EM Waves, how are they created?

[mesa]

A moving charge creates a B_f so is an EM wave created (for example) by an electron oscillating in a conductor producing a changing B_f which then induces an E_f with both perpendicular to the direction of propagation? If so I am presuming the point where the B_f crosses the axis represents the point where the electron had changed direction, is this correct?

 

[ZombieFeynman]

Light waves are produced by accelerating charges. When a charge accelerates, it creates a kink in the fields that emanate from it. This kink, starting from the location of the charge, is seen by an observer to move away from the charges in a direction perpendicular to the directions of the fields in the kink. Whats happening is that the fields nearby the charge are replaced by what the charge has just recently done. Farther away from the charge we observe what the charge has done a while longer ago.

 

[mesa]

Light waves are produced by accelerating charges. When a charge accelerates, it creates a kink in the fields that emanate from it. This kink, starting from the location of the charge, is seen by an observer to move away from the charges in a direction perpendicular to the directions of the fields in the kink. Whats happening is that the fields nearby the charge are replaced by what the charge has just recently done. Farther away from the charge we observe what the charge has done a while longer ago.

Propagation of the B_f due to a moving charge seems straightforward. The field strength is changing based on the velocity of the charge at a given time (depending on F.o.R.) so when it crosses the 'axis' that is the point when the charge changes direction, very good.

 

[ZombieFeynman]

Propagation of the B_f due to a moving charge seems straightforward. The field strength is changing based on the velocity of the charge at a given time (depending on F.o.R.) so when it crosses the 'axis' that is the point when the charge changes direction, very good.

It is crucial, I think, to note that EM radiation is from accelerating, not just moving charges.

Im also not sure what axis you are talking about.

 

[mesa]

It is crucial, I think, to note that EM radiation is from accelerating, not just moving charges.

Im also not sure what axis you are talking about.

Along x in the image below. So I am assuming the point where the B_f crosses is where the direction of velocity of our oscillating electron (at distance traveled at velocity 'c') is changing so had no velocity and therefore no B_f, is this correct?

So you are saying I should be more specific as it is the acceleration that causes the change in magnitude of the B_f and resulting induced E_f as there could be no 'wave' with constant velocity?

 

[toneboy1]

I didn't really understand your last reply mesa but to weigh in:
I thought the electron emitting a photon and absorbing was more to do with concervation of energy than B being induced then E and over and over. As I mean, when you heat something up it emitts photons. You know, like the whole band gap thing, electron moves down an energy level emitts a photon.

Though I have a question which the diagram helps, so the electron jossels about and you're saying where the acceleration changes is like the origin of the diagram, well I remeber hearing that each electron only emitts one photon, well which way does it propagate? In the x direction or -x direction?

 

[mesa]

I didn't really understand your last reply mesa but to weigh in:
I thought the electron emitting a photon and absorbing was more to do with concervation of energy than B being induced then E and over and over. As I mean, when you heat something up it emitts photons. You know, like the whole band gap thing, electron moves down an energy level emitts a photon.

Though I have a question which the diagram helps, so the electron jossels about and you're saying where the acceleration changes is like the origin of the diagram, well I remeber hearing that each electron only emitts one photon, well which way does it propagate? In the x direction or -x direction?

I am looking at an example from the standpoint of an oscillating electron in a conductor and the Bf created by this movement which produces an EM wave and does the point where the Bf and Ef cross over the axis represent the change in direction of the electron? Maybe I am missing something here but I thought EM waves could be created by oscillating charges?

 

[toneboy1]

I am looking at an example from the standpoint of an oscillating electron in a conductor and the Bf created by this movement which produces an EM wave and does the point where the Bf and Ef cross over the axis represent the change in direction of the electron? Maybe I am missing something here but I thought EM waves could be created by oscillating charges?

obviously what your sayings right concidering radio and telly but I think the electron has to move down an energy level or more to create a photon; I suspect a radio or lightbulb does this in some way. If anyone has anything to complete this picture for us that'd be good.

 

[mesa]

If anyone has anything to complete this picture for us that'd be good.

Agreed, in the meantime here is how I see it:

All three figures show the Bf created by a moving electron oscillating in the negative 'y' from the origin with an EM wave moving out along 'x' at 'c'. The magnitude (arbitrary scale of course) at each point is shown by the arrows with the point of maximum velocity being at the center of oscillation. Obviously this doesn't include the induced Ef but that is a basic relationship of cross product of the changing Bf through space so can be omitted since this is understood.

