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May 10

Several siblings, all male or female

The Boy or Girl paradox article led me off onto a vaguely related train of thought. Imagine that a couple has had six or seven children together, and all of them are girls or all of them are boys. If they have a seventh-or-eighth child, is the baby more likely than not to be of the same sex as its siblings? I'm wondering whether in such cases certain biological factors could come into play, since this isn't merely a coin flip: for example, the father's sperm production is unevenly balanced one way or the other (so it's not a 50-50 chance at conception), or the mother's somehow physically more likely to miscarry one sex than the other (so sex distribution at conception doesn't match the sex distribution at live birth). Or are such families (assuming no intentional abortions, post-birth deaths, etc.) basically just the result of chance, since approximately 1/32 of all six-children families can be expected to have children of just one sex? Nyttend (talk) 04:59, 10 May 2017 (UTC)

There are at least three mechanisms at play that may lead to deviation from an exactly 50-50 chance of either sex. First, human sex ratio is not 50-50, but rather the chance of a boy being born is slightly higher than 50%. This may be because the Y-sperm is slightly lighter (and therefore more mobile) than X-sperm, or due to a number of other mechanisms some of which are discussed in the human sex ratio article. Second, families may "want a boy" or "want a girl", in which case they are more likely to stop an the minority sex (n boys 1 girl or n girls 1 boy) rather than at the majority sex (n boys 0 girls or n girls 0 boys). Third, as you say in the question, there are genetic mechanisms that may cause substantial deviations from 50-50 in affected families. I'll see if I can find some more-or-less accurate numbers to quantify these effects. Dr Dima (talk) 07:47, 10 May 2017 (UTC)

To the extent that sex at birth is random, Dima's second point is fallacious. Say that every day you start flipping a fair coin and continue until it comes up tails. You will see sequences of T, HT, HHT, HHHT, etc., but it's still true that most probably 1/2 of the total flips will be heads. The longer the run of heads, the less probable it is. Well, similarly with births, if they are random. However, this is really irrelevant to the original poster's question, which is asking if the births are random. --76.71.6.254 (talk) 10:17, 10 May 2017 (UTC)

I appreciate your comment, but I'd like to point out that you misunderstood my statement. Consider a process where coin is tossed until both heads and tails occur at least one. For such a process, all 6-toss sequences will have either 5 heads and 1 tail or 5 tails and 1 head; indeed, having 6 heads will result in a more than 6-toss sequence, until a tail occurs. I interpreted the OP question "... since approximately 1/32 of all six-children families can be expected to have children of just one sex?" to refer to exactly 6-children families, in which case my statement above is valid, and not fallacious. Dr Dima (talk) 16:03, 10 May 2017 (UTC)

Agreed, by that interpretation it's valid. --76.71.6.254 (talk) 19:04, 10 May 2017 (UTC)

If we do a Fermi calculation, and ignore the fact that it isn't really 50-50 as noted above, let's assume it is 50-50. The answer to your question " is the baby more likely than not to be of the same sex as its siblings?" the answer is that the 8th baby has a 50/50 chance of being the same sex as its prior siblings. If there were a 9th baby, it would have a 50/50 chance of being the same sex as its prior siblings. Odds don't have a memory, so each child has a 50/50 chance of being one gender or the other. Now, the probability of one mother having, say, nine boys (again, assuming a true 50/50 chance rather than the reality which is slightly different) is 1 in 2⁹ or 1 in 512, which is only a 0.195% chance. Though not impossible; my grandmother's sister had nine children, all boys. --Jayron32 10:33, 10 May 2017 (UTC)

Whilst I applaud the rationality, I have a sneaking suspicion that a given womb-host-system may be slightly more allergic to one sex than the other. So it isn't a pure coin flip. Greglocock (talk) 10:55, 10 May 2017 (UTC)

But as the OP says, the question is whether it is 50-50 and by how much it can devy. It is a Bayesian inference problem, but we only have the observation that most children are of either sex, and the prior probability distribution (is "50-50" a good model?) is hidden in a giant heap of biological and environmental data.

If you play dice and observe that certain numbers come up more often than others, external clues can give you more confidence that the dice are loaded - the same sequence could be attributed to chance in a high-security Monaco casino, and to loaded dice in a Chicago suburb mafia-run pub. The question is how strong these external clues are in the case of pregnancy - by how much can genetic changes improve odds of either sex and how likely those genetic changes are. Tigraan^{Click here to contact me} 11:07, 10 May 2017 (UTC)

I agree that the OP is correct that the are some circumstances when the odds aren't really 50-50 (as with others, let's ignore they are never actually 50-50), but the probabilities of these are likely so low that even with 8 children of one sex these cases will still be lost in the noise i.e. nearly all such cases will just have been due to complete random chance. Nil Einne (talk) 11:29, 10 May 2017 (UTC)

Further complicating these calculations is that estimates range from 25 to 40 percent of pregnancies that result in miscarriage. (Google the subject and you'll see.) So unless the sex of the miscarried embryos is known, you can't really be certain that an apparent trend of all one sex is really a trend. It could be like XmmmXmmmXm for the case of seven successful pregnancies yielding males. If the X's were all female, that might suggest a sex-related problem, but it could also be coincidental. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:00, 10 May 2017 (UTC)

Empirical evidence: About 30 years ago I collected extant data (sorry I can't remember where) on sex of a child as a function of sex distribution of a couple's previous children. I found that there is a mild tendency for couples with more children of one sex than of the other to have their next child be of the majority sex. If I recall correctly, having several children all of one sex increased by several percent the probability that the next child will be of that sex. The context was the question of what effect perfect advance selectability of children's sex would have on number of children per couple. Loraof (talk) 14:01, 10 May 2017 (UTC)

• The article Sex ratio has a good discussion, including the passage sex ratios at birth may be considerably skewed by factors such as the age of mother at birth[3]. Also, the article mentioned by Dr. Dima above, Human sex ratio, has a good section called "Natural factors", which says These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, and parent's psychological stress. Loraof (talk) 14:12, 10 May 2017 (UTC)

• Many decades ago I came across (in the New Scientist etc.,) of couples being able slew the chances of getting a boy or girl -by diet alone. Hans Eysenck noticed that airline crew had more daughters. Well back then, little girls did not grow up to be airline pilots -like dad, so dad wanted a little boy ( or two tow) follow in dads footsteps. And hey what?!!! They fathered more little boys when their wives followed the diet. Didn’t think much of it at the time but now as the question has arisen- have googled it. Lots of references that this belief continues. (i.e., https://www.everydayfamily.com/gender-selection-what-is-your-personal-ph/) It makes some biological sense as well. Aspro (talk) 17:33, 10 May 2017 (UTC)

There's also the possibility that a couple that produces only daughters carry a mutation for an X-linked dominant disorder that is lethal in males in utero such as incontinentia pigmenti or Rett syndrome. - Nunh-huh 02:39, 15 May 2017 (UTC)

What does it mean "Negative logarithm" of the concentration of hydrogen ions.

