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Physics, Astronomy & Cosmology / Re: Gas discharge tube hydrogen

« on: 16/03/2024 23:50:19 »

Hi.

The initial excitation is by collision with cations (in this case protons) streaming from the anode.

    Some if it is but there are also always going to be some free electrons to be found.   
For one reason or another, there will be some atoms in the gas tube that are ionised so that we have a free electron and a positive ion (a "cation").   This might be a consequence of cosmic rays striking some atoms or something else.   One way or another you have a tiny number of cations and free electrons in the gas, even though it might be a very small number.

Most gas discharge tubes work by causing an "avalanche effect".   See  https://en.wikipedia.org/wiki/Townsend_discharge   for a description of the "Townsend Avalanche".



   Basically if you have a small number of free electrons to start with and then accelerate them in an electric field, they have collisions with other atoms and quickly start to liberate more electrons and leave more positive charged ions.   Under the right conditions the numbers of electrons increase geometrically (typically doubling on each collision).   So fairly quickly you have a large number of free electrons and positive ions in the gas tube.   There is a more detailed discussion in the Wikipedia page that was linked to.

Now let's go back to your ( @theThinker ) comments:

But QM requires that the electrons could only be excited by the exact frequency light

     That's half right.    If electrons were going to be moved from one orbit to another by absorbing a photon then those photons should have exactly the right amount of energy corresponding to the energy gap between the orbits.    However, QM never said that absorbing a photon was the ONLY way to move electrons from one orbit to some other  (or in this case to liberate them from the atom completely).
      It is quite possible to liberate electrons through impact between the atom and some other particle.   In our case, we have free electrons (and also some cations which can also be accelerated and get involved) these collide with the (un-ionised) atoms and liberate electrons from them.
    Actually, if you just want to completely free the electron then you CAN also do this with photons and it's a lot less restrictive then trying to move them from one orbit precisely to another.   See "the photo electric effect",   https://en.wikipedia.org/wiki/Photoelectric_effect ,   provided the photons have energy greater than the ionisation energy then the electron can be freed and the surplus energy will usually just become kinetic energy that the electron leaves the atom with.   However, that isn't what is done in gas discharge tubes - we just have electrons liberated as a consequence of collisions and impact energy.

In the gas discharge tube experiment for H2,  we have four emission lines in the visible spectrum.

    Once you have quite a few free electrons these will start to fall back into the ionised atoms.   They don't usually go from being free electrons straight to being bound in the lowest energy ground state of the atom.   Indeed they will usually start at some higher orbit energy and steadily fall further down to lower energy orbits.   Exactly as you have stated,  some of these transitions will be to the final level n=2  from higher energy orbits (this is the Balmer series of emission lines).   These are the ones which produce visible light.   More discussion and diagrams available here:  https://www.chemguide.co.uk/atoms/properties/hspectrum.html

 

I hope that helps.
Best Wishes.

Your reply is very helpful. I like to note that GDT works with AC/DC.

I have a related question on the ionization part.The first process in gas discharge is ionization. It is easy to explain classically as the electric field applied pulls the nuclues and the electrons apart in opposite direction thus separating them.

But if we want a QM viewpoint, it may be difficult. An electron could be free of the nucleus only if it absorbs a photon raising its energy beyond the transition limit. But where do such an initial photon come from?

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Physics, Astronomy & Cosmology / Re: Working of IR Laser

« on: 07/07/2023 15:37:26 »

Thanks,
 Rather complicated.

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Physics, Astronomy & Cosmology / Re: How a photon is absorbed by matter.

« on: 03/04/2023 16:12:46 »

My specific interest is whether a single microwave photon has a chance to be absorbed if it hits an isolated oxygen or nitrogen molecule or CO2 molecule. Boredchemist says no.

And, give or take the possibility of a "not quite zero probability" due to isotopic substitution, he's sticking to it.

That's why we can use microwave radio signals to send data like wifi through air.

OK. Accepted that wifi is the proof.

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Chemistry / Re: Can glucose be synthesized without organic inputs as sources?

« on: 12/03/2023 19:04:04 »

The obvious answer is that plants make glucose from CO2 and water.

Historically, the only way to prove the structure of a chemical compound was to synthesise it. Chemists got good at it.

You can use calcium carbide (made by reaction of the elements if you like)  and water to make acetylene.
On a good day you can then get this reaction to make acrolein
https://en.wikipedia.org/wiki/Hydroformylation

Or you can react acetaldehyde and formaldehyde.
Acetaldehyde is made from ethylene by addition of water catalytically (I think they use a mercury salt as the catalyst).
You can get ethylene by partial hydrogenation of acetylene (see above) and you can get formaldehyde by hydrolysis of dichloromethane which you can et by chlorination of methane which you can (just about) get by the direct reaction of carbon and hydrogen.

