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Propagation of EM-Waves, Independence of B and E, and Grav..

  1. Nov 22, 2015 #1
    Hello Forum,

    I have a lot of questions concerning Wave propagation, but I will not pose them all into one thread, because of my experience, that part of them will be overlooked.

    So lets get started:

    1. Phase shift, E-B- Independence
    Consider a plane wave generated by a distant dipole antenna, propagating in the x direction. Seemingly the phase between E and B shifts from 90deg to 0deg during the travel. And seemingly this is a contradiction to the argument, that E and B create eachother, because if one would look at one position x in space, E and B would have the same "slope", therefore it would not be possible for them to be created by oneanother. I am not sure about that, because one shall not only look at one position x. One has to look at the whole "history", I mean the whole plane wave, because otherwise, by just looking at one point I would ignore the fact, that this is actually a plane wave, which is an important information.

    Even if I look at for example an spatial interval of λ, I can not yet resolve this issue. Maybe that spatial interval is not enough, do you have an idea?

    Nevertheless, my question is actually: Does that picture of E and B creating each other hold?
    I see the fact, that E fluctuations are always accompanied by B fluctuation and vice versa and they are inseparable. So there is no way to create or absorb the E-part without creating or absorbing the B-part.
    But that does not mean at all, that they should be existentially dependent on eachother.

    If I am not straying , that would mean for the planewave in the far-field of the dipole-antenna, that the E-plane and B-plane are generated equivalently by the dipole, but not by each other. So the B-planes are not a product of the E-planes and vice versa, but both are simply a product of the radiation source. Doesnt make that sense? I mean, if you now take another antenna, to absorb that plane wave, you have to adjust it such that the E-Field is parallel to the antenna, in order to create a voltage drop inside of it. If E and B would be somehow existentially related, then I could also calculate that same voltage drop inside the antenna by just looking at the B-field. But that B-field is a plane, and it can not possibly create a voltage drop inside the same antenna.
    Similarly, and even more impressive, if you try to absorb the plane wave by using a ring-shaped antenna, then you have to adjust it such that the plane normal of the ring antenna is parallel to B. The B would then create a voltage drop inside the ring by creating an eddy E-field. Again, if that picture of E<-creating->B would hold, then that eddy E-Field which I just mentioned, that has been created by the magnetic field inside the antenna loop should also be a part of E-plane-field, which is (vainly) a product of the B-plane according to the picture.
    But this can not be the case, because the E-plane is not able to create a voltage drop inside that ring-antenna. Therefore that Eddy-E-Field is not a Part of the E-plane accompanying the B-Plane.

    Conclusion: that picture of E<-creating->B does not hold.


    EDIT:

    I thought about that phase-difference question again and actually found, that there is no contradiction. One has to link spatial variation of the one Field wit time variation of the other Field. Not spatial-spatial or time-time. Then it makes sense. Now B and E could be actually existentially linked.... But I still doubt that, because my explaination above is still active.
     
    Last edited: Nov 22, 2015
  2. jcsd
  3. Nov 22, 2015 #2

    davenn

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    considering that Maxwell's work clearly defines that a varying B creates a varying E with creates a varying B and so on

    Maybe you need to review your thoughts :wink:

    I will let those more learned, than I, on the fine details of the topic, lead you further to see where you are going astray :smile:
    @DaleSpam , @mfb

    regards
    Dave
     
  4. Nov 22, 2015 #3

    Simon Bridge

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    Welcome to PF,
    The description of E and B creating each other is kind of a yes and no thing.
    If the field you have obeys Maxwells equations then the picture is fine because that is how you get EM travelling waves out of them.
     
  5. Nov 22, 2015 #4
    Well, what is then wrong with my Ring-antenna gedankenexperiment?
     
  6. Nov 22, 2015 #5

    Simon Bridge

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    I don't understand the description.
    You seem to be asserting that, if E makes B, then the eddy E field must be aligned to the plane wave E field. Is that correct? If so, then there's your error.
     
  7. Nov 22, 2015 #6
    If that picture holds, then the B-part of the Planewave creates a lot of E-curls, which all should add up to an E-plane. So that E-plane is made up of E-curls.
    Therefore that E-Planes should also be able to create a voltagedrop inside the ring-antenna. Can they do that? I realize that the Planes have a curl (unless I only move perpendicular to propagationdirection).
    But how can... oh my god.... you are right.... OK. question Nr.1 is solved. thank you for that.
     
  8. Nov 22, 2015 #7

    Simon Bridge

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    Why should they add up to an E plane?
    It is OK for an EM field to induce another one with different properties... the field in the presence of a conductor is going to be different from the field without the conductor.
     
