Ron J said:
Hey all,
I'm curious why an uncharged capacitor can be viewed as a short circuit
by, say, an amplifier? I thought an uncharged cap would be more like an
open circuit.
Thanks!
because even though there is an open circuit between the plates of a cap
there is a force across them. This force is felt on the other side when an
electron moves onto one place. In essence it pushes off an electron on the
other plate when it arrives... the same kinda stuff happens in a wire where
one electron feels the repulsive force of another... in this case though
there is a definite force but there is a "gap" that prevents the electron
from continuing.
Thing of blowing up a water rocket with water. It requires a certain force
to push the air in and initally is pretty easy to do... the more
air(electrons) you put in the harder it gets until eventually you cannot
push any more in(or it explodes) and the forces are counter balanced. The
same thing happens in a cap except in this case we can think that we have a
reservoir of air connected to the pump(so there is a limited amount of air)
and when we pull the air from the reservoir and push it into the rocket it
is likened to a current flowing(the air being analogous to the electrons)...
if you put the reservoir and the water rocket close to each other you can
envision that there is a "hypothetical" current flowing between them where
it seems that the air is flowing continously through(ofcourse we know it
isn't but it sorta looks like that from a distance).
i.e.
(need to view this with a fixed width font)
Pump
|
|
|-----| | |-------|
| Air ==|== Water |
|-----| |-------|
|------||-----|
=== Water || Air ====
// |------||-----| \\
|| ||
|| ||
\\ //
==========|==========
|
|
pump
So as we pump it first is very easy but gets harder and harder. Also we feel
as if the air leaves the Air chamber and enters the water and we might even
suspect that some air leaves the water chamber and goes across to the Air
chamber(maybe through a little valve betwen them). This is not what happens
but if we didn't not know they were not connected and we started to pump we
would see a air current flowing from one around the "circuit".... but we
would notice it would get harder and harder to pump. We could hold the air
that we pumped into the water chamber but as soon as we released the pump
all the air would rush back into the air chamber(until it equalized).
A capacitor is very similar except that it uses conductive
plates(reservoirs) that hold electrons. When the electrons arrive on one
side of the plate they "push" off electrons on the other side and it
"simulates" a current(even though there isn't any "physical" current. The
more electrons that arrive makes it harder and harder to push more
on(because the electrons start to repel each other(ones on the same plate)
too).
What you have to understand is that just because there is a "gap"(an open
circuit) in a capacitor that it doesn't stop electrons from flowing...
Electrons can easily flow through a gap(say a vaccumn) if they have enough
energy. Since capacitors are made of conducting material electrons move
freely on on or off its plates... if, say, one of the plates were an
insulator then it would not conduct electricity.
You can also think that a battery pulls an electron off one side one
plate(making it +) and puts it on the other plate(making it -).... this
causes a potential difference across the cap and the more electrons that the
batter does this two the stronger the ptotential difference is... if you
have N electrons that we could hypothesize that the potential would be
proportial to N(which it theoretically is)... but the battery can only push
so "hard" and eventually it's voltage would be "countered" by the voltage
across the cap.
Hope that helps some,
AD