I've read on this site about the definition of pH as follow: "pH stands for potential hydrogen, and is defined as the Negative logarithm of the concentration of hydrogen ions." I understood whole the sentence except of "negative logarithm". what does it mean? 93.126.88.30 (talk) 17:05, 10 May 2017 (UTC)

Wikipedia has an article on pH which explains this. The logarithm usually turns out negative. The pH value doesn't include the minus sign, so an alternative definition is "the logarithm of the reciprocal of the concentration of hydrogen ions". Ask again if you need more help. Dbfirs 17:30, 10 May 2017 (UTC)

Thank you. Iv'e checked this article and honestly I still don't understand. There is a sentence there which says: "For example, a solution with a hydrogen ion activity of 5×10−6 = 1/(2×105)", then I don't understand how existence can be represented by minus (=no existence). 93.126.88.30 (talk) 01:16, 11 May 2017 (UTC)

Negative numbers do not mean "nonexistent". If you have a sample of pure water at STP, autoionisation will ensure [H⁺] = 10^–7 mol·dm⁻³. By the definition of the common logarithm, log [H⁺] is the exponent 10 must be raised to to give 10⁻⁷, which is clearly −7. pH is defined to flip the sign and make it +7, because most common acid solutions have nowhere near a concentration of protons of 1 mole per litre. That's why it's not the logarithm, but rather the negative (additive inverse) of the logarithm. Double sharp (talk) 01:28, 11 May 2017 (UTC)

For negative powers of ten, our article Scientific notation might help. Dbfirs 08:27, 11 May 2017 (UTC)

I'll try this from scratch. Positive logarithms are used for increasing quantities of substances: 10 balls is 1 log unit, 10000 balls is 4 log units. Negative logarithms are convenient when the quantities are really small: a concentration of 1 in 10 is -1 log units, 1 in 100 is -2 log units, 1 in 10,000,000 is -7 log units. That's because these concentration numbers would be written as fractions (1/10,000,000 = 0.00000001 in the last example, which is typical for neutral pure water.) For pH, we also assume specific units - in general, a concentration recipe could be something like 1 teaspoon in 10 cups of porridge, but for pH we're defining it to be mole (unit) per liter, i.e. molarity, more or less. (There are unimaginable levels of pedantry available about exactly how pH is defined, pH vs. p[H+], activity (chemistry) etc. - not even most scientists have any desire to know these details, but fortunately their effect is relatively small most of the time. A basic college chem course will use the definition I just gave and stay mum about the rest.) Wnt (talk) 12:49, 11 May 2017 (UTC)

Negative logarithm means -1 times the logarithm (i.e. negative refers to reversing the sign of the number.) It isn't a different type of logarithm. RJFJR (talk) 19:55, 11 May 2017 (UTC)

Note that the part of the logarithm to the right of the decimal point is always positive. See common logarithm. — Preceding unsigned comment added by 86.151.48.31 (talk) 22:36, 11 May 2017 (UTC)

I don't think that's a helpful remark in this context.

What you're talking about is a convention for calculating with common logarithms, from back when people used to do that. It doesn't apply here. If you have slightly alkaline water with a pH of 7.3, what that means is that −log₁₀[H] = 7.3, where [H] is the hydrogen-ion activity (roughly the same as concentration) in mols per liter. So that means log₁₀[H] = −7.3, not 7̄.3, which would equal −6.7. --Trovatore (talk) 22:50, 11 May 2017 (UTC)

As more and more Hydrogen ions exist the more acidic it is?

I've read the information about pH here, then can I conclude that as more and more Hydrogen ions exist the more acidic it is? the more acidic it is, or it is better to say that as more and more the cations exist the more acidic it is while the more and more anions exist the more basic it is? 93.126.88.30 (talk) 17:21, 10 May 2017 (UTC)

Read Ocean acidification. --Kharon (talk) 20:50, 10 May 2017 (UTC)

You cannot simplify it to all cations and anions, no, since not all cations and anions are involved in acidity/basidity of water. Saturating water with NaCl will have negligible, if any, effect on the pH of the water, despite a huge amount of Na+ cations and Cl- anions now in the water. Additionally, you are going to have an equal amount (charge wise) of cations and anions in water anyways, since the charge needs to balance out. Water with more hydronium ions is more acidic. --OuroborosCobra (talk) 23:01, 11 May 2017 (UTC)

Thermodynamics of a permanent magnet

Does a permanent magnet's field weaken every time it is used to pick up a paperclip from my desk? It is after all doing work in lifting a mass, thus there must be waste heat. Roger (Dodger67) (talk) 18:50, 10 May 2017 (UTC)

No. Why is it, you think, they called it "permanent"? --Kharon (talk) 20:49, 10 May 2017 (UTC)

Proton decay. Sagittarian Milky Way (talk) 21:36, 10 May 2017 (UTC)

This is a red herring. ApLundell (talk)

Nothing with such a high percent of protons in it may be permanent. Of course that is irrelevant to whether the work wears out the magnet or just come from the person lifting the magnet and proton decay takes longer than human timescales to possibly happen. Sagittarian Milky Way (talk) 22:33, 10 May 2017 (UTC)

No. This question is a based on a very common misunderstanding of how magnets work. They do not expend energy that must be replenished.

They are simply converting potential energy to kinetic energy and back. Similar to moving a weight up a heavy hill and letting it roll down. The Earth does not wear out its gravity field when things move up and down.

This common misunderstanding often leads people to believe they have an idea for a perpetual motion machine. (They don't.)

As a practical matter, some magnets do weaken over time, but that's unrelated to the 'energy' they seem to expend. (But don't really.)

ApLundell (talk) 21:58, 10 May 2017 (UTC)

Was the comment way up above about proton decay intended to answer the very reasonable question, or was it an attempt at humor? And the claim that the magnets are "permanent" is also untrue, since they weaken over time. Horseshoe magnets in science demonstration sets came with an iron "keeper" with instructions to leave the keeper across the poles to reduce the loss of magnet strength over time. If I use a permanent magnet to an iron weight some distance, as in when the object flies up from a desk to the magnet, travelling a small distance through the air, work has certainly been done by the magnet, and the energy has to come from somewhere. Edison (talk) 00:17, 11 May 2017 (UTC)

Proton decay is mostly humor. Maybe insufficient coercivity or other things like orbiting the galaxy till hitting a black hole would demagnetize anything before the protons decay so if that's proven would only be a weak disproof of permanence by contradiction. Sagittarian Milky Way (talk) 01:43, 11 May 2017 (UTC)

Please do not make jokes or other pointless statements that seem like they might be answers to the question, but definitely are not.

It is very confusing to people asking questions. ApLundell (talk) 18:57, 11 May 2017 (UTC)

I like deadpan-type humor but very well. Point taken. I could never become a physicist and I know it. Not trying to impress. Who hasn't heard of proton decay? Sagittarian Milky Way (talk) 19:33, 11 May 2017 (UTC)

The question here covers a lot of different things which are peripherally related.

• A magnet does work in picking up a paperclip, but the amount of work done is quite small, and the energy must be put back into the system from an external source if you expect to get your paperclip back.
• In theory, a magnet might not need to produce much waste heat. For example, you might set up a little plastic track near a magnet and roll a ball along it as if you were rolling it in a dish under the effect of gravity. In such a situation the ball might go back and forth quite a few times, dissipating relatively little energy in rolling friction on the track, before losing all its kinetic energy and coming to the lowest potential energy state at the closest point on the track.
• That said, a conductor such as a steel ball moving through a magnetic field is going to experience some kind of eddy current, which will produce a sort of "friction" not seen with a ball in a dish. The current will tend to oppose the motion and slow the ball sooner than it would have otherwise, I think.
• There is energy in the magnet that can be lost if it loses its magnetization. To illustrate this, consider what happens if you have a magnet you're screwing around with and you break it. I suppose if you break it perpendicular to the N-S line then the north pole of one piece will stick to the south pole of the other, but definitely if you break it the other way you'll have two N-S magnets next to each other that want to end up flipped relative to each other. Well, magnets are made of a whole lot of little paramagnetic atoms (or in theory molecules, I think) which in ferromagnetism form up into large domains where they are all lined up the same way for various reasons; nonetheless, beyond a certain scale the force of the opposing field will flip them if you simply heat up the magnet and cool it again, leaving you with an ordinary piece of metal or ceramic. You can also ruin a magnet by bashing on it hard enough, I think.
• Which brings me to the interesting center of your question: I'm not entirely sure if the small magnetic field induced in an object approaching the magnet due to eddy currents will have any incremental effect on the alignment of domains in the magnet. After all, these are very small regions which, by random chance, will tend to lose their alignment anyway over time. I suspect the effect of that and even the vibration of the paperclip hitting the magnet might truly be insignificant relative to other effects, but I don't know that. Wnt (talk) 13:06, 11 May 2017 (UTC)

Wnt's language may make someone believe that you can break the "north" and "south" poles of a magnet. You can't. Tigraan^{Click here to contact me} 16:45, 11 May 2017 (UTC)

Why do fat people have a lower pain threshold?

