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Physics, Astronomy & Cosmology / Re: Spinning a cassette player with a rope: same sound?

« on: 07/04/2023 14:24:09 »

The speed of the tape only matters relative to the magnetic head that senses the tape as it moves across. So spinning the wheels of the cassette faster to make the tape wind faster will change the pitch of the sound, but hurling the whole tape player around the room won't change anything.

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General Science / Re: Good examples of contemporary computational/theoretical engineering/physics rese

« on: 23/02/2023 15:23:24 »

I am familiar with chemistry, and there is a LOT of computational chemistry (some is based on quantum mechanical models, some on classical "force field" models, and some using statistical models). Many publications include both experimental and computational components because it is good practice to test the validity of the simulation models against the real world. There's an adage something along the lines of, "experiment can show results (what happened), but rarely mechanism (how it happened), while computation shows possible mechanisms, but rarely identifies results (how something could happen, but not if it does)"

Since it sounds like the OP is requesting purely computational papers, I would draw their attention to this open access and purely theoretical journal:

https://www.mdpi.com/journal/computation

Note: the subject matter spans a very broad range, and is highly technical, so don't expect to understand any of it on a first read :-)

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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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General Science / Re: Advent calendar maths question!

« on: 18/12/2022 16:06:06 »

If you use base six the problem is trivial; 2 dice numbered 0 to 5 will do.

Indeed.

At one time I won a (non-monetary) bet that I could count over 1000 on my fingers. The pour soul didn't say I couldn't use base 2. :-D

We can only get to 256 where I live.

I was using my fingers as a ten-digit binary display. 1111111111 (base 2) = (base 10) 512 + 256 + 128 + 64 + 32 + 16 + 8 +4 + 2 +1 = (base 10)  = 1023

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General Science / Re: Advent calendar maths question!

« on: 17/12/2022 22:08:29 »

If you use base six the problem is trivial; 2 dice numbered 0 to 5 will do.

Indeed.

At one time I won a (non-monetary) bet that I could count over 1000 on my fingers. The pour soul didn't say I couldn't use base 2. :-D

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

« on: 15/12/2022 14:19:01 »

oof! thanks for the correction--I set myself up poorly for that.

Me: "be careful about temperature and energy"
also me: "energy is defined as T =...." *facepalm*

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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 3127 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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Chemistry / Re: what are the names of 2 solids that combust when they come into contact

« on: 15/09/2022 03:46:20 »

Just a reminder not to actually try these, but for the sake of curiosity...

Manganese(VII) oxide and pretty much any organic matter will react quite violently. I believe this would also extend to solids, even at room temperature, though there may be an induction period. (also, for most demonstrations involving Mn₂O₇ the active oxide is synthesized with the aid of sulfuric acid, which is then still wetting the surface, significantly increasing the rate of reaction, especially with carbohydrate solids, like paper, sugar, starch, cotton, etc., but it also might violate the stipulation that the reaction be purely between two solids)

Few solids can undergo fast reactions at ambient temperature because being solid essentially means that it is hard for the atoms to move around, which is precisely what needs to happen for a chemical reaction to happen quickly. (this is usually a good thing) One way around this is to have at least one of the solids have a low melting temperature, and very high heat of reaction (potassium, sodium, white phosphorus, even sulfur). And another way is to use solid molecular compounds with very, very weak chemical bonds (peroxides, ozonides, malozonides, ) though many of these may just spontaneously decompose/detonate on their own even without a reactive partner (definitely DO NOT try this!!!)

For a vigorous reaction to occur between two solids with only incidental contact, they must be inherently very reactive substances, and likely each highly hazardous, even on their own. But if we allow for some energy input (shock, friction, heat, light etc.) then there are many combinations of solids that combust with each other given the input, but not without it (some combinations are so dependable, they are used commercially in matches).

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Chemistry / Re: How do I grow a spinel crystal?

« on: 25/06/2022 02:30:11 »

How about cobalt oxide spinel? Co₃O₄, which can be made by hydrothermal synthesis.

I found a paper (unfortunately the manuscript its self behind a pay wall, but both some of the experimental data and the cover art and abstract are available). Granted, their crystals were microscopic, but that's ideal for their application, and it looks like they worked out some fundamental kinetics of the crystal nucleation and growth vs temperature and other controllable variables...

