From: Ronald Wong (ronwong@inreach.com)
Date: Wed Feb 02 2005 - 00:27:24 PST
Message-Id: <l03102800be24872216ca@[209.209.14.107]> Date: Wed, 2 Feb 2005 00:27:24 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: RE: How is electromagnetic radiation generated?
A while ago, Mike Beluardi asked a question that went unanswered:
>...
>Does anyone have an internet source (or other source) that explains
>more specifically what acceleration of which charged particles
>creates the different parts of the spectrum?
***********************************
Regarding the Subject line, "How is electromagnetic radiation generated?":
An EM is created whenever a magnetic or electric field changes.
There are a number of ways to do this. One of the simplest is to take an
electrostatically charged rod and wave the charged end up and down.
This causes the charges to move in a simple harmonic fashion. The motion is
an accelerated one and, as Paul said earlier during your summer session, it
will produce an EM wave.
True, the EM wave created by this action will have next to no energy at all
and will be of the lowest frequency imaginable - that of your waving rod,
but it will be an EM wave nonetheless. It's amplitude would be half the
size of the arc of your swing, it's wavelength around 200 million meters in
size, if you waved the rod at 1.5 Hz, and it would move away from your
oscillating rod at almost 300,000,000 m/s.
Before the wiggling, there was a stationary electric field surrounding the
charged tip of the rod. The field looked like a large collection of long
pins stuck to a very small, spherical pin-cushion (the tip of the rod) or a
dandelion before it's seeds had been blown away. With the wiggling, the
electric field underwent change as the end of the electric lines of force
at the charges moved up and down with the moving charges.
Just like a wave will appear and travel down a long cord stretched between
two students when one of them moves their end of the string up and down, so
an EM wave will appear and travel down an electric field line when it's end
at the charge is moved up and down with the charge.
Maxwell's EM wave theory states that any changing electric field has a
changing magnetic field associated with it. The opposite is true as well.
The combination of these two changing fields moving together through space
and time is what constitutes an electromagnetic wave.
With that in mind, we should go back and add that waving the charged end of
the rod not only created a changing electric field but a changing magnetic
field as well.
At this point you can probably see that we could have produced an EM wave
by waving one end of a bar magnet up and down as well.
In general, an accelerated charge creates an EM wave that consists of two,
mutually-perpendicular, changing fields - one electric and the other
magnetic. It's speed in a vacuum is 299 792 458 m/s and it is less than
this when traveling through matter.
***********************************
Regarding "what acceleration of which charged particles creates the
different parts of the spectrum?":
It's important to keep in mind that the EM spectrum is continuous.
We break it down into a series of regions/"parts" based on the way
different parts of the EM spectrum interact with matter.
If you were to wave your charged rod at around a frequency of 10 giga
(billion) Hz, you would create an EM wave whose frequency was 10 giga Hz
and would find that the wave could boil water without "heating it" - in the
traditional sense (well, in a sense, you could but, as I mentioned earlier,
there's really very little energy created with the waving of your hand even
if you could wave your had this rapidly - which you can't of course). This
is because the waves radiating from the region of your vibrating rod would
have a frequency that we associate with microwave radiation.
At 10 tera (trillion) Hz, you would feel heat radiating from the region of
the vibrating rod because this is the frequency of infra-red EM waves.
From around 400 to 700 tera Hz, you would see visible light coming from the
end of your vibrating rod - the color depending on the frequency.
...and so on.
Notice that generating an EM wave doesn't entail a particular type of
charge. It could be positive or negative, electrons or protons, even
anti-electrons or anti-protons. It isn't a question of "which charged
particles...". The only requirement is that the accelerating particles have
a charge.
To create "the different parts of the spectrum", you only have to vibrate
whatever charge you have at a frequency commensurate with that associated
with the part of the spectrum you wish to create.
***********************************
Actually, you don't even need a vibrating/accelerating charge to produce an
EM wave.
If you place a voltage across a pair of parallel plates, you will create an
electric field between them. If you vary the voltage you'll vary the
strength of the electric field and thus create a changing electric field.
This, in turn, will create a changing magnetic field which, in turn, will
create a changing electric field which,... and so on - an EM wave. If the
voltage varied with a frequency of 10 tera Hz, you'd find radiant heat
streaming out between the two plates.
Actually, you don't even need a changing electric field. If you sat in the
proverbial elevator (made of non-conducting, non-magnetic material) that
was in a very large, static electric or magnetic field and you and the
elevator started oscillating up and down at 10 tera Hz, you'd suddenly find
yourself bathed in infra-red radiation (moving you up and down at 10 tera
Hz would be impossible but "theoretically"...).
Cheers!
ron
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