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another stupid capactior question

K

Ken Williams

Jan 1, 1970
0
I would like to know in common sense terms what the hell a capacitor
does. is this accurate?:

a flaky low voltage charge comes through a wire and hits a capacitor,
the capacitor will accept the charge and "charge up", when a certain
amount (specified by the farad rating) of charge is reached the
capacitor (slowly?) dissipates the charge in a more uniform and often
higher voltage.

so a capacitor can "clean up" a flaky or unpredictable line of
current/charge?

thanks.
 
B

Bob Myers

Jan 1, 1970
0
Ken Williams said:
I would like to know in common sense terms what the hell a capacitor does.
is this accurate?:

The simplest way to describe what a capacitor does is as follows:

A capacitor is a device which stores electrical energy in
the form of an electric field.

That's it; that's all there is to it. Any time you have an electric
field between two points/electrodes/whatever, it represents
a certain amount of stored energy, and the "quality" of the
space between those two points which enables this energy
storage is "capacitance."

For a more concrete visualization, imagine it this way. If
I have two flat conductive plates, separated by some distance
(the important thing being that the plates are NOT in contact,
and you can't get a conductive path between them - including
via an arc), then I can put opposite charges on the two plates
(one "+" and one "-"), and those charges will be held there
by the simple fact that opposite charges attract.

But how much charge can you put on the plates? If you remember
that *like* charges repel, you might also think that at some point,
there will be so much "like" charge on a given plate that you can't
force any more on there. Or, more correctly - it will take a little
bit MORE "force" to put this next bit of charge onto that plate than
it did to put the last one on. That "force" is voltage - in other words,
the more voltage (which can be viewed as "electrical pressure") I
have, the more charge I can force onto the plates before they won't
accept any more. The relationship between the amount of charge
you store and the voltage it takes to store that charge, for a given
physical situation, is the "capacitance" of that particular situation.
It is determined by the arrangement and size of the plates, the
properties of the physical material separating them, etc.. The
relationship between charge, capacitance, and voltage is given as

Q = CV or C = Q/V

where C is the capacitance (given in a unit called "farads"), V is
the voltage, and Q is the charge in coulombs.

Clearer?


Bob M.
 
J

JeffM

Jan 1, 1970
0
Ken said:
I would like to know in common sense terms
what the hell a capacitor does. is this accurate?:

a flaky low voltage charge
It doesn't need to be low voltage.
comes through a wire
It may also come thru a resistor or inductor or active device.
and hits a capacitor,
the capacitor will accept the charge and "charge up",
when a certain amount (specified by the farad rating)
of charge is reached
Electric charge is measured in coulombs.
It is one of the *fundamental* units of measurement.
Capacitance is measured in farads
which, as the equation Bob showed, includes a voltage variable.

What you described is an integrator.
http://www.google.com/search?q=define:integrator
An integrator includes a series resistor at its input
and is one of the many ways a capacitor can be used.
the capacitor (slowly?) dissipates the charge
in a more uniform and often higher voltage.
so a capacitor can "clean up" a flaky or unpredictable line of
current/charge?
You have described a power supply filter.
It is closely related to the integrator
and is another of the many ways a capacitor can be used.

A capacitor does NOT have gain.
You can't get more voltage out of it than you put in
(without adding more devices).

Feeding a tech term into Google thusly is usually useful:
http://www.google.com/search?q=define:capacitor

A good place to start learning about a device or phenomena is:
http://en.wikipedia.org/wiki/Capacitor
Notably, that article has a DC Circuits section
and an AC Circuits section.
 
D

Don Stauffer

Jan 1, 1970
0
Ken said:
I would like to know in common sense terms what the hell a capacitor
does. is this accurate?:

a flaky low voltage charge comes through a wire and hits a capacitor,
the capacitor will accept the charge and "charge up", when a certain
amount (specified by the farad rating) of charge is reached the
capacitor (slowly?) dissipates the charge in a more uniform and often
higher voltage.

so a capacitor can "clean up" a flaky or unpredictable line of
current/charge?

thanks.
One good way to understand how a capacitor works is to look up "Leyden
jars", which were the original capacitors. And they worked with high
voltage, not low. Charge itself does not have a voltage.

