Can a Capacitor be fully charged?

[Ratchit]

I like this answer best.

Even if it is wrong? The voltage of the capacitor will approach the source voltage asymptotically.

But, as Ratch says, it's the transfer of charge from one plate to the other.

No, I don't say that. The verb charge has no place in describing the unbalancing of electron quantities on capacitor plates.

I know you know that, but the wording was misleading.

Does he?

However, unlike Ratch, I have no problem with using the word "charged" to describe what happens to a capacitor. We should all know what the word means. Charging, after all, always involves moving charge from one place to another, in some sense.

Does it? If you send electrons along a wire, is the wire charging, or conducting?

Try saying "energized" ... and no matter how "correct" that might be, you are more likely to confuse people.

Why should that be? Unless they had the wrong conception in the first place.

There are many ways to energize something, including heating it,

Surely nobody is going to assume the capacitor is being thermally energized.

throwing it, or attaching it to a sling shot prior to launching it at a house to cause some mischief.

Surely nobody is going to assume the capacitor is being mechanically energized.

Surely everyone is going to assume the capacitor is being electrically energized because it is an electrical storage device. It is being charged with electrical energy, or in other words, energized.

Ratch

 

[MrAl]

However, unlike Ratch, I have no problem with using the word "charged" to describe what happens to a capacitor. We should all know what the word means. Charging, after all, always involves moving charge from one place to another, in some sense.

Does it? If you send electrons along a wire, is the wire charging, or conducting?
The verb charge has no place in describing the unbalancing of electron quantities on capacitor plates.
Ratch

Hi,

The wire is conducting isnt it?

I dont see why you have such a problem with the wording. Cant i be sneaky and charge one plate with positive charge and the other plate with negative charge?
Are you trying to prove that we can never say that the capacitor ever gets charged? I think the word 'charged' is used as a matter of convenience rather than complete technical accuracy. Same thing about a battery when we say we charge a battery we are really changing it chemically. But as Steve pointed out, we had to move charge to do it. The point being that to charge something you have to move charge, but just because you move charge doesnt mean you are charging something.
Maybe a couple more examples:
1. I 'water' my plants.
2. I 'hose off' the car.




Back to the original question...

We can do a lot of things in theory that we can not do in practice. However, when we do things in theory it is up to us to set the stage so we are all clear what we have to work with at the start. Therefore if we post a theoretical question we have to specify what natural laws we are allowed to break and what not. This usually involves parasitic elements like ESR or leakage current.

For a purely theoretical example, if we use an ideal source to charge an ideal capacitor, we can charge it up to the same voltage exactly as the source, and we can do it in zero time.
For a practical example of the same thing, we have a cap with ESR of 1 ohm and equivalent leakage resistance of 1 megohm at the operating voltage. We can charge this cap up to within about 0.0001 percent of the source voltage, but not any higher. Note here we have to specify what we have to work with before we can provide a reasonable answer.

Just to note, the limit of e^-t as t approaches infinity is zero. That's the damping factor in many reactive circuits. With t as little as 37 seconds we get near zero with about 16 digits of precision. Double that to 74 seconds and we are near zero with about 32 digits of precision, so you can see how fast this damps to what we call zero. For most calculations on today's computers we only use 16 digits of precision. Double that again and we are about as near to zero as a '1' with a decimal point and 63 zeros ahead of it:
0.0000000000000000000000000000000000000000000000000000000000000001

Also as a side topic, in another place we were talking about what it takes to transfer maximum charge from a source to a capacitor or from one capacitor to another with or without an inductor. That's interesting too and sort of related to this topic.

 

[Corky]

I am more than happy with 'charging' a cap, it is as far as i have heard the norm, our college teaches charging a cap as well as all the books i have read so charging it is i know it may not be correct but its accepted just as the direction of the flow of electricity is accepted, i dont want to get caught up with how i word everything to that precision because id have to research for hours to be able to write a question

cheers guys

 

[MrAl]

Hi,

Yes most people are comfortable with calling it a charging of the cap, but im sure Ratch wants to try to be as technically accurate and precise as possible so he is suggesting that we call it by a more directly applicable term rather than the accepted standard. I would be interested to hear where he first got this idea from, or want caused this idea to come about. Maybe it was a class he took or something.

