82.229.195 (talk) 14:06, 9 August 2010 (UTC)
 * I did see an article (can find it now) that said that desulphurisation caused local warming. I was wondering if this was an reasonably established fact or not. I've not seen or heard this side effect mentioned in the news etc..
 * As to Ariel's comment - I'd be interested in a enviromental cost benefit analysis - but a proper referenced one I could read for myself.87.102.35.46 (talk) 15:31, 9 August 2010 (UTC)
 * You would have to inject the sulfur into the stratosphere to cause cooling and prevent acid rain. ~ A H  1 (TCU) 22:42, 9 August 2010 (UTC)

St. Bernard dog
Why the St. Bernard dog is not recognised by the italian ENCI (Italian National Kennel Club)? 82.59.69.48 (talk) 13:10, 8 August 2010 (UTC)
 * That's a tough one for us to help you with since all the info is in Italian. Try asking on the Italian wikipedia. Also I'm not sure this question belongs in the science desk. Ariel. (talk) 05:23, 9 August 2010 (UTC)
 * The Italian national kennel club "Ente Nazionale della Cinofilia Italiana" (ENCI) is a member of Fédération Cynologique Internationale that recognizes the St. Bernard as a Molosser in Group 2, Section 2. Cuddlyable3 (talk) 12:43, 9 August 2010 (UTC)
 * Thanks. Ok, I will try to ask in the italian Oracolo too. I know, the question is border line here (is about a bureaucratic issue regarding applied biology), if you want to move it please feel free to do that, though the major experts in this matter should be here. Ok, so undirectly the ENCI recognises the St. Bernard; but why doesn't the breed appear in ENCI list? —Preceding unsigned comment added by 87.11.129.247 (talk) 16:57, 9 August 2010 (UTC)

wave analyzers
which analyzers we will use to check the frequency of the wave having frequency above 20GHZ? Is it spectrum analyzer or not —Preceding unsigned comment added by Balachandramovva (talk • contribs) 17:12, 8 August 2010 (UTC)
 * You can buy oscilloscopes that reach that frequency. Is that what you are asking? Ariel. (talk) 05:28, 9 August 2010 (UTC)

Here and here are manufacturers of frequency counters that cover the 0 to 40 GHz frequency range. Here is a 20 GHz spectrum analyzer. Cuddlyable3 (talk) 12:35, 9 August 2010 (UTC)

Yoplait has a fat-free Greek yogurt.
I thought a big part of Greek yogurt was its high fat content. What's the point otherwise? 67.243.7.245 (talk) 18:45, 8 August 2010 (UTC)
 * Taste? Regards, --— Cyclonenim | Chat 19:45, 8 August 2010 (UTC)
 * Our Strained yoghurt says that what's called 'Greek yoghurt' typically just means some form of strained yoghurt. So I would guess taste as Cyclonenim mention and texture would be a big reason why some may want 'Greek yoghurt'. Our article also notes "strained yoghurt is a traditional food in the Middle East and South Asia, where it is often used in cooking, as it is high enough in fat not to curdle at higher temperatures" which may be what you're referring to and it indeed seems likely that advantage would be lost but I guess quite a number of people aren't choosing it so it doesn't curdle at higher temperatures as you have presumed Nil Einne (talk) 19:51, 8 August 2010 (UTC)
 * For the record, several companies offer fat-free frozen yogurts, in vanilla and other flavors (I prefer vanilla). They do not lack in taste, but I suppose Greek yogurts might.  2Ð ℳǣ$₮ℝʘ  talk, sign 23:06, 8 August 2010 (UTC)
 * We have fat-free greek yogurts in our freebie kitchen at work - the actual yogurt doesn't taste of much - but they come with actual fruit at the bottom - so step #1 is stirring them up! SteveBaker (talk) 00:21, 10 August 2010 (UTC)

How does a car's wheels get out of alignment?
How does a car's wheels get out of alignment? My understanding is that driving over potholes and hitting the curb hard when parking can both cause alignment problems. What are some other causes? Also, what exactly happens when something causes your car's wheels to be misaligned? Is something in your car deformed? --173.49.16.4 (talk) 18:53, 8 August 2010 (UTC)


