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An oscillating charge

  1. May 16, 2003 #1
    What do we know about the radiation emitted by an oscillating charge ? Anyone care to share their wisdom ?
  2. jcsd
  3. May 16, 2003 #2
    Quite a lot actually -- you can find a good treatment in Jackson, or a decent one in an online book at http://www.plasma.uu.se/CED/Book .

    The power radiated away is given by the Larmor formula P=q^2*a^2 / (6*pi*e0*c^3) where a is the instantaneous acceleration (only valid for nonrelativistically moving charges.)

    In simple cases, the emitted radiation far from the source (E field strength) goes as q*a*sin(theta)/(4*pi*e0*c^2*DISTANCETOOBSERVER) where theta is the angle between the acceleration vector and the vector between the source and observer. So for a charge oscillating linearly at frequency w over a distance X, the power radiated will go as w^4*X^2, in spherical waves (falling off at 1/r), with an angular dependence of sin(theta).

    ie, dipole radiation.
    Last edited by a moderator: Apr 20, 2017
  4. May 16, 2003 #3


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    besides what damgo suggested there are some fun
    animated pictures of the field from an oscillating charge
    at a caltech site


    you get a menu of motions which you can also control
    try the circular motion, it is nice
  5. May 17, 2003 #4
    Thanx guys ... appreciate it.

    I downloaded the textbook by Bo Thidé and searched it for the word "doppler". It didn't find anything. This confirms my belief that the classical description of the radiation from an oscillating charge is incomplete, because it obviously experiences dopplereffect (as can be seem in the applet).

    I'm I way of here or what ? SHould I be searching for soething else ?
  6. May 17, 2003 #5


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    How about that! You are in Malmo and almost next door to you in Uppsala
    there is Bo T who has made maybe the best online Classical Electrodynamics book.

    It is possible that by "doppler effect" you are thinking of something different from what other people mean and so
    it COULD be (maybe) that the classical description would not
    seem incomplete to other people in the sense of failing to
    predict doppler effects.

    But what words are used are maybe not as important as what one sees. What exactly did you see in the applet that you found
    interesting? Which motions were you looking at?
    Sinusoidal? Circular? Linear with acceleration? What was happening that caught your attention and made you say:
    "because it obviously experiences dopplereffect (as can be seen in the applet)"?

    I'm asking because what I see in these animations is what I would describe as "delay". When the charge changes motion it takes a while for the news to spread out. And so the field is bent into (actually beautiful) shapes.

    I have not noticed what I would call doppler, although you and other people may see it and I just did not notice.

    BTW do you ever watch Scandinavian films? A few weeks ago I saw a really good film made in Norway---it had no violence and no visible sex but it was still very amusing. It was quite different from recent hollywood films, but was (I would say) rather civilized.
    Can't remember the name.
  7. May 17, 2003 #6
    Yup, you're off -- the classical description uses Maxwell's equations, which have all the wave properties, including not just classical 'Doppler' effects but relativistic ones as well.

    What exactly do you mean by the "doppler effect" here? Usually it is used to refer to frequency changes due to relative motion between a source and observer. You solve the EM problem for a fixed observor position, and throw in the Doppler effect at the end. If you're too close to the source relative to its range of motion, that won't work; the text deals with this nasty case. Look for the bits about moving sources...
  8. May 18, 2003 #7
    I wouldn't say next door, but at least in the same country :wink:

    Have a look at this shockwavefilm of an oscillating charge ... It shows the dopplereffect I'm talking about below.

    Yes, this is indeed what I'm talking about. Let me show you both what I mean....

    Since I'm not a mathgeenie who can put together a proof using formulas, I've tried to illustrate my point instead:
    http://hydr0matic.insector.se/oscillation.jpg [Broken]

    A: This picture just shows a charge oscillating along the y-axis and the radiation emitted (ie the standard oscillation).

    B: This is the same as A, but with added oscillation along the x-axis. The x-axis movement back and forth results in dopplereffect.

    C: This is the same as B, but here the oscillation in the x-axis has been synchronized with the oscillation in the y-axis.

    D: This picture illustrates my point. As you can see I've just rotated picture C and strengthened the actual oscillationpath of the charge. All radiation emitted by an oscillating charge experience periodic blueshifts and redshifts, except the one perpendicular to the oscillation-axis, where Θ=π/2.

