exchemist

In the visible, photons are absorbed by electronic transitions, creating excited states that relax by non-radiative means (i.e. intermolecular collisions) to populate vibrational and rotational excited states - and thence eventually translational motion. In the IR, they are absorbed by vibrational states, which then similarly cascade down by collisional relaxation. None of these radiative processes, so far as I know, can directly stimulate excitation of translational motion. The exceptions would be ionisation or bond breaking, where the absorption causes an electron to jump right out of an atom, or two part-molecules to fly apart because the bond strength has been exceeded. It seems to me you need a transition dipole moment in order to absorb a photon and I can't see how you get that in translational motion. So I think, subject to correction by one of the physicists here, that translational excitation is always the consequence of collisions, rather than radiation directly.

Surely elastic scattering, by definition, does not transfer any energy to the medium doing the scattering? If it idid, it would not be elastic, would it? If there were energy transfer there would have to be either absorption of photons or a change in frequency. This is a question of energy rather than momentum. Inelastic (Raman) scattering involves excitation of vibrational and rotational modes in the molecules and a concomitant change in frequency of the radiation. My issue is whether there is a process by which EM radiation can directly excite translational motion in molecules. I cannot think of such a process - but then I am a chemist, of course.

Oh you mean transfer of momentum by inelastic (i.e. Raman) scattering?

Can it in all cases, though? Rotational and vibrational excitation can change by absorption or emission of radiation, but translational motion? Doesn't that have to change by heat flow?

No, enthalpy would include various forms of internal potential energy associated with chemical bonding etc. One can only measure changes in that, rather than absolute values. I am thinking here of kinetic theory. What I mean is thermal energy, i.e. the famous 3/2RT at constant volume for monoatomic gases or 5/2RT for diatomic gases, due to thermal kinetic energy of the molecules. This does have an absolute value which goes to zero at absolute zero. (Though there is still some residual zero point motion at absolute zero, this is by definition unextractable.)

OK but surely one can define "heat energy" or "thermal energy" as a subset of total internal energy, meaning that portion of the internal energy due to kinetic energy of molecules, which is the same as that portion of it that can be made to flow out by means of a temperature gradient. Heat energy, so defined, is reduced to zero at absolute zero, is it not?

If it's gallium, it will turn into a puddle on a hot day in summer.

But isn't that just because we now speak of internal energy, and use "heat" to describe the flow of internal energy rather than the energy itself? I should have thought one could say that a body at absolute zero has no heat energy left in it. All that's left is is zero point energy, and various kinds of potential energy, including rest energy, none of which is extractable as heat.

Looks like ballocks to me: I don't see how a real robot, i.e. with a microprocessor capable of making decisions and mechanically executing them, survives liquefaction. But here's the abstract of the published paper (have to pay to get the whole thing): Magnetically actuated miniature machines can perform multimodal locomotion and programmable deformations. However, they are either solid magnetic elastomers with limited morphological adaptability or liquid material systems with low mechanical strength. Here, we report magnetoactive phase transitional matter (MPTM) composed of magnetic neodymium-iron-boron microparticles embedded in liquid metal. MPTMs can reversibly switch between solid and liquid phase by heating with alternating magnetic field or through ambient cooling. In this way, they uniquely combine high mechanical strength (strength, 21.2 MPa; stiffness, 1.98 GPa), high load capacity (able to bear 30 kg), and fast locomotion speed (>1.5 m/s) in the solid phase with excellent morphological adaptability (elongation, splitting, and merging) in the liquid phase. We demonstrate the unique capabilities of MPTMs by showing their dynamic shape reconfigurability by realizing smart soldering machines and universal screws for smart assembly and machines for foreign body removal and drug delivery in a model stomach. Perhaps someone else here can explain what this actually does. The reference to "locomotion speed" is intriguing.

Well, Montmartre is where the ladies of the night used to hang out.........

You mean like Evelyn, Beverly, Vivian, or Leslie/Lesley? I know Sidonie is a French girl's name, but that has 3 syllables. Perhaps Sidney for girls is a variant of that. But it sounds weird to my ears, I must admit. I suppose the geeks might have deliberately picked an androgynous name.

I suppose it's a trivial observation, compared with the scandal of not backing off from disruptive intrusion into someone's human relationships, but it also seems tone deaf to a person's likely sexual orientation, given that he is married to a woman and Sidney is a man's name. "Nul points" to the guys with spiky hair on this one.

I just wish these geeks would put half the effort they waste on this stuff into controlling the dissemination of falsehoods. Haven't they damaged society enough, without looking for new ways to do even more damage?

Yes, it doesn't bother me too much either. I lived in The Hague for a few years and came to appreciate Dutch directness. I also rowed for many years and am used to being coached, and then there is choral singing, in which coaching can also be fairly direct! I find your posts clear, knowledgeable and informative. And indeed, I think we are all here to learn, not just to pontificate.

Yes exactly. In fact one reason I hang out in these places in my retirement is to repoint the brickwork and replace loose roof tiles.

I think it is disconcerting to find that something you thought you knew is wrong, especially when it relates to a subject area that you regard as your citadel of knowledge, i.e. helps to define your self-image. I'm now old enough to tread increasingly carefully, even there, as I'm finding a false memories sometimes catching me out. This is a phenomenon I expect to become more pronounced in the years to come. But taking exposure of error or ignorance as a deliberate attempt to belittle is something else. Normally I think most of us can tell whether someone is just correcting us or whether they are using it as an excuse to indulge in patronising or point-scoring. But perhaps some are over-sensitive and imagine negative motives when there are none.

This reminds me of Spike Milligan:" Hey, who turned on the dark?" You get into trouble with the idea of "coldness" when you have to deal with absolute zero. You can't make something as cold as you like, whereas you can make something as hot as you like. So there's an asymmetry there. Just as, with light, you can make something as bright as you like but you can't make it as dark as you like.

Understood: caveat noted.

Actually I think that change served a deliberate purpose. Given that the model of the atom changed radically from the Rutherford-Bohr model to the modern (Schrödinger?) model, the word "orbit" became objectively wrong, so orbital was chosen instead, to signify the new model. P.S. Another name change in chemistry that bamboozled me briefly is from ESR, which I remember from university, to EPR, which is virtually the same thing. I suppose there is in principle a change, in that EPR implies one thinks in terms of the more general J, rather than S.

Yeah but it is handy, in a long thread that is being sensibly discussed, to be able to refer readers back to specific earlier posts. This is the only forum I've been on where the posts are not numbered.

Indeed. But with oil, which is used for cooking and to dress salads etc, one can reduce it a little without adding anything to replace it. I had been thinking that plenty of it was a good thing, because of HDL. So I can cut it back to just what is necessary. (I don't have a problem with my weight, which has been stable for years at: 66 +/-1 kg and 1.76m in height.)

Agreed. I think this would be very helpful. It seems to be available on other forums.

Yes, Carnot applied the concept of caloric in that way in his analysis of heat engines. And got it essentially right.

It wasn't his theory, though. It was Lavoisier's, which Carnot inherited as the then prevailing model and which he suspected was faulty, as some of your earlier extracts from his Appendix A showed.

Chaleur shurely? 😄 Chauler is a verb, meaning to lime or whitewash. Here is a link to Carnot's original: http://www.numdam.org/item/10.24033/asens.88.pdf Chaleur just means heat and, so far as I can see, will have long predated Lavoisier''s scientific concept of caloric. Lavoiser seems to have introduced caloric because he realised the idea of phlogiston didn't work.