Wikipedia:Reference desk/Archives/Science/2013 February 22

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February 22

What is the velocity of a body on a circular path?

Consider a body moving on a circular path, its direction of velocity is changing at every instant of time. Therefore, the velocity of the body is also changing at every instant, so, its an accelerated motion. Suppose, the body is moving on the circular path with a speed of 50km/hr. Then, what is the velocity of the body? - 50km/hr or zero. Don't forget the body is changing direction at every instant. Technologous (talk) 01:45, 22 February 2013 (UTC)

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. Someguy1221 (talk) 01:51, 22 February 2013 (UTC)

I don't see this so much as a homework request as a philosophical misunderstanding. Velocity is a Euclidean vector, which is to say that it has a directional component as well as a magnitude. As you've noted, for the provided circumstance, the magnitude of the velocity (the speed) is 50 km/hr. At any given point in time, the direction is along a line tangent to the orbital path, oriented in the direction of travel. It's well-defined and is not at any point in the orbit zero, but neither is it accurate to describe the velocity as simply "50 km/hr" -- that's only a speed, because it lacks the direction. — Lomn 02:05, 22 February 2013 (UTC)

This is not my homework, you think this as my homework question because its answer is very simple. I asked this simple question because I was confused what would be the velocity of the body when it is changing direction continuously. Should we consider the direction while talking about velocity? Again, what would be the velocity of the body - 50km/hr or zero? Technologous (talk) 02:20, 22 February 2013 (UTC)

Neither. Velocity is defined as a Euclidean vector, and accurately stating it requires both a magnitude and a direction. 50km/hr is the magnitude of the velocity, as Lomn stated. The direction is changing at all times, but at any given moment is tangent to the circular path at the position of the body. Someguy1221 (talk) 02:25, 22 February 2013 (UTC)

It's usually simpler to just think of that as a constant rotational velocity. StuRat (talk) 04:56, 22 February 2013 (UTC)

If a body is moving then it has a nonzero velocity. The net velocity over time may be zero, of course. The object has a zero velocity in the rotating frame of reference of someone riding on it who perceives it as stationary. Wnt (talk) 07:17, 22 February 2013 (UTC)

See Circular motion, this explanation, this simpler explanation with an animation. 86.163.209.18 (talk) 13:28, 22 February 2013 (UTC)

It occurs to me that you might be looking at average velocity. The average velocity of the object over a whole rotation will indeed be zero, in that the displacement ends up being zero. However, usually we find it more useful to talk about instantaneous velocity [1] [2] [3] for this sort of thing. If you haven't done any calculus yet, or haven't drawn any graphs showing velocity changing with time, you might find this confusing. Do keep thinking about it, because it sounds like your class is about to cover this in more depth. 86.163.209.18 (talk) 14:01, 22 February 2013 (UTC)

how different are the different species of manatees?

Seems like there are 3 species, but not good breakdown of differences.

• are ranges at all touching? (do animals cross ocean to Africa or do West Indian ones come in contact with Amazon ones?

• physical differention?

• can they cross breed?

• any cpntroversy if the are really differnt species versus subspecies?

Fat burning zone

I've heard doing cardiovascular exercises at 65% of maximum heart rate burns most amount of fat. But there are lot of sources in google search saying fat burning zone is a myth. I was reading this source (page 2) which says fat oxidation is highest when exercising at 65% of MHR. It further claims during 85% of MHR exercise intensity, fat oxidation falls. On the other hand, this source claims during 65% of MHR intensity, a higher percentage of fat is burned, but at higher intensity, the total fat calories burned is higher. This contradicts the claim in the first source. Which claim is true? --PlanetEditor (talk) 04:43, 22 February 2013 (UTC)

In my opinion, any such claims that are not accompanied by references to peer-reviewed papers supporting the claims should be ignored. If they had the science, they would tell us about it. --Guy Macon (talk) 09:37, 22 February 2013 (UTC)

Conditional Probability

I'm learning about probability in my statistics course, and I've learned that P(A|B) = P(A and B) / P(B). Similarly, P(B|A) = P(A and B) / P(A). Now, on a Venn Diagram, P(A|B) would be represented by the area that is within both circles as would P(B|A). However, if P(A) ≠ P(B) then P(A|B) ≠ P(B|A). If P(A|B) and P(B|A) represent the same area on a Venn Diagram, how can they ever be unequal? Have I understood some part here incorrectly? Lord Arador (talk) 04:51, 22 February 2013 (UTC)

I suggest you ask this Q at the Math Desk. StuRat (talk) 04:53, 22 February 2013 (UTC)

