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Five questions about electromagnetic radiation

  1. Aug 12, 2014 #1
    This is from Wikipedia:

    1. What is really meant by "self-propagating", how it works?

    2. How many electric and magnetic fields are in one wave?

    3. What is the strength of those electric and magnetic fields?

    4. What does -q and +q represent in the above diagram?

    5. Why would electric and magnetic fields oscillate?
  2. jcsd
  3. Aug 12, 2014 #2
    1. By self propagating it means that the change of the electric field generates a magnetic field which is changing itself. The change of the magnetic field generate an electric field and so on.

    2. The field is something that is extended all over space. There is one electromagnetic field, and it is oscillating.

    3. What do you mean by "strenght"? If you are talking about the amplitudes of the electric and magnetic fields, they are not fixed. They depend on each particular case. However, the are related as B=E/c.

    4. Two oscillating charge.

    5. Because the two charges oscillate.
  4. Aug 12, 2014 #3


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    The two charges are oscillating in response to the EM wave, they are not causing the EM wave.

    To elaborate on a few of the questions:

    1. Self propagating means that once the wave has been created, it continues to move outwards from the source without requiring anything to act on it.

    2. An EM wave is a wave within the EM field. As Einj mentioned already, there is only one field. The part of the field that is "waving" is the field vectors, which is what we use to determine how an electric charge will respond to an electric field. When the vector (the lines in the wave above) points up, a positively charged particle will be accelerated up, while a negatively charged particle will accelerate down. When the vector points down the charges will reverse their direction of acceleration. This is why antennas work. The EM waves cause the charges in the antenna to oscillate back and forth, which is detected with the electronics and amplified for processing.

    4. The +q and -q represent a positive lyand negatively charged particle respectively. They are there to show you how an electric charge responds to the wave.

    5. The vectors of each field oscillate in response to an accelerated charge(s) that creates the wave. The details of why this occurs would require study of electromagnetism and involve some math.
    Last edited: Aug 12, 2014
  5. Aug 13, 2014 #4
    Generation doesn't sound like propagation to me.

    So both the electric and magnetic field are originating around the same point?

    I mean what is usually meant by strength or magnitude. Electric field is measured in volts per meter and magnetic field is measured in teslas or amperes per meter. Is this magnitude known for EM wave? What does the amplitude represent anyway?

    You mean electric charge, like electron and proton?

    Wouldn't charges radiate equally oscillating EM wave even if they accelerated linearly in just one direction instead of going back and forth?
  6. Aug 13, 2014 #5
    That doesn't sound like self-propagation, more like Newton's first law and ordinary propagation that applies to everything else. Self-propagating sounds more like self-propelling to me, as if it has its own propulsion or acceleration system built-in.

    So the electric field has its strongest negative polarity magnitude at the top amplitude points, strongest positive polarity magnitude at the bottom amplitude points, and zero magnitude where the blue line crosses k vector? Basically it goes ON and OFF while switching polarity from positive to negative?

    They represent receiver antenna rather than emitter, even though they are on the left side of the diagram away from the direction of propagation?

    Alternating direction of an electric current in a piece of wire will emit EM waves proportional to amperes or voltage, rate of change in current direction, and maybe length of the wire? How does that compare with a direct current where charges are accelerated in only one direction?
  7. Aug 13, 2014 #6


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    No, I understand that to mean that that is how a positive and a negative point charge will respond to a passing EM wave
    Drakkith will correct that if I got it wrong

    in a DC circuit, the electrons and resulting charge are not accelerated so there is no EM generated
    Last edited: Aug 13, 2014
  8. Aug 13, 2014 #7


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    Electric fields and magnetic fields follow Maxwell's equations and the Lorentz force law:

    Those laws admit a wave solution:

    One thing that characterizes waves is that they "self-propagate". I.e. a wave here now propagates into a wave there later.

    You don't count fields. They are not discrete.

    That is related to the intensity of the wave. In the visible range of the EM spectrum a stronger EM field corresponds to a brighter light.

    Charges, presumably from a dipole antenna.

    Because they follow Maxwell's equations and Maxwell's equations admit a wave solution. See the response to 1.
  9. Aug 13, 2014 #8


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    Interestingly, in a DC circuit you can get no EM generated even when individual electrons are accelerated, e.g. in a bend in a wire carrying DC. The key point is that the current density is static so the magnetic fields are static.
  10. Aug 13, 2014 #9


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    hi dalespam

    The key point is that the current density is static so the magnetic fields are static.
    That's what I really wanted to say ... a mental block stopped me putting into words

    thanks :)

  11. Aug 13, 2014 #10
    Actually at the beginning they are what causes the oscillation of the field. This is how antennas work. But your are probably right, after the field starts oscillating their motion is sustained by the field itself.
  12. Aug 13, 2014 #11


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    Of course, you are right. An electromagnetic wave (e.g., light or radio waves) must be created by acceleration of charges (in the classical picture, to which I'd like to stick for a while in this thread).