  9. Nov 22, 2015 #8
    They must add up to a E plane. You yourself said sir, that the picture holds. therefore all of E must be a product of B. And B nothing but lots of little eddies. so it is these eddies, which in the end make up the plane. Of course we are still talking about vacuum. There is no free charge yet... wait.... I will come to that tomorrow in another thread.
     
  10. Nov 23, 2015 #9
    what is the difference of an electric field to a gravitational field? why can gravitational waves exist on their own, and why should the same be false for electric fields?

    consider the following example:

    imagine an electron somewhere in freespace. the electron creates an electric spherically symmetric field.
    now the electron is moved in the z-direction by 1 meter, with a velocity of ,say, 100 meters per second.
    now the electric field constellation has been change, since the position of that electron has been change.
    But in that transition of the electron from one position to another, also the electric field had to undergo a transition from one constellation to another.
    That transition of constellation can only occur with the speed of light. In other words: if someone, who is sitting at a distance of one lightsecond apart from the electron, and wants to estimate the position of the electron by looking at the fieldconstellation, that person has to wait 1 second until the information about the new position has reached him/her.

    And my conclusion then would be: because there is an information content in that transition field, there is also an energy transmission, because information is energy.


    PS: I really would like to see some kind of field simulation, where I can move a charge around with the cursor, and see how the field-change propagates through space. There is no other way of convincing me of the opposite. I need a simulator. Do you know something?
     
  11. Nov 23, 2015 #10

    davenn

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    because they are generated by different mechanisms

    maybe you should google some basic explanations of each ? :wink:
     
  12. Nov 23, 2015 #11

    davenn

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    google gave that answer too

    http://phet.colorado.edu/en/simulation/radiating-charge


    Dave
     
  13. Nov 23, 2015 #12
    cool, thats great. but after looking at that simulation it seems to me, that electric field waves can propagate without consideration of magnetic fields?
    Is there something wrong with my eyes?... or with my brain?
    If there wouldnt be a magnetic field for some reason, then what problem would occur? I dont see the necessity of B.
     
  14. Nov 23, 2015 #13

    davenn

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    no, its just that they are only showing the electric field for clarity of the simulation
    the moment you have a moving charge/electron, an EM ( electromagnetic) field is generated and radiated

    Dave
     
  15. Nov 23, 2015 #14

    Dale

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  16. Nov 23, 2015 #15

    davenn

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  17. Nov 23, 2015 #16

    Dale

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    What do you mean by "necessity"? If you have the E field and relativity then the B field is logically required. But does that make it a "necessity" in your view? Is the E field necessary? Is relativity necessary? Is logic necessary?

    Regardless of whether you consider it necessary, the B field is clearly a feature of the world around us.
     
  18. Nov 25, 2015 #17
    Thank you Dale for the link. yes, my first post was a little chaotically written.
     
  19. Nov 25, 2015 #18

    sophiecentaur

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    The near field arrangement of fields can have many forms in different antennae. 'Inside' a multi element array, it can be very hard to calculate and, afaik, approximations are needed. Loop antennae have different impedances from dipole impedances. But the 90° phase difference that people talk of is not exactly 90°. The departure from 90° represents the resistive component, which corresponds to an inphase component, which accounts for the energy lost / radiated. When you match the input to a transmitting antenna, the reactive parts of the input impedance are canceled and the Radiation resistance is all that's left - the V and I at the drive point are in phase. Within the antenna itself and near to it, the phases can be all over the place. 90° for a simple structure but those fields are evanescent and die out (because they cannot be radiating energy). All that's left is the in phase components.
    I would agree with other replies that you have been getting in that there's no point looking for a 'cause and effect' between E and H. They just exist together in free space but, inside a structure, one can be storing or propagating much more energy than the other so it may be regarded as existing on its own.
     
  20. Nov 26, 2015 #19

    Simon Bridge

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    ... just because the E and B feilds generate each other to propagate in air or in a vacuum does not mean the same fields will do so in a conductor. Indeed, if the conductor is ideal, there should be no E feild inside it at all.

    This is due to the interaction of the incoming wave with the conductor. Conditions outside the conductor are different to inside the conductor. There is no reason to suppose that a B field that generates a particular E field in air will generate the same field in a medium. This is why I directed your attention to Maxwell's equation so you can do physics by maths instead of by rule of thumb.

    You'll notice my original reply was actually:
    ... the link is as you explained in your edit post #1. The rest of what you need comes under the heading "boundary conditions"... which is how these equations describe different fields in different situations.

    If you prefer a microscopically detailed model, you'd need to account for the fields due to the atoms and "free" electrons of the conductor, and the fields resulting from their interactions with the incoming wave. The end result is well modelled by the usual set of boundary conditions for a classical conductor.
     
  21. Nov 26, 2015 #20

    Simon Bridge

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    An EM field with no B component is a static electric field... it does not change with time.
    This would be a problem in any physics that hopes to model a non-static electric field.
     
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