What is the evidence for the existence of protons and neutrons?

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Offline mathew_orman

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Is there an experiments proving existence of  neutrons and protons?
« Last Edit: 25/08/2015 16:45:02 by chris »

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Offline PmbPhy

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Re: Structure of an atom
« Reply #1 on: 25/08/2015 08:47:14 »
Quote from: mathew_orman
Is there an experiments proving existence of  neutrons and protons?
No. But there is endless amounts of experiments which are consistent with their existence and which therefore has led us to accept their existence as a fact. The science of physics is not about "proving" anything. For more on this please see: http://www.newenglandphysics.org/common_misconceptions/DSC_0002.MOV

Here's an example of a device which leads us to accept the existence of atoms:
https://en.wikipedia.org/wiki/Scanning_tunneling_microscope

In that page are photos in which you can actually "see" the atoms on the surface of the material being examined.

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Offline alancalverd

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Re: Structure of an atom
« Reply #2 on: 25/08/2015 11:13:54 »
If you define a proton as a stable particle of mass 1 and charge 1, and a neutron as an uncharged particle of mass 1.008 that decays into a proton, an electron and an antineutrino, then yes, the interactions of these particles with mesoscopic objects are observed. Neutron diffraction is particularly good fun as you can extract colllimated and monochromatic neutrons from a reactor by using a rotating disc with a curved channel to select only neutrons with a specific velocity.
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Offline evan_au

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Geiger & Marsden showed that there is a tiny, positively charged nucleus within the much larger atom. This is evidence for protons, which give the nucleus its positive charge. 

Chadwick received the Nobel prize for showing that neutral particles were emitted when alpha particles struck light elements like beryllium.

The existence of different isotopes of an element occurs because the identity of an element is determined by the number of protons & electrons. The number of neutrons can vary over a small range, which affects the mass of the atom, but does not change the element.

The atomic bomb dropped on Japan 70 years ago only works because the neutral neutron is not affected by the intense electrical repulsion of the many protons in the nucleus of uranium nuclei.

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Offline mathew_orman

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Re: Structure of an atom
« Reply #4 on: 27/08/2015 13:27:29 »
Yes, evidence of atoms exist...

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Offline chiralSPO

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Re: Structure of an atom
« Reply #5 on: 27/08/2015 14:00:43 »
Yes, evidence of atoms exist...

and evidence of all sorts of subatomic particles, including protons and neutrons.

I think some of the most important evidence of protons and neutrons comes from nuclear magnetic resonance (NMR, also what's used in MRI https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance ). The magnetic behavior of an atom's nucleus is very much consistent with the nucleus being composed of one or more spin-1/2 particles that each have a mass of approximately 1 au (the neutral ones are a little heavier than the positive ones). By comparing the magnetic behavior of different isotopes of a single element, it becomes quite apparent.

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Offline dlorde

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Another way to look at it is that protons and neutrons were proposed to explain experimental observations of the behaviour of the atomic nucleus. The Experimental evidence led to the idea of protons and neutrons, not the other way around.

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Offline mathew_orman

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If a neutron, which is a hypothetical particle, has no known force field, it should not be possible to detect it.
Also how are neutrons able to stay together so close if the calculated mutual repulsive force is so large?
« Last Edit: 28/08/2015 08:41:36 by mathew_orman »

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Offline Colin2B

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If a neutron, which is a hypothetical particle, has no known force field, it should not be possible to detect it.
Many chemicals and particles can be detected by their reaction to other materials.
There are quite a few reactions that will detect neutrons eg He gas bombarded with neutrons produces an isotope of He and a γ ray of a specific energy.
Neutron detection is well established in a lot of industries so it would be worth searching the net as there must be a lot of info out there
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Offline PmbPhy

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If a neutron, which is a hypothetical particle, has no known force field, it should not be possible to detect it.
Also how are neutrons able to stay together so close if the calculated mutual repulsive force is so large?
Regarding your question about proving something in science. Science is not about proving things. It's about forming hypotheses and testing what can be deduced from those hypotheses. All we get from testing is whether the observations are consistent with the hypotheses and theories formed from them. For more on this please see:
http://www.newenglandphysics.org/common_misconceptions/DSC_0002.MOV

Regarding your comment above regarding it being a "hypothetical particle, has no known force field." A neutron does have a "force field." It interacts with nucleons via the strong force. While the charge of a neutron is zero it does have a non-zero magnetic moment. It also has a tiny electric dipole moment because its composed of quarks, which are charged.