 * Joining the front steering wheels of a car there is a significant length of mechanism consisting of push-rods, threaded adjustors, gears, bearings and dampers. This mechanism is relatively light in weight.  Impacts to the steering wheels, such as impacts with potholes and the curb, can cause movement in the threaded adjustors.  However, much of the drift in the alignment is caused by wear of the tires.  Even a change in tire pressure from the pressure that existed during the last alignment can necessitate re-alignment.  Wear in the gears, bearings and dampers in the steering system also contribute to drift in the overall alignment.  Deformation of the body of the car is not a contributor to misalignment, except if the car has been involved in an accident.  There is a little information at Wheel alignment.  Dolphin  ( t ) 23:31, 8 August 2010 (UTC)

fridges
What is the reason some people say you shouldn't close a fridge door when it's turned off? Do modern fridges overcome this? 82.43.88.151 (talk) 19:53, 8 August 2010 (UTC)
 * No, this applies to all fridges and freezers of every vintage. It is almost impossible to avoid the growth of moulds and bacteria at room temperature, but normal circulation of the air with an open door helps to prevent excessive growth.  Cleaning out the fridge with bicarbonate of soda will also help.    D b f i r s   20:52, 8 August 2010 (UTC)
 * If you leave it open for a long while, and let it dry out it's OK to close it later. Ariel. (talk) 21:49, 8 August 2010 (UTC)
 * Yes, I agree that the damp conditions encourage the growth of nasties, but I'd still prefer to clean the inside before closing the door.   D b f i r s   21:54, 8 August 2010 (UTC)

So to clarify, because I'm still confused, the reason people say the door shouldn't be closed is... to stop mold growing? 82.43.88.151 (talk) 22:16, 8 August 2010 (UTC)
 * Yes, and bacteria. If you open a fridge door after it has been closed for a long time at room temperature and without drying out as Ariel suggests, you will notice a horrid stink of decay (the bacteria) and a musty smell from (usually black) patches of mould (mold in the USA) which can also attack the plastic and rubber seals of the fridge.    D b f i r s   23:03, 8 August 2010 (UTC)
 * Additionally, if a refrigerator is being removed for disposal, the door is normally removed for safety reasons, so that playing (hiding) children won't get trapped and suffocate.  Kingsfold  (Quack quack!)  16:40, 9 August 2010 (UTC)


 * I agree, it seems as though no matter how carefully you clean/disinfect the inside of an empty refrigerator - if you unplug it and close the door then it will stink horribly within just a couple of days - and pretty soon you'll see black mold spots. If you clean it and leave the door open, it'll be fine.


 * Moisture trapped inside the fridge seems to be the problem - even the smallest amount of nutrient - plus darkness and moisture produces the perfect place for mold to thrive. Remember, the refrigerator doesn't just chill the air - it also dehumidifies it.  The inside of a (running) refrigerator is a spectacularly dry place - and mold can't survive without moisture.


 * SteveBaker (talk) 23:56, 9 August 2010 (UTC)

Energy
If capillary action raises the center of mass of the water in the tube, where does the energy come from? 76.68.247.183 (talk) 19:55, 8 August 2010 (UTC)
 * I think it might come from the Gibbs free energy, but perhaps you should wait for an expert to confirm or refute this. The article doesn't make easy reading!   D b f i r s   20:53, 8 August 2010 (UTC)
 * It comes from potential energy. It takes energy to "unstick" the water from the surface of the tubes. Water, by it's nature, is at a slightly higher energy. When it sticks to something it goes to a lower energy. Note that tall trees do not use capillary action to raise the water. Instead they evaporate some water from the leaves on the top, and the lower pressure "pulls" water up from the roots. Ariel. (talk) 21:41, 8 August 2010 (UTC)
 * Are you sure? I'm sure I've seen trees  more than 34 feet high!   D b f i r s   21:52, 8 August 2010 (UTC)
 * We do, of course, have an article on the subject: Transpirational pull. If I'm interpreting it correctly, it is basically a combination of transpiration and capillary action (or something closely related to it) that makes it work. --Tango (talk) 22:37, 8 August 2010 (UTC)
 * (ec)Yes, I meant to say don't just use capillary action. There is a third mechanism they use as well, but I don't remember what it is. I'll try to look it up later. Ariel. (talk) 22:54, 8 August 2010 (UTC)
 * Yes, it is impossible for "lower pressure" to "pull" water more than 34 feet high, so the main effect must be capillary, as stated in the article. There may also be some osmotic pressure.  I am still awaiting an expert to check on my Gibbs claim, though Ariel referred to "water energy" which might be the same thing.  The OP was asking where the potential energy comes from as the water rises through capillary action.    D b f i r s   22:52, 8 August 2010 (UTC)
 * It's not something I know a great deal about, but our article says Gibbs free energy is the ability to do non-mechanical work. Lifting something against gravity is mechanical, isn't it? The osmotic pressure you refer to is root pressure, which is a factor in some circumstances. --Tango (talk) 23:19, 8 August 2010 (UTC)
 * Root pressure is feeble - it is only able to move water ~30cm upwards. The differences in water potential between the roots + shoots is a driver for water flow however. Smartse (talk) 22:31, 9 August 2010 (UTC)
 * As Dbfirs above suggests the energy comes from the interaction between the water and glass - which is favourable in terms of energetics - ie it can do work.87.102.23.179 (talk) 01:07, 9 August 2010 (UTC)


 * No, no, no! NO free energy!  Let's not even contemplate the possibility for a nanosecond!  You can't make perpetual motion machines using surface tension - although plenty of very stupid people have tried!


