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Photon-proton/ proton-electron interactions

  1. Sep 13, 2004 #1
    i was just curious to find out how photons interact with protons (virtual and real) because as we know a photon excites an electron to a new state but what happens to a proton?

    also why dont electrons collide with the nucleus. i know they have acceleration which will keep them from "falling" into the nucleus however if 2 electrons come near each other and repel each other wont one spin closer to the nucleas (in which case its possible for it to spin straight in) while the other is repelled outwards?

    ive read two ways that electrons dont collide with the nucleas. one is that photons from the proton keep them excited enough not to come to close to the nucleus (which is why i asked the first question) and if this is the case what happens if a photon misses an electron (and i know electrons arent always particles (even though a recent experiment has proven that electrons exist in 2 states at once)).

    the other way that i know of that could be possible is that electrons is an actual electron cloud... a charge spread over an area. this is fine until i think of it being spread through the nucleus. if its a charge being spread then why isnt it attracted to the positive charge of the nucleus? and if it is an electron cloud then s p d f orbitals wont really exist would they?

    so any help would be nice

    ps i know that schrodingers equation n cannot = 0
    Last edited: Sep 13, 2004
  2. jcsd
  3. Sep 13, 2004 #2
    An incident foton is absorbed by the target atom if it has just the exact amount of energy in order to trigger the excitation of the atom. This means that one of the electrons is pushed out of it's position leaving a hole. This is the excited state.

    A higher positioned (i mean higher qua energy) electron can fill this hole up by emitting some EM-radiation. This is how X-rays are generated.

    An electron will not interact directly with the atomic nucleus because of the surrounding electron cloud which yields a repulsive effect,hmm.

    The electron cloud is not attracted into the nucleus because the constituent electrons repell themselves. This leads to the equilibrium (attraction by the nucleus and repulsion by the other electrons) that leaves the cloud around the positive nucleus.

    This is just of the top of my head, i will post some more answers later on...
    till then

  4. Sep 13, 2004 #3
    in answer

    This leads to the equilibrium (attraction by the nucleus and repulsion by the other electrons) that leaves the cloud around the positive nucleus

    there must be an electron right at the bottom which is nearest to the nucleus which will be pushed into the nucleus

    why doesnt this happen?

    and how do protons interact with photons?
  5. Sep 13, 2004 #4


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    I will add just a little bit to what Marlon has said...

    You need to make sure you understand this clearly. When a photon of the right energy hits an atom, it doesn't "excite" an electron, it excites the WHOLE ATOM. The energy states that the electron can occupy is a result of the combination of the nucleus and the electron, not just the electron alone. We know this to be true because a free electron has no such energy states. So the nucleus plays a significant role in forming those energy states for the electron to occupy. It is the atom that is excited upon photon absorption, not the electron.

  6. Sep 13, 2004 #5
    A cristal clear addendum from ZapperZ

    Indeed, the electron has the energylevels described by QM because it is in interaction with the atomic nucleus.

  7. Sep 13, 2004 #6
    This is an answer i gave in another thread. It describes how electrons interact with EM-waves (photons) due to the Lorentzforce. Every charged particle in an EM will feel this force, so the following explanation is just the same for protons. Just replace electrons by protons when reading

    The EM wave (photons) exerts a Lorentzforce onto the electron. This electron accelerates and the momentum goes from p to p''. Part of the momentum of the EM wave is being absorbed by the electron (Poynting vector). Because the electron is accelerated it will emit an EM wave with wavelength lambda'. So basically the incident photon has a wavelength that goes from lambda to lambda'. The momentum of the electron them changes into p'. So we have p --> p'' --> p'

    This EM way of thinking is a local fieldtheory because their is no activity of forces "on a distance". EM-forces are being carried over by fields that fill the entire "space" and they interact with charges positioned at a specific place. This is a big difference with the Newton-way of thinking.