We can also account for their existence from nuclear reactions. See:
https://en.wikipedia.org/wiki/Neutron_detection#Basic_physics_of_neutron_detection

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Offline UltimateTheory

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Re: Structure of an atom
« Reply #10 on: 28/08/2015 13:36:36 »
Quote from: mathew_orman
Is there an experiments proving existence of  neutrons and protons?

Quantum particles can be seen in Cloud Chamber for instance.
https://en.wikipedia.org/wiki/Cloud_chamber

Charged particle is leaving trace.

We can detect kinetic energy of incoming particle, it's correlated to length of trace.
Positively charged particle spin in different direction than negatively charged particle in applied external electric and/or magnetic fields.
If particle is decaying to other charged particled, trace is "ending" and new branches are appearing from that location.

Unstable isotopes proton-rich, are decaying by emission of free proton (or positron and neutrino).
While unstable isotopes neutron-rich are decaying mostly by emission of free neutron (or electron and antineutrino).

For further info read
https://en.wikipedia.org/wiki/Proton_emission
https://en.wikipedia.org/wiki/Neutron_emission
https://en.wikipedia.org/wiki/Beta_decay
https://en.wikipedia.org/wiki/Positron_emission

Alpha particles are Helium-4 atoms. They're emitted in alpha decay process by heavy unstable isotopes, such as Uranium-238.
It's decaying to Thorium-234
U-238 -> Th-234 + He-4 + 4.27 MeV energy released

If such highly accelerated alpha particle will hit f.e. Deuterium (Hydrogen with one proton, one neutron), it'll give it missing energy 2.22 MeV, and there will be disintegration:
H-2 + 2.22 MeV -> p+ + n0
to free proton and free neutron.
Again, it can be seen in Cloud Chamber (proton).
Neutron within 15 minutes (~10 minutes half-life) will decay to:
n0 -> p+ + e- + Ve + 0.782 MeV

If neutron will be captured by stable isotope of precisely picked up element, and after A+1 it'll become radioactive,
it'll decay emitting f.e. electron,
which we will be able to detect and count.
f.e.
Deuterium turns to Tritium
H-2 + n0 -> H-3
Tritium is unstable isotope (but with quite high half-life 12.32 years)
H-3 -> He-3 + e- + Ve + 18.6 keV
it'll decay to Helium-3 atom, emitting electron.
Once again we can detect either Helium (second the lightest gas after all),
or detect electron,
if electron accelerated by 18.6 keV energy will hit some material there will be blink and photons from that location,
can be detected by photomultipliers.

« Last Edit: 28/08/2015 13:40:47 by UltimateTheory »
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Offline UltimateTheory

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You can build your own particle detector for something like 20-50 usd.
http://www.ultimate-theory.com/en/2014/6/8/how-to-build-cloud-chamber-particle-detector

There are videos showing how it works in practice.
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Offline mathew_orman

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So, if I understand correctly, neutrons stay together by mutual strong force and simultaneously make the protons also stay within together?
Also this means that strong force only acts on protons and neutrons but not on orbiting electrons?

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Offline evan_au

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Quote from: mathew_orman
strong force only acts on protons and neutrons but not on orbiting electrons?
Yes

Quote
strong force ...make the protons & neutrons stay together?
Yes. See https://en.wikipedia.org/wiki/Strong_interaction

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neutrons stay together by mutual strong force

I don't recall hearing of anyone producing a subatomic particle consisting of two or more neutrons (and no protons).
That doesn't mean it is impossible (or that I have heard of everything).

