exchemist

There are a lot of anti-vaxxers and nutjobs about at the moment, as you may be aware, that like to cast doubt on the safety and efficacy of vaccination. It looks as though some of the people here have got burnt by this in the past and may have thought you could be one of them, rather than just someone who is not well informed. Risks from the vaccines are small compared to the risks from the disease, even for younger people. Aside from the risk of death or emergency hospitalisation if you are fat, there's a lot of long Covid about. I have a nephew who is very fit (got a half blue at Oxford, rowing for the Lightweights), whose sense of taste has been permanently altered by the virus. I lost my sense of taste and smell totally for a fortnight, which became a bit frightening, though luckily it has come back. The risk of blot clots with the Oxford/AstraZeneca one seems to be about one in half a million: about a fiftieth of the annual risk of blood clots run by a woman on the pill. Nevertheless for those under 40, it is recommended to get one of the other vaccines if possible, for preference, just to avoid even this small risk. As others have pointed out, by getting vaccinated you also help reduce the incidence of Covid in the population as a whole, which reduces the chance of more, nastier, mutations coming along and setting us all back to square one. So yes, get vaccinated. I had my second shot last week (AstraZeneca).

I'm not sure I follow this. It looks as if you envisage a closed container that stretches and then suddenly bursts from the pressure of the ice and that this moves a piston. Depending on what load the piston is connected to, I suppose with a light load it might fly away from contact with the ice, leaving a near-vacuum behind. It won't be quite a vacuum as there will be the vapour pressure of water or ice at 0C present - about 4.5mmHg, so not much, admittedly. A vacuum won't cause the ice to melt. Many comets are made of ice. So there won't be any boiling. Why is this piston "gigantic", suddenly? Are you smoking exotic cheroots? 😀

I think we should leave urinary infections out of this. 😆

Strictly, yes of course, but the way this problem has been posed, you don't actually need the sign to get the right answers.

Eh? Lifting weights expands nothing. Work done in expansion is PdV. Think of it this way: if you have an expanding fluid pushing a piston, the work done is the force, F, on the piston (pressure x surface area) multiplied by the distance, d, the piston travels - which is volume change/surface area. So Fd = PA x ΔV/A = PΔV. (Since P is likely to change as V increases you need to do it as an integral, hence ∫PdV. ) So expansion volume certainly is the determinant, along with pressure, of the available energy.

The expansion on freezing is only about 9% volume, so the work done in expansion when the pressure is released is not that much - enough to bust the container but not much more. There is very little stored energy. You get a lot more stored energy in compressed gases than you do in compressed liquids and solids, because gases expand to many times the confining pressure, doing a lot more work (= energy). That's why heat engines rely on gases.

Aha! Now the rest of your calculation makes sense! I see where you get the deceleration of 2m/sec² from (12m/sec -> 4 m/sec over 4 seconds)and given that the mass is 0.5kg, that will imply a force of 1N, just as you say. So it all looks good to me.

OK. I have however one specific piece of immediate advice for you: learn to use full stops. 😀

These are only "fundamental questions of physics" if the questions have a meaning. And they only have a meaning in physics if they predict some observable result. This is where I struggle. I cannot see what observational outcomes can be dependent on this shape issue of yours. A photon is either detected, by absorption in an atom generally, or they are not. Isn't it?

Where do you get this stuff from about the limits of pumps? And why choose the worst possible type of pump for dealing with big pressure differences? You would need a +ve displacement pump, not a centrifugal one. Injector pumps can manage well over 500bar. And surely by a 2-stage process you could reduce the stresses between each stage considerably, couldn't you?

I'm not sure the "width" of a wavepacket tells you anything about the size of a photon. Surely it just tells you the shape of the probability distribution of where you may expect to detect it, doesn't it? And QM tells you that anyway, so there is no issue there. Whatever it is you are after must be something different, or we would not be having this discussion.

I can't say I had ever come across them, until a few years ago on the internet. Anyway, it's nice to know one can always annoy them by asking if it is true that the Flat Earth Society has members all around the globe. This seems to be a meme they can't kill off.

