Basic? Electronics question

[KeithB]

So imagine a capacitor being charged through a resistor.

The voltage builds on the capacitor because of the electrons on the plates.

However, if you look at the current into and out of the capacitor, they are identical. Where do the electrons come from?

[mfleming]

https://phys.libretexts.org/Bookshelves/...C_Circuits

[jhallen]

"Metallic silver is an excellent conductor. It has 5.89×10^28 conduction electrons per cubic meter." There are plenty available in the plates of the capacitor.

[ijabbott]

Capacitors block DC current.

[KeithB]

I know how RC circuits work.
I was under the mistaken belief that as it charged, the input current would be more than the output current, with the excess current supplying the charge to charge(heh!) the capacitor.

This is not the case, as SPICE readily shows. So I assume the electrons come from the dielectric, rather than the supplied current.

[ijabbott]

(08-09-2021 03:51 PM)KeithB Wrote:  So I assume the electrons come from the dielectric, rather than the supplied current.

The dielectric may be a vacuum. Electrons building up on one plate will repel electrons from the other plate.

[Nigel (UK)]

(08-09-2021 03:51 PM)KeithB Wrote:  I know how RC circuits work.
I was under the mistaken belief that as it charged, the input current would be more than the output current, with the excess current supplying the charge to charge(heh!) the capacitor.

The idea of a capacitor charging can be misleading. In fact, the total charge on a capacitor is zero, whether it is charged or uncharged! In the equation \(Q=V\times C\), \(Q\) isn't the total charge "stored" in the capacitor; rather, it is the "amount" of charge on each plate. One plate has charge \(+Q\) while the other has charge \(-Q\), giving a total of zero.

Nigel (UK)

[KeithB]

(08-09-2021 03:48 PM)ijabbott Wrote:  Capacitors block DC current.

Good thing the voltage on a charging capacitor isn't DC, then.

[Vtile]

IDEAL capacitor do block DC current as such allows AC current. Electronics as theory, like most theories them self is simplification of true world.

However, one question is that what is AC current. If something would change instantly the energy of such thing would be infinate. When capacitor charges it does not charge instantly there is a curve on voltage and current, what does happen is that in fact while charging the capacitor from DC current the actual thing that happen (slope) do have AC component (transient). The theory says AC does go through so what happens when you charge capacitor plates (electric field) from DC source?

The typical model which mostly is used as electronics and electrical analysis, is as known as centralised theory (iirc from work of Proteus Steinmetz), which assumes DC steady state (even witch AC). This is one reason GHz and above frequencies are sometimes considered as black magic. In fact many (persons who do practise the field) do say capacitors should be modeled as transmission line (one de-centralised theory).

This opinion is actually reasonable easy to rationally explain. If we assume that speed of light in vacuum is the speed limit of universe (approx. 300000000m/s), which is equivalent of velocity of 3.33 nanoseconds per meter. We also know that electricity is form of electromagnetic radiation. We also know that speed of light in another medium is always less than in vacuum (propagination factor). Now we have capacitor formed from two plates, whose dimensions are 0.01x1meters and contact point is at one end of plate. Propagination factor of plates is 1.5. In this case it takes 5 nanoseconds for the electric radiation to reach the another end of plate. This is of course simplification.

It is true dielectric can be also vacuum, then capacitor do not have dielectric absorption effect (slow dipoles in dielectric).

Electrons flow in circuit is considerably slower that electric field.

Unfortunately I notice my memory starts to deprecate on this fascinating subject.

[ijabbott]

(08-09-2021 05:28 PM)KeithB Wrote:  

(08-09-2021 03:48 PM)ijabbott Wrote:  Capacitors block DC current.

Good thing the voltage on a charging capacitor isn't DC, then.

By "DC current" I mean the steady state current (when dI/dt = 0). Of course, an ideal capacitor being charged (or discharged) through some resistance to a constant voltage will only reach the steady state in the limit as t -> infinity (treating current as continuous rather than discrete).

[KeithB]

Thanks for all the answer's guys.
I will chalk it up to displacement current and leave it at that.

[Hlib]

The electrons in the dielectric move to higher energy levels under the influence of an external field. Then they give off energy in portions that are equivalent to a single charge of an electron. This mechanism of energy storage/transfer in a dielectric is not fully understood. We have replaced the energy conversion with a simple law of conservation of charge. But the process itself is much more complicated.