We can read here: "An extra layer of fat won't provide a cushion against pain -- in fact, obese people are more sensitive to pressure pain than those who are not overweight, and they are equally susceptible to extremes of hot and cold."

The explanation offered in the article didn't sound all that credible to me:

" "It could be the case that a person who is more sensitive to pain is less likely to do physical activity and therefore more likely to gain weight and become obese," says Dr Tashani."

You would then expect the result to apply to just a fraction of the obese people, if their results then affect the average of the entire group of obese people, you would end up with a significantly larger standard deviation in that group. Count Iblis (talk) 22:14, 10 May 2017 (UTC)

I think the important comment is: "The team plan to carry out further research into the factors that make people more susceptible to pain. This includes examining the chemicals secreted by fatty tissues in the body which could affect the response of pain receptors." The team's results show a possible correlation but not a cause. Dbfirs 23:47, 10 May 2017 (UTC)

• You have given us the source. You ask us "why" and then argue "You would then expect..." It seems like you want us to give our opinions, or validate your conclusion. Is there something else you are looking for? If you want anecdote, I can tell you that having lost some 100lbs, I can now walk barefoot without pebbles and twigs being excruciatingly painful to step on, yet I find I still act as if I am at my heaviest weight. Weighing now what I did in the early nineties I am still very careful about motions that before might have caused injury, but which now are presumably much safer. (For example, I used to take two steps at a time going up stairs, now I put each foot on a step before feeling fror the next one, expecting a fall.) I notice no difference to my reaction to burns or splinters, though. μηδείς (talk) 00:16, 11 May 2017 (UTC)

If you want a hypothesis off the top of my head, well, POMC mutation causes obesity, and involves the loss of an endogenous opioid chopped up out of that protein. Perhaps that contributes to "runner's high" and also reduces pain sensation in other settings. The mutation I describe also causes red hair, which is not mentioned in your source article and might be useful to look at if the statistic is available. However, the effect of the opioid could also be inhibited if its receptor is less active, or something its receptor sets off is less active, and none of those mutations would have this particular effect. That said -- this is just a random hypothesis and the odds that this is what the researchers come up with is quite low. There are a lot of ways to invoke biochemistry to relate a particular cause with a particular effect - so many things interact with so many other things, it's like looking at a complicated roadmap and trying to say which route a car must have taken. Wnt (talk) 13:14, 11 May 2017 (UTC)

Okay, but obesity is multifactorial. My dad was 260lbs (6'2) when my parents got married, and both sides of the family had historic experience with famine. While I and my older sister have always been overweight, and she has a BMI over 45, my youngest sister was very thin. She died of a heart attack at 20; one of those "athlete drops dead" stories. I suspect my middle sister and I inherited genes from our parents which, in combination, led to obesity. But there's also the possibility that the two eldest of us got adenovirus 36 which is linked to obesity as well. In any case, the only thing I have noticed with pain sensitivity is that as I have aged I ca disassociate from it. I don't think the intensity is any less, but the attention I pay it is. I know it is normal, and will pass. Little kids live in the moment. Adults live in the lifetime. μηδείς (talk) 22:20, 12 May 2017 (UTC)

Thanks everyone. Count Iblis (talk) 19:52, 13 May 2017 (UTC)

Universe expansion

I've tried to use Hubble's law to estimate an approximate value of the size of the observable universe and found some difficulties to reach the same value (93 billion light years). If I assume the furthest point where light can still reach us regardless expansion, I realize it must be less than 13.8 Bly, specifically 13.8/e and so, the universe should then expand by a factor of e till light reach us. If I assume an initial size of 13.8 Bly, I realize light cannot reach us due to the fact that expansion is almost the same rate as light speed at that point. Is there something I am missing?Almuhammedi (talk) —Preceding undated comment added 22:35, 10 May 2017 (UTC)

The visible universe is about 93 billion light years across. In any direction we look we see the cosmic background radiation which is now a little over 46 billion light years away, this is because not only has it been moving away from us over a little more than 13 billion years, the metric expansion of space has also been increasing the overall distance. μηδείς (talk) 00:08, 11 May 2017 (UTC)

But how will you apply Hubble's law in this case? — Preceding unsigned comment added by Almuhammedi (talk • contribs) 09:23, 11 May 2017 (UTC)

See Particle_horizon#Conformal_time_and_the_particle_horizon. Dragons flight (talk) 13:13, 11 May 2017 (UTC)

May 11

Psychology study about the perception of achievement

I know that there is a specific psychology study about scholastic achievement. Basically, there are two groups of students. One group of students is assumed to do very well. The other, no response. Well, the result is that the teacher's expectations can influence the achievement of the students. What's the name of this? 50.4.236.254 (talk) 03:54, 11 May 2017 (UTC)

This paper has an overview of many such studies. --Jayron32 10:01, 11 May 2017 (UTC)

• It's the Pygmalion effect or Rosenthal effect. Loraof (talk) 14:52, 11 May 2017 (UTC)

Also note that if the teachers expect more, they may interpret the same subjective answers on a test more positively. This is a good reason to exclusively use objective tests, wherever possible, and grade tests blindly (no student names) when grading subjective tests, like essay questions. See confirmation bias. StuRat (talk) 13:50, 13 May 2017 (UTC)

Convection coefficient for Heat sink Calculation

we know about heat dissipation equation derived for a fins.see.kindly tell me where can i get a data book that specifies convection coefficient values for (aluminium -air) contact at 540^m3/h.If no such kind of data book exists,where/how can we find/calculate h(convection coefficient) value. Thanks in Advance — Preceding unsigned comment added by Sameerdubey.sbp (talk • contribs) 10:13, 11 May 2017 (UTC)

• Convection is a local phenomenon that depends mostly on the fluid properties and its speed (outside the boundary layer) relative to the solid. The convection coefficient itself is relatively independent on the solid (but see Biot number for the competition of convection vs. solid conduction effects). Your "540^m3/h" is not a speed, but probably a flow rate which you will need to divide by the cross-section.

A priori evaluation of the convection coefficient is deceptively hard. (For instance, this paper about a very rough approximation for it in a very specific case has been cited more than a thousand times.)