Figures 1 and 2 represent 2λ and fig(3) 1/2λ. So we know there will be a magnetic field creating an EM wave propagating through space but due to the oscillation of the electron the points of highest magnitude/velocity are not typically at their highest magnitude over the majority of any axis which is drawn (although through symmetry it can be drawn the way we typically see EM waves as in the previous image)

As such the magnitude will be stronger outside of that axis at any point except (assuming the 'new' axis is drawn bisecting the oscillation) 1,3,5,7,... will be accurate representations and since points 0,2,4,6... being the points of no velocity of the charge moving back to the oscillation at 'c'

An issue I see with these figures is the magnitude of the Bf dropping off at 1/r^2 from the oscillation.

Any thoughts?

 

[toneboy1]

Not to add anything to what you've said, maybe to answer my own question of which way along the axis it travels; I should probably of kept in mind that when we talk about a photon as a wavefunction it's all with respect to probability. As in, we don't actually know which way it's traveling due to H.U.P just probabilities of what it's 'expecation value' would be.
with all these different models of light for different circumstances it's easy to get confused.

 

[mesa]

Not to add anything to what you've said, maybe to answer my own question of which way along the axis it travels; I should probably of kept in mind that when we talk about a photon as a wavefunction it's all with respect to probability. As in, we don't actually know which way it's traveling due to H.U.P just probabilities of what it's 'expecation value' would be.
with all these different models of light for different circumstances it's easy to get confused.

Well then let us start with the simplest idea and break it down from there before getting into Heisenberg. How about an oscillating electron inducing a Bf? ;)

The Figures are a 2 dimensional representation of a plane of the Bf (at some point in time) created by our moving charge. There is no 'probability' to this behavior so makes drawing a representative wave, it's direction, and amplitude straightforward.

The accompanying figures should be an satisfactory 2D representation of that wave propagating outward from the oscillation at 'c' unless you see (or someone else for that matter) something I missed. I understand that the Ef is not drawn but as mentioned earlier since this can be shown as nothing more than a simple representation of cross product it can be omitted as this is understood.

It is interesting that an EM wave drawn along a straight axis is not a true representation for the maximum amplitude of our magnitude of the wave along most points of our axis although since the drift speed of electrons is negligible (and therefore only a small distance is traveled) compared to the propagation speed of our wave at 'c' I suppose to some extent it can be disregarded for now.

What do you think?

 

[toneboy1]

I think that your drawing is wrong in the same way that showing a yr 9 student an electron revolving around a nucleus is wrong (orbital theory etc)
Extrapolating on your point that drift velocity is negligable I might submit that perhaps the electron doesn't move at all for like kHz just hovers oscilating inpercievably small distances, and instead of there being some sort of physical acceleration for your zero point, that it is to do with something like molecular spin.
As Feynman said, we don't know that photons actually travel in straight lines.

Other than that I'm out of ideas, maybe the lack of response indicates that science just doesn't know yet.

 

[mesa]

I think that your drawing is wrong in the same way that showing a yr 9 student an electron revolving around a nucleus is wrong (orbital theory etc)

Is the way I understand how a B_{f ind} is created from a current in a wire wrong? We could dig deeper and look at individual electrons and the such but this is just an over view of magnetic fields created by an oscillating electron in a conductor and how the magnitude of that field permeates space resulting in an EM wave. The E_f is simply a cross product of the magnetic field and so is left out for simplification. What am I missing?

 

[toneboy1]

I wouldn't say that because I don't know, I can understand your frustration, what source did you read that indicated to you that B was created from the current?
What did you say B was crossed with to produce E ? I thought you meant that the photon was = |B| x |E|.

 

[mesa]

I wouldn't say that because I don't know, I can understand your frustration, what source did you read that indicated to you that B was created from the current?
What did you say B was crossed with to produce E ? I thought you meant that the photon was = |B| x |E|.

B_f is induced by current, or in other words any time there is a charge in motion to your F.o.R. You can find out more in any Physics texts that covers electricity and magnetism.

