On Wed, Mar 31, 2010 at 10:45 AM, Olin Lathrop
<olin_piclist@embedinc.com> wrote:
> Very briefly. =A0Voltages do not end at the ends of conductors. =A0A volt=
age
> field, called the E field permeates all space. =A0If you have two pieces =
of
> aluminum foil 1 meter apart and connect a 12V battery between them, the E
> field between will have a strength of 12V/meter. =A0Yes, it's really that
> simple.

This is the part I'm having trouble with. I understand that an
electric field is created which propagates at the speed of light. So,
is that propagating field considered the radio wave? Or is it the
propagating electromagnetic field created by an accelerating charge?
Also, if there is no closed circuit, how/why are the electrons
accelerating? Even if there is a potential difference of 12 V, there
is no conductor in between for them to move in?


> The not so simple part comes from when these voltages start changing. =A0=
If
> you were a few meters away and could sense what's left of the E field at
> that distance you would see it change a very short time after the voltage=
 on
> the plates changed. =A0That's because the E field propagates at the speed=
 of
> light. =A0So when you switch on the two plates, you've created a disturba=
nce
> in the E field that radiates out from your location at the speed of light.
> This effect drops off in magnitude quickly to the point where we can't
> measure it anymore, but it actually radiates out inifinitely and forever.=
 =A0A
> year after you did this and your aluminum foil pieces are long gone, the
> electric disturbance you created is still propagating outward at the surf=
ace
> of a sphere a light year in radius. =A0A little bit of the energy from th=
e 12V
> battery has essentially been transmitted into space.

Ok, I can understand how the field is still propagating at the speed
of light outward. But is that propagating field the radio wave?


> This thought experiment is only to show that any electric change not only
> happens in wires, but little bits of its energy leave and propagate away.
> Only a very very tiny fraction of the energy in the aluminum foil example
> was transmitted. =A0However, once you know this can happen you can get cl=
ever
> and design systems to do this deliberately, you can get a good fraction of
> the energy or your circuit to radiate. =A0That's exactly what a RF transm=
itter
> connected to a antenna is.
>
> To do this efficiently, we usually try to get the system to resonate at t=
he
> transmitting frequency. =A0Resonance is a whole other topic I don't have =
time
> for, but is usually important in RF transmitting and receiving systems.
> Again, very briefly, simple resonance requires the electric conductors to=
 be
> around 1/2 wavelength in size, or multiples of 1/2 wavelength. =A0At that
> size, the propagation delay from one end to the other is such that the wa=
ve
> sloshes back and forth. =A0That makes bigger voltages more easily, which =
makes
> bigger E fields, which means the E field disturbance can be detected at
> greater distance. =A0Radio is the art of getting all this right.
>
> I gotta go now. =A0Maybe more later.

Thank you for the explanation.

-- =

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