 

[Corky]

Hello,

I think its good how ratch doesnt like the "thats how its always been" sort of mentality and he likes to debate things he adds a bit of difference into the forum but.... i also need to get to grips with commonly used electronics language

reagards

 

[MrAl]

Hi,

Yes i agree it is good to think of things in different ways...as well as possibly raising our enlightenment it also makes it less boring

 

[steveB]

Even if it is wrong? The voltage of the capacitor will approach the source voltage asymptotically.

He said as much, in my view. But he also shows a deeper understanding.

No, I don't say that. The verb charge has no place in describing the unbalancing of electron quantities on capacitor plates.

Ok, you dont say that, but it does have a place even if you dont say it. People have been saying it for centuries and we all know what it means. Each plate gets charge is the underlying understanding.

Does he?

Yes, i think so. Otherwise he would not have revealed the most important concept needed to answer the OP's question.

Does it? If you send electrons along a wire, is the wire charging, or conducting?

I didnt say moving charge is charging, I said charging involves moving charge. The order of the statement is important. However, in fact, a wire that conducts does develop a charge distribution on it. But that's completely irrelevant to this discussion.

Why should that be? Unless they had the wrong conception in the first place.

Because there are many situations where it could happen. If they didn't have the wrong conception then the word charging will not be misinterpreted.

Surely nobody is going to assume the capacitor is being thermally energized.

Surely nobody is going to assume the capacitor is being mechanically energized.

Depends on the situation, but it would be an unusual situation. So what. You choose a more general term in place of an accepted specific term. You choose loss of information in place or your own personal view of correctness and disregard the history and everyone else's acceptance of language that works well.

Surely everyone is going to assume the capacitor is being electrically energized because it is an electrical storage device. It is being charged with electrical energy, or in other words, energized.

Not everyone, particularly someone who does not know much about electricity, and particularly someone doing an unusual experiment that involves other types of energy transfer (or energizing). Am I promoting the idea too much? Yes, surely I am, but you are promoting your idea too much too.

 

[Ratchit]

Mr. Al,

"The wire is conducting isnt it?"

Yes, it is not charging.

"I dont see why you have such a problem with the wording."

Because it is wrong.

"Cant i be sneaky and charge one plate with positive charge and the other plate with negative charge?"

Not with mobile positive charges, you can't.

"Are you trying to prove that we can never say that the capacitor ever gets charged?"

If you want to be accurate, then no.

" I think the word 'charged' is used as a matter of convenience rather than complete technical accuracy."

Why is "charged" more convenient than the correct "energize"?

"Same thing about a battery when we say we charge a battery we are really changing it chemically. But as Steve pointed out, we had to move charge to do it. The point being that to charge something you have to move charge, but just because you move charge doesnt mean you are charging something."

That sounds like doublethink. I believe you are confusing what is being done with how it is being done. One can energize a battery. That is what is being done. You can do it by electrolysis. That is how it is being done.

"Maybe a couple more examples:
1. I 'water' my plants.
2. I 'hose off' the car."

Idiomatic expressions should not be used in scientific definitions, and used only in teaching situations when there is no ambiguity.

"We can do a lot of things in theory that we can not do in practice. "

Doing things in theory is imagining, do something in practice is realizing it. Theory and practice are not related as far as implementation. One is mental, the other is physical.

"However, when we do things in theory it is up to us to set the stage so we are all clear what we have to work with at the start."

I think you are saying one has to explain one's theory.

"Therefore if we post a theoretical question we have to specify what natural laws we are allowed to break and what not. "

Natural laws are never broken.

" This usually involves parasitic elements like ESR or leakage current."

Those items follow laws also.

"....... Double that again and we are about as near to zero as a '1' with a decimal point and 63 zeros ahead of it:
0.0000000000000000000000000000000000000000000000000000000000000001"

It is an asymptotic relationship. That says it all. Two values can approach as closely as you can finitely define, but they never equal each other.

"Also as a side topic, in another place we were talking about what it takes to transfer maximum charge from a source to a capacitor or from one capacitor to another with or without an inductor. That's interesting too and sort of related to this topic"

It is a problem for differential equations.

Ratch

 

[Ratchit]

Corky,

"I am more than happy with 'charging' a cap"

Are you happy with "walking" in space like NASA does? Would you walk away if your tether broke?

"it is as far as i have heard the norm, our college teaches charging a cap as well as all the books i have read so charging it is i know it may not be correct but its accepted just as the direction of the flow of electricity is accepted, i dont want to get caught up with how i word everything to that precision because id have to research for hours to be able to write a"

Yes, a consensus of opinion does not make it correct. Why does changing a word or two make it such a chore?

