The amplitude of EM radiation

  • #1
Disconnected
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Aight, so I have this problem where I think about something and then I understand it, but then instead of stopping thinking and understanding it, I keep thinking and stop understanding it.
Unfortunately then my poor brain is so confruzed that I am unable to backtrack to understanding, and I have to come full circle and get back to understanding it. The good thing about this is that I generally have a better understand of why something is.

So my latest problem has come from EM radiation, and particular it's amplitude.

So AM radio alters the amplitude of the radiation, so it can be altered. I know that the ratio between the amplitude of the E and the B parts is c, but I have confused myself as to what the amplitude is, and how you would change it at the source.

I mean, an electron hops down an energy level and a photon is released. Great. Bigger distance hopped? Higher frequency. Perfect. So, what do you change to change the amplitude?

So, uh, yeah. I googled it, but alas no cigar.
 

Answers and Replies

  • #2
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Hi "Disconnected" :-)
what level of question is this, please?
If you want to know about how amplitude modulated signals are generated that's not so hard for me to answer - the voltage fed to the aerial is modulated by electronic circuits so that more or less energy is fed to the aerial. I can tell you as much as you want to know about that.
If on the other hand you want a quantum physics explanation of near-continuously varying electron levels I can only point to the general subject of lattice energies, about which I know nothing.
It may also be that you want to know about electric fields.
Can you clarify, please?
 
  • #3
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Hi,
I am not 100% sure. But amplitude may be related to the amount of photons. More flux more amplitude.
Let us wait for expert's answer.
cheers.
 
  • #4
Disconnected
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Hi "Disconnected" :-)
what level of question is this, please?
If you want to know about how amplitude modulated signals are generated that's not so hard for me to answer - the voltage fed to the aerial is modulated by electronic circuits so that more or less energy is fed to the aerial. I can tell you as much as you want to know about that.
If on the other hand you want a quantum physics explanation of near-continuously varying electron levels I can only point to the general subject of lattice energies, about which I know nothing.
It may also be that you want to know about electric fields.
Can you clarify, please?
Sorry for the ambiguity, I am considering a single photon, and what it's amplitude represents, and how it is altered at the source.

As far as the radio signal goes, I was just mentioning it as it's what made me start thinking about the amplitude of a photon. However, I am curious as to why altering the energy fed into the aerial would alter the amplitude of the photons (correspondence principle and all that).
 
  • #5
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Amplitude could be related to the probability of finding the photon at some point.
 
  • #6
sophiecentaur
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The energy of a photon just depends upon the frequency (E=hf). You can't have a high or low amplitude photon; it's a quantum of the energy. The Power of an EM signal is related to the Number of Photons emitted per second. You are trying to appreciate this problem by considering the Classical and the Quantum description at the same time. This is not really possible. All you can say is that the Amplitude of the Fields associated with an EM signal is related to the probability of a photon being available to interact with some receiving mechanism.
 
  • #7
Disconnected
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The energy of a photon just depends upon the frequency (E=hf). You can't have a high or low amplitude photon; it's a quantum of the energy. The Power of an EM signal is related to the Number of Photons emitted per second. You are trying to appreciate this problem by considering the Classical and the Quantum description at the same time. This is not really possible. All you can say is that the Amplitude of the Fields associated with an EM signal is related to the probability of a photon being available to interact with some receiving mechanism.
Cool. This makes sense.

So now I am going to ask about where the property of amplitude in EM waves comes from from a classical perspective. If more energy simply means higher frequency, then how can alteration in the energy in the aerial simply result in a changes of amplitude but not frequency? Also, how are the radio waves produced, by electron excitation/falling or Bremsstrahlung/something?
 
  • #8
sophiecentaur
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I think you need to forget about electrons in Hydrogen-like orbits. There are no electrons when EM waves are passing through space and, when em waves do interact with an antenna, it is with the whole antenna and not just one electron. The distribution of energy levels in such systems is more of a continuum than the discrete levels in an isolated atom.

