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How can a layman visualize an electric field?

  1. Apr 30, 2007 #1

    AJH

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    I have read that an electric filed is a vector of electric charge and this explains its effects and boundaries. How, in practice, is the charge mediated accross the field from the source charged particle so that the particle can interact with any other charged particles that come within the field?

    I have read that electrons have a wave/particle duality like light - is this how tha charge passes through space within the field? If so, why does the electron wave carry no charge beyond the boundary of the electric field?
     
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  3. Apr 30, 2007 #2
  4. Apr 30, 2007 #3

    robphy

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  5. Apr 30, 2007 #4

    AJH

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    Thanks for the illustrations, Rob. I have has no science education and am trying to resolve something that has always mystified me.

    I guess the nub of my question was.... when charges move from a source point through an electric field are they mediated by waves OF the source particle (eg electron waves) or waves of smaller particles (eg photons/radiation) FROM the source particle.
     
  6. Apr 30, 2007 #5
    You sure ask really deep questions for someone with no science education. :rolleyes:
     
  7. Apr 30, 2007 #6

    jtbell

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    The electric field is a vector but it's not "made of" electric charge. In classical electromagnetism, there's no good answer to the question "what is the electric field made of?"

    A charged particle produces an electric field which fills the space surrounding it. This field exerts a force on another charged particle which is placed in it. No charge flows between the two particles.

    Now you're heading towards quantum electrodynamics, which is a very different description of electromagnetism. But here it's not the waves associated with the electrons that carry the interaction. In QED, the electrons interact with each other by exchanging virtual photons.
     
  8. Apr 30, 2007 #7
    Here is an answer with a classical background: the electric field is something fundamental. It is a property of nature, and it is mediated by that nature.

    For a layman to understand it? Perhaps use the thin rubber membrane model (a fun one). A negative charge deflects the membrane in one direction (like a weight dropped on sheet of rubber stretched out); a positive charge deflects in the other direction. If you add a new charge, it will be attracted or repelled based on the other charge's deflection.

    So, that's an explanation. In a classical mindset, it is fundamental. However, a person with quantum field or string theory knowledge may be able to give you a deeper model. But, do not hold your breath. :wink:
     
  9. May 1, 2007 #8
    How are virtual photons different than real photons? Different functions describe them? Since everything in quantum land seems like a function, how is one thing virtual and another not?
     
  10. May 1, 2007 #9

    AJH

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    Thanks for the rubber membrane analogy, Deep Q. When you say that an electric field is fundamental, a property of nature, are you inferring that it a property of space itself - similar to how matter bends space and creates gravity? Can the nuclear forces also be explained in this way?

    JT, I apologise for posting a differently worded version of the question in the general physics forum - a bad habit picked up from Yahoo!Answers where pretty much all answers are received within the first 10 minutes of posting.
     
  11. May 1, 2007 #10
    Yes. But, I used the word nature rather than space, because I did not want to imply only three dimensions. Much of what comes about involves time. Without time, a lot would be missing.

    Yes, those are fundamental - in the classical mindset. Then the issue becomes: how are they related? (If they are at all). Does one affect the other and in what situations? In other words: is there something even more fundamental going on?
     
  12. May 2, 2007 #11
    I visualize an electric field as a bunch of little blue arrows in space. But I know that's not the real question. I hope you'll like my answer; it's based on plain ol' classical electromagnetics and no high-falutin' stuff. :wink:

    If the question is "how does the electric field get from the charge to the space around it", then my answer is electromagnetic waves. The waves only happen when the charge is moving, but those waves "set up" the electric field in front of the charge as it moves, and "tear it down" behind it.

    After everything has stopped moving, the field stays where it is, where the electromagnetic wave last left it. You can think of the wave as rolling from the moving charge itself out into space, propagating itself because of the rules of electromagnetics, and eternally setting up the electric field behind it, which indicates the charge's new position. If a charge scoots into place a light-year away from you, it will take you a year to see its electric field. This is because you have to wait for the electromagnetic wave (which travels at the speed of light of course) to reach you and set up the field there.

    If the question is "why are the rules of electromagnetics the way they are", then I'd point out that it's hard to imagine many other ways they could possibly be. The laws of electromagnetics are exactly what they need to be to make relativity true (that is, the results of electromagnetic experiments appear to be self-consistent and make sense from any point of view, even a moving point of view), and to make certain other things true (such as all three spatial dimensions being identical in every way), and to make a non-trivial universe (if you want to invoke the anthropic principle, which is always fun).

    Did I answer it? I hope??
     
  13. May 3, 2007 #12

    AJH

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    Thanks for the message, Xezlec. I may be getting a bit confused because your answer seems to contradict the answers from JT and Deep Q. My understanding was that a field was local to a charge (maybe around a proton) and not being carried through space by radiation (which I thought was just energy) - can light and other waves carry a charge? I would be grateful for any clarification.

    The "little blue arrows" image is, I think, a block to understanding as surely an electric field is created around a single charge and the "blue arrow" interaction/behaviour is the product of TWO (oposite) charges. Further confusions seem to stem from the similarity in name of quite different (though connected) phenomena: electric field, electromagnetic radiation, electric currents etc.
     
  14. May 5, 2007 #13
    The proton is surrounded by a field that we think of as being due to that charge, but it isn't local to the charge. The field extends out infinitely far. Your proton's electric field can be felt (very weakly!) at the edge of the universe. Well, after you give the field time to propagate there, of course.

    It also turns out that charges are not the only things that produce fields. Changing magnetic fields create electric fields all by themselves. And changing electric fields produce magnetic fields all by themselves, without the need for any charges. Fields, it turns out, are very "real" things which exist in their own right.