    The sources (and the only sources) of the electromagnetic fields are charge and current distributions. If these are time dependent (and not just the Lorentz boost of a static charge distribution to another inertial frame, where they move with a constant velocity) electromagnetic radiation is produced.

    These fields are dynamically propagating not only within matter containing charged particles but also in free space. That's what Wikipedia might mean by "self propelling". It's a bad wording, however, because there is nothing "self propelling" here. It's just like a ball you through, and it's moving further without being in contact with you anymore. In free space, i.e., without gravity and air friction, according to Newton's Laws it would just go ahead in uniform motion. It's not self-propelled. In a similar way also the free em. field travels on in free space and is not self-propelled.

    It is also a wrong picture you get when it's said that the changing electric field causes a magnetic field and vice versa. You cannot interpret the corresponding homogeneous Maxwell equations (i.e., Faraday's Law and the Ampere-Maxwell Law) in a causal sense. There is only one electromagnetic field. It's even an observer dependent statement what you call "electric" or "magnetic" fields. The electromagnetic field as a whole is a frame-independent concept (described by an antisymmetric 2nd-rank tensor in Minkowski space, called the Faraday tensor).

    The "strength of the field" in a certain sense can be associated with it's energy density, which is given (in Heaviside-Lorentz units) by
    It depends on how vigorously you wiggle the charges, labeled with [itex]\pm q[/itex] in the figure, which is, however, very misleading, if not entirely wrong. What seems to be depicted is the most simple example of a radiation source, the Hertzian dipole (which is realized approximately by, e.g., a linear antenna with a harmonic current; it becomes exact in the limit of vanishing length of such an antenna). The field looks entirely different from the plane wave plotted in the picture. A much better picture (even animated) can be found here:


    The 5th question cannot be answered other than by "that's how nature is". The Maxwell equations which describe electromagnetic phenomena to high accuracy (as long as quantum effects can be neglected) are fundamental laws which have been found after some centuries of observations and just summarize in an elegant mathematical model these observations. They canno be derived from simpler principles and are fundamental in that sense. The existence of electromagnetic wave fields has been in fact predicted by Maxwell and have been found later in experiment by H. Hertz. This is a typical example for the way how science works: You start from a lot of observations, doing quantitative measurements and then find a fundamental law (which is a very rare event, by the way) which leads to further predictions about phenomena that may never have been observed before (or as in the case of electromagnetic radiation have not been realized as such; nowadays we know that visible light is nothing else the electromagnetic radiation in a specific range of wave lengths our eyes are sensitive to). Then you can try to do new experiments to check, whether electromagnetic waves really exist. If not, you have to modify your mathematical model or create a completely new one. If you find them, as is the case here, you have consolidated the validity of the model, and so on.
  13. Aug 13, 2014 #12


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    I agree completely here. Causes always come before effects, and Maxwell's equations show the relationship of the E and B fields at the same time. So at most you can say that they are related or associated as described by Maxwell, but the "causes" language is incorrect.

    However, I think that you are going a little too far here. Classical EM is perfectly consistent with EM waves that exist without ever having been created by a charge moving, (i.e. as a boundary condition of the universe or even as advanced potentials). I think that it is true that the EM waves that we deal with on a daily basis were all created by acceleration of charges, but classical EM is broader than that.

    Just as you can say that an EM wave here now was caused by charge acceleration there yesterday, you can also say that an EM wave here now was caused by an EM wave there yesterday (ad infinitum).

    I agree that we should stick with classical EM here, but I don't think that we need to make a broad sweeping statement about classical EM that is not completely required in classical EM and would have to be revised in QM and cosmology.
    Last edited: Aug 13, 2014
  14. Aug 13, 2014 #13


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    The reason or explanation for wave propagation is that it takes time (a delay) for an effect, in one place to reach another place. As you change the positions of the two charges (as they oscillate together and apart, for instance) and plot how the effect varies as you get further away, you will get a snapshot of the way oscillation varies in time, and that reveals itself as a 'graph' over the distance. The wavy graph will be moving away from the source at the speed (wave speed) that the effect can travel through the medium (space / air etc)
    This is the same for EM and mechanical waves - it's just the mechanism that's different. The overall effect is that energy gets carried away from the source by the waves.
  15. Aug 13, 2014 #14


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    Indeed. If you solve Maxwell's equations, you'll find the retarded potentials in the Lorenz gauge and then by the appropriate derivatives the gauge-independent field equations which are also retarded quantities with respect to the sources [itex]\rho[/itex] and [/itex]\vec{j}[/itex]. No field components occur as sources of other field components (that's what's known as Jefimenko equations in the literature; see also the Wikipedia, which has a good article on it).