A single neutron by itself is unstable. I am guessing that two neutrons together might also be unstable?

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Offline mathew_orman

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Now, if neutrons do not affect electrons then what stops an electron falling down onto the nucleus as being attracted by protons?
« Last Edit: 30/08/2015 12:01:12 by mathew_orman »

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Offline evan_au

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Quote from: evan_au
I don't recall hearing of anyone producing a subatomic particle consisting of two or more neutrons (and no protons).
Now I am in a country which permits access to well-known search engines, I see that a dineutron consisting of two neutrons is a very short-lived state which has been shown to exist transiently in some nuclear interactions.

So the strong nuclear force is not enough, by itself, to hold neutrons together.

Particles consisting solely of more than 2 neutrons are currently not thought to exist (or at least, claims of their existence have not been able to be reproduced).
See: https://en.wikipedia.org/wiki/Neutronium#Neutronium_and_the_periodic_table

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Offline evan_au

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Quote from: mathew_orman
Now, if neutrons do not affect electrons then what stops an electron falling down onto the nucleus as being attracted by protons?
Niels Bohr proposed that electrons can take on only certain specific energy levels. There is a minimum energy level for each atom, and the electron cannot approach the nucleus closer than this.

This simple model of an atom was later superseded by more accurate models of an electron cloud surrounding the nucleus, developed by Heisenberg & Schroedinger.

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Offline mathew_orman

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Proposed models would not hold an electron from falling onto the nucleus...
Orbiting electrons act as repelling force for atoms and there is no force system that would hold electrons in stable orbits for what ever reason or to satisfy the proposed models...

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Offline chiralSPO

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Do not try to think of electrons as little particles orbiting a nucleus that is another particle--It's a trap!

You must think of these systems in a quantum mechanical way, not a classical mechanical way. On this scale, it is best to think of electrons in terms of their wave-like nature. Because electrons are so light (low mass) their de Broglie wavelengths are much, much larger than the wavelengths of the nucleus, and therefore, even when the electrons and nucleus share the same center of mass, the electron is much more diffuse, and the nucleus much more concentrated.


I think it is best to think of every electron in an atom as "stuck to the nucleus." The electrons are trapped in the atom, and can only be removed with a significant amount of energy, or can be stolen by another nucleus (or collection of nuclei), given the right circumstances.

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Offline mathew_orman

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Quantum mechanics does not support continuity of motion and there is no evidence of instantaneous transfer of matter...

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Offline chiralSPO

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Quantum mechanics does not support continuity of motion and there is no evidence of instantaneous transfer of matter...

Actually, there is plenty of evidence for quantum tunneling (if that's what you mean by instantaneous transfer of matter), your computer wouldn't work without it! Here is one example that I am quite familiar with, that involves comparing the tunneling rate of a hydrogen atom vs that of a deuterium atom: https://en.wikipedia.org/wiki/Kinetic_isotope_effect#Tunneling

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Offline evan_au

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Quote from: mathew_orman
Proposed models would not hold an electron from falling onto the nucleus...
On the contrary, these quantum models of the atom & electron were developed to explain the observed fact that the electron's average distance from the nucleus doesn't fall to zero.

In Heisenberg's model, the mass of the electron had an equivalent wavelength. Like vibrations on a guitar string, the electron could only exist in certain orbits which were an integer number of wavelengths. The smallest orbit is 1 wavelength long, ie the electron cannot fall into the nucleus. (Schroedinger's model gives an even better explanation, but you have to be a supercomputer to solve it for most atoms.)

But there is another case which is instructive: the Muon has the same charge as an electron, but has 207 times the mass. In Heisenberg's model, this means that the Muon wavelength is 207 times smaller than an electron. It is possible to replace an electron in an atom by a Muon; the minimum orbital radius of the Muon is 207 times smaller than an electron - but it still has a minimum average radius around the nucleus.

