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Physics, Astronomy & Cosmology / Re: Is there a substance where entropy decreases at higher temperature?

« on: 19/12/2022 05:10:37 »

It does seem to hinge around whether a supercooled liquid, or @chiralSPO 's suggestion of any chemical reaction that is only proceeding imperceptibly slowly,  really is in an equilibrium state.   Certainly in the case of a slow reaction, it isn't.

I wouldn't call either of these cases as being in an equilibrium state.  I suppose we could say that an uninitiated heat pack could be in thermal equilibrium with another system (ie when it's on the shelf, it should have the same temperature as the surrounding air, and will roughly track the temperature of the air as it rises and falls during the day, absorbing and releasing heat to "re-equilibrate" as long as the energy involved is not enough to disrupt the metastable state and kickstart the transition.)

As far as "very slow" vs 0 reaction rate goes... Because molecules are discrete there can be reactions that are not strictly impossible (even thermodynamically favorable), but are still so improbable (at a given temperature) that even with moles of interacting molecules the molecular reactions would not take place even once in millions of years (or any arbitrary timeframe). But increase the temperature by a couple hundred degrees, and suddenly it's a fast reaction (temperature is usually a term in the exponent of the rate equation).

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Physics, Astronomy & Cosmology / Re: Is there a substance where entropy decreases at higher temperature?

« on: 18/12/2022 16:20:07 »

I believe it would be impossible for strictly positive temperatures to satisfy the constraints in the OP, if we want to talk about thermodynamic equilibria (I haven't done the math,  but I'm pretty sure one would quickly run afoul of the second law of thermodynamics.)

But, I think petrochemicals post shows an interesting solution when one also accounts for kinetic barriers.

One could imagine a chemical reaction (or just change of state, like melting/freezing) which is thermodynamically spontaneous at a given temperature, T₁ (ΔG < 0), with ΔH < 0 and ΔS <0). If there is an activation barrier such that the rate of reaction is imperceivably slow at T₁, but progresses at a higher T, such that ΔG, ΔH, and ΔS are all negative, then one would observe a system that becomes more ordered as it's temperature increases.

Supercooled water is a good example of this, but perhaps there are even better examples, such as supercooled sodium thiosulfate or sodium acetate solutions. (commonly used for instant heat packs. they crystallize and get hot after the crystallization is initiated--typically done mechanically, but I'm sure increasing temp would work too, if done right).

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Physics, Astronomy & Cosmology / Re: Is there a substance where entropy decreases at higher temperature?

« on: 15/12/2022 02:17:57 »

I think you need to be careful about specifying whether you mean temperature or thermal energy.

There are plenty of systems one can imagine (or even observe!) in which increasing energy of the system results in there being fewer available micro-states, and therefore lower entropy. The first example that springs to my mind is the electronic structure of a transition metal ion.

Screen Shot 2022-12-14 at 8.57.56 PM.png (27.42 kB . 964x342 - viewed 1611 times)

If some light is absorbed, the resulting system has both more energy and less entropy than it did before (it's not a closed system).

More broadly, we can imagine any system made up of bits that can be in two states (one high energy, one low). Imagine all the bits start out in the low energy state, and you start feeding energy into the system. Initially the entropy increases with increasing temperature. But once the 50/50 state is reached, any additional energy going in is going to decrease the entropy of the system, until finally it is 100% energized, and has regained its initial low entropy.

Think this isn't realistic? It's how lasers work.

Also, going back to my initial remark about temperature: I never said anything about the temperature of the system of bits, only that you are adding energy. One reasonable definition of Temperature is: T = ∂U/∂S (constant volume). So ... if the energy is increasing, and the entropy decreasing, then the system has a negative temperature (ie below absolute zero) 

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General Science / Re: Why is gold gold?