    It is in complete accordance with the local fieldequations of Maxwell that electrons are pointcharges. This is a consequence of the fact that Maxwell equations need to be relativistically invariant. The only question remains as to why the entire EM-wave is absorbed as one single quantum. The answer to that question is ofcourse the wave-mechanics of Schrödinger...as we all know...(first quantization)

    So we have fields, and particles and it is the second quantization that gives us force carriers (viewed at as particles not as waves) and the fermionic matterfields (like the Diracfield being the general solution to the Dirac-equation)that yield the elementary massless particles of the Standard Model.

    marlon :cool:
  8. Sep 13, 2004 #7
    i still dont understand how the whole atom is excited when its just one electron rising a shell then dropping a shell once it releases the photon. the atom doesnt change just the electron.

    if a photon does not interact with a electron directly but does with a proton, will the fact that a proton is so large mean that it will just reflect the photon?
    also why doesnt the electron hit the nucleus? :P dat was da main question

    if the orbital of an electron is slightly changed due to repulsive forces from another electron wont that be enough for the electron to spiral into the nucleus (just like a satellite). in addition to the attractive force of the proton (which is quite large)

    so 3 question in summary :D
    1) how do proton directly interact with photons (reflect or absorb?)
    2) why dont the electrons collide with the nucleus in any case senario?
    3) how is the whole atom effected when a single electron is excited and de-excited

    thanks :D
  9. Sep 13, 2004 #8


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    Those "shell" that you talk about came about when you solve the schrodinger equation for a central, spherical potential. This is the NUCLEUS of the atom! Without the atom, there are no shells, no energy states, etc.

    There are no "absorption" of a photon by a free electron. There are scattering of a photon by free electrons. Thus, the electron in an atom are "incidentals". It is the whole atom that gained energy, not just the electron making the transition. When you exercise, you say that your whole body becomes warmer. Yet, technically, only certain parts of your body that are exerting most of the effect. Yet, you make no such distinction of what parts of your body are warmer and more "excited". You say your whole body systemically are warmer/hotter.

    Same thing with the atom....

  10. Sep 14, 2004 #9
    so in other words the reason for an electron not hittin the nucleus is the fact that it is always excited because of constant photon interactions?

    and if we were able to stop these photons then what?
  11. Sep 14, 2004 #10
    I don't think so. The electron and the nucleus underdo many interactions. They are both charged so they we exert a Coulomb-interaction onto eachother. If you would look at the corresponding potential, you will see that this potential has a minimal value when plotted in function of the distance r between the two interaction particles. This minimum in potential energy denotes the binding-state because everything in nature will strive to be in the state of lowest potential energy. You could say that nature is as lazy as possible. Now, the distance r corresponding to this lowest potential energy is NOT 0 but has a certain finite radius (the Bohr-radius for example) , thus the electron has a minimal distance to the nucleus that is equal to this r-value.

    Keep also in mind that two things have to be taken into account. First you have the "attraction" between the electron-cloud and the nucleus, but the electrons themselves are repelled because of the many electrons that make up the cloud surrounding the nucleus.

    An atom does not need photons hitting it in order to be in this stabe equilibrium.

    Finally, stopping a photon (or changing it's path) would require extern magnetic fields in order to exert a Lorentz-force on it. ofcourse this also brings a change in the energy-levels of the atom (e.g. The Starck and Zeemann-effect). Let's not do that... :smile:

  12. Sep 15, 2004 #11
    ok thanks ... still doesnt seem to logical to me but i guess ill have to deal with it
  13. Sep 15, 2004 #12
    What is it, that maken this look unlogical to you...

    I will try to explain to you...

  14. Feb 28, 2008 #13
    Hello, I got the same question. But for simplicity, lets talk about hydrogen atom, with one proton and one electron (so there is not electron cloud, just 1 electron). Why electron doesn't collide with nucleus (proton), but keeps orbiting around it? (There is an acceleration, but lets assume that sooner or later it is gone be externaly disrupted in a way that electron is gone be pushed towards nucleus).
  15. Mar 1, 2008 #14
    Check out the answer to your question : https://www.physicsforums.com/showpost.php?p=862093&postcount=2 [Broken]

    Last edited by a moderator: May 3, 2017
  16. Jul 20, 2009 #15
    How does this relationship work in say, Hydrogen, when there is only one electron?
    If the electron exists as a smeared cloud, precluding it from contacting the nucleus indefinately, then neutron stars could not exist, as this process of annihilation between the protons and electrons would be necessary for that. If the electrons do not exist as a solid tangible mass, succeptible to the laws of physics, how do they succumb to pressure and gravity, eventually "falling" into the nucleus in the collapse of a neutron star?
    Bear in mind I am not a physicist, so please forgive me if my questions seem unintelligent.
    Last edited: Jul 20, 2009
  17. Jun 24, 2011 #16
    OMG, thank you all so much!
    I've been doing research because I am writing a science fiction book about superheroes, and I wanted to use some form of realism in my story so I started doing some research. My character uses electromagnetic radiation for her super powers, and I needed to know what would happen if certain elements collided with her energy force fields, or if she shot the energy off as a beam, what the effects would be. I needed to know more about photons and electrons, and how everything involved in EM radiation worked. I think this topic gave me a very good idea of the results and how her powers should be. For the most part I understood everything that was posted. I think I'm going to bookmark this site and lurk around a bit. I'm now very interested in learning more about science....
  18. Jun 24, 2011 #17


