Discussion in 'Physics' started by davebee, Dec 28, 2012.

1. ### davebee Thread Starter Well-Known Member

Oct 22, 2008
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I gave it my best answer, but I still don't understand the radiation part very well.

Can anyone explain how a dipole radiates EM radiation? In the dipole, charges slosh back and forth like a swinging pendulum. During that charge oscillation, energy is exchanged between the electric and magnetic near fields. That much I feel pretty clear about.

But how does the phase of the radiated energy relate to the phase of the moving charges? For example, does the peak of the radiated energy phase correspond to the point where the electric and magnetic fields are equally strong (one growing and the other decaying)? That would be my guess, but I can't back it up. And if so, why is that?

I could guess that if the electric and magnetic fields are sinusoidal in strength over time, the point that they are equally strong would correspond to the point at which they both are changing most rapidly; would that be related to emission of radiation? That would argue that EM radiation comes from the fields themselves, the changing fields, not the charges. Is there any sense to that?

2. ### Wendy Moderator

Mar 24, 2008
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2,536
They must be 90° apart, if they aren't it isn't EM. It is part of the coherence that is light, on an individual photon level. It is also the basis for Polaroid lenses, light also being EM radiation. It is a fundamental part of what makes a photon, a core definition if you will.

My personal visualization of a photon is where the two charges meet and combine in specific way, creating something that resembles a particle. The synergy between the two also means they do not disperse, and a wave only would. Just as with electrons, it is possible to have a single photon, it is a point particle of energy. A photon has a specific polarization too.

3. ### davebee Thread Starter Well-Known Member

Oct 22, 2008
539
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I was more wondering about the phase in time of the EM energy in relation to the motion of the charges themselves and the local E and M fields, and what specifically makes an electromagnetic wave appear.

Like, can we point a finger to one particular mechanism and say that is what generates the EM wave? Accelerating charges? Changing E or M fields?

4. ### Wendy Moderator

Mar 24, 2008
20,765
2,536
It is all of that, a antenna causes certain conditions to be met, it has both magnet and electric fields that couple in a specific manner, part of which is the polarization.

5. ### vortmax Member

Oct 10, 2012
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I believe two charges oscillating is the most fundamental starting point without hitting quantum electrodynamics which then calls into question the nature of 'charges'.

It is important to remember that magnetic fields are just a special case of the electric field. They are inherently linked to one another. Things like coherency in phase and polarization are simple extensions of the geometry of the problem. For instance in a dipole with a single degree of freedom, the acceleration vector for the charges will be limited to a line.

I think this is one of those cases where suffering through the calculus brings better understanding.

6. ### davebee Thread Starter Well-Known Member

Oct 22, 2008
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I remember reading about magnetic fields being related to electric fields through relativity, but the funny thing is that whenever I've studied electronics, that relation always seems to be ignored. I'd be curious why you suggest it's important in this case.

I'm not very mathematical; explanations that consist only of a page of complex formulas somehow seems unsatisfying. I'd really like to try to see an explanation in plain English, if possible.

I found something called the Larmor formula which seems to answer the question, in part, at least:

http://en.wikipedia.org/wiki/Larmor_formula

It states that accelerating charges radiate. As simple as that. I like it!

The page includes a "simple" derivation, and also points to the Liénard–Wiechert potential

http://en.wikipedia.org/wiki/Liénard–Wiechert_potential

for a fuller derivation.

Liénard–Wiechert looks like it's not for the faint of heart, I'd say, but on the other hand, it does look like it is an essential part of the answer to the question I was asking, "How does a dipole radiate."

I'll have to mull it over and try to make sense of it.

7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,782
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http://en.wikipedia.org/wiki/Electromagnetic_tensor

another avenue to explore on this quest. The ties to Maxwell's equation are shown and a link is provided as well.

(I understand the not liking a page full of mathematics as an explanation, but the words are not in existence to 'describe' some of these processes. Only a mathematic description can be presented)