« on: 19/11/2022 23:56:52 »

Multiple effects can be blamed for the colors of metals--sometimes it is from the electronic structure of the metal itself (as in the case of gold, freshly polished copper, bronze, etc. see below), and sometimes it is due to the electronic structure of what is on the surface (often dull colors due to "tarnish" "rust" "patina" etc., like red iron, green copper/bronze, dark silver), and sometimes it is due to the thickness of an otherwise transparent oxide layer on the surface, creating colors due to interference, just like with soap bubbles or petroleum residues on the surface of water (creating the rainbow colors observed on bismuth crystals, or can be done intentionally, as when anodizing aluminum to achieve certain desired colors). There can be even more unusual phenomena involved when the metal is in very small particles (nanoparticles), where the electronic structure can become non-metallic (at least at "defect sites", which become more densely present in smaller and smaller particles, and then there's plasmonic resonance, which is how gold or silver particles in stained glass can adopt so many colors)

So, with that out of the way, I will now just focus on answering why gold is gold:

Gold is metallic because it has a partially-filled "conduction band" that spans the entire lattice. The electrons in this band can be considered delocalized across the whole "piece" of gold, and are very much free to be anywhere, or "moving" anywhere given a potential. There are ALSO bands that are completely filled (lower in energy than the conduction band) AND bands that are completely empty (higher in energy than the conduction band). Photons of suitable energies (frequencies, wavelengths) will be absorbed, promoting an electron from one band to another. And in the case of gold there are a few absorbtions in the blue part of the visible spectrum, so we perceive it as yellow.

(being able to calculate the electronic structure of gold is no small feat--it requires QM principles, modified by general relativity, because the electrons close to the gold nucleus experience both relativistic time dilation and mass dilation, compared to electrons far from the nucleus!)

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Physics, Astronomy & Cosmology / Re: Does Jupiter's radiation make life on it's moons impossible?

« on: 14/10/2022 14:18:05 »

Just to add on to evan_au's response:

Radiation at the "surface" of Europa, is likely quite inhospitable to anything we would recognize as life (which is at least in part due to the significant radiation from the interaction of solar wind (or other charged particles) and Jupiter's magnetic field).

But the interface of the solid ice and tenuous gaseous atmosphere isn't the only "surface" available. Europa appears to have an icy crust that is on the order of a dozen km thick (if I recall correctly), and then several hundred km of liquid, salty water on top of a rocky core. It is this liquid-rock interface that many people believe is potentially suitable for life.

There is ample evidence that Europa is a geologically active world, and that it has active hydrothermal vents that are likely to be quite similar to those found on Earth in terms of composition. We have more direct evidence of this from Saturn's similar moon, Enceladus, which frequently has visible geysers (the ejecta of which make up Saturn's E ring). We have even directly sampled the geysers and analyzed them by mass spec to see that it has a tantalizing mix of small organic compounds (containing C, H, N, and O 10.1093/mnras/stz2280 ), salts (Na⁺, K⁺, Cl^–, OH^–. HCO₃^–, CO₃^2–, NH₄⁺--it appears to have a pH of 11 or 12 10.1016/j.gca.2015.04.017), and even possible energy-containing ("food") molecules (H₂, CO, H₂S, NH₃, CH₄, CH₂O 10.1038/nature08153).

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That CAN'T be true! / Re: hydrinos!

« on: 11/06/2022 19:42:50 »

It's 100% nonsense, as far as i'm concerned, that's not in question. How has he kept this going for 30 years? I'm beyond baffled.

Well, there are still people out there selling (and buying!) "free energy" devices... (and horoscopes for that matter!)

People are easily fooled into believing something they dearly wish to be true. And that goes for multi-zillionaire investors as well as regular folks.

It is obvious (and mathematically derivable!) to anybody who understands college-level chemistry and physics that there is no energy level below the 1s. But many people have never gone to college, and even those who have never took (or passed) any serious chemistry or physics classes, and even those who have may not really remember after a few years... Anybody without such an intimate understanding would only accept that the 1s is the lowest energy level on faith, or based on oversimplified models, either of which can be overcome by a smooth-talking charlatan. (especially since the reality is quite counterintuitive and not easily tested or examined by a typical person.)

