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Understanding the nature of the electric field

Discussion in 'General Electronics Discussion' started by flippineck, May 25, 2014.

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  1. flippineck

    flippineck

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    Reading a discourse on the nature of the electric field, I keep running into discussions of discrete charges 'creating' an electric field in the space surrounding them.

    http://www.physicsclassroom.com/Class/estatics/u8l4a.cfm
    https://van.physics.illinois.edu/qa/listing.php?id=414

    Is it such that a region of space with absolutely no charges within it or significantly near it, does not contain an electric field - i.e. that an electric field is only present near to some particle giving rise to it, and outside some defined boundary, there is no actual field?

    Or, is there a single, all-pervasive and essentially stable and uniform electric field throughout the universe, and what is often referred to in basic texts as 'the electric field' of any given system, is simply the local disturbance caused by that system, within that larger pan-universal electric field?

    Whichever, surely the field must be composed of something tangible. What is that something?

    Same questions apply to magnetic fields

    Any why and how, are the two fields electric and magnetic, linked?

    If anybody has any good links that help understanding for the mathematically dyslexic layman like myself could you let me know..
     
    Last edited: May 25, 2014
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    These are physics questions. For the answer to the last one, I suspect you should google for something like "unification theories for beginners", or similar.

    You need to be careful with "why" because depending on what you mean, science might not answer that question. Even "how" questions can be dicey when you're talking about what are currently considered fundamentals.
     
  3. BobK

    BobK

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    The electric field is a mathematical construct that has no real existence. All of the interactions attributed to the electric field (and magnetic field) are actually caused by exchange of particles called virtual photons.

    Bob
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Well, that's the current theory...
     
  5. flippineck

    flippineck

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    According to wikipedia these seem to pop into existence when maths makes them convenient, and disappear again when not needed by the maths.. seems like a modern day equivalent of "Here Be Dragons" on old maps! I guess I'm best forgetting the how and why and just accepting that things just are.

    I'm trying to understand aerials again, basically
     
  6. BobK

    BobK

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    Yes, we won't even talk about strings.

    Bob
     
  7. Ratch

    Ratch

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    Yes, charges do cause static electric fields.

    A propagating electromagnetic wave through space contains both magnetic and electric fields with no particles. https://www.google.com/search?q=electromagnetic propagation&biw=1920&bih=914&tbm=isch&imgil=pVGeBiUSdftqvM%3A%3Bhttps%3A%2F%2Fencrypted-tbn2.gstatic.com%2Fimages%3Fq%3Dtbn%3AANd9GcSaznUZMZ-z3YDsoiDIVCjkMgw9Cvi8m6cO9TSTpRXi_Td3V3JX3w%3B1814%3B853%3BnBG2Ib11vewrqM%3Bhttp%253A%252F%252Fmysite.du.edu%252F~lconyers%252FSERDP%252FFigure5.htm&source=iu&usg=__JDz2oyQcqBfYRYBsftonXcFLds8=&sa=X&ei=UaKDU43oJo21yASeooGoDg&ved=0CGEQ9QEwCg#facrc=_&imgdii=_&imgrc=pVGeBiUSdftqvM%3A;nBG2Ib11vewrqM;http%3A%2F%2Fmysite.du.edu%2F~lconyers%2FSERDP%2Fem_electric_magnetic_propagating_waves.jpg;http%3A%2F%2Fmysite.du.edu%2F~lconyers%2FSERDP%2FFigure5.htm;1814;853



    What would cause and sustain such a permanent all inclusive uniform field?

    It is another form of energy. Of what is energy composed?

    Same answer applies to magnetic fields.

    Haven't you heard of Maxwell's laws?

    Maxwell's laws are presented and discussed extensively in textbooks and the web.

    Ratch
     
  8. flippineck

    flippineck

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    Sep 8, 2013
    So there isn't a constant 'zero value' field everywhere, like a 3d net, then? I thought that if you could think of it like that, then it would be logical to visualise that a 'twang' in one corner of the net could propagate out through the lattice to infinity, even though you didn't quite know what the lattice was actually made of..