You have understood incorrectly. The area on the Venn Diagram that is part of both circles represents P(A and B). P(A|B) and P(B|A), on the other hand, are probabilities within a different set of possible outcomes, namely B and A, respectively. Someguy1221 (talk) 05:21, 22 February 2013 (UTC)

To expand on this: P(A+B) is the probability of being in the overlap of the two circles, given that you are somewhere in the whole diagram. P(A|B) is the probability of being in the overlap, given that you already know you are in circle B. P(B|A) is the probability of being in the overlap, given that you already know you are in circle A. If circles A and B are not the same size, then these probabilities will not be the same. 86.163.209.18 (talk) 09:48, 22 February 2013 (UTC)

See the Venn diagram in our article Conditional probability. P(A|B2) = P(A and B2) / P(B2) is numerically 0.12 / (0.12+0.04) = .12/.16 = 3/4. But P(B2|A) = P(A and B2) / P(A) is numerically 0.12 / (0.10+0.30+0.12) = .12/.52 = 3/13 < 1/4. Duoduoduo (talk) 17:51, 22 February 2013 (UTC)

Thanks so much, this clears it up perfectly. I also apologize for posting at the wrong desk.Lord Arador (talk) 22:50, 22 February 2013 (UTC)

Alternate wheel arrangement

I've seem some carts, for laundry and such, which have wheels in an unusual diamond arrangement. Here's a top view:

+------------------+
|        □□        |
|                  |
|□□              □□|
|                  |
|        □□        |
+------------------+

1) What is the name of this arrangement ?

2) Do we have an article on it ?

3) What are the relative advantages and disadvantages of this arrangement ? StuRat (talk) 05:13, 22 February 2013 (UTC)

This site[4] might help. Unfortunately it doesn't name the arrangement. Dncsky (talk) 05:40, 22 February 2013 (UTC)

The website provided by Dncsky has the key to it but doesn't spell it out. The key is that when this layout is used, all 4 wheels are fixed, i.e., are not swivel castors, and the two centre wheels are installed slightly proud. This means that the cart sits with weight on the 2 centre wheels and only one end wheel - the other end wheel is clear of the floor by 5 to 10 mm. By applying minimal force, you can rock the cart transfer weight to the other end.

Disadvantage: As weight is only on 2 wheels when turning, the load carrying capacity is reduced by 50% compared to other 4-wheel arrangements.

Advantage: By pushing, the cart moves reliably in a straight line, but by a bit of up/down force you can get the weight off both ends simulataneously and easily turn the cart. Other layouts that permit turning will not stably follow a straight line when pushed along.

Advantage: Because it is symetrical, the cart is equally manuverable pushing or pulling from either end.

You most often see this layouts on carts used by the stocking staff in supermarkets, where a) the aisles are narrow and long, b) there's not much room for manuevering, and c) the weight carried is not great, due to retail packaging.

Wickwack 58.170.140.138 (talk) 06:42, 22 February 2013 (UTC)

Another aspect: Look at each of the wheel arrangements an imagine a very heavy object placed in the corner. Some configurations will tip, others will not. --Guy Macon (talk) 06:54, 22 February 2013 (UTC)

Having used such carts in a hospital laundry, I found that they are able to negotiate certain small obstructions a lot easier than a conventional shopping trolley arrangement. Consider a sliding door that runs on a track. A shopping cart needs to be "forced" over the rail by rocking it side to side or just plain brute strength. With the above wheel arrangement, the leading wheel can be lifted off the ground simply by using your body weight to push the trailing wheel down, and so pass over the obstruction. Once the center axis wheels reach the obstruction, it is a lot easier "lift" the cart over the obstruction. It is not necessary to lift the center axis wheels off the ground. The leading wheel will take most of the weight - almost like a wheelbarrow. 196.214.78.114 (talk) 09:19, 22 February 2013 (UTC)

I once used a cart like that with large enough wheels that the deck cleared a standard curb. This made it somewhat easier to maneuver. --Guy Macon (talk) 09:34, 22 February 2013 (UTC)

Also, over time the corner arrangement will tend to cause the cart to sag substantially in the center, whereas the diamond configuration will not (as much).165.212.189.187 (talk) 16:24, 22 February 2013 (UTC)

regarding 3: You can roll it straight stable and easy with middle pair fixed wheels and alike easy turn it on the spot, if front and back wheel can turn. You cannot get the same resultcombination with classic four wheel layout in any combination of fixed or turnable wheels. --Kharon2 (talk) 23:27, 23 February 2013 (UTC)