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Physiology & Medicine / Re: Can You Define What a Woman Is ?

« on: 28/05/2022 18:28:54 »

Why oh why do some people struggle to define what a woman is ?

It depends on what the context is. The simple, men and women model is appropriate for most considerations.... But in some contexts, this oversimplified model must be extended. There are many different ways in which it can be extended, depending on what the question at hand is:

• Including age: boys, men, girls, women. And then, we have to wonder what defines the cutoff. Is it a certain age that applies across the board? (ie legal adulthood definitions for voting, driving, drinking, etc.) Perhaps sexual maturity (for which multiple thresholds could be defined), which would make sense when considering sexual activity/risks etc.

• For sports. It might make more sense to look at hormone levels. (Even without administered hormones, genetics alone doesn't necessarily dictate which hormones are produced and at which levels, and even then, the bodies might have atypical responses to hormones. For example, some people are born with XY genes, but no testes, and therefore develop as female https://rarediseases.org/rare-diseases/swyer-syndrome) If we consider administered hormones, then does a female who transitions to male compete as a female or a male? What about someone who has developed as a male and then transitions to female? Should there be more than two categories, or maybe only one? The rules should be decided for each competition, well in advance of the actual competition.

• What about for medical treatment? Just M/F is terribly oversimplified, especially when it comes to treating diseases related to sex. Ideally, doctors should know the genetic makeup, hormonal history etc. of each patient. Think of it this way: a flashlight (torch) can be on or off (simple enough, no?), but if it is off, one must know whether the switch is off, if it has batteries in it, if the batteries are installed with the correct polarity, if the circuit is broken etc. It is similar for human bodies. There are so many interconnected systems, and an apparent error can have many different causes (disease, mutation, toxic exposure, injury, etc.)

• Definitions base on body parts suffer from complications such as surgeries (as alancalverd pointed out). And sometimes, people are born as intersex, with genetalia that are not obviously assignable as strictly male or strictly female. (going off of neilep's tagline, some people are only partially inside out) https://www.healthline.com/health/baby/what-does-intersex-look-like

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Physiology & Medicine / Re: Can You Define What a Woman Is ?

« on: 28/05/2022 18:08:24 »

Just stick with the XX chromosomes. The reproductive bits don't always work and are sometimes removed if they go very wrong, but every cell of a woman's body contains two X chromosomes and every adult member of homo sapiens with two X chromosomes is a woman.

well... there are some people who have two X chromosomes and a Y (XXY). As I understand it, they present as male, and are considered male with Klinefelter syndrome. https://kidshealth.org/Nemours/en/parents/klinefelter-syndrome.html

But yes, we should definitely avoid requiring body parts to define womanhood.

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Plant Sciences, Zoology & Evolution / Re: Have animal reactions to mirrors and windows been studied?

« on: 27/05/2022 04:19:41 »

Well, this cat certainly appears to perceive motion in this static picture:

Perhaps just like we do:

rotsnake.jpg (613.75 kB . 1024x768 - viewed 4751 times)

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Question of the Week / Re: QotW - 22.05.23 - Where does the potential energy of a spring go in acid?

« on: 23/05/2022 15:12:22 »

Good question!

The compressed spring will probably decompress as it corrodes. So, for the most part, the answer is, "wherever energy goes when a compressed spring is released without any load on it." The mechanical energy is dissipated, with some of it turning into sound, some into heat, and possibly some other minor components.

However, this doesn't answer the core question here (at least not directly). Some of the spring must have dissolved from the "compressed state". What happened to that energy?

So, the act of compressing spring increases the potential energy of the spring. Because energy is conserved, there must be a way to account for it. So there are a few possibilities:

• Either dissolving the compressed spring results in a solution with the excess energy stored in it somehow (probably as heat, so it would be warmer than a solution otherwise identically prepared from the uncompressed spring.)

• Or, the solutions resulting from dissolving the spring in either state are identical, and the energy was released (again, probably as heat).