[1] gives a correlation between the convection coefficient and air speed: $h[W/m/K]=10.45-v[m/s]+10{\sqrt {v}}$ (for speeds between 2 and 20 m/s). But I have no idea where it comes from and what are the conditions for validity, so... Tigraan^{Click here to contact me} 11:38, 11 May 2017 (UTC)

• Perry's Chemical Engineers' Handbook, 7th edition, section 5-12 suggests using the Nusselt number N_Nu = h*k/L to get h. It then notes that N_Nu = a*(N_Gr*N_Pr)^m where N_Gr is the Grashof number and N_Pr is the Prandtl number and gives values of a and m for a few geometries.--Wikimedes (talk) 20:14, 11 May 2017 (UTC)

Elephants in a balanced ecosystem

What kept historically the population of elephants limited? What were their natural predators? --Hofhof (talk) 12:09, 11 May 2017 (UTC)

Elephant#Ecology_and_activities mentions several predators (primarily taking calves). Also note their slow reproduction. Henry^Flower 12:34, 11 May 2017 (UTC)

Predators don't typically limit a prey population much - either the prey can sustain the predators even in lean years, or else the predators would die. (The exception is if the predators are sustained by something else and the prey is really bad at surviving - I mean, if you could let loose a flock of dodos in the woods the foxes and coyotes would figure out something to do about them; then they might be restricted, say, to a few local back yards where the predators are chased off by the owners' mutts. Or in this case, poachers who spend most of the time sticking people up on the road or something but who aren't averse to going in after an elephant if they happen to see one)

The usual problem is food availability: see [2]. The ecologists talking about culling elephants (!) are of course trying to prevent nature from taking its course by converting the ecosystem to the point where very few can survive; in former times perhaps bands of elephants roamed around looking for fresh forests to destroy. The implication from that source seems to be that the overall ecology of Africa (forest vs. grassland) has been impacted by the reduction in elephants! Wnt (talk) 13:24, 11 May 2017 (UTC)

"Predators don't typically limit a prey population much" - careful. Trophic cascades go both ways. Some systems are under top-down control. Prey switching is the real and common phenomenon where predators seek alternative prey when primary prey is scarce. What is true is that most predators are not highly specialized, and can switch among many prey species. One famous example where single-predator control of single prey species is obvious is the lynx and hare system [3].

Anyway, one class natural controls on elephant populations that is often overlooked is parasites e.g. ([4]) and disease (e.g. [5]). SemanticMantis (talk) 15:36, 11 May 2017 (UTC)

Let's not forget that elephants are considered keystone species - a species that has a disproportionately large effect on its environment relative to its abundance. DrChrissy ^(talk) 19:12, 11 May 2017 (UTC)

@SemanticMantis: What you say is all true, but ... if you look at that figure you linked, the number of hares fluctuates over a ten-fold range or so. This is important, but in terms of understanding whether hares are common or rare, it's not all that important. The other thing I have to note is that the data doesn't really show a rigid cause and effect relationship -- in 1907, the lynx harvest drops to nothing, but the hares take years more to recover; in 1886, both lynx and hares reach an all time peak and then all decline together - yet just a few years before that, the lynxes were at near peak levels relative to other cycles, and yet, the hares were about to see their population size explode. I don't deny there is an effect, overall, and that statistically significant results can be obtained to prove it, but I doubt that it is really very much compared to the kind of population differences we see between sparrows and eagles. I think mostly, the number of animals is determined by the amount of their food source. Wnt (talk) 01:24, 12 May 2017 (UTC)

In Britain, deer used to be culled. They were recently made a protected species. Now the population is higher than it's been for a thousand years. 62.30.204.247 (talk) 13:00, 12 May 2017 (UTC)

That's interesting, but an artificial situation. On one hand, some people deliberately feed deer; on the other, some hunt them with weapons and tactics they never evolved to contend with. This provides far more potential for the population to vary in either direction than a natural environment and ecosystem. Wnt (talk) 16:16, 12 May 2017 (UTC)

• Sabertooth cats preyed on the smaller and older/sicker individuals (as do lions still). Elephants also die from toothwear and parasite load. μηδείς (talk) 22:05, 12 May 2017 (UTC)

Why are combustion motors noisy?

Could a combustion motor be silent? Why are fires (kind of) silent, but combustion in a motor noisy? --Clipname (talk) 17:28, 11 May 2017 (UTC)

Thermoelectric generators are silent. Internal combustion engines are noisy because they have rapidly moving parts moving under relatively high pressure of the combustion products, and because the combustion itself is rapid and periodic rather than continuous. Dr Dima (talk) 17:41, 11 May 2017 (UTC). NB we also have a good article on Aircraft noise, you may want to read that one as well. Dr Dima (talk) 17:47, 11 May 2017 (UTC)

(edit conflict) Most rely on explosions to provide energy (except for gas turbines, jet engines and rockets which have different sounds). See Internal combustion engine. Dbfirs 17:52, 11 May 2017 (UTC)

Would an imaginary frictionless combustion engine make no or little noise? Clipname (talk) 18:37, 11 May 2017 (UTC)

There would still be the sound of the explosions (or turbulent flow). Dbfirs 18:41, 11 May 2017 (UTC)

Technically deflagrations but by that definition gunpowder also doesn't explode. Sagittarian Milky Way (talk) 18:55, 11 May 2017 (UTC)

True, but see Explosion. It's just a matter of speed. Dbfirs 19:15, 11 May 2017 (UTC)

Fundamentally the engines work on sudden, repeated changes of pressure as the fuel burns, pushes down the piston, and the waste flows out the exhaust.

Those pressure changes are almost certainly going to result in noise unless something is done to stop it from happening. Mufflers help, but the vibrations from all those pressure changes is going to be hard to eliminate entirely. ApLundell (talk) 19:03, 11 May 2017 (UTC)

Internal combustion engines extract work from a hot fluid, usually heated by a rapid chemical reaction - the oxidation of fuel - constrained inside a combustion chamber and compelled to do pressure-volume work against a piston. There are a few variations on the theme, and some definitions that we could stretch, but the general idea is a hot fluid inside a metal can. So, why is it noisy?

Work is wasted by the working fluid for at least a few reasons:

1. The fluid collides inelastically with the walls of the combustion chamber: this transduces pressure and heat into sound-vibration of the chamber itself, and the wave propagates as sound in the material and thence to the ambient air.
2. At the exhaust valve, the fluid is not in perfect equilibrium with the ambient pressure and temperature. This transduces pressure and heat into sound-vibration of the parts (and creates a pressure-front that directly impinges upon the ambient air), and thence propagates as a convective flow and as a supplemental sound wave into the ambient air.
3. The moving parts - valves, cams, drive-shafts, have metal-to-metal contact friction. These movements transduce kinetic energy into sound-vibration of the material - microscopic and usually-reversible metal deformation - and these sound-waves thence propagate into the ambient air.

Tackling the first: how do you make a perfectly elastic collision? Well, you can build the engine's combustion chamber out of an infinitely-stiff material! (That's impossible in the real world). You can come close by building the engine out of very heavy metal.

Tackling the second: you could perfectly match the exhaust velocity (and temperature and pressure) to ambient air. This is what rocket engine nozzles try to do: optimally expand the working fluid. This is very hard - it is, actually, rocket science - to design a mechanical contraption that causes fluid to lose pressure and heat at the perfectly-calibrated rate so that it exits the tube at an exactly-desirable thermodynamic condition. It is not commonly seen in conventional reciprocating engines, although - in aviation - you will sometimes find exhaust expanders, and salesmen who tell you that they allow the engine to extract a few extra horsepower out of the fuel. For example, here are some after-market mods you can perform to upgrade your airplane. Those are not the standard tubes, for any readers who are unfamiliar with the normal tubes that stick out of an airplane engine, those tubes are specially-crafted to be more thermodynamically efficient. Supposedly, it'll give you a little extra kick - maybe a few percentage points faster. Would you buy that physics (for a few thousand dollars)?

Tackling the third - well, nobody in engine-land wants metal-to-metal contact. There's supposed to be a film of lubricant preventing contact, essentially turning every mechanical activity into a perfectly-elastic collision. Review point number one for more on this detail. If you could have perfect, frictionless, interactions - and if your reciprocating parts never deformed even a millionth-of-an-inch when subjected to very high dynamic and kinematic forces, you'd lose zero energy transducing metal-on-metal contact deformation into acoustic noise that propagates into the engine (and thence, as a wave, into the ambient air).