However, with front and back wheels turnable (castors) it does not inherently run in a straight line when pushed, and so requires a varying lateral steering effort, which may be quite large if the floor is not dead flat. The standard supermarket style trolley, with all 4 wheels fixed, requires no lateral steering effort to run in a straight line, and is thus easier to use, particularly in supermarket aisles, which generally are pretty narrow. Wickwack 121.215.50.17 (talk) 01:41, 24 February 2013 (UTC)

That is true for the common supermarket trolley indeed but the questioned example was a cart for laundry. Imagine your efford pushing that supermarket trolley around a corner if its loaded with 500 Kg to understand the difference. --Kharon2 (talk) 10:49, 24 February 2013 (UTC)

Thanks, all. So we don't have an article on this ? StuRat (talk) 03:57, 25 February 2013 (UTC)

No, unusual wheel arrangements are to special to fulfill minimum relevance for articles. --Kharon2 (talk) 21:35, 25 February 2013 (UTC)

Blast wave

How can I calculate the minimum "safe" distance from an explosion involving a known number of carloads of TNT? Thanks in advance! 24.23.196.85 (talk) 06:39, 22 February 2013 (UTC)

No distance is "safe" - law enforcement has already seen your post and if anything happens, you're pretty easy to track. 86.101.32.82 (talk) 10:02, 22 February 2013 (UTC)

I'd look for a military safety manual and find the safe distance in peacetime for civilians or unprotected troops. Some googling found me this www.af.mil/shared/media/epubs/AFMAN91-201.pdf from the Air Force. There are probably OSHA standards or something similar for blasting work but I doubt that they include safe distances for those amounts of explosives. There might be regulations concerning the storage of large amounts of explosives, but I don't know where. Sjö (talk) 12:44, 22 February 2013 (UTC)

I don't think there is any one "safe" number you can calculate. For example if the explosion kicked up a rock, that rock can fly very very far, much farther than the blast wave itself. If you are able to calculate the size of the pressure wave then I can tell you that 5 PSI overpressure is normally considered the tipping point for structure damage. People can tolerate more than that - but are those people near a building? Other factors like that make it impossible to answer this question. For example, how high is the explosion off the ground? Does it have a surface behind it to reflect the blast wave? Ariel. (talk) 20:09, 22 February 2013 (UTC)

Just FYI, I'm not trying to blow anything up -- it's for one of the scenes in my WW2 thriller, where the good guys have to blow up a German ammo train at the station, but they specifically DON'T want to kill the signalman, so they tie him up and drag him away (normal Maquis SOP for this situation). So the reason why I'm asking is so I can figure out exactly how far they have to drag him. 24.23.196.85 (talk) 02:34, 23 February 2013 (UTC)

I'd drag him to the nearest ditch or other really substantial obstacle. You can survive surprisingly close to ground zero of an explosion if you've got something to shadow you from the shock wave and debris. --Carnildo (talk) 03:42, 23 February 2013 (UTC)

Good idea, Carnildo! Are irrigation ditches plentiful in the bocage? 24.23.196.85 (talk) 21:38, 23 February 2013 (UTC)

No, there aren't generally aren't irrigations ditches in France, the climate doesn't require them. There are drainage ditches instead, which are exactly the same ditches, running the other way. :p--Argarthar (talk) 02:02, 24 February 2013 (UTC)

Very well then, I'll drag him to a drainage ditch (preferably immediately behind a hedgerow -- the Normandy hedgerows can also provide limited protection from the blast). Thanks! 24.23.196.85 (talk) 00:29, 25 February 2013 (UTC)

Checked out the link that Sjo provided; that Air Force manual actually provides some very good methods for calculating the safe distance. Based on that, it looks like my "part-time commandos" will have to drag that signalman the better part of a country mile to ensure his safety. Thanks, Sjo! 24.23.196.85 (talk) 03:19, 23 February 2013 (UTC)

In which case, maybe they should just toss him in the back of their truck and drive away, and then put him out by the side of the road for the field-greys to find once they're at a safe distance? 24.23.196.85 (talk) 03:27, 23 February 2013 (UTC)

It would probably be wise to preface questions like this with "this is for a novel about WW2". That way, nobody will waste time informing you that what you are discussing is dangerous, and as a bonus answers will not assume technology that wasn't available before 1950. --Guy Macon (talk) 07:02, 23 February 2013 (UTC)

Yeah, I should've done that right off the bat. 24.23.196.85 (talk) 21:40, 23 February 2013 (UTC)

Time period of two identical pendulum

There are two simple pendulums and both are identical i.e., their strings are of same length (4 meters) and their bobs have same mass (m). One is kept in water and the other is kept in normal surrounding i.e., in air. Both pendulums are on the earth surface. What would be the ratio of time period of the two pendulums? 106.209.211.104 (talk) 10:20, 22 February 2013 (UTC)