• Or, there is a range of intermediate cases, in which some of the energy goes into the solution, and some is released.

So that's the pure thermodynamic approach. But how can we explain it in a more concrete way?

On an atomic level, the energy stored in the spring can be thought of as strain on many different bonds between atoms—the bonds are either too short (compressed) or too long (extended) compared to the optimal bond length. Either way, this means that the bonds between atoms will be easier to break. Because dissolving the spring in acid requires breaking the bonds, it will take less energy to do this part for the "energized" spring, leaving more energy for motion of the resulting ions (heat).

A similar approach can be taken for thinking about springs that are in the same state of compression, but different temperatures (or one that is molten, and one that is solid).

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Chemistry / Re: How well understood is the Chemistry of the trans-uranic elements?

« on: 16/05/2022 17:16:37 »

Is it possible to slow the decay down by other methods?    A few years ago, school-level physics would have stated that nuclear reactions are unlike chemical reactions -  nuclear decay is a random process and the decay rates are un-affected by environmental conditions like pressure and temperature.  The general explanation being that the nucleus is dominated by the effects of nuclear forces (the strong and weak force) and effectively independent from whatever else is going on outside the nucleus.   It was a good explanation but like the semolina pudding they served at school lunchtime, it was just so wrong.
   This belief was seriously adjusted in my life time when a type of nuclear change called electron capture was studied.     (  https://en.wikipedia.org/wiki/Electron_capture  ).   In electron capture it is possible to adjust the nuclear reaction rates just by ionising the atom  (for example the nuclear change  ⁷₄Be⁺  →  ⁷₃Li⁺   proceeds more slowly than ⁷₄Be  →   ⁷₃Li   ).   You can get a smaller (but statistically significant) difference just by bonding the atom to certain things.

    I've not seen any information about it - but it begs the question that you might be able to adjust the rates of other nuclear changes.   For example, alpha emission may be reduced if the unstable nucleus can be surrounded with ligands that are positively charged and create a potential barrier against the emission of another positively charged particle.     Maybe just putting the unstable nuceii under pressure is enough to slow the decay (since the appearance of more particles tends to increase pressure PV = nRT   etc).    I don't know and It's important to point out that this is just speculation:   I have a personal belief that nuclear reactions are much more like chemical reactions than we had first imagined (just in terms of their reaction kinetics, obviously chemistry goes on outside the nucleus but nuclear change doesn't).

Neutrinos can also influence the rate of reverse electron capture (https://iopscience.iop.org/article/10.1086/305343/fulltext/34468.text.html)

And, obviously, nuclear fission chain reactions such as those crucial (critical?!? sorry...) to atomic bombs, show that nuclear decay can be influenced by the environment.

That said, because of the great energies typically involved in nuclear reactions, I don't think that most changes in pressures and temperatures that could be highly influential for chemical reactions would do much for nuclear ones. But the stability of neutrons in neutron stars is significantly greater than for lonely neutrons—and I think it might be due to the insane pressures within a neutron star that favor single neutrons over proton/electron pairs (and neutrinos!).

I think the case of electron capture in ⁷₄Be⁺ vs ⁷₄Be can be explained by looking at how much electron density is within the cross-section of the electron capture of the nucleus. However, I am not so sure that adding positive ligands around an atom would significantly reduce α decay. The electric field at the "surface" of the nucleus will be almost entirely dominated by the protons within the nucleus For example, a ²¹⁰Po nucleus, which typically undergoes α-decay with a half-life of about 140 days, has 84 protons in a sphere with a radius on the order of a few femtometers—adding a handful of singly (or doubly) positively charged ligands at a radius of 150 picometers (almost 1 million times as far away) likely won't change much. In terms of the electric potential at the nucleus, I think this will be a similar story, but might have a greater effect (but, I would expect it to be the opposite of your prediction—more positive ligands leads to a more positive potential at the nucleus, making it more favorable for the nuclear charge to decrease).

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Physics, Astronomy & Cosmology / Re: Is there a limit to how hot things can get?

