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Response of a Capacitor to AC source

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beenuseren

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Well I have a basic doubt.

If we consider the instantaneous voltage across a capacitor, it keeps increasing to a point after which the voltage keeps decreasing..

First question: Does decreasing of voltage across the capacitor meant it started sourcing the current?????

Second question: If we compare the source voltage and the capacitor voltage, capacitor voltage starts decreasing even when the source voltage is still higher than the capacitor voltage. How is it possible the higher source voltage allowed lower capacitor voltage to source the current?

Please refer to the waveform I have attached..
 

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Well I have a basic doubt.

If we consider the instantaneous voltage across a capacitor, it keeps increasing to a point after which the voltage keeps decreasing..
Depends on the circuit.

First question: Does decreasing of voltage across the capacitor meant it started sourcing the current?????

Yes.

Second question: If we compare the source voltage and the capacitor voltage, capacitor voltage starts decreasing even when the source voltage is still higher than the capacitor voltage. How is it possible the higher source voltage allowed lower capacitor voltage to source the current?
That's what happens in another universe where the cap voltage
leads the source voltage with an RC circuit, or simply when we
draw the cap voltage incorrectly as leading instead of lagging :)
If the cap voltage was drawn correctly as lagging it would make
more sense. Try that next and see what happens.
 
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Thanks MRAI,

What you said is right. The capacitor voltage lags the supply voltage.

Another doubt, the same wave forms hold good for a RL circuit.
My previous questions apply for this too.
How come the inductor voltage starts reducing, when the supply voltage is increasing and above the inductor voltage???

Please check the waveform I have attached...
 

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  • 1.jpg
    1.jpg
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There will always be a source resistance in series with the source voltage, no matter how small. If an ideal voltage source (no series resistance) is suddenly placed accross a capacitor, an infinite current will flow, since Ic = dVc/dt, and dt = 0. This cannot occur, although some very high currents could be induced, as would be the case with a 1.2 V submarine battery that can vaporise a spanner (almost zero source resistance). So, the voltage on the capacitor can only be different to the source voltage if there is some resistance.
 
Reply to myselfe

I'me thinking of a step response, not a sinusoidal response. Doh! With a sinusoidal voltage source, the resistance is the impedance which is 1/(2(pi)fC). I havn't realy answered your question.
 
Thanks MRAI,

What you said is right. The capacitor voltage lags the supply voltage.

Another doubt, the same wave forms hold good for a RL circuit.
My previous questions apply for this too.
How come the inductor voltage starts reducing, when the supply voltage is increasing and above the inductor voltage???

Please check the waveform I have attached...

Hi again,


That's a good question really, as the inductor voltage would lead
which looks a little strange at first, but only if you think of the
inductor as completely 'passive'.

The inductor is considered passive in some types calculations,
but for this kind of question the inductor has to be thought of as
an energy storage element, which is capable of delivering energy
as well as receiving it. It's when the inductor is delivering energy
that we see that it is definitely not passive, but active in that
it can supply current and voltage to a circuit just like a battery
would, at least once it has some energy stored in it.
The cap does this too, only it expels its energy in the form of
a current, while the inductor does so in the form of a voltage
most basically, as when it is trying to maintain a constant
current it forces the voltage to some value to satisfy this
condition. We could also look at the cap current and ask
a few questions too.

To best understand the inductor over time we have to look at
this little equation:

v=L*di/dt

where

v is the voltage across the inductor
L is the inductance value
di is the change in current over time increment dt
dt is the change in time.

Note one thing here: that the voltage (which the waveform shows)
depends not on the supply excitation voltage, but the value of the
inductance L and the rate of change of the current di/dt.
Thus, it is the rate of change of current that determines the voltage,
not the supply voltage by itself.

What this means is that if you draw the current wave and then look
at the inductor voltage wave and think about the derivative of the
current wave you will see the inductor voltage follow this relationship.
Where the peaks of the current are flat the derivative is zero, so
the voltage will pass through zero at those points, and where the
current wave is changing the fastest, the current will reach its peaks.

Another way is to simply plot the derivative of the current, multiply
by the inductance L, and there you have the exact voltage wave.

Oh yeah, BTW, it's unfortunate that some fonts render an upper case
"i" as l as well as lower case "L", but with a better font this is my
screen name:

Code:
MrAl

or in all upper case:
Code:
MRAL

This is a problem that comes up now and then.
 
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