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About the photon carrying the electromagnetic forces

  1. Nov 5, 2014 #1
    I’ve read that the particle responsible for the transmission of the electromagnetic force is the photon. Another way to say this is that the photon is the carrier of the electromagnetic force.
    Having that in mind, let’s suppose that we have an electron resting in vacuum, and suddenly an electric field centered at a distance of C from the electron appears (for example with the help of aVan de Graaff generator). My questions are:
    1) Does it take 1 second for the electron to feel the electric field / attraction force?

    2) Is it necessary the emission/reception of photons from/to the electron or the Van de Graaff generator for this interaction to take place? (when I say interaction I mean the simple fact of the electron feeling an attraction force)

    3) Lets imagine that there is some sort of mechanical barrier that prevents the electron from accelerating towards the center of the electric field, allowing them to stay at rest separated by the same distance C at all times. I understand that the electron does not ceases to feel the electromagnetic pull, which for me implies some sort of continuous communication between the source of the EF and the electron. What is this “mechanism of communication”? Is there something happening here in the quantum scale? It the conservation of energy respected in this scenario?
    (It comes to my mind that maybe a “mechanical barrier” could be a contradiction in the quantum scale. If it is, please elaborate a little bit why)

    Thanks in advance and best regards,
  2. jcsd
  3. Nov 5, 2014 #2
    Your "C" is 1 light-second and is not the same as "c", the speed of light.
    To answer your questions as you posed them:
    1) Yes.
    2) Yes.
    3) That "mechanical barrier" will be a non-conductive, probably solid material that is held together by electrostatic forces. The electron will only move so close to that barrier before being repelled electrostatically by the other electrons in that insulator. Consider what happens in a capacitor. When electric current is applied, charges build up on either side of a thin insulating material. When the electric current stops, there is still an attraction between the electrons on one side and the unmatched protons on the other. The force carriers in this situation are photons, although in an entirely static situation such as this, they would be virtual photons. They are your "mechanisms of communication". It is a quantum mechanical "happening", although on a scale larger than commonly considered a "quantum scale". And conservation of mass and energy is definitely "respected".
  4. Nov 5, 2014 #3
    However, after the electron gets electrostatically repelled the first time, another photon from the EF source will "hit" it again, causing attraction force again. Then the electron will be repelled again by the barrier. This process will continue forever, meaning that the EF source will have to be sending photons forever... Now:

    1) What if the EF source is just a bunch of protons together? How is it possible that they will be continuously sending photons forever? Where does that energy comes from?

    2) Also, I think that the EF acts upon all the universe, so the EF needs to be sending infinite amounts of photons in all directions at the same time and forever? What is really happening here?

    Also, in the example of the capacitor, what do you mean by
    Could you please elaborate better the example of the capacitor, let’s say, from (t =0) the moment when you close the circuit, (ta-b) while the charge is building up in the plates, and (tb) when the charge is at the top value and you open the circuit. What is going on with the photons in this scenario? Who is the emitting them?

    In which direction do the photons flow? Who emits and who receives? Do electrons emit photons too?

    Thanks again!
  5. Nov 5, 2014 #4
    I think one of the "real QM physicists" could answer this better, but here goes. The maintenance of a static charge can be described in terms of a continuous exchange of photons among the charged particles. Or it can be described as a wave phenomenon. Or you can just go with a simplified Math model.

    If you use the particle model, bear in mind that these particles do not have specific locations and so thinking about an electron getting hit by these photons as if they were machine gun bullets doesn't work. It is probably better to think of these photons as being consequences of the QM Math that don't become real evident until the particles start moving around and the photons actually carry energy into or away from the system.

    I realize I haven't directly answered all your questions. I don't want to attempt a description of how a charged particle transmits an electromagnetic field. As soon as motion gets involved, you need someone handier with Maxwell's equations than I.
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