Ratch

 

[Ratchit]

He said as much, in my view. But he also shows a deeper understanding.

Ok, you dont say that, but it does have a place even if you dont say it. People have been saying it for centuries and we all know what it means. Each plate gets charge is the underlying understanding.

Yes, i think so. Otherwise he would not have revealed the most important concept needed to answer the OP's question.

I didnt say moving charge is charging, I said charging involves moving charge. The order of the statement is important. However, in fact, a wire that conducts does develop a charge distribution on it. But that's completely irrelevant to this discussion.

Because there are many situations where it could happen. If they didn't have the wrong conception then the word charging will not be misinterpreted.


Depends on the situation, but it would be an unusual situation. So what. You choose a more general term in place of an accepted specific term. You choose loss of information in place or your own personal view of correctness and disregard the history and everyone else's acceptance of language that works well.


Not everyone, particularly someone who does not know much about electricity, and particularly someone doing an unusual experiment that involves other types of energy transfer (or energizing). Am I promoting the idea too much? Yes, surely I am, but you are promoting your idea too much too.

SteveB,

"He said as much, in my view. But he also shows a deeper understanding."

"Asymptotically" says it all in one word. What deeper understanding are you referring to?

"Ok, you dont say that, but it does have a place even if you dont say it. People have been saying it for centuries and we all know what it means. Each plate gets charge is the underlying understanding."

No, only one plate receives charge, the other plate loses charge.

"I didnt say moving charge is charging, I said charging involves moving charge. The order of the statement is important. However, in fact, a wire that conducts does develop a charge distribution on it. But that's completely irrelevant to this discussion."

Lots of things involve moving charges. That still does not make energizing something a charging operation. As long as there is a conduction path, a wire does not have a charge distribution.

"Because there are many situations where it could happen. If they didn't have the wrong conception then the word charging will not be misinterpreted.

Depends on the situation, but it would be an unusual situation. So what. You choose a more general term in place of an accepted specific term. You choose loss of information in place or your own personal view of correctness and disregard the history and everyone else's acceptance of language that works well."

Why not use the correct word in the first place? Then you would not have to rely on context, tradition, inference, or consensus to define the correct meaning.

"Not everyone, particularly someone who does not know much about electricity, and particularly someone doing an unusual experiment that involves other types of energy transfer (or energizing). Am I promoting the idea too much? Yes, surely I am, but you are promoting your idea too much too.[/QUOTE]"

The correct definition will survive no matter what experiment is done or what the skill of the experimenter is.

Ratch

 

[steveB]

There are more important things in my day than this Ratch. I dont mean any disrespect in not discussing further, but the important points have been made. This minutia (both yours and mine) is much less interesting.

 

[MrAl]

Hi Ratch,


Thanks for correcting me on every single point i was trying to make in my last post
It is interesting to see that you believe that i was wrong on 99 percent of my thoughts on charging a capacitor.

I get the feeling you are trying to prove something beyond the terminology being used that we had been talking about here.

Im not sure where to begin here so i'll keep it short...

"Natural laws are never broken"
You did not consider the context, which is "theory". In theory we often ignore certain things in order to show how other things work which are more important to us at the time. An infinite current can not exist, yet i can say "infinite current". That's theory. This cant happen in practice, but math is bigger than the universe.

I'll try to stick to the main point of the discussion though...
You seem to want everyone to change from saying we "charge" a capacitor to we "energize" a capacitor. That i can guess is from an examination of the physical phenomena involved where energy transfers from some place to the capacitor. That's great. But you also have to realize that it is not as descriptive to say "energize" as it is to say "charge" when everyone knows what charge means already. For example, i can "energize" components that are not storage devices, yet i can not really say that i can charge devices that are not storage devices. Thus using your terminology, we cant tell what is going on if i state the following:
"I energized the device"
but if i say:
"I charged the device"
you have a very good idea that i had to replenish something that was lost in the device.
If we had only two devices, a cap and resistor, from the first statement you cant be sure if it was a capacitor or resistor,
From the second statement you can be sure it was a capacitor and not a resistor because we dont charge resistors but we do energize them.

The convenience comes in the form of not only being more descriptive, but also using the same terminology that is used in many books on the subject.

Also, stating that "to charge a device you move charge but if you move charge that does not mean you are charging anything" is not doublethinking or whatever you call it. It's a matter of exclusive or.
If i say i move charge that does not mean that i am charging something, but if i say i am charging something then you know i am moving charge. There's nothing wrong with this statement.

