AC flowing through a cap. What actually happens?

Discussion in 'General Electronics Chat' started by qtommer, Feb 25, 2011.

1. qtommerMember

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we all know DC gets blocked and AC passed when going through a capacitor. Assuming an AC signal propagating through a cap, can anyone explain specifically the flow of electrons acting on the capacitor which hence allows the signal to be passed through? just curious all this while. Couldn't find any book/site which specifically explains why this happens.

Thanks=)

2. ydewitNew Member

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Hello there,

That was an interesting read and well worth it i think, so i guess next time i go to analyze my little RC circuit i instead of the simple 1-e^-t/RC i am now going to have to analyze the complete electric field inside the capacitor in order to get to the solution...boy that sounds like fun doesnt it?

Everything in life is an abstraction of reality. There are some things that are considered to be closer to reality than others, but that doesnt mean that the things farther from the truth can not be very very useful in everyday life, and when we go to prove what the truth really is we often find that it's extremely difficult or even impossible to prove anyway!

Would you say this:
"I went to the store to buy some milk and eggs".

or would you rather say this:
"I went to the door of my house, turned the door handle, opened the door, stepped out, closed the door, walked to the car, opened the car door, got in the car, put the key in the ignition as i closed the door, turned the key to start the car, put the car in gear, drove down the street to the store, went in the store, bought eggs and milk, got back in the car, drove home, went back in the door, closed the door."

Both are abstractions but one is much simpler than the other.

But i dont want to bore you with long overly descriptive sentences, and i dont have to because nobody said it better than Hayt and Kemmerly in the FIRST chapter of an engineering circuit analysis text book of which i have attached a brief but informative excerpt. To see the other side of the coin read these two wonderful paragraphs that puts it like it really is.
Note that this is one of the first things mentioned in the book.

(Legal note: this is ok to post because it falls within the fair use act).

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4. DaveNew Member

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I have it figured out. The attached explains everything.

Ron

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6. ydewitNew Member

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LOL... indeed the best ever explanation I ever read!

7. RatchitWell-Known Member

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qtommer,

A capacitor is a specific electrical energy storage device. It stores energy by separating charge, which in turn causes an electric field to form. Since all electric fields are a form of energy. It is wrong to say that a capacitor is "charged". You should instead say the capacitor is energized when the charge on the two plates are unbalanced, and a voltage appears across the capacitor. Current exists in the series circuit that contains the capacitor, but no charge goes through a capacitor unless there is leakage. The charge accumulates on one of the plates and depletes equally on the other to make a net charge of zero. Again, the capacitor does not charge, it instead energizes. When the voltage appears due to the charge accumulation and depletion of the plates, it opposes the energizing voltage. When the energizing voltage equals the capacitor voltage, no more charge flows in the capacitor circuit. That is why the charge flows temporarily in a capacitor circuit when a DC voltage is applied, and later slows up and finally stops when the opposing voltage equals it. Therefore, an AC voltage can cause a current to exist back and forth in a capacitor indefinitely, while a DC voltage is stopped after the capacitor is energized to its applied voltage.

Ratch

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Good luck with that!

I think your sig line says it all:

pe·dan·tic

1.
ostentatious in one's learning.
2.
overly concerned with minute details or formalisms, especially in teaching.

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This is close to the worst explanation of a capacitors operation I have ever seen.!

You state that a capacitor is energised, yet you explain the operation of the circuit using charge to do so.???

10. DaktaNew Member

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awesome diagram!

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Hello,

The point that gets missed here though is that we usually prefer to use the simplest most straightforward abstraction, not the most real.

12. Mr RBWell-Known Member

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I was taught as a simple understanding that when you charge a cap current flows through it one way, then you discharge the cap current flows the reverse way.

It you attach it to AC it is constantly charging and discharging, and constantly allowing that AC current through it both ways.

And an AC signal (like audio) passes through the cap (as current) for exactly that reason.

13. RatchitWell-Known Member

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ericgibbs,

What is wrong with the explanation? Especially when it is true. It takes energy to change the charge density of the two capacitor plates. The net charge does not change, the net energy does.

Ratch

en·er·gize (enÆÃr jzÅ), v., -gized, -giz·ing.
v.t.
1. to give energy to; rouse into activity: to energize the spirit with brave words.
2. to supply electrical current to or store electrical energy in.
v.i.
3. to be in operation; put forth energy.
Also, esp. Brit., enÆer·giseÅ.

