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Thread: electrons and protons

  1. #1 electrons and protons 
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    Hi,
    what happens if we shoot a beam of electrons on a bunch of protons? all protons become atoms of H or some electrons hit the protons and get stuck?


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  3. #2  
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    Quote Originally Posted by monalisa View Post
    Hi,
    what happens if we shoot a beam of electrons on a bunch of protons? all protons become atoms of H or some electrons hit the protons and get stuck?
    I suppose it depends on the energy of the electrons. If it is < the ionisation energy of hydrogen then I would think yes you get H atoms. If more then you won't, as the protons will I imagine diffract the electron beam but will not be able to capture it.


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    How about expanding the technique to create Oxygen as well, then we will have a continuing supply of water.......There are untold zillions of both electrons and protons hanging around everywhere which could be brought into the process! jocular
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    Quote Originally Posted by exchemist View Post
    . If more then you won't, as the protons will I imagine diffract the electron beam but will not be able to capture it.
    That is amazing, exchemist, the electrons should be attracted by and onto the protons , like an asteroid on the earth, why so? what is the difference?

    If you accelerate an electron it escapes the proton, right?, if you slow it down would it never crash onto the proton?
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    Individual sub-atomic particles just don't act like billiard balls on a table-top. Many other effects come into play, not the least of which is that we cannot "look at", or observe the absolute behavior of a single, discrete particle. jocular
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    Quote Originally Posted by monalisa View Post
    That is amazing, exchemist, the electrons should be attracted by and onto the protons , like an asteroid on the earth, why so? what is the difference?
    The difference is that an electron is a quantum object; it therefore obeys the rules of quantum mechanics rather than classical mechanics. In that regard, an electron is most emphatically not a small ball of matter whizzing around the nucleus on an orbit, like in a miniature solar system. It is better to picture it as a 3-dimensional standing wave; that way it is immediately obvious that it can only occupy discreet orbitals around the nucleus, more specifically those orbitals that accommodate integer multiples of the wave length. What I am trying to point out here is that the electromagnetic attraction of the electron to the proton - while it does play a role - is not the only ( or even most important ) factor here.
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    Thanks, Markus, But a beam of electrons is not a standing wave: in a cathodic tube it smashes onto the cathion, why shouldn't it smash on protons?
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    With sufficient energy it can. More likely is however just scattering on proton`s potential. A lot of things can happen in such interactions.
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    Quote Originally Posted by Gere View Post
    With sufficient energy it can. More likely is however just scattering on proton`s potential. A lot of things can happen in such interactions.
    what happens when an electron hits a proton, does it form a neutron?
    Isn't sufficient energy always provided by Coulomb-law acceleration?
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    Quote Originally Posted by monalisa View Post
    Thanks, Markus, But a beam of electrons is not a standing wave
    True, but I was actually referring to a single electron in the vicinity of a nucleus. In any case, the main point is that electrons are not classical objects; therefore there are quantum effects to be considered here.
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    No. You have to realize that proton is of size about 1 fermi (femtometer). For incoming pointlike electron it is very improbable target. Electron with high enough kinetic energy will see just EM potential and will scatter on it. Electron with low energy will probably be catched by proton EM field which will result in hydrogen atom. If electron will indeed hit the proton directly with high enough kinetic energy to get close weak interaction may take place. Simplest of weak processes here would be -> . I think its called inverse beta decay or electron capture or some like this.

    edit: that no wasn`t to Marcus he just posted faster >D
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    Quote Originally Posted by monalisa View Post
    Quote Originally Posted by exchemist View Post
    . If more then you won't, as the protons will I imagine diffract the electron beam but will not be able to capture it.
    That is amazing, exchemist, the electrons should be attracted by and onto the protons , like an asteroid on the earth, why so? what is the difference?

    If you accelerate an electron it escapes the proton, right?, if you slow it down would it never crash onto the proton?
    Markus Hanke has given a bit of insight into the wave-particle duality of particles like electrons.

    But even if we stick to your asteroid example for a moment, an asteroid with enough kinetic energy will not be captured into orbit by a planet it goes close to, (though of course if it actually scores a direct hit, it will crash on the surface). If its path takes it on a near-miss trajectory, and its speed exceeds the escape velocity of the planet's gravitational field, it will swing by, get a kink in its path from the gravity of the planet and zoom on back out into space. It would be on what is called a hyperbolic orbit.

