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Discussion in 'Physics & Math' started by Tylor, Feb 1, 2013.

  1. James R Just this guy, you know? Staff Member

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    A couple of minor corrections:

    In an EM wave, the electric and magnetic fields are always in phase, as I understand it. So, I'm not sure what this assumption is about.

    Electromagnetic induction is not the same thing as propagating EM waves.
     
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  3. leopold Valued Senior Member

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    i'm not too sure either, it's been like 30 years or so.
    apparently they are 180 out, not 90.
    http://en.wikipedia.org/wiki/Electromagnetic_radiation
    well . . .
    you sure?:bugeye:
    i believe it's identical to how radio waves propagate.
    how do inductors induce power into other inductors, or any other metallic object?
    by EM fields.
    wiki page on induction:
    http://en.wikipedia.org/wiki/Electromagnetic_induction
     
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  5. Q-reeus Valued Senior Member

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    In the far - i.e. radiation zone, propagating E and B fields are always temporally in-phase (or anti-phase depending on one's convention), but spatially the two are mutually orthogonal. It's in the near-zone (so-called static and induction zones) where r<<lambda that E and B fields are essentially in phase quadrature. When it comes to interfering waves - counter-propagating waves forming a standing wave pattern - then one has E and B being 90 degrees apart in both time and space.

    It was astonishing to find this shown wrong in both editions (1966 and 2000) of a standard and much referenced tome on microwave theory - Foundations for Microwave Engineering, by R.E.Collin. In pp104-105 of 1st ed'n there is illustrated and described the E and B fields for a propagating wave in rectangular waveguide having pattern relationships appropriate for standing waves. Thankfully afaik a non-propagating basic conceptual error that didn't impact on the rest of the excellent material therein though. An error stemming in part from his conceptual belief that E and B fields 'caused' each other (although that concept, rightly applied, would still imply E and B having an in-phase relation). And I have come across similar errors in other well known textbooks that survive from one edition to another - one bit in Jack Kraus's book on Antennas (the 'bible of antenna engineering) being a case in point.
     
    Last edited: Feb 11, 2013
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  7. James R Just this guy, you know? Staff Member

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    Hmm... well that would just depend on some convention or other. 180 degrees is experimentally indistinguishable from 0 degrees in this case (I think).

    Yes, but an EM field is not the same as an EM wave. An EM wave is a self-propagating combination of EM fields. Many EM fields do not self-propagate, including those of inductors driven by DC currents. AC currents are another matter...
     
  8. Aqueous Id flat Earth skeptic Valued Senior Member

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    6,152
    Think of electrons in terms of emission (field generation) and photons in terms of transmission (field propagation) and you'll get closer to the source of your confusion.
    The right hand rule will help you get started understanding vector math. Next you need to know the vector equiations that describe the principles you're trying to understand.
    You obvously don't understand that there are two conventions used.
    Photons propagate the electromagnetic radiation regardless of the type of particle that is generating the field.
    Electrons absorb and emit photons. This is the most basic principle of this thread. Energy and frequency are equivalent (inversely proportional) and energy is conserved, so frequency is conserved in the process.
     
  9. leopold Valued Senior Member

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    17,455
    i believe you are correct.
    an inductor generates a magnetic field not an EM field.
     
  10. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    You make it sound like a crime.

    Are you thinking talking about baby universes is crank?
     
  11. AlphaNumeric Fully ionized Moderator

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    It would be 'crank' (are we now using 'crank' in a manner other than a noun, 'he cranks', 'you crank', 'we crank', 'she cranked' etc? Is it 'I was crank' or 'I cranked'?) to unilaterally refuse to discuss any and all concepts outside of verified science, as such an attitude would preclude novel idea generation and scientific development, but tentative ideas should be treated as such. Baby universe formation and development might be something a cosmologist initially ponders, which might then make a string theorist wonder if there's some way of formalising such things, which then might cause him/her to devote a little bit of time to messing with various equations to see what might arise but until such time as something concrete comes out of the formalism there's no point in drilling down into making claims about the specifics of baby universes.

    Otherwise it's a bit like religion (wait, let me explain!). Most religions not only attest to the existence of their god or gods but they also claim to know what this god or these gods like or dislike, what their preferences are, what their opinions on food, sex, laws, behaviour etc are. But it is ridiculous to start embellishing this god or gods with opinions and desires when you haven't even shown it or they exist yet. To use an example often brought up on the Atheist Experience, it's akin to talking about the fashion preferences of the society of Bigfoots when you haven't even shown they exist yet.