    An EM wave can be thought of as a changing magnetic field producing a changing electric field producing a changing magnetic field producing a...........

    No, light and other EM waves do not carry charge. Yes, light and other EM waves do "carry" electric and magnetic fields. In fact, An EM wave is made out of nothing but electric and magnetic fields. That's all EM waves are. Oscillating fields propagating out into space.

    Radiation is "just energy" in the sense that it is not matter. But it has to be "some kind" of energy, doesn't it? Specifically, it is the energy stored in moving electric and magnetic fields. Because it is just a bunch of moving fields. No more, no less.

    No, you've got it a little mixed up. A field has a magnitude and a direction at every point in space. Every point in the universe has an electric field of some magnitude pointing some direction, and a magnetic field of some magnitude pointing some direction. A field is a vector (a little arrow), not a scalar (a simple number).

    But you are correct that the "interaction" of a field with a charge (the force that results) is also a vector. In fact, whenever you have a charge in an electric field, the charge will feel a force pointing the same direction (if the charge is +) or the opposite direction (if the charge is -) as the electric field. Also, the strength of the force is equal to the strength of the field times the magnitude of the charge. This is one possible "definition" of electric field, actually.

    I know I just said this, but it's worth repeating: electromagnetic radiation is actually made out of electric fields and magnetic fields. So EM radiation = fields. No confusion there :smile:

    Electric currents are caused by electric fields (or magnetic fields). Also it turns out that in certain ways, an electric current is equivalent to a changing electric field, but that's kind of a special topic.

    Let's see if that's any better.
     
  15. May 5, 2007 #14

    AJH

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    Hooray! This is the missing link between fields and EM radiation that I have been clumsily trying to understand since I started this thread - disturbance/movement in fields creates radiation; radiation propogates fields. I am happy to take this as gospel as I will never have the maths or understanding of waves to delve any further. Just one little thing is still puzzling me...:rolleyes:

    What's happend to the original charged particle in all of this? Surely charge is a property/symmetry of matter like protons, electrons. How is their charge translated into waves/fields? I understand that electrons can emit radiation due to their energy levels and that they have waves of their own...oh dear, I feel as if my question has come full circle. Does the presence of a charge in the source point make waves be emitted?

    Would be grateful to Xezlec or anyone else who could tie up the loose ends.
     
  16. May 5, 2007 #15
    OK, I see the problem. Perhaps I shouldn't have said "a field isn't local to a charge". I guess a field can be thought of as localized if a charge is moving. I'm used to saying that the field extends out forever, when in fact that's only true once you've given the wave time to propagate out.

    Charge is indeed a property of certain particles, and it creates fields and responds to fields. A moving charge creates a magnetic field, and a charge standing still does create an electric field. Before, I was trying to explain that an EM wave propagates those fields from the region right around the charge to every other point in space. I guess you can sort of say that only the fields "right on the surface of the particle" (if that even means anything) are due to the charge, and that the fields further out have to propagate there as a wave. Sort of.

    The problem is that I'm trying to give intuitive explanations of a precise (but complicated) mathematical situation. Whether my wording is "correct" is open to debate (in fact I'm surprised it hasn't been criticized yet by anyone).

    But it is definitely true that moving charges give off EM waves. And it's definitely true that as that wave propagates, a field appears behind the wave that accurately represents the "force" that the charge "wants" to exert on any other charges that come nearby.

    I might have to do some numerical calculus and put together a cute little animation or something.

    EDIT: I notice in your post you may be also trying to mention "electron waves" in quantum physics. These have nothing to do with our subject of conversation. There be dragons.

    Also, I notice you may not be clear on the following: fields create fields, but charges also create fields. There are exactly two things that can cause an electric (or magnetic) field. That's why moving charges can create a wave, and the wave can "create a copy of itself" (move).
     
    Last edited: May 5, 2007
  17. May 6, 2007 #16

    AJH

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    Time for a re-cap

    OK, let me try and organise logically what I've learnt about electric fields from physicsforums.com and the terse responses I got from YahooAnswers...

    1) Charge is a property of particles (connected to their "symmetry" within a particle system - not quite sure what this means).

    2) Charged particles are the source points of a vector.

    3) Charged particles always have a (electric) field of influence.

    4) As a vector, the little blue lines of influence always point in a specific direction (the field is not just a vague fog of charge).

    5) The electric field is a property of nature/the fabric of the universe itself and its composition can't be explained by classical pysics.

    6) Fields and waves are definitively linked (if anyone asks how I can say they're the same thing viewed differently): the state of the field creates a wave; waves propogate fields.

    7) Waves "carry" replicating fileds accross space - therefore the field as a whole has no boundary.

    8) The aforemention waves are electromagnetic waves, the product of fields (which are the product of charged particles) - they are definitely NOT electron waves.

    9) Changes to fields create more waves/fields

    10) The influence of the charged particle diminishes with distance due to inverse-square rule.

    11) Any charged particle within the field can react with the field (no direct interaction between charged particles) through exchange of "virtual photons". This will allow attraction/repulsion and, for instance, the attraction of electrons into a current.

    This is good fun, I'm gonna sound like einstein down the pub. If the above is all nonsense I'm throwing in the towel.:rolleyes:
     
  18. May 6, 2007 #17

    jtbell

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    Electric and magnetic fields on the one hand, and virtual photons on the other hand, are alternative descriptions of the interaction between charged particles. The e&m fields are the classical description, and the virtual photons are the quantum description. In either case, the charged particles do not interact directly; the fields or the virtual photons mediate the interaction, depending on which description you're using.
     
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