I think it depends on two factors; first the density of the bob (because its effective weight is reduced according to the mass of water it replaces) and second the aerodynamics if water friction must be taken into account. - Lindert (talk) 10:29, 22 February 2013 (UTC)

Conflict - the period of a pendulum is independent of the mass of the bob. 196.214.78.114 (talk) 10:58, 22 February 2013 (UTC)

The period is normally independent of the mass only because the (effective) gravitational force is proportional to the mass of the bob. When the bob is immersed in water, that is no longer true. The period of a pendulum depends on the gravitational acceleration, and the latter changes when objects are immersed. For example, if the bob has a density equal to that of the water, it would not swing at all. - Lindert (talk) 11:57, 22 February 2013 (UTC)

The gravitational force doesn't change. Rather, there are new forces at play that didn't exist before: buoyancy and water resistance. Actually, buoyancy and air resistance exist in air, but their effect is so small that they can usually be neglected. --140.180.254.250 (talk) 19:21, 22 February 2013 (UTC)

I don't think you could come up with a "ratio of time period" in the above scenario. The friction on the pendulum in water would bring it to a standstill rather quickly - it would oscillate with a significantly shorter period each time. Remember that "The simple gravity pendulum is an idealized mathematical model of a pendulum." (see the wikipedia article). In real life one would strive for a frictionless environment to achieve a constant period - like in a pendulum clock - which uses a wound spring to offset friction and keep the pendulum going. 196.214.78.114 (talk) 10:52, 22 February 2013 (UTC)

I guess it depends on the size of their bobs too i.e. they would have different Reynolds numbers. Sean.hoyland - talk 15:25, 22 February 2013 (UTC)

It might be interesting to consider this question in superfluid liquid helium - apparently a torsion pendulum was used to prove ultra low viscosity in the first place, though I'm not so sure about pendulum bobs. Would certainly be funny to see. Wnt (talk) 23:54, 22 February 2013 (UTC)

Buoyancy and friction require knowing volume and cross-sectional area, respectively. For teensy bobs with tiny periods, the pendulums would swing exactly the same. Interesting question though. SamuelRiv (talk) 17:09, 24 February 2013 (UTC)

Matter wave diffraction

Suppose you fire electrons through a slit onto a screen, so it causes a pattern along the screen. If you have the probability distribution over the momentum for an electron (something that can be deduced from a given wave function) how do you then calculate the distribution of the electrons impacting on the screen? — Preceding unsigned comment added by 150.203.115.98 (talk) 13:27, 22 February 2013 (UTC)

Where did the mercury in coal come from?

Howcome coal contains mercury, selenium, and arsenic in the first place? is it that the nature when it formed were polluted with these substances? Then those plants and aninmal ought to have a higher tolerance for this? Electron9 (talk) 14:18, 22 February 2013 (UTC)

Firstly, look at the amounts involved: According to [5], burning coal in the US releases 50 tons of mercury per year. This source [6] says that the US mines more than a billion tons of coal per year - and burns 90% of it (the rest is exported). So the amount of mercury in coal is roughly 50 parts per billion. Coal forms from rotting plant material that turns into peat. According to this [7] about half of the mass of a tree is carbon - and that's pretty much all there is in coal - so the plants from which coal is formed had to have about 25 parts per billion of mercury in them. According to mercury poisoning the USDA limit for mercury in food is one part per million - so 500 parts per billion. Hence, we may deduce that the plants that formed coal had about 5% of the USDA limit...the animals of that era had no risk from mercury poisoning. SteveBaker (talk) 14:34, 22 February 2013 (UTC)

I meant more, was the overall mercury level in the environment higher when it formed than it is now? and in particular how high then and now? Electron9 (talk) 15:34, 22 February 2013 (UTC)

I searched google scholar for /paleoclimate mercury/. One of the top hits was this paper that appeared in Science in 1999: "Mercury in a Spanish Peat Bog: Archive of Climate Change and Atmospheric Metal Deposition"-- you can read the abstract (and whole article if you can get access through a library) here [8]. Anyway, I haven't read the whole article, but this sentence- "Anthropogenic mercury has dominated the deposition record since the Islamic period (8th to 11th centuries A.D.). "- leads me to believe that mercury levels were generally lower prior to the anthropocene. The paper only analyses samples up to 4k years before present, which is too recent for coal from the carboniferous, but there are probably papers in the references (or that cite this one) that go back further in time. SemanticMantis (talk) 17:12, 22 February 2013 (UTC)

The key thing to remember here is that the coal deposits we burn weren't laid down in the few years it has taken us to run through them. They represent the accumulated debris of millions of years, and whatever survives the trip up the flue ... comes back down to haunt us faster than it can wash out to sea. Wnt (talk) 00:01, 23 February 2013 (UTC)

BCNT - what's the deal with 11B?