If you really want to describe an AM signal in terms of quanta of em radiation then the density of photons arriving (each with its characteristic energy) changes as the amplitude of the RF signal changes from modulation peaks to modulation troughs. This is not helped by the fact that, when you modulate a signal, you produce sidebands with different frequencies to that of the carrier frequency - so you will have a mixture of quanta with Different Energies from that of the Carrier frequency. These quanta / photons will add together (in a way that is associated with the classical wave / fourier behaviour) to produce varying bursts of energy as the modulation 'goes up and down'.

I would seriously suggest that you get the classical description of rf / em signals, modulation and how they interact with antennae, sorted out to a reasonable level and also the way that quanta of em energy interact with macroscopic systems before trying to get the two ideas connected. Or just open another bottle of beer. :smile:
 
  • #9
Disconnected
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I think you need to forget about electrons in Hydrogen-like orbits. There are no electrons when EM waves are passing through space and, when em waves do interact with an antenna, it is with the whole antenna and not just one electron. The distribution of energy levels in such systems is more of a continuum than the discrete levels in an isolated atom.

If you really want to describe an AM signal in terms of quanta of em radiation then the density of photons arriving (each with its characteristic energy) changes as the amplitude of the RF signal changes from modulation peaks to modulation troughs. This is not helped by the fact that, when you modulate a signal, you produce sidebands with different frequencies to that of the carrier frequency - so you will have a mixture of quanta with Different Energies from that of the Carrier frequency. These quanta / photons will add together (in a way that is associated with the classical wave / fourier behaviour) to produce varying bursts of energy as the modulation 'goes up and down'.

I would seriously suggest that you get the classical description of rf / em signals, modulation and how they interact with antennae, sorted out to a reasonable level and also the way that quanta of em energy interact with macroscopic systems before trying to get the two ideas connected. Or just open another bottle of beer. :smile:
Well, this is 90% cleared up. The photon density is where I was at around 4 hours ago.

So, now I am just curious as to how radio towers work.
 
  • #10
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I suggest the ARRL Handbook, which provides quite accessible explanations.
 
  • #11
sophiecentaur
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  • #12
Disconnected
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Yes that's fine, unless he wants a QM explanation. Just a tad more difficult!!
Hah, unsurprising.
Thanks for the advice on the manual. I think this thought has taken up wayyyy too much of my study time already (the course barely involves EMR).
 
  • #13
sophiecentaur
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Yes. Get yer head down lad and do some exam preparation!
(Do as I say and not as I did! :cool:)
 
  • #14
Disconnected
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Yes. Get yer head down lad and do some exam preparation!
(Do as I say and not as I did! :cool:)
Hah! Yeah, somehow I always find myself becoming hugely interested in non-examable stuff days before exams...

But, yeah. I thought about it while walking home, and here's where I am (If I am wrong, please correct me). Please note that I am aware that I am giving this WAY TOO MUCH THOUGHT.

Aight, so just as sound waves can be described as sinusoidal waves with position on the x axis and air pressure on the y axis, EMR can be described in the same way but with photon/energy density on the y axis.

The frequencies of the wave function of the photon and of the wave are the same cuz where there is low probability of finding a photon, there will be low energy.

The concept of negative photon density is fooling with me a bit, but it's pretty PG-13.

Anyways, thanks again for the thought-guidance etc.
I should *probably* go learn some properties of matter now...
 
  • #15
sophiecentaur
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Why "negative photon density"?
A.M. just drops to zero amplitude during the troughs of modulation (zero energy).
 
  • #16
Disconnected
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Why "negative photon density"?
A.M. just drops to zero amplitude during the troughs of modulation (zero energy).
Yeah, that dawned on me right after I typed that.
 
  • #17
sophiecentaur
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It's the same with sound waves - the pressure is never less than zero.
 

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