    So there is a bit of a parallel, even if we think of the electrons as just particles (which they're not).
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    Thanks, exchemist, the point is that if you aim a beam of electrons in a cathotic tube they flight on a straight line like bullets, if you aim it at free protons, not a single one hits a proton, unless it has an enormous KE.
    What are the QM effects that cause different behaviour? Coulomb force works on a straight line, what forces make the electron curve?
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    Quote Originally Posted by monalisa View Post
    Thanks, exchemist, the point is that if you aim a beam of electrons in a cathotic tube they flight on a straight line like bullets, if you aim it at free protons, not a single one hits a proton, unless it has an enormous KE.
    What are the QM effects that cause different behaviour? Coulomb force works on a straight line, what forces make the electron curve?
    I'm not sure I'm quite with you but I'll try. An asteroid passing close to a planet or star will be deflected in a curve as it passes, due to the force of gravity. If it has enough KE it will be able to escape the gravity and fly on into space. The deflected path it follows is hyperbolic. If it does not have enough KE to escape the attraction, it will be captured into an elliptical orbit.

    If an electron passes close to a proton it will be deflected in a curve by the electrostatic attraction. If it has enough KE ( > ionisation energy of hydrogen) it will escape, just as the energetic asteroid does. If not, it will be captured into an orbital, which is the quantum-mechanical analogue of the asteroid orbit. That much is common to both electrons and asteroids.

    The only differences in behaviour arise because the electron can also have wave-like properties. For this reason, an "orbital" is not quite the same thing as an orbit. An orbital has to correspond to a standing wave pattern for the electron and this is possible only for certain energies, whereas an orbiting asteroid can have any energy it likes, provided it is less that that needed to give it escape velocity.

    Also, if you have several protons deflecting electrons that have enough energy to escape, the electrons may not just be deflected but also diffracted, producing an interference pattern, in the same way that waves are diffracted round a set of obstacles.

    If this does not answer your question please feel free to come back and ask further.
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    Thanks, but I am asking why even if you aim electrons precisely at the center of a proton not a singl one ever hits it, the orbit/orbitals are not an issue here.
    In what way its wavelike qualities prevent that? Can it be an interference-interaction of magnetic forces ?
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    Quote Originally Posted by monalisa View Post
    Thanks, but I am asking why even if you aim electrons precisely at the center of a proton not a singl one ever hits it
    I'm not sure why you think that...
    electron-proton collision - Google Scholar
    ei incumbit probatio qui dicit, non qui negat
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    Ah, OK.

    Well, the problem is that the lowest energy state of a electron in a hydrogen atom (which is what you get if you put an electron and a proton together) is the 1s orbital. Due to its wave properties, no lower energy state is possible. In the 1s orbital the electron actually goes right up to the nucleus - can be thought of as touching it from time to time - but that's as far as it can go.

    There is a process for larger nuclei called electron capture, whereby nuclei with more protons than neutrons can in some circumstances capture an electron from an inner orbital, but that is a nuclear reaction, whereby a proton changes into a neutron (+ a neutrino, I think). There is more about this here:
    Electron capture - Wikipedia, the free encyclopedia
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by monalisa View Post
    Thanks, but I am asking why even if you aim electrons precisely at the center of a proton not a singl one ever hits it
    I'm not sure why you think that...
    electron-proton collision - Google Scholar
    Good point. Monalisa, my parallel reply deals with the case where the electrons have low enough energy to be captured to make atoms.

    The sort of process Strange refers to needs more energetic ones (I think).
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    Quote Originally Posted by exchemist View Post
    Good point. Monalisa, my parallel reply deals with the case where the electrons have low enough energy to be captured to make atoms.