Boron neutron capture therapy works by ¹⁰B absorbing a neutron to give ¹¹B that decays, releasing destructive energy in its vicinity, killing cancerous cells. At least I think that's how it works. The catch is that ¹¹B is apparently rather stable nucleus, comprising 80% of boron in nature. So is the initially formed ¹¹B derived from ¹⁰B some sort of labile excited state?--Smokefoot (talk) 14:23, 22 February 2013 (UTC)

Apparently a metastable state. I can't find a ref for a distinct ^11mB nuclear isomer, but I do see some comments that there is some excess energy from the ¹⁰B + n → ¹¹B reaction. I don't know if it's just momentum from the collision that winds up being dissipated by fission rather than some other emission or increased vibration? How does the actual energy balance work?--is there a change of nuclear mass defect to indicate that that some of the incident neutron's mass winds up as available energy? DMacks (talk) 16:36, 22 February 2013 (UTC)

Wouldn't you expect the newly created ¹¹B to emit a gamma ray? μηδείς (talk) 15:27, 23 February 2013 (UTC)

An alloy that expands on heating

My question is on alloys.

Name an alloy which expands on heating.

Thanks for your help Ssmagic (talk) 16:58, 22 February 2013 (UTC)

Almost every alloy expands on heating. Looie496 (talk) 17:01, 22 February 2013 (UTC)

Are there any at all which don't? Not including unverified claims such as those for YbGaGe--Gilderien Chat|List of good deeds 19:59, 22 February 2013 (UTC)

Invar has a coefficient of thermal expansion close to zero. 24.23.196.85 (talk) 03:13, 23 February 2013 (UTC)

There are a number of materials that have a negative thermal expansion coefficient over a temperature range. Water, for example, contracts when heated from 0C to 4C, and zirconium tungstate contracts over almost its entire solid temperature range. --Carnildo (talk) 03:52, 23 February 2013 (UTC)

Expanding alloys

My question is on alloys

Name an alloy that expands on coolingSsmagic (talk) 17:02, 22 February 2013 (UTC)

Sorry, my earlier question was incorrect

Thanks

We have an article on this, actually. You can check negative thermal expansion for some answers. Livewireo (talk) 17:54, 22 February 2013 (UTC)

This sounds like a homework question. Woods metal and Field's metal are two very common alloys that would both fit the bill.--Aspro (talk) 17:58, 22 February 2013 (UTC)

See an example of it used for Movable type#Type-founding where the expansion of the metal makes strong sharp edges in the letters. It's an allow of lead tin and antimony. Ariel. (talk) 19:22, 22 February 2013 (UTC)

Search for some bismuth alloy. OsmanRF34 (talk) 19:46, 22 February 2013 (UTC)

A sealed ecosystem that thrives for 40 years???

All around the web this story is going strong, here's one example [9]. I updated the article about Ecosphere_(aquarium) and noted that according to the article Every manmade closed system inevitably degrades. Has Latimer allegations been tested? Where can this conundrum be further researched? Zarnivop (talk) 18:21, 22 February 2013 (UTC)

Well, the bottle isn't a closed ecosystem in that it relies on outside light. I also wonder about the carbon dioxide being released by rotting vegetation. I wouldn't expect 100% of the plant's CO₂ needs to be fulfilled by that mechanism. I would think you'd need animals to convert free oxygen back to carbon dioxide. Perhaps microscopic animals are present. StuRat (talk) 19:11, 22 February 2013 (UTC)

It's a matter of time scale. 40 years may seem like a long time, but it really isn't, insofar as the nitrogen cycle, water cycle, and carbon cycle of that bottle are concerned. Predicting exactly what will cause it to fail/die is difficult, but "inevitably degrades" is true of this bottle too. SemanticMantis (talk) 19:39, 22 February 2013 (UTC)

If you wait long enough, the earth will "inevitably degrade" when the sun becomes a red giant. Many of our our pages in this area are a mess. I would encourage anyone who has the time and interest to gather some citations and rewrite portions of the articles

If you look at http://www.synergeticpress.com/wp-content/uploads/2012/04/Bioscience-1993-using-a-closed-system-to-study-the-Earth.pdf ("Previous research in closed ecological systems" section) some systems are talked about that were closed in 1968 and were still functional as of the 1993 date that the paper was published (25+ years). As far as I know, there is no upper limit to how long a Winogradsky column can operate