14. RatchitWell-Known Member

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MikeML,

We all have our faults and proclivities. At least I announce one of mine, and give you fair warning.

Ratch

15. RatchitWell-Known Member

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MrAl,

I believe the point I was trying to make was that the descriptive word used was a misnomer.

Ratch

16. RatchitWell-Known Member

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MrRB,

You were taught a simplified explanation which is not quite true. Charge does not flow through the capacitor unless leakage is present. But current does exist in the circuit where the capacitor is present due to the accumulation and depletion of the charge upon the plates of the capacitor. So a rising and falling voltage causes the capacitor to energize and de-energize.

Ratch

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Hello again,

Well that's funny because you are arguing that "charge" is not as good a word as "energize" yet "charge" is less ambiguous than "energize" is.
We can "energize" a component but that says nothing about whether or not it will hold that energy or dissipate that energy. So which is it?
Are you going to try to stop me from "energizing" my light bulb?
Charge a capacitor and then disconnect it. Is it still energized?

In each of these tell me what i am really doing...

I am going to charge my car battery. (pretty clear)
I am going to energize my battery. (am i going to electrically energize it or mechanically energize it, or chemically energize it?)

It just so happens that i have a black box with two wires coming out. I am going to energize it. (did i make it do something like move, or did i store energy in it?)
It just so happens that i have another black box with two wires. I am going to charge it. (did i make it move or did i store something in it?)

Another black box known to be an electrical component with two wires. I energize it. Later, i connect the two wires to a light bulb. Does the light bulb light up or not?
Another black box known to be an electrical component with two wires. I charge it, or even "i charge it up". Later, i connect it to a light bulb. Does the bulb light up?

When we say "charge" we know pretty much what is happening. When we say "energize" we cant be quite as sure.

Let us now "charge" a capacitor. Current flows THROUGH the capacitor. In the integral, we show that something accumulates (sums). We then disconnect the capacitor and connect it to a resistor. What flows out of capacitor? We can say energy, we can say current, or we can say charge. If we say "energy" we dont know how much current is flowing. If we say "current" we know right away what is happening. If we say "charge" we really mean to say "moving charge" but we drop the 'moving' for short.

The capacitor stores energy in it's field, yet we say it is charged for short. The inductor stores energy in it's field, but we dont refer to that many times and we also call that charged too.

Agreed, or disagree with any of this?

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18. RatchitWell-Known Member

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MrAl,

By "not as good", I hope you mean that "charge" is not as descriptive as "energize" when referring to a voltage across a capacitor. I don't see how energize is ambigous.

The ability to store or dissipate energy is irrelevant to the word energize.

Certainly not. Turning on a light bulb is certainly energizing it.

Certainly, as long as it has a voltage across it.

OK.

Clear because it has become an accepted description that you mean you are going to electrically energize it. Not really true, however, because it does not contain any more charge after energizing it that it did before. But, it does contain more energy.

From knowing what it is and the context, most folks would assume you mean electrically energize.

Could be either one or both. If you said "charge", what would that tell anyone?

Who knows?

It depends on the component, the state of the component, and the circuit.

It depends on the component, the state of the component, and the circuit.

I think both words are precise when used in the proper context. I aver that "charge" is often not used correctly. Often "energize" is the better word to use.

Negative. Current does not exist through a capacitor, and charge does not flow through a capacitor, unless there is leakage. Your next statement says as much.

That is correct. Charge accumulates on one plate and depletes equally on the other plate. If the charge passed through the capacitor, then it would not accumulate or deplete. So no charge would separate, no electric field would form, and no energy would be stored. That proves that charge does not go through a capacitor.

You asked what is flowing. Charge would be correct, and flowing means moving charge. Current is also correct, because moving charge defines current. Energy is also correct because energy changes when charge moves through a resistance.

Yes, the electric field does store energy, but the descriptive term is false for this phenomena. The net charge does not change, but the energy certainly does.

I have never heard of a coil being called charged, but if so, that would not be true either. Coils can store energy.

Some of the above I do agree with and some I don't

Ratch

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How did I know this thread would take this route? Oh wait, possibly the transistor thread from hell of maybe a year ago? I thought it would never die. This is like Deja-Vue all over again.

