Capacitor DC charging question.

[Ratch]

pebe,

If, by 'charge imbalance', you mean the two plates have different charges, would not that be implied by their voltage difference?

Yes, any cap whose plates don't have the same charge on them will automatically show a voltage between its terminals. By charge imbalance, I mean that the surplus of charge on one plate will be exactly the deficiency of charge on the other plate.

Ratch

 

[pebe]

An unusual choice of word. I would refer to the charges on two plates that were exactly equal and opposite, as balanced.

 

[Ratch]

pebe,

An unusual choice of word. I would refer to the charges on two plates that were exactly equal and opposite, as balanced.

Unlike p-type semiconductor material, a cap does not have any mobile positive charges. Its mobile charges are only negative electrons. One side has a surplus of electrons which could be called "negative", and the other side has a deficiency of electrons which could be called "positive". If there are more electrons on one side than the other, I don't see how that could be called "balanced". Balanced to me means the same number of electrons on each side like you would get if the cap were shorted for a significant time. However, I am open to new explanations of definitions provided they don't conflict with established and agreed upon nomenclature.

Ratch

 

[pebe]

Ratch,

…….If there are more electrons on one side than the other, I don't see how that could be called "balanced". Balanced to me means the same number of electrons on each side like you would get if the cap were shorted for a significant time. However, I am open to new explanations of definitions provided they don't conflict with established and agreed upon nomenclature.

A state of balance can be considered as being ‘in equilibrium’, or stable. If I balance my budget it ensures that my finances are stable; that my income equals my expenditure – two opposite movements of finances. An imbalance only occurs when one exceeds the other.

But all this is a red herring. Assuming for a moment that charges exist on the two capacitor plates that are equal but opposite in polarity, ie. +2C and –2C. They can only be considered as opposite relative to some fixed datum point that has no charge. The ground or earth (depending on which country you live in) is such a body. So if the negatively charged plate is connected to ground it now has a charge of 0C. No charge has left the capacitor because the voltage between the plates is unchanged, so the charge on the positively charged plate must have increased to +4C.

It should be obvious that the absolute charge on each plate is not important – it is the differential charge that matters. That is what constitutes the charge, C.

So I maintain that when a voltage exists between the plates of a capacitor, a charge exists in it, ie. it is charged.

 

[Ratch]

pebe,

A state of balance can be considered as being ‘in equilibrium’, or stable. If I balance my budget it ensures that my finances are stable; that my income equals my expenditure – two opposite movements of finances.

No, in accounting, balancing a budget means that an income or expenditure is accounted for, not that money spent is replaced. Accounts can be balanced and still show a surplus or debt.

An imbalance only occurs when one exceeds the other.

That is true for caps. When one side has an excess of electrons, the other side has a deficiency of electrons, and the cap is unbalanced.

But all this is a red herring.

No, I believe it is important to understand.

Assuming for a moment that charges exist on the two capacitor plates that are equal but opposite in polarity, ie. +2C and –2C.

As I said before, a cap has only one mobile charge of negative polarity. If you say it has a +2 C charge on one plate, that means that plate has 2 C fewer electrons than the other plate.

They can only be considered as opposite relative to some fixed datum point that has no charge. The ground or earth (depending on which country you live in) is such a body.

No, they are considered relative to what they were when the cap was at equilibrium with no voltage across its terminals. The earth is nothing special, except a common point of conduction. If a cap is hanging out somewhere isolated from earth, no voltage can be measured and no current will exist if only one plate is connected to earth.

So if the negatively charged plate is connected to ground it now has a charge of 0C. No charge has left the capacitor because the voltage between the plates is unchanged, so the charge on the positively charged plate must have increased to +4C.

Nothing is going to change on the plate if it is connected to ground with no conduction path to the other plate. Try this. Take a cap and energize it to say 10 V. Disconnect the voltage supply and verify 10 V across the cap. Measure the voltage from the + terminal to ground leaving the - terminal isolated. Nothing, right? Now only touch only the + terminal to a good ground. Then measure the voltage across the cap again to read 10 volts as before. Nothing has changed as long as there is no conduction path between the cap's terminals.