In case it's not obvious, all of these improvements depend on "magic materials" with perfect properties. In the real world, we do the best we can - trading off against real, available material properties; and balancing agains other constraints like cost, weight, reliability, and so on.

Are there other places in the engine where kinetic energy is transduced into sound? Sure! Imperfect thermal expansion, imperfect kinematic impedance-matching, imperfect manufacturing tolerances, ... if you were the kind of person who used physics and math to design and analyze engines ... (that would be an engineer)... , you could make a lucrative career out of budgeting these energy losses, tracking them down to actionable design-changes, and trying to solve them. The most awesome engines are the ones that don't need sound-insulation or muffling. It's one thing to reduce the noise by suppressing it - it's a lot more amazing if you can prevent the sound from being generated in the first place!

Nimur (talk) 19:52, 11 May 2017 (UTC)

• An internal combustion engine produces a high pressure inside the cylinder, then opens the exhaust valve(s) and allows these gases to escape. If the valves open whilst there is still a high internal pressure, this exhaust is noisy. If it opens at a low pressure, it is quiet(er).

In the 1920s, Daimler were fitting their prestigious luxury cars with a version of the Daimler-Knight engine, which is a sleeve valve engine. As a sleeve valve can have much greater area than a conventional poppet valve, it can open later (after the pressure has dropped) and still provide adequate scavenging. Sleeve valves also have a reputation for high oil consumption and difficult lubrication. Daimlers gained a reputation for departing quietly in a blue haze of oil smoke, but this low noise was considered important enough to persist with the sleeve valve, despite the oil burning. Andy Dingley (talk) 23:48, 11 May 2017 (UTC)

In a loose sense, a combustion motor does not need to be noisy at all -- see fuel cell vehicle. It is possible to take ethanol, react it in a fuel cell, extract electricity, and use the electricity to turn a quiet motor. That may be too far from the intent of the question, but I see no theoretical reason why there should not be something which is halfway between an engine and a fuel cell, i.e. taking fuel and reacting it somehow on a small to microscopic scale to harvest energy with little if any detectable noise, then combining that energy somehow to turn a shaft, or otherwise provide propulsion. There ought to be thousands of independent designs within those broad specifications waiting to be invented. Wnt (talk) 16:26, 12 May 2017 (UTC)

Fish do it. Blooteuth (talk) 19:06, 13 May 2017 (UTC)

Could the US or Latin America have lots of wild elephants if someone had released breeding pairs?

Or would that have caused so much ecological and/or agricultural and/or ivory-related trouble that they would've been hunted to near-extinction like wolves? Sagittarian Milky Way (talk) 23:30, 11 May 2017 (UTC)

Maybe not elephants, but you should check out United States Camel Corps... --Jayron32 00:43, 12 May 2017 (UTC)

One breeding pair would not be enough (inbreeding would set in almost immediately). (I can't remember what article to link to for about how many you need.) When did you want to let them loose? If you did it now they'd be hit my a car almost immediately (i.e. killed by humans) but they might have had a chance earlier in history. RJFJR (talk) 00:46, 12 May 2017 (UTC)

If President Lincoln didn't decline the Thai king's offer to release a number of male elephants and a number of female elephants, wait for multiplication, then have a free supply of beasts of burden to catch. Sagittarian Milky Way (talk) 01:10, 12 May 2017 (UTC)

The multiplication and catching them later was part of the offer, not implied. Sagittarian Milky Way (talk) 12:21, 12 May 2017 (UTC)

Found that article: Minimum viable population. RJFJR (talk) 00:48, 12 May 2017 (UTC)

And for the record, the U.S. and Latin America used to have elephants, or at least close cousins. See Mastodon, Columbian mammoth, etc. --Jayron32 00:56, 12 May 2017 (UTC)

Apparently, Germany has acquired a population of Nandus from just three breeding pairs escaping from a farm in 2000. The population is s now in the 100s, and farmers are calling for controlling measures, as they eat their crops. --Stephan Schulz (talk) 06:07, 12 May 2017 (UTC)

Elephants are mentioned in the Book of Mormon in connection with the Jaredites. They were noted as being among the most useful animals. The Jaredites are estimated ^{[by whom?]} to have arrived in the New World between 2600 and 2100 BC. And they also had horses, and asses, and there were elephants and cureloms and cumoms; all of which were useful unto man, and more especially the elephants and cureloms and cumoms. (Ether 9:19). Whether Evidence for the survival of the elephant can be found in Native American myths depends on whom you believe. Blooteuth (talk) 08:47, 12 May 2017 (UTC)

• See rewilding. μηδείς (talk) 22:02, 12 May 2017 (UTC)

May 12

Understanding the Wheels

It's easy to get the concept of rear-wheel drives - using the rears to transfer engine's motion to overall movement while keeping the front ones free to be steered as needed. But as far as front-wheel drive is concerned, along with passing serious motion of the engine, the critically precise steering movements needed to be translated as importantly. Without letting the two to interfere with each other. How it's done, please ? 210.56.127.224 (talk) 05:46, 12 May 2017 (UTC)

Here is somewhere to start, and something similar it seems to be facilitated by the use of universal joints. Richard Avery (talk) 07:18, 12 May 2017 (UTC)

Also note that the rear-wheel drive is not as simple as it seems - to avoid slippage during turns, it needs a differential, to distribute power to the wheels. The outer wheel in a curve has to travel a longer distance than the inner wheel, so if both would turn at the same speed, at least one of them would need to slip against the ground, with corresponding lack of control. --Stephan Schulz (talk) 07:54, 12 May 2017 (UTC)

Normally a Constant-velocity joint for front wheel drive vehicles.Phil Holmes (talk) 08:34, 12 May 2017 (UTC)

The "Without letting the two to interfere with each other" is not completely straight-forward, nor is it always completely achieved. See Torque steering. In my "boy racer" days I was well aware of the sudden tug huge you could get on the steering wheel if the tyre on the inside of a bend lost traction, which used to happen often when I pulled out of bends on full power in a front-wheel-drive car. -- Q Chris (talk) 08:45, 12 May 2017 (UTC)

Crude mid-century four-wheel drive front axle, with exposed Hookes joints

• Four-wheel drive is older than front wheel drive and has some of the same problems to solve. Such vehicles began as slow off-road vehicles, so could cope with many drawbacks. The first used heavy, complicated front axle drives that required a lot of maintenance. The 1924 Thornycroft Hathi had a system of vertical shafts through the steering pivots, with lots of bevel gears to the axle. Most early four wheel drives (Land Rovers and Jeeps) used a rigid front beam axle with a Hookes joint each side to allow steering.

The difficulty with the common Hookes joint is that when bent and rotated at a constant input speed, the speed of the output shaft varies up and down through the rotation cycle. This is fine on a slow off-road vehicle, but makes handling poor for a faster vehicle on a tarmac road.

Rzeppa CV joint

The solution is the constant velocity joint (CV joint). This is more complicated to make than a Hookes joint, but rotates at a constant speed, even when bent. The first of these was the Belgian Tracta joint, used to make their front wheel drive cars in the 1920s, and remaining on military vehicles into the 1960s. Most modern cars now use the Rzeppa joint instead, which resembles a sphere with grooves in the surface and balls rolling in these grooves. A simple idea, but requiring very precise machining to manufacture it.

A modern FWD car has a transverse engine and gearbox alongside it, with a jointed shaft out to each wheel. These shafts may be the same length, or one may be longer than the other, in order to position the gearbox more efficiently inside the engine compartment. As the outer joint of the shaft needs to bend more with the suspension, and also permit steering rotation, that is a sophisticated CV joint with a lot of articulation. The inner joint doesn't move as much, so can either be an identical joint, or a simpler and cheaper type. Sometimes the shafts are of fixed length, and may form part of the suspension linkage too (like a Chapman strut, although these are rarely used at the front), otherwise suspension links control the position of the wheel and the drive shaft may be slightly telescopic to allow its length to change with suspension movement.