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    Electrons are neither small balls orbiting the nucleus nor are they "clouds". They exist as a wafe form with a probability of being detected in certain spots around the nucleus based on the energy level they currently occupy. Oddly enough, there IS a small probability of the electron being INSIDE the nucleus. The electron isn't captured by the proton because that would result in then proton turning into a neutron, which would be less favorable for the nucleus than simply keeping the proton and the electron seperate. The full description would involve lots of math and formulas, and unfortunantly I'm not familiar enough with them to explain.

    But be aware that electrons follow the laws of physics 100% to the letter. The way you think of the world at our scale is NOT the way it actually is when you get down near the atomic scale.

    Good luck with your book! Just be aware that you need to take EXTREME liberties with physics in a book like this usually lol. Otherwise where's the fun at?
  19. Jul 21, 2011 #18
    Hello, everyone!

    I made a post a few weeks ago about how I was writing a book and I was using this forum as reference. I've been doing a lot of research on EM waves, protons, and photons, but I am not a physicist. I've only been studying for about six to seven months now (because that was when I started writing my story). My character is able to manipulate, absorb, and alter electromagnetic energy that is around her, and can bend it/shoot it in a variation of ways.
    Such as focusing it into plasma shields, plasma beams, or force fields.
    I've been having trouble trying to figure out what all she can repel with her EM waves, and so I started testing it out in different ways.

    So before I actually started writing the book I've been role playing my character out on another site to test out her powers. Below is a Role play battle I am having against another guy and his hero. I did not include his post though.
    Now I've been reading about photon interactions and EM waves, and I would like you to read my post and to tell me if what I wrote is "logical".

    I know superheroes are not realistic, but I would like my hero's powers to make sense. I want my book to include some form of realism to it. I am going to continue to study and learn, but because I do not know any scientist in real life, I would like your feed back on what I came up with. Does the below post make sense with what she did and how she repelled the force of energy?

    I think you can figure out what had happen prior to her attack based on how I wrote it out. Thanks in advance.

    PS: I will probably be asking follow up questions about EM waves and photons as you post to better understand everything. Small bits of realism in books always makes those daring escapes all the more fun when people can follow the chain of logic.Otherwise it is just nonsense.
    Last edited: Jul 21, 2011
  20. Jul 21, 2011 #19


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    Why is her gravity forcefield repelling things? Gravity is only attractive.
  21. Jul 21, 2011 #20
    Alright, perhaps I worded it wrong? Should I have said magnetic force?
    But I still run into a problem...

    I have tried doing research but as I said, I'm having trouble when it comes to her powers repelling things. If gravity can only attract, what force would be required to repel?
    I tried going to youtube to see if I could find something that was forcing objects away, but that was no good. I just found this, but I don't think it is real.

    The whole thing is complicated because the guy in the fight is shooting some type of gravity ball at her, so lets take that out and replace it with something more simple.
    Lets instead replace it with a person.

    I have never seen a real life force field so I'm not sure what would happen. I read that plasma can work as a shield to keep particles out, like on a spacecraft, but it wouldn't actually repel the object, the plasma would just destroy the object trying to pass through, right?

    So hypothetically speaking, how would she go about reflecting matter?
    Is there a real life machine that can do what I am asking?
    On magnets the same poles repel each other (like north and north), but how would this be applied to other objects that are not magnets, like people?

    So lets say that there are two giant magnets lodged in the ground, both are of the same "north" magnets and facing each other and repelling away. If these magnets were extremely powerful, what would be the effect of something (a person) moving in the center of the field?
    Would they be smashed from the force, or just hit a barrier?
    Or perhaps the waves would have no effect on a person at all?

    I guess what I am asking is, would it be possible for her EM waves (or field) to repel photons and particles in way that can work on large objects like people and nonmetal projectiles?

    If I am to write the book I'm going to need to understand how this works.
    Last edited: Jul 21, 2011
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