It should be obvious that the absolute charge on each plate is not important – it is the differential charge that matters. That is what constitutes the charge, C.

Yes, but the differential charge is always an equal surplus and equal deficiency. If you say a surplus constitutes a charge, then I can say the deficiency constitutes a discharge. So when a cap has a voltage across it, is it charged or discharged? That is an ambiguity that the word "energized" eliminates.

So I maintain that when a voltage exists between the plates of a capacitor, a charge exists in it, ie. it is charged.

And I could just as logically maintain that when a voltage exists, the cap is discharged due to its deficiency of electrons on one plate.

Ratch

 

[pebe]

As I said before, a cap has only one mobile charge of negative polarity. If you say it has a +2 C charge on one plate, that means that plate has 2 C fewer electrons than the other plate.

A normal atom has an equal number of protons and electrons and since a proton and an electron have equal and opposite charge, an atom on the whole is electrically neutral; ‘normal’ being the state of most things around us. So a deficiency of electrons would leave the atom with a positive charge.

Therefore, the plates of the cap are negatively and positively charged plates, respectively.

No, in accounting, balancing a budget means that an income or expenditure is accounted for, not that money spent is replaced. Accounts can be balanced and still show a surplus or debt.

I emailed my son to get his opinion on the matter. He is a Fellow of the ACCA – the top drawer of accounting associations in the UK – and here is his reply which I quote verbatim:

“When talking of budgeting, then you are right Dad. He is I think referring to a Trail Balance, which is a summary of all credits / debits prior to producing the P&L and Balance sheet. This indeed does allow accounts to be balanced regardless of whether there is a profit or a loss for the year.”

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I feel that to continue this ‘discussion’ would just provoke your further nit picking; your sole aim being to just be pedantic.

I have better things to occupy myself with. So I will leave you with your unusual views and continue, along with most of other people, to accept that a capacitor with a voltage across its terminals is ‘charged’.

QED

 

[Ratch]

pebe,

A normal atom has an equal number of protons and electrons and since a proton and an electron have equal and opposite charge, an atom on the whole is electrically neutral; ‘normal’ being the state of most things around us. So a deficiency of electrons would leave the atom with a positive charge.

Therefore, the plates of the cap are negatively and positively charged plates, respectively.

As I pointed out before, only the negative charge carriers (electrons) move in a capacitor. The other positive ions are locked in place as part of the ionic core of the metal and dielectric. Their numbers are always the same on either side of the plates, no matter what the voltage. Therefore, it is the movable electrons that determines whether a surplus or deficit negative charge exists on each plate.

I emailed my son to get his opinion on the matter. He is a Fellow of the ACCA – the top drawer of accounting associations in the UK – and here is his reply which I quote verbatim:

“When talking of budgeting, then you are right Dad. He is I think referring to a Trail Balance, which is a summary of all credits / debits prior to producing the P&L and Balance sheet. This indeed does allow accounts to be balanced regardless of whether there is a profit or a loss for the year.”

So I was right in interpreting that for accounting at least, a balance does not have to be a surplus or deficit? Doesn't matter anyway. We are discussing electronics, not accounting.

I feel that to continue this ‘discussion’ would just provoke your further nit picking; your sole aim being to just be pedantic.

My aim is to be correct. I happen to be pedantic on the way to correctness.

I have better things to occupy myself with. So I will leave you with your unusual views and continue, along with most of other people, to accept that a capacitor with a voltage across its terminals is ‘charged’.

You are in good company. NASA thinks their astronauts "walk" in space, whereas I observe them floating. I have never heard of the broadcast media or any reputable scientific organization call them out on that. So, although you cannot find fault with my discription of an energized capacitor, you are going to call it something you must know is incorrect.

Ratch

 

[pebe]

QED........................