As four wheel drive began to be replaced by permanent all wheel drive, and the cars became faster and also usable on roads, vehicles like the Range Rover also switched to using CV joints rather than Hookes joints. Andy Dingley (talk) 08:47, 12 May 2017 (UTC)

From the point of view of the tire the traction forces are longitudinal and the steering forces are lateral. Therefore an important part of the suspension designer's job is to minimise the influence of traction forces on steering by optimising the geometry. Essentially this means aligning the centre of the contact patch with the steering axis of the suspension, so tractive forces cannot generate a torque around the steering axis. That is called the scrub radius. There is a separate torque created by the halfshaft torque, because the halfshaft is not truly perpendicular to the steering axis. Various elaborate solutions such as Revoknuckle attempt to minimise that. Greglocock (talk) 08:15, 14 May 2017 (UTC)

May 13

Medicinal side effects

During a clinical trial for a new medicine, a significant number of patients report experiencing a certain unexpected effect that wasn't happening before the trial, and when they go off the medicine, they find that it stops. This is definitely going to be considered a side effect. But what if the unexpected effect only happens in certain situations, e.g. when the medicine's taken before bed but not when the patient will be standing for many hours, or when taken on an empty stomach but not when taken with food? Is such an effect still classified as a side effect, since it's a situation for which the medicine is a necessary cause (i.e. it won't happen without the medicine), or is it not so classified, because the medicine isn't a sufficient cause (i.e. the medicine by itself can't produce the effect)? Nyttend (talk) 04:06, 13 May 2017 (UTC)

Yes. It is a side effect as long as the medication is a cause, even if it is not by itself a sufficient cause. Side effects are often classed broadly with terms like "common", "uncommon", and "rare". A rare side effect may well have multiple preconditions (which may or may not be known). Dragons flight (talk) 06:29, 13 May 2017 (UTC)

It is improbable that all side effects would be discovered in the testing phase of a medicine. In the UK we have the Yellow card system to record any possible side effects after the issuing of the drug. Richard Avery (talk) 07:40, 13 May 2017 (UTC)

(aside): Is there an equivalent to the Yellow Card, for recording when a drug is 'too effective', such as for example what was sold as mild-pain relief, inducing total oblivion to pain, or would this just be recorded as a side effect, albeit one that's not necessarily harmful?ShakespeareFan00 (talk) 12:32, 13 May 2017 (UTC)

ShakespeareFan00, if the medicine deserves more than the yellow card, it gets a red card. Nyttend (talk) 00:17, 15 May 2017 (UTC)

A medication's strength is considered where a pharmaceutical regulator limits the size of the dose of the same medication sold both on prescription and non-prescription. Examples of two common pain relievers sold in prescription/non-prescription doses respectively ^{[do not assume these limits apply in all countries]} are Ibuprofen 600mg / 200mg and Diclofenac 100mg (as Voltaren) / 25mg (as Volterol). Blooteuth (talk) 18:08, 13 May 2017 (UTC)

Evolution of sexual selection in humans

In most mammals, the very basics of reproduction starts with sexual selection. But why did humans evolve to select based on personality, love etc and become monogamous. What advantages does this bring in practice looking purely from a scientific point of view? 82.132.231.84 (talk) 13:11, 13 May 2017 (UTC)

Monogamy is in large part a cultural rather than a biological phenomenon. In practice humans (of both sexes) frequently ignore it – if they weren't inclined to, there would be no need for so many social customs and legal sanctions trying to enforce it. {The poster formerly known as 87.81.230.195} 2.122.60.183 (talk) 13:16, 13 May 2017 (UTC)

Note that there is an evolutionary advantage to monogamy, in that it prevents the spread of venereal disease. AIDS, for example, would wipe out a larger portion of the population if we were completely polygamous and did not adjust our behavior to prevent the spread of the disease. The same is true for many other such diseases. StuRat (talk) 13:35, 13 May 2017 (UTC)

An interesting idea, but I'd need convincing. After all, a *lot* of people don't follow the model anyway. I'm not sure if preventing diseases in this way would have been relevant in an age of small independent tribes of people. After all, HIV seems to have smouldered on for at least a century before modern transportation, mobility, and convenience allowed it to hit the big time. Wnt (talk) 15:42, 13 May 2017 (UTC)

But AIDS is only one of many historically deadly or at least incapacitating venereal diseases. StuRat (talk) 19:14, 13 May 2017 (UTC)

True but what about selection by personality, love etc. 82.132.231.84 (talk) 13:25, 13 May 2017 (UTC)

Love or pair bonding, in scientific terms, is the mechanism by which monogamy is achieved. As for personality, it's important for survival to have complementary interests and skills. For example, if one partner likes to care for kids while the other looks for food, that works, but if both want to do the same thing, that won't work out as well. Of course, in modern society, all evolutionary rules are off, as kids can survive having bad parents now. StuRat (talk) 13:37, 13 May 2017 (UTC)

And we've also evolved beyond caring for kids and hunting food. So I wonder what the basis has become these days. 82.132.231.84 (talk) 13:47, 13 May 2017 (UTC)

Evolution is a very slow process, and our evolutionary pressures have only changed very recently, in terms of human history, so don't expect any changes soon. In a few decades we will take control of evolution by genetic engineering, so all bets are off then. StuRat (talk) 13:53, 13 May 2017 (UTC)

Intersting. Slightly different topic but there also seems to be a lot more singles in modern society and people seem to be settling down later. Could this be part of the evolution? 82.132.231.84 (talk) 14:29, 13 May 2017 (UTC)

I cannot see any evolutionary mechanism there, and, as StuRat writes above, the time scale is far too short, so I would put it down to social and financial pressures. Perhaps someone can point to some research? Dbfirs 15:13, 13 May 2017 (UTC)

Evolution can exhibit strong effects on a population in as few as 10-20 generations (see e.g. experimental evolution) given strong enough selection forces. That's too long to see an effect from "modern" society, but longer-term changes such as the rise of agriculture and cities probably have had an impact on the characteristics of the human population. Dragons flight (talk) 20:46, 13 May 2017 (UTC)

What do you mean by "we've evolved beyond caring for kids and hunting food"? Having children is a choice. Hunting food is not. No food, you die. And if you have kids, you care for them, assuming you're a civilized human being. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:16, 14 May 2017 (UTC)

See also here. Count Iblis (talk) 19:48, 13 May 2017 (UTC)

• The term sexual selection is being used entirely incorrectly here. Sexual selection explains otherwise non-adaptive traits, such as bright colors and showy displays by male birds like the Northern cardinal or the Bower birds that otherwise put them at a survival disadvantage. Because females favor these traits, the males exhibit them, even though they are metabolically wasteful or put them at a greater risk of predation.