« on: 12/05/2022 15:14:52 »

excellent questions!

I don't believe that there is any hard limit (that we know of) on the maximum possible temperature. In general, big bang energy tends to dissipate rather than concentrate, so a reasonable first approximation of the maximum temperature would be the temperature in the earliest moments of the big bang. According to this source (https://lco.global/spacebook/cosmology/early-universe/), in the first second, temperatures were on the order of 10³² Kelvin.

As a society, we are still learning about the earliest moments of the universe, so this may be subject to change.

Also, temperature can be difficult to define in extreme situations, especially if it only involves a few particles. So be aware that any discussion of "temperature" in particle collider experiments, is probably some sort of "effective temperature."

Humans are primarily made of (and dependent on) liquid water. This puts pretty significant restrictions on the range of temperatures and pressures that we can survive at. However, this apparent "specialness" is likely explained by the anthropic principle. Having arisen on the surface of the Earth, it makes sense that our existence is tuned to those conditions. I would not be terribly surprised if there were some form of plasma-based "life" that could have arisen in stars. They might not even be recognizable as alive to us (and we to them).

Finally, one thing to note is that, in some sense, negative temperatures are possible. The most common example given is lasers. https://en.wikipedia.org/wiki/Negative_temperature

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General Science / Is 2 really prime? If so, why isn't 1?

« on: 28/04/2022 16:41:40 »

Got in a spirited debate the other evening (no pun intended), about whether the numbers 1 and 2 are prime.

The definition, as I know it from schooling, is that a prime number is "any number that is only divisible by itself and 1." ie if you can divide the number by any whole number that is not itself or 1, there must be a remainder or fractional component.

This can also be through of geometrically: if you have n things (let's say dots), then the only "rectangular" array of those n dots must be 1×n or n×1.

For example, in the image below, we can see that 3 dots can only be arranged in a rectangular array of 1×3 or 3×1. If we try making a 2×2, there is an incomplete edge. On the other hand, 4 is not prime because 2×2 works.

Screen Shot 2022-04-28 at 11.29.38 AM.png (85.49 kB . 990x1302 - viewed 5339 times)

1 is often left out from the primes because it has only one factor, itself (or 1). So it cannot be formed from an array of 1 by itself. Maybe this is a straw man argument (if so, please put me right). But this seems specious. A 1×1 array is still an array.

It seems to me that 2 should be treated the same as 1 because there isn't a choice among proportions of rectangular arrays. Only 1×1 for 1. Only 1×2 or 2×1 for 2. Only 1×3 or 3×1 for 3. etc. And really, there is no geometrical difference between a Only 1×2 and a 2×1 rectangle. The orientation is arbitrary (these aren't matrices).

Only when we get to non-prime numbers is there a choice of multiple types of rectangular arrays. One can arrange 4 dots as 1×4 (4×1) or 2×2. One can arrange 24 dots as 1×24 (24×1) or 2×12 (12×2) or 3×8 (8×3) or 4×6 (6×4).

There are many options for how to arrange 3 or more dots (only some of which. are rectangular arrays). There is only one option for 2 dots, and only 1 option for 1 dot.

So the real question I have is: if we count 2 as prime, should we not also count 1? Or, does the reason we don't count 1 also extend to 2?

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Just Chat! / Re: A Short puzzle with dogs.

« on: 12/04/2022 16:47:55 »

Are these dogs point particles that must be co-localized at the endpoint, or do they have some nonzero radius that is less than the "1" specified as the side length?

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Chemistry / Re: How well understood is the Chemistry of the trans-uranic elements?