    You can, of course, integrate also, e.g., the Ampere-Maxwell Law to express the magnetic field components in terms of the current and the magnetic field components. This, however is not a causal law, i.e., it's not expressing the solution as a retarded expression if you interpret [itex]\vec{j}[/itex] and [itex]\vec{E}[/itex] as sources.

    You are right, from just locally observing an electromagnetic radiation field in free space, I cannot conclude anything on the sources in a unique way. For this I'd need the field in the entire space in order to be able to use the Maxwell equations to get back the charge and current distributions.

    Concerning the advanced solutions, you are also right. They are valid solutions of the Maxwell equations. The only trouble with them is to realize them in practice.

    As an example take the usual thing of an antenna with some AC radiating off radio waves. It's to a good approximation dipole radiation. This is the retarded solution, which is easy to realize in the real world.

    Now, of course you can just apply a "time reversal transformation", leading to the advanced solution. Since the em. interaction is time-reversal invariant you get a valid solution of Maxwell's equations, and there is nothing telling us that this is not possible to occur in nature. However, it's pretty difficult to realize in practice. What you'd need to do is to create a dipole field somehow in some distance from the antenna precisely such that it gets absorbed by the antenna causing the (time reversed) AC current we started from to create it. This is practically impossible to realize, and that's why we usually only consider the retarded solutions.

    Another thing are attempts to get rid of the fields alltogether to establish an interacting theory between point particles without the mediation of a field, i.e., a kind of action-at-a-distance model which does not violate special-relativistic causality. The most famous example is the absorber theory by Feynman and Wheeler, which they invented originally as a starting point for a relativistic quantum theory of a system of point particles without the fields in the hope to solve the problems with divergences in perturbative QED. As we all know, this idea has not survived, and Feynman finally solved the QED problem in terms of renormalization theory (on the way inventing path integrals and Feynman diagrams :-)).
  16. Aug 13, 2014 #15
    How about if the circuit was turned on and off, so electrons accelerate to their drift speed and then decelerate back to normal, repeatedly? How about if voltage supplied is increasing rapidly, or increasing and decreasing repeatedly? How about if the wire was a loop, would electrons following a curved path count as acceleration? Or how about if electron beam is bent with magnetic or electric fields, does that count as acceleration and do they emit EM waves then?
  17. Aug 13, 2014 #16
    I read those equations say something like "mutually inductive", but nothing that says "self-propagating". I've also never heard of any other waves being regarded as self-propagating, but simply propagating and subject to Newton's first law as well. As far as I know we don't say photons are self-propagating, just propagating, also supposed to follow Newton's first law, aren't they?

    You count their sources and sinks, the points of attraction and repulsion. A single electric field at a single point in time can be either a source or a sink, that is either of positive or negative polarity. A single magnetic field must have both, a source and a sink, it has to be a dipole. Isn't that what Maxwell's equations say?

    Can you print down that equation or provide some reference about it?
  18. Aug 13, 2014 #17


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    If the voltage or current is changing, then the circuit will generate EM waves. This can occur in DC circuits when you switch the circuit on or off or when you have variable resistors or other circuit components.

    Steady current flowing through a loop of wire generates no EM waves since the magnetic field is static. Bending an electron beam will indeed generate EM waves. This is how the free electron laser works: http://en.wikipedia.org/wiki/Free_electron_laser
  19. Aug 13, 2014 #18


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    That is fine. The English terminology is imprecise. The solution to a wave equation has a certain mathematical form. If you want to describe that form as "propagating" rather than "self propagating" that is fine as long as you are referring to the same equation.

    Even that seems problematic to me, but even if you have a countable set of sources you still don't count the fields.

    If you think this is a meaningful procedure then please provide a reference. I personally have never seen any professional reference count the number of fields.

    Ceetainly. I will post it this evening when I get home.
    Last edited: Aug 13, 2014
  20. Aug 13, 2014 #19
    Wikipedia says "self-propagating". It is only me who said that sounds as "self-propelling", because to me neither makes sense in the same way.

    So is electric field in EM wave negative or positive, or both negative and positive in the same time, or little bit negative and then a little bit positive?

    Maxwell didn't just accidentally find a fundamental law or blindly attempt to formulate the wave equation. It is interesting then his theory was discarded while the equations were kept. It's like developing a theory to make pudding just to end up with the recipe for making a perfect sausage. Mmmm.
  21. Aug 13, 2014 #20


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    The electric field at any one point alternates from + to - and back over time.
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