Ron

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Quotes from Ratchit

MrAl,

By "not as good", I hope you mean that "charge" is not as descriptive as "energize" when referring to a voltage across a capacitor. I don't see how energize is ambigous.
>I thought i made that clear with the light bulb. The light bulb being energized does not charge up, while the capacitor does. Thus, if we say energized it doesnt seem as clear.

The ability to store or dissipate energy is irrelevant to the word energize.
>Yes, that's what i am saying. That's why it is ambiguous. With 'charge' we know right away.

Certainly not. Turning on a light bulb is certainly energizing it.
>But it's not charging so i couldnt say charge. It's not going to be holding anything, while the cap does hold something.

Clear because it has become an accepted description that you mean you are going to electrically energize it. Not really true, however, because it does not contain any more charge after energizing it that it did before. But, it does contain more energy.
>So you dont accept the term 'charge' that is known by the world over. Remember this for later.

From knowing what it is and the context, most folks would assume you mean electrically energize.
>Most folks? So now we can go with the normal human assumptions but for the word 'charge' we can not? What if i pour acid into the battery?

Could be either one or both. If you said "charge", what would that tell anyone?
>Charge implies that something is being stored. Energize does not make that distinction. If i say that i energized a circuit you wont know if it is storing any energy. If i say i charged a certain circuit you know something is being stored.

It depends on the component, the state of the component, and the circuit.
>For 'energize' yes, we dont know.

It depends on the component, the state of the component, and the circuit.
>For 'charge' however we know right away.

I think both words are precise when used in the proper context. I aver that "charge" is often not used correctly. Often "energize" is the better word to use.
>Ok if you want to say that that's up to you. Im not saying that it is totally wrong either, just that 'charge' is not only accepted, it is descriptive if you look at it as charge being stored and then dumped later. It acts like it stores charge so we say 'charge'. There are a few different views on what really happens too including virtual particles. Im not sure i want to get into that one though

Negative. Current does not exist through a capacitor, and charge does not flow through a capacitor, unless there is leakage. Your next statement says as much.
>Well, depending on the view i guess. External measurements would say otherwise too and the external measurements are important in analyzing circuits. If we connect a constant current generator up to a capacitor we couldnt have a complete circuit unless there was the closure of the path so it would not start to store energy if there was not a complete circuit there. It's hard to say that no current flows 'through' the capacitor. Who cares really? If it goes in one side and appears to come out the other side and always works that way we're all set.

That is correct. Charge accumulates on one plate and depletes equally on the other plate. If the charge passed through the capacitor, then it would not accumulate or deplete. So no charge would separate, no electric field would form, and no energy would be stored. That proves that charge does not go through a capacitor.
>That view seems fine. But it's just that view.

You asked what is flowing. Charge would be correct, and flowing means moving charge. Current is also correct, because moving charge defines current. Energy is also correct because energy changes when charge moves through a resistance.
>Yeah but when we think of energy we think of E=1/2 C*V^2, and although that is very descriptive of the energy it doesnt tell us what is happening an infinitesimal distance from one of the plates.

Yes, the electric field does store energy, but the descriptive term is false for this phenomena. The net charge does not change, but the energy certainly does.
>Ok.

I have never heard of a coil being called charged, but if so, that would not be true either. Coils can store energy.
>Yes inductors are also said to be 'charged' quite commonly. We dont have to talk about the field though when we talk about inductors until we get to the finer points of the inductor.

Some of the above I do agree with and some I don't
>Ok that's cool. I think i may have deviated a little from the main point of the thread anyway

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Ron,

Well it appears that we have another interesting discussion. I happen to think it is fairly interesting really.
Next topic open for discussion? Here's one...

A capacitor blocks DC current. True or false?
Again we usually fall back to the more common understanding of what DC current is, and most of us would blurt out a resounding, "TRUE".

But then one day we are looking at some theoretical circuit and happen to connect a DC current source up to a capacitor, and guess what happens?
You guessed right The DC current flows through the capacitor (the external view as per above discussion) and continues to flow at the exact same level as it did when we first started the simulation.

The idea here is that most of us will assume that DC refers to the steady state solution for DC, which means not only the current is DC but the voltage is also DC. In fact, we should really be saying just "DC" rather than "DC current", but again we choose the shorthand version and assume everybody would know that a constant current generator has to force current through any circuit element (in theory) at a constant rate so that is one exception to the rule.

Last edited: Feb 26, 2011