If you do a Muon substitution in heavy Hydrogen (Deuterium gas), the two Deuterium nuclei are brought close enough so that tunnelling sometimes causes nuclear fusion events, despite the Muon's minimum orbital radius!

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Offline mathew_orman

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There is no model of motion of such structures as proposed by QED...
At present time we can created 3D simulations of any structures in motion but not a single atomic structure containing electrons spinning around the nucleus exists...
Not even a gold foil at room temperature...

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Offline evan_au

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In my simple understanding of modern quantum theory, it describes the energy that particles can have, and the probabilities of finding them in some particular location, but it does not tell you how they get there. Stronger than that, it states that you cannot define an exact path by which the particle reached that position.

This supersedes simple high-school analogies such as "An atom has electrons circling the nucleus like planets orbit around the Sun". This analogy provides a reasonable mental image of a massive nucleus surrounded by low-mass electrons. But it can also produce incorrect images such as thinking that an electron spins around the nucleus. Like any analogy, it has its limitations.

 

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Offline chiralSPO

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not a single atomic structure containing electrons spinning around the nucleus exists...

That's true. Because, as others have pointed out, electrons DON'T spin around the nucleus. Therefore it is a good thing that models based on that assumption fail.

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Offline chiralSPO

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PS: I don't intend to sound hostile--it's just that there have been a number of threads on this forum in the last year or two, in which some members have pointed out repeatedly that quantum can't be correct because it doesn't make any sense--and I'm fatigued of this discussion.

Because I know that you cannot be held responsible for the aggregate behavior of several other people, I apologize if I was too short in my responses on the matter.

Please understand that quantum mechanical models are actually quite good, and are based on reams of evidence. Nobody sat around and theorized, "what if everything was made of waves and couldn't be measured without disturbing the system?" Instead people assumed that they were studying particles in ways that could give definitive answers, and after decades of high-quality experimentation by hundreds of experts across the globe, analysis of the data collected revealed that the world is actually a very strange place on the molecular scale and smaller.

I think reading a history of the development of quantum mechanics might clear up some of the confusion. There are plenty of members here who would be happy to point you in the direction of some good references on the subject.

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Offline alancalverd

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Quite simply, physics is an attempt to derive predictive mathematical models of what actually happens. The simple Bohr atom doesn't predict what actually happens, so we use quantum mechanics which gives us a much better result.

Thing only go wrong when people anthropormorphise the inanimate and ask unanswerable questions like "why doesn't the electron spiral into the nucleus?" You have to take a much more Zen approach to make sense of physics. Start with the observation that it doesn't, then derive a model that begins with that fact, and before you reach graduation day, you will have derived orbitals, explained stereochemistry, and proved that anthropogenic global warming is bunkum!

Eddington said that the student of physics must become accustomed to having his common sense violated seven times before breakfast. I'd substitute "intellectual vanity" for "common sense" in that sentence, or, as my navigation instructor said "start from where you are, then you won't get lost before you take off".
« Last Edit: 02/09/2015 17:05:48 by alancalverd »
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Offline mathew_orman

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So they do not spin but do  motion sequences  declared by QM but not able to define or simulate...

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Offline alancalverd

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We describe and predict what they do, by use of a vector called "spin". It happens that the spin vector of subatomic particles behaves in some ways like the angular momentum vector of a mesoscopic object so it's a very convenient term.
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Offline mathew_orman

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So there is a vector which descries the motion of an electron which suggests that that temporal position is known and if so then the motion cycle should also be known and animated model would show the electron's motion... But why there is no such visualization available?

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Offline PmbPhy

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Quote from: mathew_orman
At present time we can created 3D simulations of any structures in motion but not a single atomic structure containing electrons spinning around the nucleus exists...
Quote from: chiralSPO
That's true. Because, as others have pointed out, electrons DON'T spin around the nucleus. Therefore it is a good thing that models based on that assumption fail.
Both of you have used the term "spin" incorrectly and which may have caused some confusion in this thread.

