All About Circuits Forum Capacitors on AC
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#1
05-15-2014, 07:51 PM
 tadm123 Junior Member Join Date: Nov 2013 Posts: 43
Capacitors on AC

I know the capacitor blocks DC and passes AC.
My question is in AC, The capacitor charges in the positive cycles and discharges in the negative cycles? Or the discharging begins from the peaks towards 0 and similarly in the other cycle from the negative peak to 0?﻿

This concept seems a little bit tedious to understand and I'm probably looking too much into it. Can anyone explain me the process in a simpler way? Thanks a lot.
#2
05-15-2014, 08:21 PM
 #12 Senior Member Join Date: Nov 2010 Location: 15 miles west of Tampa, Florida Posts: 9,048 Blog Entries: 9

Take one circuit at a time because mixing the two concepts makes it more complicated than it needs to be.

A capacitor can be charged with a DC voltage.
When you use it as a connection between an AC source and a load, the capacitor has an impedance related to frequency. If the impedance of the capacitor is small compared to the impedance of the load, very little voltage will be observed across the terminals of the capacitor. In a sense, the capacitor does NOT charge up in this AC circuit.

If the impedance of the capacitor at today's frequency is large compared to the load, the voltage across the capacitor terminals will be significant and it will be a simple sine wave if the load is a resistor. Whenever there is a voltage across a capacitor, you can say it is "charged" to some amount, but an AC circuit will have the charge continuously changing.

Continuous. The capacitor doesn't wait for a while to discharge. It is a passive component and it allows current to flow continuously, depending on the frequency, the load, and the wave shape you apply to it.
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#3
05-15-2014, 08:22 PM
 Veracohr Senior Member Join Date: Jan 2011 Posts: 240

Q = CV

Q = charge
C = capacitance
V = voltage

A capacitor with 0V across it has no charge, no stored energy. A capacitor with other than 0V across it has charge, has energy stored in an electric field, regardless of the polarity of the voltage.
#4
05-16-2014, 02:09 AM
 Brownout Senior Member Join Date: Jan 2012 Posts: 1,653

Quote:
 Originally Posted by tadm123 I know the capacitor blocks DC and passes AC. My question is in AC, The capacitor charges in the positive cycles and discharges in the negative cycles? Or the discharging begins from the peaks towards 0 and similarly in the other cycle from the negative peak to 0?﻿ This concept seems a little bit tedious to understand and I'm probably looking too much into it. Can anyone explain me the process in a simpler way? Thanks a lot.
Think of an hour glass. If you turn it on one side, sand will flow through it until one end is empty and the other end is filled. This is the DC equivalent, where charge flows into a capacitor until one end is full and the other is empty. Now, consider an hour glass which is continually turned over. Sand will flow as long as you keep flipping it. This is the equivalent to the AC case, where current flows continuously.
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#5
05-16-2014, 10:14 PM
 profbuxton Junior Member Join Date: Feb 2014 Posts: 27

Please note, I do not believe that current "flows through a capacitor". It is a "open circuit". When volts are applied a charge build up on the plates in a particular direction (depending on polarity of volts). When volts are reversed(as in AC case) charge is reversed in polarity. This occurs via the external circuit loop.
You can charge a cap with DC through a resistor and measure charging current. If you then switch the charging volts polarity cap will discharge and build up charge in other polarity. If you do this very rapidly and measure charging current it will appear as if cap is passing current. This is what happens in AC circuits.
 The Following User Says Thank You to profbuxton For This Useful Post: MaxHeadRoom (05-16-2014)

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