The capacitor will present a voltage when it is charged. The charge is
flowing in the wire because of an applied voltage. The flow will stop
when the charge on the capacitor equals the applied voltage. There is
then no potential difference to drive a current (flowing charge). The
basic equation is Q= C*V, so that tells you how much charge (Q) must be
stored in any given capacitor, C, to get a given voltage.
 
A

Alan

Jan 1, 1970
0
I would like to know in common sense terms what the hell a capacitor
does.   is this accurate?:

a flaky low voltage charge comes through a wire and hits a capacitor,
the capacitor will accept the charge and "charge up", when a certain
amount (specified by the farad rating) of charge is reached the
capacitor (slowly?) dissipates the charge in a more uniform and often
higher voltage.

so a capacitor can "clean up" a flaky or unpredictable line of
current/charge?

thanks.

If you look at a capacitor in terms of voltage and current in a
graphical way, set the current to vertical access momentarilly.
Assumming the capacitor is at zero charge, initially, when the switch
is closed the voltage is applied, but momentarilly short circuit
current flows through the capacitor. This current reduces exponetually
by the natural logarithm or 2 over time until the current aproches a
very small static value almost zero depending upon the capacitor. This
is how Capacitors behave in electric circuits.
 
C

Charles

Jan 1, 1970
0
Ken Williams said:
I would like to know in common sense terms what the hell a capacitor does.
is this accurate?:

a flaky low voltage charge comes through a wire and hits a capacitor, the
capacitor will accept the charge and "charge up", when a certain amount
(specified by the farad rating) of charge is reached the capacitor
(slowly?) dissipates the charge in a more uniform and often higher
voltage.

It is a rate of change issue. When the voltage is changing rapidly, the
capacitor says "No, you can't do that." When the voltage is steady, the
capacitor says "I could care less."
so a capacitor can "clean up" a flaky or unpredictable line of
current/charge?

Yes, a capacitor can smooth voltage change. Filter capacitors help diodes
change AC to steady DC.
 
Z

z

Jan 1, 1970
0
I would like to know in common sense terms what the hell a capacitor
does.   is this accurate?:

a flaky low voltage charge comes through a wire and hits a capacitor,
the capacitor will accept the charge and "charge up", when a certain
amount (specified by the farad rating) of charge is reached the
capacitor (slowly?) dissipates the charge in a more uniform and often
higher voltage.

so a capacitor can "clean up" a flaky or unpredictable line of
current/charge?

thanks.

the plumbing equivalent of a capacitor is a pipe with a rubber
diaphragm across the middle, blocking it. No net steady current could
flow through, obviously; but a short pulse of pressure (voltage) would
go through, as a little bump of current. If a pressure is applied to
it then current flow blocked, it will retain the pressure internally
until it is allowed to release it with a small movement of current.
The wider the pipe, the bigger the capacitance, and the more current
it will soak up until it reaches an equal back pressure.

That above describes a capacitor in series; I suspect you are talking
about a shunt capacitor, in parallel; the same analogy still holds. It
won't affect steady current flow, but short pulses of pressure or
random noise will have a path through the capacitor that bypasses the
circuit further downstream. Same idea as a muffler on a car exhaust.
 
B

Bob Myers

Jan 1, 1970
0
Anon said:
Doesn't a capacitor in series smooth out voltage spikes in the
amplitude of a sine wave, if not the frequency?

The short form answer to this is to note that a capacitor represents
a component whose impedance decreases with frequency.
"Spikes" and other such noise are high-frequency components,
so a capacitor which appears "across" such signals will decrease
their amplitudes more than they affect the low-frequency content
(which is what you're trying to preserve).

Note that that is not how "capacitor in series" would normally
be interpreted, though.

I appreciate that alternate formula. When pairing capacitors up in
series like this, I seem to recall the effect of doubling the voltage
rating? Scott

Yes, but don't push your luck with that. "Doubling the voltage
rating" assumes that the voltage across the pair is exactly
split between them, which is generally not the case. And it's
definitely not the case if the capacitors are of different values.

Bob M.
 
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