 * This is no different from taking a stretched elastic band - releasing it and asking where the energy came from to contract it. The energy was put into the elastic band when you stretched it - it contracted in order to get into a lower energy state by releasing that energy.  It happened because the rubber molecules like being coiled up - it takes energy to uncoil them - and they give up energy when you let them coil back up again.  Same deal here.  Water molecules like being stuck to glass more than they like being stuck to other water molecules.  Not all molecules are like that...Mercury, for example prefers the company of other mercury atoms than it does glass and has an upside-down meniscus and tends to sink down a capilliary tube rather than rushing up it.  It's all to do with hydrogen bonding and Van der Waals forces.


 * The water (when it's at the bottom of the tube) is in a higher energy state than when it's at the top of the tube - because (like a stretched rubber band) it is in a configuration it doesn't want to be in. It actually loses energy in moving up the capilliary tube...even though it's moved upwards against gravity (and therefore has more gravitational potential energy).  It gave up more energy than that by clinging to the glass than it lost in the upward motion - just like the rubber band un-stretching.


 * Once the water is stuck to the glass, you have to add energy to get the water to go back down to the bottom of the tube again. That's why cars need windshield wipers.  The raindrops like being stuck to the glass - you have to put energy into wiper blades to get persuade it to fall off.


 * You can't extract energy from the height of water in the capilliary tube because you have to use energy to unstick it from the glass. Kinda like pulling a magnet off of your refrigerator.


 * SteveBaker (talk) 23:46, 9 August 2010 (UTC)
 * There is, of course, such a thing as free energy. --Trovatore (talk) 23:50, 9 August 2010 (UTC)
 * Of course it's free as in speech, not free as in beer. --Trovatore (talk) 00:36, 10 August 2010 (UTC)
 * It was "free" as in "available" (like a stretched band), not "free" as in "perpetual motion", but I agree with Steve's analysis. There is an official recommendation to remove the word "free".  Perhaps our article should just be "Gibbs energy".  It's probably not a very helpful concept here because it is more usually applied to chemical reactions rather than to physical attractions between molecules, but it does emphasise the fact that all substances have internal energy in their atoms and molecules.  Smartse's water potential link is better, but I suspect that matrix potential alone is not sufficient.  Does the tree also input energy to create osmotic differentials to implement capillary action in stages?  There is a limit to the height attainable in any capillary tube (otherwise we run into Steve's objection of perpetual motion).   D b f i r s   02:08, 10 August 2010 (UTC)


 * No, I think the article should stay where it is. IUPAC and similar organizations can recommend all they want; we should change when common practice actually changes, not because some body made something "official". --Trovatore (talk) 02:24, 10 August 2010 (UTC)


 * OK. I understood the meaning of "free", and I don't suppose many non-specialists read the article anyway.  I agree that we should reflect general usage.    D b f i r s   02:41, 10 August 2010 (UTC)
 * Yes, it is "free energy" in the technical sense (not in the perpetual-motion machine sense). Understandably, this nomenclature is confusing (and reeks of violation of the second law of thermodynamics) - but that is a matter of language-use.  No actual physicist or chemist thinks "Gibbs free energy" comes from nowhere.  It's "free" in the sense that it is not inherently bound to the internal process that is moving energy from a source to a sink; but energy still has to come from the source.  Nimur (talk) 18:07, 10 August 2010 (UTC)
 * No one said it came from nowhere. I was just needling Steve on the no free energy comment. --Trovatore (talk) 18:31, 10 August 2010 (UTC)