You do not say how old you are, but you could perhaps start with something like this: https://www.ducksters.com/science/chemistry/. If it's the wrong level let me know and I'll try to find something else. Also, if you are interested in chemistry you can learn a lot by looking at things in the Periodic Table. Here is one I use, that links to Wikipedia articles on each element and is quite informative: https://ptable.com/#Properties Obviously, people on a discussion forum are not going to be able to provide a course on-line just for you. That would be a lot of work. We can maybe help you with specific issues, though, as you encounter them.

I'm going to start by saying I'm a mere chemist who learnt some quantum chemistry 40 years ago, so I am not an authority. However I have read a bit about this, as I found it confusing. What follows is my understanding. I'm also going to ignore the instruction to start with yes or no, as I think you pose a bit of a false antithesis. Sorry. I have to take issue with your assumption that to be force carriers these entities need to be "pockets of energy". I think that is wrong at at least two levels. First, energy is not stuff: you can't have a jug of energy. Energy is a property of a system of some kind. Things "have" energy. They can't "be" energy. Second, I think it is wrong to imagine that virtual particles need to have energy in order to be force carriers. An entity does not have to experience energy gain or loss to experience a force. As to whether virtual particles are "real", I'm going to risk annoying you by saying I think it depends what you mean by "real". What seems to be the case is that virtual particles are not particles. They are - so I gather - disturbances in various fields that can be modelled using some of the same mathematics as particles. But the disturbances in the field are "real", in that there are observable consequences of them. A year or two ago I came across a very good article about virtual particles by Matt Strassler, here, which may help you. It helped me, anyway: https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/

Your attitude seems to betray a conviction that if only we could somehow "see behind" QM, we could restore the Newtonian world of exact, deterministic knowledge of physical systems. Einstein thought the same, so you are in good company. However, every attempt at restoring a deterministic universe, via "hidden variables" and so forth, has failed, to date. Einstein was wrong, apparently. Most physicists seem to think the QM picture, in which there are things that are intrinsically indeterminate (to do with Fourier transforms, non-commuting operators for the observables in question, and all that jazz) looks correct. As I understand it, Heisenberg's QM was deliberately built on modelling only observables - and avoiding what may or may not "go on" besides, because whatever it may be, it is not observable! So I do not think it is fair to say QM is built on classical mechanics, really. And it's not like creationism, because it is a model that works, experimentally. All theories come up against a limit at some stage, beyond which we can only shrug and say "that is just how nature seems to be. Sorry."

The overall reactions appears to be Mg + H₂O + 1/2 O₂ -> Mg(OH)₂. There is a description of how it works here: https://pubs.rsc.org/en/content/articlehtml/2014/mh/c3mh00059a The clever bit, it seems to me, is the "air cathode". This seems to be a multi-layer structure, incorporating a conductive layer (obviously) bonded to a catalyst layer, which seems to involve silver as the material that encourages oxygen to react. I have not read this in detail, but it looks as if optimising this part of it is an active area of research.

Agree with @studiot that a piece of information seems to be missing from the problem, in the way you describe it. You don't say over what period of time the deceleration occurs. Without that, you can't know if the deceleration was gradual, due to a small force applied for a long period of time, or sudden, due to a large force applied over a short period.

Well by now you know my view: I suspect these are just semi-classical fictions, with no observable consequences.

I think the technical term for this is "clutching at straws."😀

I was sort of hoping my example of Chicago might deter him. I have actually seen that example used by flat-earthers elsewhere, as "proof" the lake is flat. So I thought I would pop that bubble before he had a chance to refer to it. The psychology of these people is what intrigues me. I suppose it may just be contrariness: a determination to be agin everyone else, just for the hell of it. But it is very strange. Makes QAnon look normal - almost. But I digress.......

Yes it is now just about entirely populated with nutcases and cranks- and not even interesting cranks, at that.😄

How do you account for the momentum of a photon in your model? Does that belong to the wave (difficult with a transverse wave, I'd have thought) or to the photon "corpuscle" - which does not move?

That's interesting. In the desert I'd expect an inferior mirage, e.g. the sky being seen as at ground level, so I'd have thought you would each see the other lower than they really were. Or was this at dawn, with cold ground?

OK I see, thanks for the explanation. Well, it's been almost a month but maybe our poster will return and enlighten us....

That's rubbish and I've already explained why. Twice. If you still haven't seen the point, I'm not going to spend more time on this.