    The sort of process Strange refers to needs more energetic ones (I think).
    That is my understanding as well. It is not a "normal" situation.
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by monalisa View Post
    Thanks, but I am asking why even if you aim electrons precisely at the center of a proton not a singl one ever hits it, the orbit/orbitals are not an issue here.
    In what way its wavelike qualities prevent that? Can it be an interference-interaction of magnetic forces ?
    Again. Diameter of proton is about 1 fm. You can`t aim at that. If it does collide however weak processes will take over and many things can happen. Read Stranges reference.
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    Just to run through everything at once: If you shoot electrons of various energies at a proton, the overwhelming number of processes you will see are electromagnetic. No direct nuclear processes will occur, because electrons don't feel the nuclear force. The three main electromagnetic effects will be elastic scattering of the electron off the proton, inelastic scattering of the electron into another lower-energy state, and radiative capture of the electron into one of the bound states of the hydrogen atom. To get the correct elastic scattering of the electron you must take into account the internal charge distribution of the proton due to its being composed of 3 quarks plus an indefinite number of quark-antiquark pairs. This charge distribution affects the scattering by producing functions called electromagnetic form factors. The difference of these form factors from what you would get from a point charge [a constant set equal to one] tells you the effect of the electron specifically hitting the proton. They are a major part of the scattering distribution, so there is no question that the electron "hits" the proton in a quantum-mechanical sense.

    There are also contributions to the inelastic scattering due to exciting internal states of the quarks in the proton, making higher-mass baryons which don't in fact last very long. These contributions are perhaps an even more intuitive "striking" of the proton, but they are indirect. They involve not only the electromagnetic effect of the electron but also the existance of strong forces internal to the proton.
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    Quote Originally Posted by mvb View Post
    plus an indefinite number of quark-antiquark pairs
    That's interesting, and immediately jumped out at me when I read your post - would you be able to elaborate on this a little, I was not aware of the existence of such pairs within the proton in addition to the usual three quarks.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by mvb View Post
    plus an indefinite number of quark-antiquark pairs
    That's interesting, and immediately jumped out at me when I read your post - would you be able to elaborate on this a little, I was not aware of the existence of such pairs within the proton in addition to the usual three quarks.
    The three uud quarks forming a proton are known as valence quarks as they contribute by their quantum numbers to overall numbers of proton (eg. electric charge). They are bound together by gluon field. In language of perturbation theory they exchange (virtual) gluons. While this virtual gluon "flies" from one quark to another it can annihilate into (virtual) quark antiquark pair that subsequently annihilates and creates gluon. These are higher order (4+ in coupling constant) processes that naturaly result from coupling of gluon field to quark field (it is interaction of gluon with quark vacuum). Since these quark antiqark pairs are virtual they do not contribute to quantum numbers of hadron. If these processes are accounted for in gluon propagator this results in running coupling constant of strong force. Hadrons in general along with real valence quarks contain indefinite number of virtual quark antiquark pairs. However the fun begins when one delivers energy (by collision). Then these quark antiquark pair may go on-shell becoming real and creating mesons. If collision energy is great it results throu color confinement in creation of great many mesons that form so called jets.
    Last edited by Gere; October 4th, 2013 at 05:22 AM. Reason: h
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    Quote Originally Posted by Markus Hanke View Post
    . It is better to picture it as a 3-dimensional standing wave.
    We know that the orbital H1 is equal to the wavelength of the electron, does anyone know the amplitude of that wave? Is there a hard-and-fast rule to calculate it?
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    Quote Originally Posted by monalisa View Post
    We know that the orbital H1 is equal to the wavelength of the electron
    What???



    where is electric charge and
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    Quote Originally Posted by Gere View Post
    The three uud quarks forming a proton are known as valence quarks as they contribute by their quantum numbers to overall numbers of proton (eg. electric charge). They are bound together by gluon field. In language of perturbation theory they exchange (virtual) gluons. While this virtual gluon "flies" from one quark to another it can annihilate into (virtual) quark antiquark pair that subsequently annihilates and creates gluon. These are higher order (4+ in coupling constant) processes that naturaly result from coupling of gluon field to quark field (it is interaction of gluon with quark vacuum). Since these quark antiqark pairs are virtual they do not contribute to quantum numbers of hadron. If these processes are accounted for in gluon propagator this results in running coupling constant of strong force. Hadrons in general along with real valence quarks contain indefinite number of virtual quark antiquark pairs. However the fun begins when one delivers energy (by collision). Then these quark antiquark pair may go on-shell becoming real and creating mesons. If collision energy is great it results throu color confinement in creation of great many mesons that form so called jets.
    Interesting, that makes sense, thank you
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    Quote Originally Posted by Gere View Post
    What???
    Quote Originally Posted by Markus Hanke View Post
    it can only occupy discreet orbitals around the nucleus, more specifically those orbitals that accommodate integer multiples of the wave length.
    Isn't the ground orbital H1 just 1 x wavelength?
    If I got it wrong I apologize, but do you know anything of the amplitudes?
    Last edited by monalisa; October 4th, 2013 at 09:08 AM.
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    Quote Originally Posted by monalisa View Post
    Isn't the ground orbital H1 just 1 x wavelength?
    No, the ground state is not just one wavelength - it depends on the exact form of the potential in question, among other things. Generally you need to find the orbital states as a solution to a QM wave equation, such as the Schroedinger equation.
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    So,... not integers?
    But what about the amplitudes, do they vary according to a formula?
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    Quote Originally Posted by monalisa View Post
    So,... not integers?
    Yes, integers, but not all integers. The allowable values depend on the specific problem at hand.