 

[JimB]

What is the definition of the expression " déja vu " ??

It is knowing as soon as you see the title of this tread, it will soon degenerate into another willy waving contest over the meaning of the word "charge" and what happens to a capacitor when a voltage is impressed across its terminals.

JimB

 

[Ratchit]

Mr.Al,

When you charge a energy storage device, you have to ask what are you charging it with. Electrical energy of course. So you might as well say energizing. It makes no sense to say you are charging a resistor because it is a dissipative device and cannot hold an electrical charge. To say you are moving a charge to energize a capacitor describes how you are doing something to a capacitor. It does not tell what you are doing, specifically energizing it.

With respect to charging, I sure don't see its description being more clear or convenient.

Ratch

 

[MrAl]

Hi Ratch,

Mr.Al,

When you charge a energy storage device, you have to ask what are you charging it with. Electrical energy of course. So you might as well say energizing. It makes no sense to say you are charging a resistor because it is a dissipative device and cannot hold an electrical charge. To say you are moving a charge to energize a capacitor describes how you are doing something to a capacitor. It does not tell what you are doing, specifically energizing it.

Im sorry and i am not trying to be rude but you sometimes seem to miss the point.

With respect to charging, I sure don't see its description being more clear or convenient.
Ratch

I agree with you there totally, you don't see it


Despite our disagreement on some issues i still appreciate your usually well-informed input.

Take care,
Al

 

[MrAl]

What is the definition of the expression " déja vu " ??

It is knowing as soon as you see the title of this tread, it will soon degenerate into another willy waving contest over the meaning of the word "charge" and what happens to a capacitor when a voltage is impressed across its terminals.

JimB

Hi,

He he, de/ja V(u) again

This topic comes up now and then for some reason. Some people want to change the world somehow to hopefully make it a better place.
Newton tried that with language long ago to make it more logical, and failed miserably. If he didnt, we'd be talking like computers today.

As to this thread at this point to quote a saying from a good movie:
"All our words have been said...safe journey."

 

[nsaspook]

Mr.Al,

When you charge a energy storage device, you have to ask what are you charging it with. Electrical energy of course. So you might as well say energizing. It makes no sense to say you are charging a resistor because it is a dissipative device and cannot hold an electrical charge. To say you are moving a charge to energize a capacitor describes how you are doing something to a capacitor. It does not tell what you are doing, specifically energizing it.

With respect to charging, I sure don't see its description being more clear or convenient.

Ratch

I'm on your side with explaining that 'charged' in this context really means 'energized' in a electronics capacitor as it's not like a balloon or cup to be filled with charge but more like a pressure vessel with a diagram in the middle that moves from displaced charge but idioms don't die easily.

 

[Ratchit]

MrAl,

I guess we have a divergence of the minds on some things. No harm done.

nsaspook,

Thanks for your support on this item.

Ratch

 

[steveB]

This is the original question from the OP is as follows.

so the cap would never actually reach full charge? or would it eventually reach its max?

The correct answer is given by nyaknyan , as follows

charge of real capacitor consists of electrons and you can't have charge less than one electron's charge. So when the "last" electron will go to cap, if next electron will try to move to cap, voltage on cap will be bigger than supply voltage, so it won't do that.

nyaknyan is correct to point out that the asymptotic solution is one that results from a "continuum" assumption and continuous time math, but the real world has discrete charge in units of "e" only. (let's forget about quarks, please). So the ideal solution is that solution that allows for the discrete nature of charge, and it will reveal a full charge in a finite amount of time.

nyaknyan was also wise enough to point out that the ideal solution under discrete charge assumption is also limited because of real world noise or variations.

One can always nitpick, but that is usually a waste of time. The points about "energizing" and what physically happens when we "charge" a capacitor are perhaps useful in an introductory classroom, but not here time after time after time when it detracts from obtaining the simple goal of answering a simple good question.

 

[nyaknyan]

I just thought that i'm not really right. When cap is energized potential difference between cap's plate and voltage source decreases, so speed of last electrons decreases to, so they will move slower and slower infinitely? Or no? Even if charge is discrete, electric force acting last electrons can vary depending on distance from charge stored on cap's plate, so last electrons will slow down infinitely while moving to cap (or from it on other side, but let's forget about other plate)? Last electron will newer get to cap? If space-time or energy is discrete itself it will stop after some time, but i don't know if it is. However this small movement will became indistinguishable from thermal noise much earlier.