As far as the E_f is concerned it is nothing more than the result of the B_f but perpendicular to it and to that of the direction of motion. I feel like I was able to answer my original question and am happy with the results shown here unless someone on the forum has a reasonable objection.

The only thing I would add is it would be better to view the EM wave from a standpoint far away from the oscillating electron as the waves amplitude decreases by 1/r^2 and therefore would be more similar to a conventional representation (like the first diagram above) when viewed at far distances from this oscillation. Anyway, it's been fun, see you around the forum.

 

[toneboy1]

I would disagree with your assertion that the B created from the electricity is the same B present in the propagation of a photon, that's more conventional classical physics, photons are a different Quantum relm; but if you're happy than good.

 

[nsaspook]

I would disagree with your assertion that the B created from the electricity is the same B present in the propagation of a photon, that's more conventional classical physics, photons are a different Quantum relm; but if you're happy than good.

Explain your disagreement please.

 

[toneboy1]

I'm tempted to ask what you don't understand about the distinction already made. Where to start; the B created in an electric circuit is vertual photon field due to a flow, not oscillation, it is not comprised of an E component nor is it's magnitude related to the frequency of electron oscillation. Again the diagram posted doesn't illustrate any usable concept of B (or E) because photons are described more by de broglie waves. *insert reiteration of HUP*...look up Biot–Savart.

 

[ZapperZ]

obviously what your sayings right concidering radio and telly but I think the electron has to move down an energy level or more to create a photon; I suspect a radio or lightbulb does this in some way. If anyone has anything to complete this picture for us that'd be good.

This is incorrect. In a synchrotron light source, free electrons are made to oscillate back and forth as they pass through insertion devices such as an undulator or a wiggler. This can be accurately described via straightforward classical E&M without invoking any "energy level".

As far as I know, the description of antenna and EM wave being generated by transmission sources are done using classical E&M, not via quantum mechanical "energy levels".

Zz.

 

[toneboy1]

correct me if I'm mistaken but I think that's misleading as the synchrotron must create a photon and positron if it's creating EMR from free electrons. Which isn't a classical electromagnetism explanation.

my background on the matter is merely second year QM and semiconductor phys.

 

[ZapperZ]

correct me if I'm mistaken but I think that's misleading as the synchrotron must create a photon and positron if it's creating EMR from free electrons. Which isn't a classical electromagnetism explanation.

A synchrotron must create a photon and positron? Since when?

I suggest you look up insertion devices that I mentioned earlier. I don't just read about synchrotron centers. I worked at one.

Example:
http://www.synchrotron.org.au/index.php/synchrotron-science/how-is-synchrotron-light-created

Zz.

 

[toneboy1]

I see what you're saying, I read what you said like the device converted them.
I very much doubt that classical electromagnetism explains this operation though, but you'd know better than me, if you can explain it a bit that'd be interesting.

Where you worked, Victoia I'm assuming?

 

[ZapperZ]

I see what you're saying, I read what you said like the device converted them.
I very much doubt that classical electromagnetism explains this operation though, but you'd know better than me, if you can explain it a bit that'd be interesting.

Where you worked, Victoia I'm assuming?

I gave you an example of a synchrotron light source. I didn't imply that I work at that location. I am not even in Australia.

You know, if you simply look up things such as cyclotron, etc... when all you have is electrons going round and round in circles and producing EM radiation, you should be able to satisfy yourself of what you've been told. You can doubt all you want, but you need to remember that you are making yourself look very foolish by saying such things when you don't know any better.

Zz.

 

[toneboy1]

Excuse me, but you're the one not substantiating what you're claiming, everytime I've put forward a view-point like about your synchrotron, I've explained what I was basing it off like 'well if those electrons were circulating like that the EM would be related to something like quantum spin IMO'. So I think what you just said is rather hypocritical, especially given all I've done is invite explanation from others, yourself included (which was apparently declined).
I was going to ask you if the devise you worked with used a magnetic Hallbach array but I don't wish to continue correspondence with you.

 

[mesa]

Excuse me, but you're the one not substantiating what you're claiming, everytime I've put forward a view-point like about your synchrotron, I've explained what I was basing it off like 'well if those electrons were circulating like that the EM would be related to something like quantum spin IMO'. So I think what you just said is rather hypocritical, especially given all I've done is invite explanation from others, yourself included (which was apparently declined).