    But what about the amplitudes, do they vary according to a formula?
    Again, the amplitude of the wave function will depend on the boundary conditions of the problem at hand. Remember that a physically meaningful wave function should be renormalisable, which means simply that ( in position space ) we expect to find the particle somewhere. For example, for a simple 1-D wave function this condition would look like



    This determines the amplitude.
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    The standing wave idea is long gone. Bohrs old quantum theory was based on assumption that

    where N=1,2,3...

    This doesn`t really works. When you want to solve hydrogen atom you have to solve Schrodinger equation

    where

    which gives you discreete energy levels and respective wavefunctions .

    If you are masochist or have bad luck of needing even better solutions you have to solve time independent Dirac equation



    where , are Dirac matrices in standard represenation, is fully relativistic energy and is bispinor.
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    Gah I hate LaTeX. That "Unknown Error" is too frequent.
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  34. #33  
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by mvb View Post
    plus an indefinite number of quark-antiquark pairs
    That's interesting, and immediately jumped out at me when I read your post - would you be able to elaborate on this a little, I was not aware of the existence of such pairs within the proton in addition to the usual three quarks.
    Well, they are just virtual pairs, but in principle they matter for the electromagnetic form-factor. At any time one of the three original "valence" quarks can emit a virtual (off mass-shell) gluon which can then disappear, emitting a quark-antiquark pair. Since energy is not conserved in this process, the time that the gluon or quark can be around is fairly short, but the quark mass is small enough that the time isn't negligibly small. At the time that confinement was still a live issue, there were a number of estimates of the average number of gluons and antiquarks that would be present in a nucleon at any given time. The estimates varied, but they all gave significant numbers, and many suggested that there are more gluons in a nucleon than there are quarks. It's a real mess in there.

    Interestingly, when I found a way to calculate the electromagnetic form factors in the MIT bag model if I could ignore the extra quarks, I got very good fits to the experimental data. Apparently confinement ensures that the distribution of quark matter inside the cavity is not significantly changed by extra, virtual quarks and gluons. They must have much the same distribution as the quarks would have if there were only three quarks inside occupying the ground state of the system.
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    Quote Originally Posted by Markus Hanke View Post
    the amplitude of the wave function ....
    How can you detect the wave of the electron, it is not electromacnetic, i suppose, is it the trajectory sinusoildal or what?
    What make DeBroglie conclude that it is a wave?
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    Quote Originally Posted by monalisa View Post
    What make DeBroglie conclude that it is a wave?
    By sending a beam of electrons through a diffraction grating - what you get is a phenomenon called electron diffraction, which shows its wave nature.
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    I suppose that is also the method to determine the length of the wave?
    Does that mean that the trajectory of the electron is a wave?
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    Quote Originally Posted by monalisa View Post
    I suppose that is also the method to determine the length of the wave?
    Yes, you can determine that from the diffraction pattern.

    Does that mean that the trajectory of the electron is a wave?
    No, it means that the electron itself has a wave aspect to it.
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    Quote Originally Posted by Markus Hanke View Post

    No, it means that the electron itself has a wave aspect to it.
    Could that be due only to KE, which is electrmagnetic, and not to the electron itself?
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    Quote Originally Posted by monalisa View Post
    Could that be due only to KE, which is electrmagnetic, and not to the electron itself?
    No. The diffraction process happens only to waves, not particles; this is how we know that the electron ( and all other particles ) have a wave-aspect to them as well.
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