Articles that need improvements and/or sections about how long they can last:
Closed ecological system
Controlled Ecological Life Support System
Spome
Wardian case
Winogradsky column
Eden Project
Biosphere 2
BIOS-3
MELiSSA
Biosphere
--Guy Macon (talk) 20:20, 22 February 2013 (UTC)

My feeling is that the odds of degradation also should depend on the number and type of species present - specifically, any closed system should be subject to Island biogeography. For example, if you look at that plot in the article for the number of lizards on Caribbean islands, you can predict an intercept somewhere around the size of a football field to keep even one species alive. Below that and sooner or later you'll have a tragic mishap and end up with all males or something. (I really should review the theory of single species metapopulation dynamics - there are lots of studies like [10] based on a 1970 theory by Richard Levins that do real-life games of life to see what happens) A bottle garden with a plant is harder to picture becoming unstable - nonetheless, there's no reason to suppose that the plant's lifespan will be infinite, and it is possible that when the whole plant decays that the parameters inside the bottle will go outside the range that any potential seedling can tolerate. Wnt (talk) 23:04, 22 February 2013 (UTC)

I gather from this discussion that the statement as presented in the article is false. I edited the article. I also try to convince my wife to try spiderwort sphere, and I may get back to you in 20 years or so. Zarnivop (talk) 16:00, 23 February 2013 (UTC)

ips cells

How do ips cells work? Clover345 (talk) 19:19, 22 February 2013 (UTC)

You might find the Induced pluripotent stem cell article to be helpful. Red Act (talk) 19:35, 22 February 2013 (UTC)

does pwm (pulse width modulation) led's hurt eyes?

does applying pwm to 'dim' LED's in fact hurt eyes, as they are on full-blast, albeit a shorter period of time? 86.101.32.82 (talk) 21:55, 22 February 2013 (UTC)

It would all depend on how the actual light level changes. If they go from black to full brightness and back, then it might cause a problem. However, I doubt if they do that. More likely, they just get a bit dimmer and brighter going through the cycle. Also, even if it did go completely black in the cycle, if it did it quickly enough (say 60 or 70 times a second), our eyes would only see the average, not flickering, so it would be OK. StuRat (talk) 22:03, 22 February 2013 (UTC)

This is what I mean - why would our eyes "see the average"? I mean, if you imagine a laser that is bright enough to burn a retina, is it "ok" as long as it's only on for 1/1000th of a second? What I mean is that there must still be a difference between every photon that is going to come through in 1 ms coming through at an even pace throughout that ms, or instantaneously at the start of that ms...don't you think? 86.101.32.82 (talk) 23:15, 22 February 2013 (UTC)

Well, chemical reactions take time, including those which would cause damage to the retina. For example, if the LED is going to raise the local temperature to a point where the retina will be burnt, it will take some time for the temperature to build to that point. Now, if the photons were coming in quickly enough, that time period might be extremely low. However, the level of photons coming off an LED probably isn't enough for that. So, this gives the retina a chance to cool down when it is switched off. This continual heating and cooling results in a temp close to the average.

A useful analogy here might be the Earth's day and night cycles. If the Earth didn't rotate relative to the Sun, then the side facing the Sun would be too hot for us to live, and the dark side would be too cold for us to live. But, with it rotating, it starts to heat up during the day, then starts to cool down at night, never getting to the extremes it would reach if stationary. StuRat (talk) 23:23, 22 February 2013 (UTC)

It sounds from [11] like a pulse laser takes maybe 40x less energy to burn the retina if it produces "giant" pulses with a high peak power than "long" pulses with lower power, but I'm afraid I'm missing some of the context to understand it fully; in particular I missed how often these pulses were. Not really my thing - this was a top hit from a Google search; probably you can find better studies. Wnt (talk) 23:31, 22 February 2013 (UTC)

In the case of the laser, the total power is too low to cause damage, but because the laser beam is concentrated in a small area of the eye, the short pulse heats that area faster than the heat can dissipate, while the long pulse doesn't. --Carnildo (talk) 03:58, 23 February 2013 (UTC)

Humidity

A question asked earlier piqued my interest. Why would one humidify a room? I live in Ireland and can't really imagine what a "too dry environment" would feel like. — Preceding unsigned comment added by Stanstaple (talk • contribs) 23:25, 22 February 2013 (UTC)

In parts of the world where it gets very cold and dry in the winter (the American midwest, for example), humidity in the living space feels better than dryness. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:33, 22 February 2013 (UTC)

