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Question on electromagnetic interactions

  1. Sep 2, 2012 #1
    Hello all, I have a question relating to charge. I understand that when a positive charge is near a negative charge it creates an attraction. This attraction is explained by the electromagnetic force and by the exchange of electrons. My question is this, how can a new particle such as a photon come out of another? Is a photon simply a carrier of energy and that is why it happens? Also can an electromagnetic attraction ever lose its attraction by removing all its energy via the transfer of photons?

  2. jcsd
  3. Sep 3, 2012 #2


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    Yes, the two particles feel a force that pulls them together.

    No, there is no exchange of electrons. You may be thinking of an exchange of virtual photons, but this is a far more advanced concept than what is necessary to understand electric charge. Classically two charges of opposite polarity experience a force while two like charges experience a repulsion.

    I won't delve into where new particles come from, as that is way beyond my level. However photons are the quantized interaction of an electromagnetic wave with matter. That means that when an EM wave passes over an electrically charged particle it will interact with it in "chunks" of energy we call photons. This is very difficult to grasp if you have no idea what the classical explanation is, so I suggest that you browse the following links and see if they help you.

    No, the electric charge of a particle is an intrinsic property. This means that it is "built in" to the particle, much like mass and spin are. The force isn't "emanating" from the particles, it is simply a reaction that they have to each other. None of the 4 fundamental forces of nature can "run out" in this manner.
  4. Sep 3, 2012 #3
    E=mc squared. Particle accelerators like the large hadron collider create pure energy out of high velocity proton collisions. As the energy from these collisions "cools" particles manifest or are created, including photons. Most of these particles decay almost immediately, such as the heavier leptons, and the the more stable particles just go on their merry way.

    The only way a body can lose its EM attraction is to remove charge from that body in units of quanta. This would mean indiviudal electrons or protons. I don't think photons play a role in this process.
  5. Sep 3, 2012 #4
    Hello, thank you for that answer but it still dosent really answer my question.

    Would anyone be able to explain how photons can come out of electrons and travel to protons or is it the opposite way, that being photons come out of protons and go to electrons.

    I understand this is difficult to understand but such understanding would give a better understanding to the electromagnetic force.
  6. Sep 3, 2012 #5


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    Photons do not "come from" electric charges. An electric charge that is accelerated creates an electromagnetic wave. This wave then interacts with objects using photons.

    HOWEVER, this is not required to understand the EM force itself. I highly recommend you do not think of the force in terms of virtual photons coming and going anywhere. Stick to the classical description for now. Virtual particles will only confuse you.

    If you really decide to look into it, have a look here: http://en.wikipedia.org/wiki/Virtual_particle
  7. Sep 3, 2012 #6
    Hello, thank you for answering. I want to understand the interaction in terms of the exchange of virtual particles. Also would an electromagnetic wave be a "stream" of virtual photons? are waves a manifestation of energy in the form of virtual particles?
  8. Sep 3, 2012 #7


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    Do you understand "classical electromagnetism" yet?
    Edit: I only ask because if you don't understand at least the basics of classical EM theory then it's nearly impossible to accurately explain virtual particles. For example, if you don't know what a "field" is, then there's no real point in explaining Quantum Field Theory to you, which is where the concept of virtual particles comes from.
    Last edited: Sep 3, 2012
  9. Sep 3, 2012 #8
    Yes in a brief sense.
    Edit: I understand fields and such but not on a advanced level but good enough to understand when you are talking about a field
  10. Sep 3, 2012 #9


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    Well, I'm probably going to butcher this explanation, but I'll have a go at it.

    In developing Quantum Field Theory, it was discovered that we can mathematically model the various forces as interacting with particles by saying that "virtual particles" are exchanged between two particles. Imagine two ice skaters skating parallel to each other. One tosses a ball to the other one. The force of the throw causes the first one to move away, while the catch does the same thing to the other skater. Thus both skaters were "repulsed" from each other by throwing a ball.

    QFT describes interactions in a way similar to this, only it's way more complicated since we obviously cannot explain an attraction using the same analogy. Virtual particles can cause attractions as well as repulsion. (Imagine the thrown ball where the math works in reverse. The thrower and catcher move towards each other)

    From wiki:

    The short version is that the math describes both real and virtual particles equally well. It just boils down to which ones we can directly detect. For example, if a photon is detected by your eye or a camera, that is a "real" photon, not a virtual one. Virtual ones carry momentum between particles that interact via electromagnetism.

    I'm sorry if that's a terrible explanation, as my understanding of virtual particles is not very good lol.

    Edit: Before you ask I want to tell you that the interpretation of whether virtual particles "actually exist" or whether it is simply a "mathematical concept" is a VERY heated one that has happened several times since I've been here on PF.
  11. Sep 3, 2012 #10
    The analogy of repulsion makes sense however can we say that the kinetic energy of the emitted photon causes a repulsion which can either repel or attract? for instance is simple EM we describe as positive "reaching out" and negative "pulling in" could this be describe by the positive emitting these photons and the negatives doing something else?

    Haha sorry this got confusing I'm just trying to understand how virtual particles actually work
  12. Sep 3, 2012 #11


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    I don't think so. The math of QFT is far beyond my abilities, so I don't know exactly how it is calculated, but I doubt that the different charges are modeled in such a way. After all, how can two negative charges interact if neither are "emitting" virtual photons?
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