One thing that seems to have caused some confusion in this thread is the use of the term "spin" which refers to an intrinsic property of a particle. What you both spoke of here is orbital motion. The two quantum mechanical quantities having to do with angular momentum are spin angular momentum and orbital angular momentum. Classically spin refers to a body revolving about an axis through it's body while orbit refers to something moving around a point like when the moon orbits the earth. So the Earth "spins" about its own axis and the moon "orbits" around the Earth.

Hans C. Ohanian wrote an article in the American Journal of Physics called What is spin? that you might want to read. It's online at: http://people.westminstercollege.edu/faculty/ccline/courses/phys425/AJP_54(6)_p500.pdf

Ohanian shows that spin ... can be regarded as an angular momentum generated by a circulating flow of energy of the wave field of the electron.

Quote from: mathew_orman
If a neutron, which is a hypothetical particle, has no known force field, it should not be possible to detect it. Also how are neutrons able to stay together so close if the calculated mutual repulsive force is so large?
That's wrong. Neutrons can be detected in other ways since it has mass as well as a magnetic moment. Since it has mass it contributes to the mass of the nucleus but not to its charge. Therefore when an atom is ionized and shot into a magnetic field the atoms with the same number of protons but different number of neutrons will move in different trajectories due to their varying mass and therefore the mass of the nuclei can be measured. The charge of the nucleus tells you how many protons are in the nucleus so once you know that you need to account for the rest of the mass of the nucleus and that's from neutrons.

To see how they were discovered please read the following: https://en.wikipedia.org/wiki/Neutron#Discovery
Quote from: mathew_orman
So they do not spin but do  motion sequences  declared by QM but not able to define or simulate...
If by "they" you're speaking of electrons then an electron has spin by virtue of it contributing to the total angular momentum of an electron as well as it having a magnetic moment.

Quote from: mathew_orman
So there is a vector which descries the motion of an electron which suggests that that temporal position is known and if so then the motion cycle should also be known and animated model would show the electron's motion... But why there is no such visualization available?
In  quantum mechanics its incorrect to think of electrons in motion, so no. That is incorrect. No visualization of an electrons motion is possible because electrons don't have motion in the classical sense of the term, i.e. an electron does not have a position as a function of time.

Mathew - I've noticed that you made a lot of claims about quantum mechanics

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Offline mathew_orman

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So, using QM one cannot model CRT beam of electrons which could be focused, diverted scanned, expanded and ect. ?
I am not interested in evaluating QM and its is other posters who try explain the problem using QM only to find out that i generates many other questions for which no logical answer exists...

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Offline chiralSPO

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QM can and does model the behavior of populations (large numbers of) particles, as in a CRT beam. It gives the same answer as classical physics, so people usually use that because the calculation is easier. QM really only gets "funny" when one tries to consider what only one or two particles are doing, especially if one is concerned with what happens on a very small scale (either spatially or temporally).

Think of it this way: a river flows downstream at some velocity that is determined by the depth and width of the river, the change in elevation, and the fluid properties of the water. But if one were concerned about how an individual water molecule is moving, the answer would be very different. Sure, the net movement of water is downstream at a speed of 2 m/s, but each of the molecules is moving at a different speed (all about 10 m/s) and bouncing around off of one another. Almost all of them are moving upstream about half of the time! It would be very difficult to predict or measure the exact path taken by an individual water molecule (especially if we get into the tricky question of how one considers the motion of molecules that are exchanging protons/hydrogen nuclei, as happens quite readily in water). But once a sufficiently large group of molecules is considered, the average or net properties become very classical.

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Offline mathew_orman

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CRT ray of 16kV, 1mA, 60cm between cathode and anode... What is the resistance, inductance, skin effect, self capacitance, velocity of electrons, power dissipation, radiation effects and etc.?
Where are the models in QM or classical format?