Your skin gets dry, starts to itch, and eventually cracks. The mucous membranes in your nose start to dry out, which is uncomfortable and makes you more susceptible to sinus infections. Very dry air is also bad for musical instruments. thx1138 (talk) 23:44, 22 February 2013 (UTC)

(See winter itch) Wnt (talk) 23:59, 22 February 2013 (UTC)

You might try half-an-hour of mouthbreathing in a restaurant's walk-in freezer. For the first time this winter I plugged in a humidifier last night. I have a sore-throat from the dryness. μηδείς (talk) 23:52, 22 February 2013 (UTC)

In my case I get chapped lips, which will crack open and bleed if I don't apply lip balm, and deep cracks in the callouses on the soles of my feet, which would also crack open and bleed, if I didn't apply bag balm to them each night. Fortunately, I have generally oily skin, so the rest is OK. On the plus side, I can just toss my grapefruit peels in the garbage, and they dry out, whereas they would get fuzzy if I tried that in (humid) summer. StuRat (talk) 00:01, 23 February 2013 (UTC)

Chapped lips - deep cracks in the callouses?!!! See your doctor. This is symptomatic of a lack of Vitamins A, B's , C, and E. Especially B2. Is your skin ever scaly etc.? By a rifle (no, not an AK 47 off off eBay but a good one) and hunt your own food. Eat everything, liver, kidneys, brain, lungs, etc., and let you wife fry up the gonads. Eat with freshly ground wholemeal bread. Vitamin deficiency gone -leaving you with lips that a moose would love to kiss and feet like Fred Astaire.--Aspro (talk) 01:01, 23 February 2013 (UTC)

I don't think so. This happens every winter. Due to the poor insulation in this house, the relative humidity inside stays around 20% all winter. I have a healthy diet and take vitamins supplements, and don't have any problems, so long as I remain properly em-balmed. StuRat (talk) 08:19, 24 February 2013 (UTC)

<peeve> calluses, no o. Callous is an adjective; callus is a noun. </peeve> --Trovatore (talk) 04:06, 23 February 2013 (UTC)

How callus of you to make that wise-crack. (Now don't you feel like a heel ?) :-) StuRat (talk) 08:14, 24 February 2013 (UTC)

I'm not sure about the relation of chapped lips to weather. I don't find much about it at all in PubMed; one paper that mentions it is a study in military recruits during the summer. I've seen the claim in textbooks and other sources citing old studies that there is a relationship between these and riboflavin levels (which causes angular cheilitis when deficient) [12]. Wnt (talk) 00:16, 23 February 2013 (UTC)

Low humidity definitely has an effect on my lips. I don't know why they have difficulty in determining this. Why wouldn't the extremely thin skin on the lips allow moisture to escape, and why wouldn't this rate of evaporation be dependent on the atmospheric humidity ? I've also noticed that salty foods make my lips chapped. StuRat (talk) 00:36, 23 February 2013 (UTC)

Well, personally I've never used a lip balm, including in Midwest winters; since I read about the vitamin issue as an undergrad I've taken any roughness at all as good reason to take a multivitamin - but in truth I do that very rarely. Wnt (talk) 08:06, 24 February 2013 (UTC)

I don't understand. You think chapped lips are only caused by vitamin deficiencies ? StuRat (talk) 08:14, 24 February 2013 (UTC)

I shouldn't say that. As I said above, I really don't know what causes them - there's not much literature to go by. All I'm saying is that I'm skeptical that they are the ordinary result of dry weather. Given the various stuff on the web about people so skeptical that lip balms are "addictive" that they actually have an urban legend about "ground glass" in them, I'm getting the feeling that people who use balms are not really reaching the root of the problem. Wnt (talk) 08:50, 24 February 2013 (UTC)

I suppose the root of the problem is that humidity is too low, but it's not very realistic to change that in some places (like outside), so treating the symptom with lip balm seems like the way to go. At $1 a tube, I find it solves the problem quite nicely. StuRat (talk) 04:05, 25 February 2013 (UTC)

I wonder if the poster comes from that south western part of Ireland where the only dry parts are were the Garda Síochána na hÉireann have convisicated all the poitín stills. Take a holidy in the Arab Emirates. 105 Fahrenheit in the shade, relative humidity 20% and you'll drink a gallon a water an hour... as the whole land is tea total – then you'll under stand dry--Aspro (talk) 00:31, 23 February 2013 (UTC)!