The relevant concept here is mate selection. In humans, large penises, permanent breasts, relatively hairless skin and manes are believed to be the products of sexual selection. Affection, and selection by smell (people prefer mates whose body odor does not match their own, as it is a sign of immunological diversity) have to do with mate selection. Mate selection can be considered a form of sexual selection, but it differs from traits which are showy, but otherwise put the individual at a survival disadvantage. μηδείς (talk) 22:19, 13 May 2017 (UTC)

Lactose gene

Why exactly LCT gene for lactose digestion turns off by itself in mammals after weaning causing lactose intolerance? There doesn't seem any strong evolutionary pressure against it. Besides, given our long sequence of evolution from previous mammals, one might assume that the gene should have firmly rooted itself in the genome without turning off later. Yet it turns off in other mammals too after some time. Brandmeister^talk 18:51, 13 May 2017 (UTC)

Just a thought, but could this have helped to wean the young ? After all, until this happened, if the mother couldn't have another litter, it would be important to move them along to solid foods. Also, milk is somewhat incomplete, nutritionally, so solid foods would also be important for the survival of the first litter. StuRat (talk) 19:09, 13 May 2017 (UTC)

Lactase has an evolutionary history in bacteria and humans alike of being "inducible". It is a fairly rare energy source, so the gene is usually around somewhere, but usually off. Such is the case in humans -- I looked into this a while back and it is very common for lactose intake to increase lactase production; I think I personally have experienced lactose intolerance after going a long time without milk products, and then induced it back up to the point where I could consume inadvisably large amounts of ice cream and/or cheese without intestinal issues. (True, some of that is doubtless a function of intestinal bacteria; and I can't swear it wasn't all that). It is common to portray "lactose intolerance" as an all-or-nothing phenomenon, but I think there are many shades of gray like this.

Note that "firmly rooting itself in the genome" actually is something of a thing (transgenes notoriously can lose activity in a mouse strain over time, as genomic 'spam filters' hone in on it by gene silencing or heterochromatin formation perhaps, so a history of a few generations for a gene actually is somewhat relevant. But on the evolutionary time scale we can assume all genes are stable. But one sign of an old gene is a complex pattern of regulation, which often means developing more reliable ways to turn it off! And lactase is a very old gene indeed. Over very long evolutionary time scales - and sometimes, even in a long term microbe culture in a very careful laboratory - the reduction in optimal growth and evolutionary fitness caused by supporting a single unnecessary gene can actually be measured. Wnt (talk) 20:13, 13 May 2017 (UTC)

• Lactase is a protein, and protein synthesis is one of the major metabolic costs of life. There is no good reason for organisms not getting lactose to produce lactase, for the exact same reason cave fish don't waste energy to grow eyes. μηδείς (talk) 03:09, 14 May 2017 (UTC)
• We have a whole Lactase persistence article that addresses the history and possible evolutionary advantages. DMacks (talk) 04:30, 14 May 2017 (UTC)

…one might assume that the gene should have firmly rooted itself in the genome without turning off later. You seem to be implying that if a gene is required at some point in an organism's life, its expression wouldn't ever be switched off, but that's not true. Just because a gene is highly conserved doesn't mean it won't be subject to regulation. The obvious example: all of your somatic cells with nuclei contain the exact same DNA (with minor exceptions). Genes that a particular cell type doesn't need are silenced as part of cellular differentiation. Your nerve cells don't produce hemoglobin, and your bone marrow cells don't produce myelin, even though they have the genes for those proteins. Some genes are normally never expressed after early life, like fetal hemoglobin. Regulation of gene expression is complex and there's still a lot we don't know about it. --47.138.161.183 (talk) 09:43, 14 May 2017 (UTC)

May 14

Is Faraday's law of inductance properly applied to Wiki's article on LC circuits?

The article uses Faraday's law, to explain why a decrease in magnetic field causes and increase in EMF.

"... Eventually all the charge on the capacitor will be gone and the voltage across it will reach zero. However, the current will continue, because inductors resist changes in current. The current will begin to charge the capacitor with a voltage of opposite polarity to its original charge. Due to Faraday's law, the EMF which drives the current is caused by a decrease in the magnetic field, thus the energy required to charge the capacitor is extracted from the magnetic field." LC circuit

The correct way of dealing with floating references is to use the template

References

. This puts the refs in a neat box and keeps them with the original posting. For example, DrChrissy ^(talk) 15:19, 14 May 2017 (UTC)

DrChrissy, You just placed "This is a bogus ref to illustrate the template that prevents floating refs" at the bottom of this page, in the wrong section. Was that your intent? --Guy Macon (talk) 16:52, 15 May 2017 (UTC)

@Guy Macon: Hi. Another editor has already raised this on my Talk page - I suggest you read that thread to put this into perspective. DrChrissy ^(talk) 16:59, 15 May 2017 (UTC)

Faraday's law of induction applies to magnetic INDUCTION. Magnetic induction occurs when a magnetic FIELD produces magnetic energy within a superconductor, or both magnetic energy and EMF within a non-superconductor.

In an LC circuit, the capacitor does not contain a magnetic field. The capacitor gains its EMF when magnetic energy directly converts into electrostatic energy.

Note: The article uses an inductor's resistance to change in current to explain why current begins to flow to a capacitor after a polarity change.

However, the inductor does not resist a polarity change. Current INSTANTLY reverses direction during the two polarity changes that occur within each cycle.

In my opinion, a better explanation of current flow to the capacitor after a polarity change is the fact that:

WITHIN AN LC CIRCUIT, RELATIVE POLARITY DETERMINES THE DIRECTION OF CONVERSION OF ENERGY.

For example, electrostatic energy cannot convert into magnetic energy without proper magnetic versus electrostatic polarity. Exhaustion of electrostatic energy results in the instability of having an inductor with the entire circuit's stored energy. A polarity reversal makes it possible for magnetic energy to convert into electrostatic energy.

Overall, relative polarity drives the direction of energy conversions. Capacitance and inductance determine the rate of energy conversion. Current and volts are the result of the energy conversions within an LC circuit. Vze2wgsm1 (talk) 01:15, 14 May 2017 (UTC)

You're referring to the article LC circuit. You linked to it in an obscure way, so I hope this helps. Akld guy (talk) 02:06, 14 May 2017 (UTC)

You state that the inductor does not resist a polarity change, and you refer to "the two polarity changes that occur within each cycle". The only way to read that is that you're referring to an LC circuit connected to an external signal source, but the part of the article that you're dissatisfied with is discussing only a capacitor and inductor connected together and to nothing else. It explains what happens when the capacitor, which is charged, begins discharging through the inductor. There is no signal from an external source involved at this part of the article. Akld guy (talk) 02:44, 14 May 2017 (UTC)

An example of capacitor charge and discharge without polarity reversal within a capacitor is when a discharging battery adds electrostatic energy to a capacitor. Afterwards, (after adding extra charge) the capacitor could charge the battery, without changing the polarity of the battery or the capacitor.

Let a battery substitute for a capacitor within an LC circuit. After the battery completely discharges into the inductor, the energy within the inductor would begin trying to charge the discharged battery, in a direction that would make the anode negative, and the cathode positive.

The inductor's attempt to reverse the polarity of the battery requires a reversal of polarity of the energy within the inductor.

Vze2wgsm1 (talk) 22:50, 14 May 2017 (UTC)

• @Vze2wgsm1: Just a sidenote, "Wiki" is not a correct way to refer to Wikipedia.--Jasper Deng (talk) 22:58, 14 May 2017 (UTC)

ceramic capacitor vs tantalum capacitor

Between a ceramic capacitor and a tantalum capacitor of the identical capacitance and rated voltage, let's say 100 µF and 10 V, which one would have a smaller leakage current? ECS LIVA Z (talk) 03:06, 14 May 2017 (UTC)

The leakage of the ceramic is typically a thousand times lower. Typical leakage for a 100 uF, 6.3 VDC rated tantalum-polymer capacitor is 60 uA. A ceramic capacitor of the same capacitance and voltage would have an equivalent leakage of 6.0 nA. Source: Technical Update – Comparison of Ceramic and Tantalum Capacitors --Guy Macon (talk) 03:54, 14 May 2017 (UTC)

Holy crap that's a big difference. Thanks a lot! ECS LIVA Z (talk) 04:08, 14 May 2017 (UTC)

It's an unrealistic comparison though. Tantalums are electrolytics, where many drawbacks (polarisation, leakage, low voltage rating, sometimes accuracy of their value) are sacrificed in order to achieve a high capacity or a small physical size. 100μF is a typical value for tantalum, but would be exceptional for a ceramic. 0.1μF is a more practical upper limit for an off-the-shelf commodity disc ceramic capacitor, 1μF for newer MLCC devices. You can go higher with recent devices (my usual suppliers peak at 22μF] in similar packages to tantalum but the price gets steep (although the voltage rating is high enough to be board-design limited).

