Electric and magnetic fields of an electromagnetic wave

  • Thread starter bdkeenan00
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  • #1
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Main Question or Discussion Point

I have simple question here that I've been wondering about for sometime now, and here it is. In classical physics when an charge is accelerated it creates an oscillating electric field which creates an oscillating magnetic field, a electromagnetic wave, and each field carries energy with it. So my question was does that energy come from the charge/electron? Does the electron give up some of it's kinetic energy? If so why would the electron loss energy if were to speed up?
 

Answers and Replies

  • #2
A preliminary question:

Why is an electron accelerating? What is the energy shift associated with that acceleration?
 
  • #3
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Let's say the source of acceleration was an electromotive force that accelerated the electron, so classically it would emit an electromagnetic wave. Where did the energy of the wave come from in this situation?
 
  • #4
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Erm... the point to ponder is that acceleration doesn't mean "speeded up". It means a change in the velocity vector. If it was accelerated linearly then what you say is true - you basically have inefficient addition of energy, and the energy is taken from the KE.

The really interesting stuff of this is when the speed doesn't change, but the direction does, such as when in a magnetic field the electron moves in a circle. This phenomenon then explains how we see the electron "spiral inwards".
 
  • #5
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sorry to say "speed up" what I really meant was any change in it's velocity
 
  • #6
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The whole point I gather is that it may not be an electron that is accelerating at all. New research into Oliver Heaviside's theories , seem to prove that it is photons that are accelarated in a current and not electrons.
 
  • #7
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The fields around a charged par-
ticle at rest are static. No EM
waves are created untill the
charged particle is put into
motion one way or another.

The answer to your question, if
I understand what you're asking
correctly, is that the energy in
the EM waves coming from a charged
particle in motion, is coming
from whatever force is putting the
particle in motion. The particle
is acting as a transducer, chang-
ing one form of energy into another. The particle itself is
not suffering any net loss of
mass or energy.

-zoob
 
  • #8
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Thank you everyone! I'm starting to get the idea now(I think), but what if we talked about the quantum description of electromagnetic radiation? Photons are emitted in a atom when an electron drops from higher energy level to a lower energy level. In that case the electromagnetic radiation(photon) emitted was from the electrons energy ,because the emitted photon's energy was equal to the energy transitions that the electron made.
 
  • #9
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My understanding of this situation
is that the electron won't be in
that higher orbit to begin with
unless extra energy has been pump-
ed into the atom forcing the elec- tron into the higher orbit. It drops immediately back to a position that will bring the atom into equilibrum, releasing the photon. So here again, the elect-
ron has not lost anything that
wasn't artificially added to
it.

-zoob
 
  • #10
Ivan Seeking
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Science Advisor
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Originally posted by zoobyshoe
The particle is acting as a transducer, chang-
ing one form of energy into another. The particle itself is
not suffering any net loss of
mass or energy.

-zoob
This is a nice analogy; acting as a transducer.
 
  • #11
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Originally posted by zoobyshoe
My understanding of this situation
is that the electron won't be in
that higher orbit to begin with
unless extra energy has been pump-
ed into the atom forcing the elec- tron into the higher orbit. It drops immediately back to a position that will bring the atom into equilibrum, releasing the photon. So here again, the elect-
ron has not lost anything that
wasn't artificially added to
it.

-zoob
I see what you mean but my point was for a short time that electron had more energy and it gave it up by emitting radiation when it decayed back to the lower energy level.
 
  • #12
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1,277
As far as my understanding of the
subject goes, your last statement
is correct.

-zoob
 
  • #13
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1,277
Originally posted by Ivan Seeking
This is a nice analogy; acting as a transducer.
Thank you.
 

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