Here is a description of what living in low humidity is like: http://livingintok.wordpress.com/2011/12/14/the-dangers-of-low-humidity/ --Guy Macon (talk) 07:10, 23 February 2013 (UTC)

Because we in the British Isles are surrounded by water and most of our weather comes either from the Atlantic or the North Sea, it never gets really dry here. The humidity in both London and Dublin today is 81%. Alansplodge (talk) 12:28, 23 February 2013 (UTC)

Something else bad about low humidity is it's effect on wooden furniture, which can warp or crack. Applying oil to it helps, but you really need to keep the humidity up, too. (Technically it's the change in humidity which is the problem, so, if you grew a tree in low humidity, and always kept it's wood in low humidity, it would be OK.) StuRat (talk) 16:52, 23 February 2013 (UTC)

what's the essential difference between capacitor and a battery?

what's the essential difference between capacitor and a battery? What I mean is, with the recent "breakthrough" of the guy printing a dvd of carbon capacitor, like, could it be that it stores 1 kilowatt-hour on it and discharges it over 5 minutes? Meaning you just draw 12,000 watts for 5 minutes continuously? (The example could be reread at 1/10th rate: store 0.1 kw-hours on it, meaning drawing 1200 watts for 5 minutes conntuously). The reason I ask is that the NORMAL way i think of capacitor is "instant discharge" (<1 second). But aren't there slow discharge capacitors?

In this case what is the essential (ESSENTIAL) difference or line in the sand between a (slow-discharge) capacitor and a battery? The carbon printing on CD guy mentioned charging a capacitor briefly and then powering a led on it for a long-time? THen isn't it technically a battery and not a capacitor? Thanks. Also motor capacitor. Thanks. 86.101.32.82 (talk) 23:55, 22 February 2013 (UTC)

A capacitor stores electrons and/or holes without alteration, such as in the conduction band of a metal. There is no chemical reaction. A battery reacts to uptake those electrons into some compound or another, which means that more electrons can come in without being repelled by those already stored, until the amount of material practically available to react is used up. Wnt (talk) 00:19, 23 February 2013 (UTC)

Also, a capacitor stores a net charge. Batteries are electrically neutral. Batteries work by chemical reactions that maintain electrical neutrality, but work by moving electrons from areas of high potential to areas of low potential, and replacing the electrons so moved by moving ions into their place. See Galvanic cell for a nice, idealized picture of how all batteries basically operate. The key is, at no point and nowhere in the battery is there a net build up of charge (or indeed, anywhere in the circuit). In a capacitor, you have a net buildup of both positive and negative charges on the two poles, batteries don't work that way. --Jayron32 00:47, 23 February 2013 (UTC)

To avoid confusion: a capacitor has no net charge overall. It has 2 plates, one of which has a charge of Q, and the other has a charge of -Q. The two sum to 0. --140.180.254.250 (talk) 00:49, 23 February 2013 (UTC)

I remember reading an article about work on this sort of capacitor shortly after 9/11. One problem was they could discharge so fast they were essentially little bombs. μηδείς (talk) 00:51, 23 February 2013 (UTC)

That's true, 140, but there is a localized buildup of charge on each pole of the capacitor. No such localized buildup of charge exists in a battery, which from my point of view, is the essential difference. --Jayron32 01:12, 23 February 2013 (UTC)

The distinction between a capacitor and a battery can sometimes be blurry nowadays. In particular, one type of supercapacitor is a hybrid capacitor, which stores a charge both electrochemically and electrostatically. Red Act (talk) 02:02, 23 February 2013 (UTC)

Well, true, but the very existence of a hybrid category only highlights the clear distinctions in the traditional devices. --Jayron32 02:20, 23 February 2013 (UTC)

There are two fundamental differences between batteries and capacitors. 1) A battery works by storing electrochemical tension, and a capcitor works by electrostatic tension, as explained by some of the posts abeove, and 2) for a constant current load, a battery delivers essentially constant voltage over time until it reaches a point of exhaustion, whereupon the voltage drops rapidly; A capacitor with a constant current load delivers a voltage continously and linearly decreasing with time.

It is perfectly feasible (though not gnerally cost effective) to use a capacitor of sufficient size to run an electrical device (motor, radio, etc) however allmost all devices require a near constant voltage. A battery does this inherently, but a capacitor will require a power conversion circuit (switch mode power convertor) in order to do this.

Capacitors, because they work by electrostatic tension, inherently are precise devices if used for timing purposes. As batteries work by means of electrochemical tension, and chemical reactions are very much temperature dependent, The time to discharge in a battery is generally an innaccurate thing.

As mentioned above, you do get devices that differ to some extent from what I've listed above. For example, electrolytic capacitors do not have very accurate capacity, and some capacitor types show a degree of electrochemical storage.

Keit 120.145.29.214 (talk) 05:55, 23 February 2013 (UTC)