Jumbo Jet vs. Mortal
Is it possible for a human being to move a Jumbo Jet by pulling it ? -- Jon Ascton    (talk)  21:03, 8 August 2010 (UTC)
 * Never mind a jumbo jet (by which I guess you mean a 747) which is old (1997) news, the record appears to be for a 416299 lb CC-177 . Nil Einne (talk) 21:19, 8 August 2010 (UTC)
 * It depends only on the friction and nothing else. It does not depend on how heavy the jet is. I think jets have reasonably low friction on the wheels but I'm not sure. Ariel. (talk) 21:39, 8 August 2010 (UTC)
 * (ec) For an object on wheels, you just need enough force to overcome the friction in the bearings, etc., which isn't necessarily that great. Once it is moving, it will accelerate very slowly, so you need to be very strong if you want it to move a significant distance in a reasonable amount of time, but moving it isn't actually that hard. The difficult bit is probably getting enough grip with the ground so you don't just slip backwards rather than pulling forwards - if you apply too much force, your shoes will just slip against the ground, regardless of how strong you are. --Tango (talk) 21:41, 8 August 2010 (UTC)


 * The difficulty in moving a rubber-wheeled vehicle, such as a jumbo jet, comes not from friction in the bearings which is very minor. The difficulty is deforming the rubber in the tires.  Rubber is not perfectly elastic so it displays elastic hysteresis.  Much more energy is absorbed by the tires as they reshape during rolling than is absorbed by friction in roller bearings.


 * It is useful to contemplate moving a locomotive on steel wheels, or better still moving a ship through water. In the era prior to power-driven tugboats, ships had to be moved up and down rivers, and maneuvered to and from the dock, by rowing boats powered by nothing more than half a dozen men with oars.  So large vehicles can be moved by manpower, but it doesn't help to be impatient!  Dolphin  ( t ) 22:56, 8 August 2010 (UTC)


 * If a human were in a space suit in orbit holding onto the International Space Station and a space shuttle, he/she could move them quite easily by pushing or pulling, since there is no friction. The speed would start out ever so slight and increase to a meter per second in under 4 minutes, if the person pushed with 500 Newtons (112 pounds force). A jumbo jet standing on the runway would require considerable force to start it moving on a perfectly level surface, probably more than anyone with normal strength could exert. Exerting that same force  for a considerably longer time would likely leave the jumbo jet right where it started. Edison (talk) 03:37, 9 August 2010 (UTC)
 * How do you expect your astronaut to produce a force of 500N without separating from the space station? It's like the point I made above about your shoes slipping against the ground, but taken to the extreme. --Tango (talk) 03:50, 9 August 2010 (UTC)
 * Easy, the SS and Shuttle would be, say, 5ft apart and the astronaut would crouch between them and push orthagonally to move them apart: obviously the scenario of moving them "horizontally" relative to one another could not work using only surface friction, though foot and handholds on which to engage might enable a horizontal component. (87.81 posting from . . .) 87.82.229.195 (talk) 13:57, 9 August 2010 (UTC)
 * But you can only do that for a second or two. You can't do it for 4 minutes to get them up to 1 m/s (assuming constant acceleration, they would be 120 metres apart by the end of the 4 minutes). --Tango (talk) 16:43, 9 August 2010 (UTC)
 * Many adult humans of normal strength could exert a 112 pound force for 3 or 4 minutes. Lots of adults who should weigh 130 pounds actually weigh 240 and engage in all sorts of activities. People sometimes carry heavy burdens long distances, or push or pull heavy things. Edison (talk) 19:20, 9 August 2010 (UTC)
 * You can only do it for a second or two because after that the spacecraft will be out of reach. --Tango (talk) 22:17, 9 August 2010 (UTC)
 * I have some ambivalence about calculations versus perceived reality. I would not think that an angry astronaut could attack the space shuttle or space station and destroy it with his bare hands. But it seems that if he had his feet hooked around a stanchion on the ISS, and held a rope attached to the shuttle, he could in a few minutes generate enough relative velocity to crash them together disastrously. And if he started to push or pull with 500 Nt force, would folks on the 100000kg space shuttle feel the "jerk?" Edison (talk) 02:07, 10 August 2010 (UTC)
 * This image seems relevant. --Sean 15:57, 9 August 2010 (UTC)


 * It's obviously possible - there are lots of videos of it being done ( for example). The problem of moving a plane is basically four-fold:


 * Overcoming static friction. Most surfaces produce more friction when they are stationary than when they are moving.  Once you get them moving (even very slowly), static friction goes away.
 * Overcoming regular dynamic friction. Traditionally, weight plays a big part in the amount of friction there is - but we're not talking about one surface rubbing against each other - this is rolling friction which is mostly due to tyre deformation and such like.  There is a certain amount of traditional friction in the bearings of the wheels - but that's really very small.
 * Overcoming aerodynamic drag. This is a non-problem at the super-low speeds we're taking about - planes are also really well streamlined!
 * Producing an acceleration. Acceleration equals Force divided by Mass - so mass is really important here.  But acceleration is another non-problem.  No matter how small the acceleration you are able to produce, the speed will gradually build up until all of the force you're able to supply is overcoming friction and drag.  At the low speeds that a person could reasonably pull at, drag is negligable - so once you've got it moving, friction is 100% of the problem - and it doesn't change much with speed...so once the plane is moving, you can gradually speed up until you're pulling the plane pretty quickly.
 * Since the sticktion is bigger than the friction - this becomes only a matter of whether you can get the thing initially rolling. Once it's moving the distance and speed are largely irrelevant.