It's not pretty, but it's possible (and there are probably better ways)
I think magnesium carbide gives an interesting set of hydrocarbons (including propyne) on reaction with water; that might be worth looking into if you are looking for acrolein.

OK ! My bad. I missed the acetaldehyde and formaldehyde to acrolein part in the wiki.

I have now confirmed the correctness of your post. Now if we accept the 1887 article of Nature how Drs. Emil Fischer and Julius Tafel in the chemical laboratory of the University of Würzburg synthesized glucose from acrolein, then it would be true that we are able to synthesize food (glucose) from inorganic starting compounds. This would be a great surprise to me as we have successfully mimic photosynthesis in plants - a living thing.

I have a question. When I search google with : "laboratory" "synthesis" "glucose", why is that the only relevant link I get is still the 1887 article in Nature. Why is there not any other more recent articles about lab synthesis of glucose? 

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Physics, Astronomy & Cosmology / Re: Why does "aether" still haunt alt science?

« on: 14/01/2023 14:15:54 »

In the end, physics is a matter of opinion. You are free to decide what is evidence and what is not evidence.

I don't think it's the end yet.
There are constraints to our opinions, either physically or socially.
Rational persons accept or reject evidence with reasons.
We can reject experimental results by declaring that they are generated by tricks or experimental errors, but it requires supporting evidence.
We can accept experimental results, but reject an interpretation of those results, also using supporting evidence.

Of course, what you brought up is correct. I just use one word, "opinion" instead of boring the readers.   

Ultimately, science means application of the scientific method, but people do dispute about such things. It all comes down to good science and bad science, good scientists and bad scientists, con-scientists, pseudoscience, etc. It is here that we can have great quarrels, i.e. it comes down to which side we take - or our opinions.

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New Theories / Re: Why light changes its speed and direction during refraction?

« on: 12/01/2023 15:31:51 »

Dear Hamdani,

I am late to see your original post.

To be honest, I think contemporary physics cannot yet explain the actual physical mechanism that gives the property of light refraction - the change in direction as found in Snell's law. It cannot be properly explained because we still almost have no real understanding of what light is; there may be claims otherwise by some people, but I don't buy them.

I personally is very interested in the answer. I will go through the earlier posts to see what have been written.

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Physics, Astronomy & Cosmology / Re: How long would a mile-long capacitor take to discharge?

« on: 24/01/2018 21:29:59 »

I have committed an embarrassing mistake - a big one!   What everyone can see as obvious, I make a mess out of it.

Some clarifications. I don't have a B.Sc physics, dropped out of first year Engineering. Did some further reading all on my own and know some rudiments of EM theory. I did not do the basics thoroughly. I vaguely understand something about the relation between current, drift velocity, carrier charge density, current density, etc. I don't know what happened to my thinking going haywire and off the track.

There is no excess electrons traveling the full 2 miles from the lower A end to the upper A end. It is all about current flow - flow of charge carriers electrons and nothing else. It must be the diagram  of chiralSPO - the three dark circles - that make me remember that current flow is just a movement of a "train of electrons". The analogy of water flow in a pipe is correct.

I just made a mental slip-up concerning current flow. There are two ways to view current flow, as coulomb/sec or as actual motion of the electrons at a drift speed v. In water flow, it would be volume/sec or speed v of water flow. Either way is ok.     

Wiki :"Copper has one free electron per atom, so n is equal to 8.5×1028 electrons per cubic metre". A typical drift speed of electron of 1 Amp current may be  2 x 10¯⁵ m/s. With such drift speed, in 1 sec, the  electrons move a distance of 0.02mm, but the charges passing a section of the wire (due to high N/m³)may be of the order of Q, the initial charge of the capacitor. So a slight distance shift in the "train of charge carrier" electrons would have enough electrons to pass R and to neutralize the upper +Q charge fully discharging the capacitor. None of the electrons do a full 1 or 2 mile trip.

Some other considerations. At any moment where the discharge current is i(t) in R, the current in the wires is higher nearer the end B and dropping gradually where it is near zero nearer the end A. This is because near end A, only a small amount of charge flow is enough to neutralize the excess charge imbalances.

Electric field considerations (may be wrong). Before discharge, only the gap between the plates have a uniform electric field E perpendicular to the plates. At the very moment of shorting the capacitor, the field lines in the air gap disappears altogether (?).  The only field lines would now be within the conductors giving the electrons motion due to : E = ρJ where J = current density, ρ = resistivity. At end A, J is small and E weak; at end B, J is big and so E is strong. I assume a sudden change in the electric fields based on instantaneous-action-at-a-distance for electrical interactions.