Specialist HV ceramics have always been around in large capacities, but those get expensive and obscure. They're more usually made these days from polypropylene capacitors (look at the Tesla coil people and 'MMC' banks). This highlights another issue - at higher frequencies it might not be a simple leakage current that is the problem, but losses within the dielectric. Andy Dingley (talk) 12:22, 14 May 2017 (UTC)

On Digikey I'm finding ceramic capacitors significantly cheaper than tantalums at 0.1 μF, 1 μF, and even 10 μF. The two only reach price parity (with tantalum still trailing badly in performance) at 100 μF, but I don't use anything that big (nor do most hand-held consumer electronics). Seems like the latest technology advances and price drops in MLCC have made tantalum virtually obsolete.ECS LIVA Z (talk) 21:10, 14 May 2017 (UTC)

Here's a ceramic capacitor at 100 μF[6] that's actually cost-competitive against the tantalum equivalent. ECS LIVA Z (talk) 21:18, 14 May 2017 (UTC)

Thanks, those haven't reached my end of the workshop yet. Note the low voltage ratings, which are characteristic of MLCC and indicative of the thin layer trick that allows them to get such a high capacity into a small package. Andy Dingley (talk) 08:45, 15 May 2017 (UTC)

Capacitors can be wired in parallel to increase the total. For example, three ceramic 10 μF units in parallel result in 30 μF, with the same voltage rating as the lowest rated one. Akld guy (talk) 02:33, 15 May 2017 (UTC)

Resolved

Data and maths of man-made climate warming

If a non-climate scientist got basic climate and weather data (like temperature, CO2 concentrations, floods and hurricanes) and he wanted to know whether it's man-made or not, what maths would he need? What steps would he need to take? That is, without appealing to authorities or experts, how could he come to this conclusion? --Clipname (talk) 11:50, 14 May 2017 (UTC)

From an inhuman point of view, the best test would be to get rid of all the men (inclusive) and observe subsequent weather data. Apologies for that answer. Dbfirs 11:56, 14 May 2017 (UTC)

• Attribution of recent climate change and the sources therein may help. Loraof (talk) 14:03, 14 May 2017 (UTC)

First you need to determine whether the CO2 increase was man made. There are multiple lines of evidence that show this is indeed the case, but perhaps the most straightforward is the change in relative concentrations of carbon isotopes in atmospheric CO2. See Seuss effect.

From this point onward you can choose between approaches that run from the simple to the very complex. Possibly the easiest would be to use one of the simplified formulas for radiative forcing as a function of CO2 concentrations. You could then take this answer and use a reasonable value for climate sensitivity to get the resulting temperature change. For this calculation it would be more appropriate to use a value for transient climate response rather than equilibrium climate sensitivity.

You have to also keep in mind that the observed warming is not solely due to CO2. Radiative forcing due to methane is also substantial (a bit more than half as large as the forcing due to CO2).

There's a whole range of approaches beyond this, from fairly simple radiative-convective equilibrium models to full-blown climate system models such as the CCSM. Shock Brigade Harvester Boris (talk) 15:17, 14 May 2017 (UTC)

The thing is, while quantifying the exact amount of past and future global warming takes a lot of effort and calculation, the simple fact that manmade greenhouse gases are warming the planet really doesn't. The greenhouse effect of gases like CO₂ and methane can be demonstrated in a laboratory. We know human activities like intensive agriculture and burning fossil fuels releases these gases into the atmosphere (again, demonstrable in a laboratory). So either the planet should be warming, because we're dumping these greenhouse gases into the atmosphere, or some unknown process is precisely compensating for these manmade alterations to the atmosphere. Occam's razor suggests the former. --47.138.161.183 (talk) 07:43, 15 May 2017 (UTC)

That is a poor argument. If the effect is small, say 1 deg C per doubling (about what you get in the lab) then it will be hundreds of years before we see another 1 deg C, other than short term spikes due to El Nino etc. Who knows what will happen in several centuries? Maybe some actual science as opposed to guessing games and alarmism. Greglocock (talk) 12:10, 15 May 2017 (UTC)

If you mean 1 deg C per doubling of CO2 it won't take centuries to happen. Since the 50s it has not only doubled, but went up by a factors of 8. And that's discounting other greenhouse gases, a chain effect (more heat -> more water vapor in the atmosphere -> even more heat) and other side effects like increased acidity of oceans, disturbing ocean currents, triggering the release of methane crystals. Hofhof (talk) 13:12, 15 May 2017 (UTC)

What has gone up by a factor of 8 since 1950? Not CO2 concentration. On another note, Greglocock's value of 1 deg C per doubling is roughly the effect solely of CO2 in an isolated, static system; it is not plausible to apply such a value to the Earth. Shock Brigade Harvester Boris (talk) 13:47, 15 May 2017 (UTC)

Not a proof, but the "hockey stick curve" is dramatic enough that, if you see it graphed out, you don't have to be an expert to see that something unusual has happened to our weather in the last century. (example)

The caveat there is that all the temperature data older than the late 1600s is based on pretty intense research that could only be done by experts.

... but that's going to be true no matter what. There's no way something as big as this could be deduced entirely from personal knowledge and personal calculations. At some point, you're going to have to use someone else's data. It's just a question of which data you use or don't. ApLundell (talk) 13:51, 15 May 2017 (UTC)

There's also the problem that science is collaborative and cumulative. Scientists are not hermits collecting data and creating theories from it in isolation. There are literally thousands of climatologists around the world that have been working together on this subject, and one person is unlikely to have the time or resources to recreate it all on their own from scratch. --Jayron32 14:43, 15 May 2017 (UTC)

May 15

Meaning of "above" in geology

I know by the law of superposition in geology that generally younger rocks lie above older ones. Reading Moine Supergroup I see the phrase "tectonically above" used twice and, although I can't find anywhere a definition, I suspect this means that the rocks physically above have been forced there and are, in fact, older. Am I understanding this properly? Would you ever say "tectonically above" if the higher strata were younger – both situations occur in the area of the Moine Thrust Belt? In the article "structurally above" is used twice and I think I can infer from the article that this phrase has the same meaning as "tectonically above". Is this right? When the word "above" is used without qualification in a geological context does it have a definite meaning to a geologist? I'm wondering whether "above" is ever used to mean "younger". Thincat (talk) 08:29, 15 May 2017 (UTC)

You're correct in your assumption - this is tectonic stratigraphy. Thrust faulting has the general effect of placing older rocks on younger rocks. We would use the term tectonically or structurally above (they are synonyms) when the upper sequence lies physically above a lower sequence with a tectonic (faulted) contact, rather than a normal stratigraphic contact. Mikenorton (talk) 10:55, 15 May 2017 (UTC)

Good, thank you. That seems clear now. Thincat (talk) 12:45, 15 May 2017 (UTC)

Induction and Convention of thermoelement

Does induction and convention of thermoelements are depends on their geometry? What geometry of thermoelements is more profitable, it is a complex geometry or simple geometry?--109.252.29.219 (talk) 15:30, 15 May 2017 (UTC)