 * I used to know someone who did a charity jumbo-jet pull with about 20 co-workers (they got sponsorship from the airline for the distance they could pull it...which was kinda silly given the physics of it all...once they got it moving, they could have pulled it around all day!) The biggest concern of the company that runs these events was mostly concerned over people being run over by the plane once it was moving!  The other little 'gotcha' is that you always assume they are doing this on level ground - but my friend said she could see that they'd picked a very gentle slope to do it on...so gentle it was hard to spot - but she spilled the bottle of drink she was carrying and could see the water rolling away ahead of her efforts to pull the plane.  It only takes a very gentle slope to overcome friction on something like a plane.  So this is very easy to fake - and since everyone is doing this for publicity and for a good cause, they all have incentive to cheat if they can get away with it.  So we have to be super-careful about what we do and don't believe.


 * SteveBaker (talk) 23:15, 9 August 2010 (UTC)
 * An additional help is that the tires on aircraft are inflated to a much higher pressure then on a car. Something like 150-200 psi if I recall, so tire deformation should be lower.  Googlemeister (talk) 13:33, 10 August 2010 (UTC)
 * No, the weight is higher, so the deformation is about the same. Pounds of plane / PSI of tire = Square inches of contact patch (not including what the sidewall strength of the tire holds). In theory a tire is a circle with a contact area of zero. So the contact patch represents deformation of the tire. This is true in a car as well. Ariel. (talk) 17:08, 10 August 2010 (UTC)
 * I wouldn't be surprised if they overinflate before one of these stunts. APL (talk) 00:33, 13 August 2010 (UTC)

Food poisoning, even after cooking?
Can dead bacteria and viruses or other related compounds cause food poisoning, even after the food has been thoroughly cooked?

To put it differently, can food that's extremely old always be made safe for human consumption by merely cooking it? &#9775; Zenwhat (talk) 21:43, 8 August 2010 (UTC)
 * I think our article on Bacterial toxin answers your questions. I don't advise trying it out on yourself.    D b f i r s   21:50, 8 August 2010 (UTC)
 * Putting it simply, it's not just the biological agents that cause food-borne illness, but also the toxic chemicals they leave behind. These chemicals can remain after cooking, so no, cooking old food doesn't make it safe.--el Aprel (facta-facienda) 00:15, 9 August 2010 (UTC)
 * I don't know why you felt the need to repeat what had already been said...Anyway, there are also bacteria/viruses that can survive normal cooking. The longer the food has been left, the more of these bacteria there will be...obviously. 90.195.179.60 (talk) 01:46, 9 August 2010 (UTC)
 * Your snotty tone is uncalled for, and a reference is always helpful. Edison (talk) 03:18, 9 August 2010 (UTC)
 * It hadn't already been said... A link had been provided to a page that included the information, but it can help to provide a summary here. --Tango (talk) 03:52, 9 August 2010 (UTC)
 * Another relevant article is Danger zone (food safety).. Some food can be far from "spoilt" but no amount of cooking will make it safe to eat if it has been in the "danger zone" for a few hours. Vespine (talk) 04:43, 9 August 2010 (UTC)
 * It's not food poisoning, and has nothing to do with how old the food is, but there's no way known to destroy prions and leave the food intact. Paul (Stansifer) 05:09, 9 August 2010 (UTC)


 * A good example is ciguatera. From the article: "Ciguatoxin is very heat-resistant, so ciguatoxin-laden fish cannot be detoxified by conventional cooking." Viriditas (talk) 05:17, 9 August 2010 (UTC)
 * Thanks for the help and the example! &#9775; Zenwhat (talk) 21:49, 9 August 2010 (UTC)
 * Salmonella bacteria can often affect cooked meat, while Bovine spongiform encephalopathy requires 48 hours of cooking at 500C(?) to get rid of the prions. ~ A H  1 (TCU) 22:39, 9 August 2010 (UTC)