I believe I could make a similar argument, however this contention is not related to the thread and does nothing to get the answers we are looking for.

ZapperZ has been on the forum for a long time and has experience in the field. I would suggest taking a closer look at his links before disregarding them, I certainly have.

 

[toneboy1]

I believe I could make a similar argument, however this contention is not related to the thread and does nothing to get the answers we are looking for.

ZapperZ has been on the forum for a long time and has experience in the field. I would suggest taking a closer look at his links before disregarding them, I certainly have.

THERE WAS NO LINK when I took issue with the way I thought it worked (not that it mattered when he did post a link, the issue I take with it's opperation is still along the same lines).

If I was arguing that it was definitely not explainable classically for no reason and not open to hearing how then objectively I could see how instead of a physics explanation I'd receive a condescending repremand about assuming where they worked, however I wasn't, hence I find this community rude and boarderline ignorant.
If you think, that specific field duscussed here is relevant, without explanation or the address of questions about it answered, fantastic. I'm not here to build up a reputation on the forum which apparently holds much sway with you, it doesn't benefite me, I regret reading this thread.

 

[mesa]

THERE WAS NO LINK when I took issue with the way I thought it worked (not that it mattered when he did post a link, the issue I take with it's opperation is still along the same lines).

Then present a more solid argument than 'i think', 'i believe', etc.

...I find this community rude and boarderline ignorant.

I can understand that frustration, for example:

I think that your drawing is wrong in the same way that showing a yr 9 student an electron revolving around a nucleus is wrong

If you think, that specific field duscussed here is relevant, without explanation or the address of questions about it answered, fantastic.

Considering your input on the topic of this thread it may be advisable for you to do a similar review of basic mechanics before spending so much time on an area of physics that is orders of magnitude more difficult.

I'm not here to build up a reputation on the forum which apparently holds much sway with you, it doesn't benefite me

If you believe I am worried about a 'reputation' on the forum I would suggest reading some of my posts in other threads. A systematic and scientific approach to gaining knowledge is what I care about, making connections with people that understand complex problems is an added bonus.

Either way this latest development on this thread has gone way off topic and I feel the original question has been answered so none of this really matters. I am done here.

 

[tim9000]

Then present a more solid argument than 'i think', 'i believe', etc.

Why did you want him to make something deliberately theoretical, objective? That wouldn't just be counter productive, it would be unscientific, mesa I don't think you grasp the point of scientific objectivism, quite unwelcoming.

 

[Drakkith]

obviously what your sayings right concidering radio and telly but I think the electron has to move down an energy level or more to create a photon; I suspect a radio or lightbulb does this in some way. If anyone has anything to complete this picture for us that'd be good.

Think of it this way. Do you know of any energy levels where the difference between them is on the order of micro, nano, or pico electron volts? That's the energy of each photon in the various radio frequency ranges.

The reason that hot objects radiate across a broad spectrum is because the creation of light does NOT required an electron to drop energy levels (Although that is one way to do it), it only requires that a charge, of any kind, be accelerated. The oscillation, vibrations, and other movements of atoms and molecules in a bulk material allow an infinite number of wavelengths to be created, with the temperature of the object determining the distribution of energy and thus the spectrum emitted.

correct me if I'm mistaken but I think that's misleading as the synchrotron must create a photon and positron if it's creating EMR from free electrons. Which isn't a classical electromagnetism explanation.

my background on the matter is merely second year QM and semiconductor phys.

Since we already have an electron, the creation of a photon obeys all conservation rules. Thus no other particles need to be created. This would be different if we were talking about something such as two photons annihilating and creating an electron-positron pair. The creation of those particles is necessary in order to conserve momentum and charge at minimum.

 

[tim9000]

Think of it this way. Do you know of any energy levels where the difference between them is on the order of micro, nano, or pico electron volts? That's the energy of each photon in the various radio frequency ranges.

The reason that hot objects radiate across a broad spectrum is because the creation of light does NOT required an electron to drop energy levels (Although that is one way to do it), it only requires that a charge, of any kind, be accelerated. The oscillation, vibrations, and other movements of atoms and molecules in a bulk material allow an infinite number of wavelengths to be created, with the temperature of the object determining the distribution of energy and thus the spectrum emitted.

Drakkith best post of the thread.