Re: DC and AC input's influence on the capacitor

From: John C. Polasek (jpolasek_at_cfl.rr.com)
Date: 03/04/05 

Date: Fri, 04 Mar 2005 15:10:50 -0500

On 4 Mar 2005 08:58:39 -0800, "PD" <pdraper@yahoo.com> wrote:

>jason wrote:
>> Hello All
>>
>> I wish to know what will happen for the case of 1)DC and 2)AC input
>> power or signal to the capacitor in a circuit.
>> I heard that capcitor can act as a short or open circuit depending on
>> DC or AC(in high or low frequency input power signal or signal
>itself).
>>
>> Can anyone help to explains this clearer in the point of view from
>> physics and electronics?
>>
>> Kindly share
>> Thank you
>>
>> rgds and thanks
>> Jason
>
>Sure. The impedance of a capacitor increases as it "fills up" with
>charge.
Not at all.
>That is, there is a certain amount of steady-state charge that
>can sit on a capacitor with a certain voltage on it: Q = C*V. If V
>changes suddenly,
The voltage on a capacitor cannot change suddenly.
>then the charge will run off one plate and run onto
>the other plate freely
How does it get to the other plate?
>-- the effective impedance is zero.
The impedance is only defined with a known AC frequency.
>As the
>plates fill up to quota allowed by the new voltage, there becomes no
>room for more charge on the plates, and the effective impedance becomes
>infinite.
It does no such thing.
>The higher the AC frequency, the more quickly the voltage is switching,
>and so the lower the effective impedance -- there is always room on the
>capacitor for charge to flow on the plates, because the voltage is
>swinging back and forth so rapidly.
You were able to find room on the plates?
>For DC, the voltage is steady, the capacitor fills up with charge and
>there is no room for any more, and the impedance is infinite.
NO NO NO.
>Note that for a DC circuit with a switch that is suddenly opened or
>closed, it's kind of state in between these two. The effective
>impedance changes over the course of the RC exponential curve.
What the heck have you been reading? You were all wet in sentence one,
and did not help yourself by extrapolating thereafter.
>PD
John Polasek

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