A question about capacitor

[FvM]

In my view, the original question is simple. I would suggest to understand it based on common sense, "charge" of a capacitor is Q = ∫I dt = C V. With an unconnected terminal, no current flows into the capacitor (sparking should be considered as a kind of connection, isn't it?), so the capacitor is keeping it's initial charge (respectively energy) state.

 

[Ratch]

FvM,

...that a capacitor literally doesn't store charges...

It does in a way. It stores negative electrons on one plate and positive ions on the opposite plate for a NET charge of zero. Most folks think that the whole cap or a battery becomes full of charges when "charged". In a cap, the charges are only separated. In a battery, the chemicals are renewed through electrolysis.

..because the current sum through it's terminals is zero..

Current does exists through the capacitor terminals for a transient time, but no current exists through the capacitor unless the dielectric is leaky. The sum of the current in and out of a resistor is also zero.

...this viewpoint apparently contradicts common explanations of capacitor operation.

It sure does. I have seen explanations that say "positive" charges move toward a plate. That does not happen because the positive charges are ions locked in the core of a material, and do not move.

"Charging a capacitor" (or a battery) seems to be a clear technical description at first sight.

Only if you mean charging with energy. Then you might as well say "energizing".

Having this in mind, we can keep on talking about capacitors in usual terms, including "charging" and "discharging", I think. .

Just like NASA talks about "walking" in space.

Sloth,

Ratch does have a point. If you calculate the net charge for plate A and B in respect to some middle point, you could say the net charge remains constant. I haven't dug into the meaning of charge. But I'd say there's also a electric charge between the plates. That would mean that there really is a charge in "charged" capacitor.

It doesn't happen that way, otherwise you could not make a vacuum capacitor. A vacuum contains no material. Only the electrostatic field and the dielectric (vacuum in this case) exists between the plates. The electrostatic field contains the energy. The proper operation of an ideal capacitor requires that the dielectric be a perfect insulator.

Ratch

 

[Sloth]

FvM:

In what I was explaining the capacitor plate would have been considered as a floating piece of conductor. Not an actual capacitor. But yeah, I guess that went a bit off topic...

Ratch:

I didn't mean there would be something conductive in between the plates. Electrostatic field exists even in vacuum, and there will be a force affecting the plates. So I consider that there is a charge in the plates in respect to each other. With that "middle point" I meant that the point to which you compare the potentials/charges should be chosen carefully. In most calculations the point is chosen to be the other end of the capacitor. And that would be a wrong way to think in this case. I hope you get what I meant.

 

[Ratch]

Sloth,

I don't think a voltmeter would register any voltage between the plates of a vacuum capacitor, even though there is an intense electrostatic field present. The ends of the cap are the only place you can measure voltage.

Ratch

 

[Sloth]

Not going into measuring of electric fields, I just say that if there's a field, there's a charge.

 

[Ratch]

Sloth,

Not going into measuring of electric fields, I just say that if there's a field, there's a charge.

The field extends out from the charge, so the charge may not be where the field is.

Ratch