    Unfortunately a lot of hacks don't realise that when someone in the mainstream talks about such things as universe formation or baby universes they at least have some semblance of a formal model to explore, it isn't completely random supposition, unlike the hacks.
     
  12. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    Thank you.

    You cought my post before an edit.
     
  13. Prof.Layman totally internally reflected Registered Senior Member

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    I thought cranking could be a good verb for the word crank. For example, I was really cranking ideas one day and thought that there could be pocket universes created by time changes trying to fullfill causality that directly violated the causality of another time change, so there would only be a section of a universe that would exist for a short period of time so that it wouldn't interfere with the other causality loop of another time change. Some scientist in the distant future could actually discover and prove such a theory, but it may not be recognized because some crank already took the word pocket universe or baby universe so then it could no longer mean a universe that isn't actually an entire universe. So then he could never discover this theory and use a word that is appropriate for descriptions of things in that theory without sounding like a total crank themselves. Then preventing the discovery of actual science. I think we should reserve words being used if they don't really have a foundational basis for using that word. Baby universe and pocket universe shouldn't be used to describe a whole universe in its entirety, who knows if it could more accuratly describe some far off theory of the future?
     
  14. origin Trump is the best argument against a democracy. Valued Senior Member

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    Are you saying that E field and the B field are 90 degrees out of phase for EM radiation that is very close to the source?
     
  15. Q-reeus Valued Senior Member

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    It is never strictly the case for arguably the most fundamental EM radiator; that of a short dipole oscillator. But it gets awfully close the further in one gets to the source. Which accords with that power density close in is almost purely reactive, hence the time averaged Poynting vector ~ ExH must have E and H essentially in quadrature phase. it's interesting to study the mathematical form of the various E and H field components. This wiki article section is good in that respect: http://en.wikipedia.org/wiki/Dipole_antenna#Elementary_doublet
     
  16. origin Trump is the best argument against a democracy. Valued Senior Member

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    Still don't know what your point is. You seem to be saying that either spatially or temporally the E field and B field change depending on the distance from the source. Is that what you are saying?
     
  17. Q-reeus Valued Senior Member

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    Of course. And it's explicitly present in those field expressions that I linked to last post. You know how to read off the temporal and spatial components shown?
     
  18. origin Trump is the best argument against a democracy. Valued Senior Member

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    Interesting, I dd not know this. So if I understand this correctly at r<<\(\lambda\) the interfernce from the antenna essentially sets up a standing EM wave which has the E field 90 degrees out of phase with the B field.
     
  19. brucep Valued Senior Member

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    Good analogy. An example of such a model is Eternal Inflation.
     
  20. Mazulu Banned Banned

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    As I passionately crank to you, all baby universes should be interconnected to each other somehow. That's what's important. One of those baby universes is now a full grown universe with a faster speed of light, c'. We would want to build a starship that can force itself into this other universe in order to travel at superluminal speeds (relative to our universe). Every universe should have it's own timeline and speed of light. It's like taking the subway while everybody else has to walk. But how do we insert our spaceship into another universe and interface with it's laws of physics? That's what I can't figure out.
     
  21. Q-reeus Valued Senior Member

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    Yes the phase relations go to 90 degrees in the limit as r -> 0 (but it's not quite correct to call it standing waves, which are the result of reflected traveling waves as in a cavity resonator say). And it makes perfect sense because what you have very close in is in effect an inside-out capacitor putting out an alternating electrostatic field + alternating magnetostatic field owing to the 'charging current'. This situation of reactive fields close in gradually transitions through to what's termed the intermediate zone and finally into the far radiation zone, where power is entirely real and E and H are always in phase. In the intermediate zone where all the various 'static', 'induction', and 'radiation' components are comparable in magnitude, a real fruit-salad situation exists. Some text books caution the reader of the 'paradoxical results' one gets by working out the Poynting vector flows there. For instance - formally real power flowing transverse to the radius vector. It's only by integrating everything over all space and a full cycle that only the expected radial acting radiation power emerges unscathed.
     
  22. Aqueous Id flat Earth skeptic Valued Senior Member

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    Gawd you do love to BS.
     
  23. Q-reeus Valued Senior Member

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    2,497
    Talking to me again? Ignorantly not quoting anyone, so just a guess. Back up your accusation - flaming/trolling accusation, with cogent facts, or learn to shut up!
     

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