« on: 28/02/2022 01:17:25 »

Trans-uranic elements are much harder to study (due to their radioactivity, rarity, and cost), but the chemistry of actinide elements (including trans-uranics) is an active field of study.

https://pubs.acs.org/doi/10.1021/acs.inorgchem.8b03603

These f-block elements don't have chemistry similar to carbon, but many of them do have quite complex chemistries. They can have many different oxidation states, and can be used in compounds that serve as catalysts (Fox, A.R.; Bart, S.C.; Meyer, K.*; Cummins, C.C.*, Towards Uranium Catalysis, Nature, 2008, 455, 341 - 349), fluorescent compounds (https://pubs.rsc.org/en/content/articlelanding/2006/cp/b607486c), and thereputics (https://www.lanl.gov/discover/publications/actinide-research-quarterly/pdfs/arq-2019-01.pdf)

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Physics, Astronomy & Cosmology / Re: Can sand/salt permanently molecules absorb resonate frequency?

« on: 22/02/2022 15:31:33 »

The patterns formed on Chladni plates are a result of the properties of the plates themselves, and has nothing to do with the sand/salt/sugar/dust/etc. placed on top for visualization purposes. The grains that are arranged into the pattern are "agnostic" of why they are in the particular place that they are in.

The plate vibrates in such a way that parts of the plate have significant displacement, and other parts hardly move at all (the "nodes"). Grains that happen to be on parts of the plate that are moving get tossed up and land on another part of the plate, while grains that happen to be on the nodes where the plate is hardly moving get to stay put. This means that after a brief period, almost all of the grains end up on the parts of the plate that are hardly moving. The pattern that the nodes make is a function of the frequency of the vibration and the size/shape of the plate.

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Physics, Astronomy & Cosmology / Re: Has the distance between the sun & earth changed?

« on: 23/01/2022 23:42:32 »

Im the one that asked the question; and thank you for posting this Origin.

I would like to know if this distance is a big cause of Climate change.

I see from posts that the distance does change, though is there an on going change that is climbing (a distance change or orbit change), and not staying nominal?

It can have an effect to some extent, and there are some cyclical changes in earth's orbit (see more here: https://www.fs.usda.gov/ccrc/education/climate-primer/natural-climate-cycles)

But the current (fast) changes in the climate are driven primarily by greenhouse gases (mostly carbon dioxide and methane).

We can see the effects of the cycles, as well as the recent trends (the practically vertical line at the far right) by looking at atmospheric carbon dioxide concentrations found in ice cores, the record of which goes back almost a million years (https://keelingcurve.ucsd.edu/)

co2_800k.png (110.13 kB . 1000x600 - viewed 8117 times)

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Physics, Astronomy & Cosmology / Re: Whats a branch of physics with high potential that has unjustified disinterest

« on: 10/01/2022 22:11:01 »

I may be biased (as a chemist), but I would recommend physical chemistry. It has all the mathematical rigor and knowledge of "standard" physics, but it's also much more easily explored experimentally than high energy physics, astrophysics, cosmology etc.

Physical chemistry (or chemical physics, or materials science, depending on what you're looking at) involves a lot of quantum mechanics (because it largely deals with collections of photons, electrons, atomic nuclei, and how they interact with each other). It also involves relativity, when considering very heavy atoms (like gold or uranium), in which electrons that get close to the nucleus have enough kinetic energy that reletivistic considerations must be taken.

Bridging the gap between physics and chemistry means that it is very useful, and often overlooked by purist physicists and chemists alike. But physical chemistry is critically important for "sexy" topics like (among other things):
• quantum computing (and regular computing)
• LEDs (and OLEDs)
• Photovoltaics
• Drug design
• Protein folding
• Sensing
• Spectroscopy
• Making new dyes (for aesthetics, cellular imaging, light emission, light harvesting)

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General Science / Re: Is this a feasible system for recycling CO2?

« on: 02/01/2022 18:57:56 »

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Physics, Astronomy & Cosmology / Re: How can we see ultraviolet light in Balmer series?

« on: 02/01/2022 17:59:12 »

I think it is also important to note that the Balmer series (and Lyman, and Paschen etc.) are for atomic hydrogen, not molecular hydrogen (H vs H₂). H is not stable at standard temperatures and pressures, but is stable at temperatures where electronically excited H atoms are thermally accessible.