    Without the idea of an all-pervading electromagnetic 'aether' like that, I find it difficult to understand/visualise how the wave could travel in a completely dark vacuum

    Aware dimly of Maxwell but unfortunately, I think I was born without the maths part of my brain, so I'll just have to settle for accepting that EM travels on little tiny dragons ;-)
     
  9. Ratch

    Ratch

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    If a field had a zero value, then it would not be a field, would it?

    What is this "lattice" you are talking about? You have to define your terms. Propagating fields can move toward to infinity until they become so weak that for practical purposes they are nonexistent.

    Complete emptiness is called "free space" by physicists. Energy is able to transverse free space, like the sun's rays when they shine on Earth.

    Maxwell's insights were the crowning achievement of 19th century physics, akin to relativity in the 20th century.

    Ratch
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sure, you can think of it like that. Gravity can be visualised as a distortion in space-time, and is imagined as a flat rubber sheet on which masses make indents. On that surface light travels in a straight line, the indents mean that a straight line is curved.

    There is no such surface (rubber or not) so this model doesn't reflect reality.

    I am pretty sure you can borrow spacetime as your canvas :)

    You could use a particle model to describe it. It's perhaps easier to imagine particles travelling through nothingness. However, consider that empty space is not nothing. It has time and space. And then we're back to space-time again.

    They're the preferred method of transport for Maxwell's demon.
     
  11. davenn

    davenn Moderator

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    and that hadn't so far been touched on

    flippineck,
    The Classical Physics view
    in a really basic way, you do realise that the propagation of an EM wave is the propagation of photons ( or precisely packets of photons) ?
    Then you get into the whole physics of photons and the particle function and wave function .... but we wont go there ;)

    doesn't matter if its radio waves, IR, visible, UV light, or X-rays and as such they have no trouble propagating through a vacuum

    But on a basic level .... accelerated charged particles, in this case an electrons ( a flowing AC current) generates a moving magnetic field, in turn generating a moving electric field and so on
    The accelerated electrons results in the emission of photons aka an electromagnetic wave

    cheers
    Dave
     
    Arouse1973 likes this.
  12. flippineck

    flippineck

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    OK I'm now happily just accepting that the EM wave travels no prob through free space, basically by magic. I'm hoping that to form a basic working understanding of antennas, that's all I need, and that I don't need the brain of Hawkins; just to accept that there is such magic and it does actually work.

    I'm familiar with the O-level physics lesson about how a generator works - there's a wire which is moving in a permanent magnetic field and Fleming's right hand rule shows the current induced in the wire. So I can easily visualise a moving magnetic field, as a wave propagating past my antenna, moving electrons in the antenna just like a generator.

    Is that the whole story of an antenna's pickup of RF current; or does the electric field part of the EM wave directly contribute to the displacement of electrons in the antenna as well as the magnetic part?
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The electric field predominates close to the antenna, the magnetic when further away (or at least that's my understanding).
     
  14. Arouse1973

    Arouse1973 Adam

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    I think it depends on the type of antenna you use. Whips will respond better to the electric part of the EM wave. Loop antennas will respond better to the magnetic part of the EM wave. At the end of the day it doesn't matter if you think of it as virtual photons or as mentioned strings. The fact is energy is transported by this EM wave and received by the other end, the antenna. They are in phase and have a ratio of 377 Ohms.

    An example I can think off is take a piece of wire say 1m long and take say 6 saucers. Drill a small hole in the centre of the saucers pass the wire through the saucers and fix them to the wire. Stand the wire up. The saucers will now show the propogation of the magnetic field.

    Remove the saucers and now take two dinner plates an place the together along the length of the wire. Stand it up again. This shows the electric field propogation. Of course in an EM wave you have both and they are not solid.

    So with this you can see a two whip antennas one tx and one rx would respond better to an electric field. Because they both would be pointing upwards.

    So how does this energy get to the other side. Well my take on it is this. In a d.c circuit there can only be a reactive field because the charged particles which we will call electrons have the same speed. They don't accelerate and can't create a EM wave. They can't radiate.

    When an electron accelerates the electric field lines linking to other charges bend and as this field lines try to became straight again it needs energy and this is supplied by the source. It is this energy that appears on the other side at the antenna.

    Take a piece of string and connect two charges together. Wiggle the string at one end and it will wiggle the other end moving the other charge.

    Probably got this wrong.

    Adam
     
    Last edited: May 30, 2014
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