EM Waves: Understanding w/o Quantum Mechanics?

[kcodon]

A test charge some distance away, won't feel the effect of a given oscillation instantly - this is a reflection of the fact that EM waves have a finite speed.

Yes, photon flux is proportional to irradiance - so two charges sitting in points of space with equal irradiance will feel the same magnitude of force.

Ok let me reword my question. I am trying to determine whether the EM wave itself can be thought of as simply a ripple in the EM field of the initial source. This would explain why a lesser force is felt at a distance (EM field is weaker i.e. 1/d^2 and EM radiation is less probable/weaker by 1/r^2...they are equivalent), and seems a much simpler view of things. Or is it that the EM radiation is separate from the field itself. This is why I asked the following:

However when you say it will cause a test charge to feel a force, I take it this is not due to the actual alternating electric field around the dipole is it? I mean at some distance close to the rotating dipole, the test charge will be in its electric field, and will thus feel force not due to the EM. Does this mean that when the test charge is far away, outside of the influence of the electric field from the rotating dipole, that it will feel a force from the EM radiation?

In other words, is the EM radiation a separate field, or the same field as that of the source (changing as the sources does). If so what evidence for this is there? Do you understand my question? Sorry I'm just struggling to put it into words, even though in my head the concept is so simple

Yes, photon flux is proportional to irradiance - so two charges sitting in points of space with equal irradiance will feel the same magnitude of force.

So this implies that the force felt by one photon (i.e. same irradiance) will get weaker as radius gets bigger? Kind of goes with what I was saying above.

Photons do not have to be absorbed to exert a force, for example a wave transmitted through a transparent medium, will cause that medium to polarise momentarily, thus the medium experiences a force, even though the photon continues on its merry way.

Ok this is interesting. Somehow the photon gives up energy to cause the force, yet travels away with the same energy? Or in the case of the transparent medium, is this to do with the resonance in the phonons etc, so energy is still conserved?

An oscillating field would not be caused by something like this, the field would not be sinusoidal, it would be some other shape (Gaussian maybe). Using Fourier analysis, we can decompose any waveform into its spectrum, so while the total field may not strictly be oscillatory, we can describe any field as a sum of oscillatory components.

Ok I looked up Fourier analysis and it looks a bit over my head...but are you saying that the shape of the field (whatever it may be) of a linearly accelerating partice, can be represented as the sum of other fields that area sinusoidal? That seems like cheating almost, if you know what I mean. Mind you how else you manage to physically get a sinusoidally oscillating EM field from a linearly accelerating charge I have no idea...if anyone else does it would be much appreciated if you shared it.

Hmmm I have just read some stuff from other posts, and it looks pretty confusing as to why accelerating charges emit radiation:

https://www.physicsforums.com/showthread.php?t=183074"

(I like the idea stated here by Andrew Mason that charges can only be accelerated by an EM force etc)

And then also it gets confusing with relativity and gravity etc:

https://www.physicsforums.com/showthread.php?t=160533"

No! Any inertial frame of reference will "see" the charge being accelerated.

I said uniform velocity relative to the (accelerating) charge...so you are accelerating also...not an inertial frame.

Polarisation is not complicated, it is just the direction the electric field pushes the hypothetical test charge. A rotating polarisation will cause the test charge to move in a circular (or corkscrew) motion.

Thanks for the heads up...I looked into polarisation and you're right it wasn't too challenging

I was considering starting a new thread about accelerating charges and radiation, however it doesn't actually look like anyone knows, so I think I'll leave it. Thinking about it you can kind of intuitively see how the strength of the electric field changes...it "builds up" in the direction of acceleration (I'm just guessing here), but not how a sinusoid is generated.

Thanks again for your help Claude,

Kcodon

 

[Claude Bile]

An EM wave is part of the EM field, yes.

Regarding the force applied by a photon - The quantity "Photon flux" is in units of photons per second per unit area. If two charges experience the same Photon flux, they feel the same amount of photons on average over time. Obviously there will be an element of Quantum randomness here - note that the relationship between Photon flux and Irradiance is a classical relation.

If we want to look at the effect of single photons, then we need to move into the QM regime. In this case, photons have an energy that is dependent on frequency, and a momentum that is dependent on wavelength.

With regard to photons in a medium - When the photon "uses" energy to polarise a medium, where does the energy go? Back into the EM field! (The energy you use to pull apart two opposite charges goes into the EM field that is generated as a result) While we can (and often do) decompose the total field external field and the internal response field, the distinction is irrelevant from the photons point of view. What matters is the presence of an EM field oscillating at some frequency, and that is preserved in this case.

Onto EM radiation caused by accelerating charges - I agree that it is highly confusing! It's one of those topics I try to veer away from, as my own experience in sorely lacking - the only case I am reasonably confident in discussing is the case of an oscillating dipole, and only then from a phenomenological perspective.

I said uniform velocity relative to the (accelerating) charge...so you are accelerating also...not an inertial frame.

My apologies, I must have misread something - Once your reference frame is non-inertial, then you need to invoke GR (and all bets are off as far as I'm concerned, GR is another one of those topics I know little about).

Claude.

 

[Riogho]

Can you suggest any books (other than the Feynman Lectures) which might help me do that?

Read Maxwell's papers.

Or, better yet, read some Faraday, and his wonderful lines of force :D



"An EM wave is part of the EM field, yes."
by Claude

To make things nice and simple for this guy, try to perceive the EM field radiating from the source outward in all directions in a 2D plane.
Of course this field extends forever and it gets weaker at the inverse square of the distance. However, the strength of the field (the density of the photons if you will) can change depending on the location of the source.

A simple experiment you can do to try this is get a magnet, and some iron fillings. At a certain distance there is a point where the magnetic attraction of the magnet is strong enough to attract the iron to one of it's poles. Once you find the distance needed to start attracting them you can move the magnet there quickly. If you notice it takes time for the filings to sense that the magnet is there. That is because the strength of the field is changing due to the position of the source, there are more little info photons moving to the iron to say, Hey, There is a magnet there, and it is exerting a force on you. Thus, the iron responds and attracts itself to the magnet.

 

[kcodon]

An EM wave is part of the EM field, yes.

Ok I feel so amazingly silly at the moment...yet so enlightened! These few words have basically revolutionized my view of EM radiation and it seems so much simpler now - thank God! Thanks so much Claude for persisting!

With regard to photons in a medium - When the photon "uses" energy to polarise a medium, where does the energy go? Back into the EM field!

Touche! Oh this is a good day!

Haha yup I think I will steer clear of linearly accelerating charges, for the meantime anyways. For now I suppose I move on to the next question...back to the topic of the thread to some degree...what is an electric "field". I can imagine there are mathematical descriptions, but I don't think they stem from a physical description...more so they create a variety of interpretations. I think this then leads to the "what is charge" question, and into a whole lot of metaphysics that will likely get messy...especially for a laymen like me, without the math skill to understand the mathematical descriptions.

To Riogho

If you notice it takes time for the filings to sense that the magnet is there.

Somehow I get the feeling that the speed of light would make this a somewhat miniscule time However I appreciate the description of the EM field.

Thanks again to all,

Kcodon