Abstract: The hydrothermal growth of cobalt oxide spinel (Co3O4) nanocrystals from cobalt acetate precursors was monitored with in situ powder X-ray diffraction (PXRD) in combination with ex situ electron microscopy and vibrational spectroscopy. Kinetic data from in situ PXRD monitoring were analyzed using Sharp–Hancock and Gualtieri approaches, which both clearly indicate a change of the growth mechanism for reaction temperatures above 185 °C. This mechanistic transition goes hand in hand with morphology changes that notably influence the photocatalytic oxygen evolution activity. Complementary quenching investigations of conventional hydrothermal Co3O4 growth demonstrate that these insights derived from in situ PXRD data provide valuable synthetic guidelines for water oxidation catalyst production. Furthermore, the ex situ analyses of hydrothermal quenching experiments were essential to assess the influence of amorphous cobalt-containing phases arising from the acetate precursor on the catalytic activity. Thereby, the efficient combination of a single in situ technique with ex situ analyses paves the way to optimize parameter-sensitive hydrothermal production processes of key energy materials.

https://chemistry-europe.onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1002%2Fchem.201801565&file=chem201801565-sup-0001-misc_information.pdf

https://chemistry-europe.onlinelibrary.wiley.com/doi/epdf/10.1002/chem.201886961

https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.201886961

https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/chem.201801565

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Chemistry / Re: In what fundamental forms are energy released when hydrogen burns in oxygen?

« on: 20/06/2022 03:51:00 »

NOTE: I think this thread is more suitable for General Science as it involves energy.

I think chemical energy to kinetic energy seems problematic.

Let's assume 2 H + 1 O atoms with negligible KE in the CM frame. Somehow, they combine to give H₂O. There is no way the single H₂O molecule could acquire any kinetic energy relative to the center-of-mass frame. This would require an external force acting on the H₂O, but any forces during the formation of this single molecule may only be internal forces.

How to solve this classical mechanics paradox?     

I don't think that purely internal forces causing changes to vibrational motions would be prohibited by any conservation laws.

By analogy, perhaps could imagine an arrangement of magnets (or springs) with negligible KE in the CM frame, that when allowed to combine and "react" with each each other would result in a system with significant internal kinetic energy. This extra KE could well be in the form of a symmetrical vibrational mode, in which there is no net motion of the center of mass.

The forces involved in chemical reactions are often viewed very abstractly, but I think this is a mistake. On a molecular level, chemical reactions are extremely violent events (series of events, really), involving lots of smashing, and rending, and ricocheting. More like cars crashing into each other (repeatedly) than building with legos...

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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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Chemistry / Re: How to extract Fluoride from Mouthwash?

« on: 10/06/2022 15:31:37 »

you can buy calcium fluoride (fluorite) crystals fairly cheaply. Many of them are very pleasing octahedral shapes (for example, a quick google search shows:


Screen Shot 2022-06-10 at 10.28.34 AM.png (841.44 kB . 1466x792 - viewed 2364 times)

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

« on: 10/06/2022 15:16:56 »

There is very little functional requirement nowadays for most of the human male anatomy. The likely longterm outcome is that, like some fish and spiders, evolution and food shortages will favor smaller males, gradually diminishing to genitals on legs.

You are forgetting that not all traits that are selected for evolutionarily are for survival. Attracting mates is also quite important, and many phenotypes are selected for purely for mate attraction, even when they are detrimental to survivability (see: bird plumage)

https://www.nature.com/scitable/knowledge/library/sexual-selection-13255240/

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General Science / Re: What takes more energy walking or skateboarding?

« on: 06/06/2022 21:17:16 »

More information is needed to answer the question: same speed? I assume flat means level (no grade) and with no bumps large enough to impede the wheels (is this correct?) how smooth/slippery is it? is it windy? if so, how much, and which direction compared to direction of motion? how well-oiled is the skateboard? (one could imagine under perfect conditions a single push would allow the skateboarder to eventually make it to the end.)

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Chemistry / Re: quick question for bored chemist

« on: 04/06/2022 01:47:41 »

Thanks for the clarification.

Yes, diazomethane has a very rich chemistry, beyond mere methylations. It is extraordinarily hazardous too: in addition to its substantial toxicity, it is prone to detonation if it gets too hot, or if it is treated roughly. There is a silylated version which is slightly easier to handle (trimethylsilyl diazomethane), being a liquid rather than a gas, but even then... yikes!

One of my colleagues in grad school referred to diazomethane as "Angry Yellow" because of its apparently ill temper and its bright yellow color.

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Chemistry / Re: quick question for bored chemist

« on: 03/06/2022 18:56:03 »

I'm not Bored chemist, but as a chemist, would be happy to offer some insight.

I would imagine it to be equally hazardous to one's dna.

Any methylating agent with a sufficiently good leaving group will be able of damaging DNA, unless it is so good it reacts with water first or is not bioavailable (polymer supported methylating agents would be much harder to get into cell nuclei).
https://pubs.acs.org/doi/pdf/10.1021/ol025633s
http://www.arkat-usa.org/get-file/29478/

diazomethane

diazomethane is only useful as a methylating agent under acidic enough conditions to protonate the methyl group (for example, methylating carboxylic acids).

What are you hoping to methylate?

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Chemistry / Re: How can I find the optimum ΔH and ΔS for passive T control?

« on: 01/06/2022 12:30:01 »

thank you! it will take some time to read, but it looks like you made more headway than I did, so this should be very informative!