    So why am I so interested in the dopplereffected radiation of an oscillating charge ? Well... I've had this crackpot-theory for quite a while now and I can't seem to shake it. The idea is to replace quantum theory with an oscillating charge model. ... wouldn't that be something ? :wink:

    The challenge for classical physics is to explain blackbody-radiation, quantic atomic spectra (hydrogen lines) and the photoelectric effect. Although they're not very likely to be correct, I've come up with explanations for these phenomenas based on the oscillating charge model. Let me begin with the hydrogen lines....

    If the oscillating charge is moving with a very high speed, the amount of dopplereffect will be significant, resulting in blueshifted radiation with wavelengths a lot shorter than the one at Θ=π/2. Now, will all of the wavelengths radiated by the charge turn up in the spectrometer ? No, they won't. The charge is oscillating with a frequency corresponding to a wavelength λ (http://hydr0matic.insector.se/wavelength.jpg [Broken].

    Now, be honest... If this was the end of the nineteenth century, what would you consider more likely ? - that the lines is a result of the structure described in our current atomic model, or that the lines is simply a result of the hydrogen atoms' movement.

    There is a catch though (probably more than one), if this was correct the lines would shift depending on temperature. I know that the Lyman, Balmer, Paschen (etc) series are all produced at different temperatures, but I don't know any details. Could experimenters have missed the hydrogenspectrums dependence on temperature ? - Not very likely :smile:.

    Could the dopplereffect perhaps explain the photoelectric effect ? I haven't analysed this possibility enough to make any qualified guesses, but at a first glance - it seems possible. The blueshifted part of the wave is less distributed in space, which gives more energy per unit volume. So in general, the high frequency parts of the radiation have higher intensity. If one were to calculate the energydistribution of the radiated spectra, perhaps Planck's constant would turn up somewhere ? :smile:

    I made a pathetic attempt to derive Planck's blackbody radiation formula with the oscillating charge model and the dopplereffect. I haven't studdied advanced math so the lameness of my effort was overwhelming. My intuition tells me it could be possible though. Anyone care to give it a shot ? :smile: ... Since the dopplereffect is dependent on temperature it could give the required frequencyshift in the Planck curve. In general, I think the oscillating charge model explains the appearence of the Planck curve beautifully. I'm just not able to formulate it mathematically.

    No, not very often; once in a while there's an OK movie produced, but in general - they all suck .

    You're right, this thread now belongs in the theory development forum.
    And yes, I know this idea is very crackpotish. :wink:
    Last edited by a moderator: May 1, 2017
  9. May 18, 2003 #8


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    I looked at the film you call "shockwave"

    It is beautiful. I have saved the URL so I can recommend it to other people and I have looked at it several times.
    It is similar to the Caltech movie which I recommended to you,
    but covers a different type of motion.

    You say scandinavian movies suck. (1) you may be right, I cannot say since I have seen only a dozen or so (2) we may have very different taste in movies. Some of the scandinavian films I have seen are among my most favorite pictures of all time.

    You raise the issue of crackpotism. Crackpotism is a worthy undertaking which is necessary for the honor of mankind and the advance of civilization. Go for it!

    Your concept of doppler is totally wacko, in my opinion, but this is all right. Do not apologize. Your pictures ABC and D are not what the field would do---they are tracings of the motion the particle on a moving strip of paper and do not represent the field. I am not going to argue about this.

    I certainly never said that this thread belongs in Theory Development! I do not understand your PS :
    "You're right, this thread now belongs in ...."
    Your ideas are more sensible than half of what gets posted
    in Astronomy, Physics, and Theoretical Physics. Plus, you
    have illustrations, pictures, links!!! Also you have modesty and
    as sense of humor. I have no authority here, just being an ordinary poster like yourself, but I think your speculations are right at home and normal for PF Theoretical Physics.
  10. May 18, 2003 #9
    I agree, the appearence of my concept of doppler is wacko. Unfortunately for me, there is no "electromagnetic fields"-tool i photoshop, and I don't feel like spending hours trying to replicate one, so please give me a break :wink:. I must not have given the impression of knowing very much about physics :smile:, but trust me, I'm familliar with the concept of doppler.

    That's because it wasn't directed specifically at you, but for anyone reading this thread thinking it does not belong here.

    -> Marcus, why not point out the flaws in my idea instead ? That is, after all, the reason why I posted :smile:. Just because I acknowledge the fact that the idea is very crackpottish does not mean I don't want you to argue your case.
  11. May 21, 2003 #10
    Ok, someone else then ? .. Does anyone even understand my idea ? :smile:

    Perhaps this illustration can clarify what I'm talking about: http://hydr0matic.insector.se/oscillator2.jpg [Broken]
    Last edited by a moderator: May 1, 2017
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