Brian,
I really liked Nick Rouse's explanation for completeness. I'd like to
extract maybe a simplification of the "self inducing" part.
It was explained to me that E-M fields are sourced from charges that
accelerate. When charges are stationary or when they are moving with
constant velocity, they produce a static field. However, when charge
accelerates, there is a "kink" produced in its static field. This kink is
mathematically described as "displacement current" by Maxwell's (don't know
if he's any relation) equations.
The acceleration produces a time-varying "kink", which produces the
displacement current which then starts a time varying electric field and a
corresponding time varying magnetic field. I don't want to argue whether
the electric field starts the magnetic field or vice-versa because it's a
chicken and egg type of argument. We'll be here forever.
The fact that they propogate through a vacuum is a matter of boundary
conditions. Once the field is started, they are self sustaining because the
electric field supports the magnetic field and vice versa. The field will
propogate until it meets a material which will either absorb/reflect/cancel
the field (lossy dielectrics and conductors). At any rate, the fields won't
end until they meet another charge to accelerate or decelerate (the eddy
currents which cause losses in lossy dielectrics and conductors).
A good analogy would be to think of marbles. (Stay with me here, I haven't
lost mine). Let's consider all of the charge and energy in space to be
marbles and lets assume that they have no friction. A DC current can be
thought of a group of marbles moving with constant velocity. As long as
they're all moving together, we have no kinks. If we accelerate one of
them, we start off a chain reaction. Marbles start knocking into one
another (analogous to the propogating field). If one, or some of the
marbles get knocked off into space (field propogating through space) these
propogating marbles won't stop until they meet another group of marbles (a
material to either reflect or absorb the field). The collisions between
the propogating marbles would produce some marbles that bounce back
(reflection) or the propogating marbles may impact into another group and
transfer its energy to the marbles within the group (absorption).
>From simple minds, come simple explanations :-)
PS. if you want to apply this to EMC. Think of ground planes and short
conductors as methods to minimize charge acceleration and hence radiated
fields.
Chris Maxwell, Design Engineer
GN Nettest Optical Division
6 Rhoads Drive, Building 4
Utica, NY 13502
PH: 315-797-4449
FAX: 315-797-8024
EMAIL: chr...@gnlp.com
> -----Original Message-----
> From: brian_kunde [SMTP:brian_ku...@leco.com]
> Sent: Thursday, November 30, 2000 3:51 PM
> To: emc-pstc
> Subject: How does RF travel through outer space?
>
>
>
>
> Hello,
>
> I'm sorry if this is too simple of question... "How does RF travel through
> outer
> space?".
>
> I will be teaching a class in which this question will come up. I want to
> be
> prepared with all the basic science behind this principal. I need an
> explaination that is simple and easy to understand.
>
> People seem to have no problem understanding how waves can travel through
> mass
> such as a body of water but can not understand how it can travel where
> there is
> no mass. I also understand that there is a lot of debate over how Light
> travels
> through space (photons and all).
>
> Also, I understand that RF signals degrade at a rate of
> 1/distance(squared).
> What force is causing this attenuation?
>
> Try to keep it simple for my audience it not all that technical.
> Appreciate the
> help. Please forgive any improper punctuation or word misuse.
> Brian
>
>
>
>
>
>
>
>
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