This is how I mentally picture a capacitor:

When current goes into one leg it goes out of the other. When it does the=20
voltage over the dielectric also rises more and more until it prohibits any=
=20
more current flowing (causing the capacitor charge/discharge curve). No=20
electrons are actually moved from one side to the other though (electrostat=
ic=20
repelling forces of charges of equal polarity).
=20
You can compare a capacitor in an electric circuit to a barrel with a flexi=
ble=20
water tight membrane in the center (dividing the barrel in two parts) in a=
=20
water flowing circuit. When water pushes in on one side, the membrane flexe=
s at=20
the same time as water is pushed out on the other side. The membrane flexes=
 as=20
much as the water pressure allows. To high pressure and the membrane breake=
s=20
causing a short. It is also the membrane that holds the energy in the barre=
l,=20
allowing it to push back water when the pressure is lowered. Also, the more=
=20
flexed the membrane is, the more work is needed to flex it even more.

This model also makes it easier to understand why a high frequency can pass=
=20
easier through the capacitor than a low frequency (where the extreme is DC)=
..

/Ruben
=20

> Capacitors:
>=20
> I'm trying to (fully) understand the principle of capacitor reactance and
> high pass filters. I'm having trouble forming a mental picture of how a
> capacitor in series with a signal blocks DC and has the lowest impedance
> for the highest frequency signals on a voltage and charge-on-the-plate
> level. I can understand how a capacitor tied to ground acts as a buffer a=
nd
> smoothes out high frequencies by averaging the voltage. But I don't reall=
y get
> how the reverse is true for a capacitor in series with a signal. I can se=
e how
> it blocks DC after it is fully charged.
>=20
> 1. How does a capacitor in series work to block a DC signal, but pass a
> high frequency signal?
> 2. If you apply a voltage to a capacitor in series (keeping in mind that =
it
> takes time for the voltage from the power source to rise), I understand t=
hat the
> current before the capacitor is NON-zero. But what about the current AFTE=
R (on
> the other side) of the capacitor? Is it zero or non-zero (considering tha=
t a
> capacitor is just two parallel plates)?
>=20
>=20
> LC circuits:
>=20
> 1. Consider a parallel LC circuit where the bottom is tied to ground and
> the top is tied to a mixed-frequency signal source. I understand that the
> parallel LC circuit will shunt all frequencies to ground other than those
> around its resonant frequency. After the signal source is removed, the
> circuit will eventually lose its energy. How do I make an oscillator out =
of such
> an LC circuit that self-starts (that is, I can simulate in LTSPICE which =
doesn't
> take into account external noise coming into an oscillator circuit, which=
 is
> required for some oscillator designs)? I've looked at LC oscillator circu=
it
> types like the Hartley oscillator, but I don't see how this can self star=
t in
> theory under ideal conditions. -- http://www.piclist.com PIC/SX FAQ & lis=
t
> archive View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>=20
>=20
> -----
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>=20

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

--=20
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