Electric Potential Across Capacitors

sophiecentaur

You seem to be implying that each electron has its own identity and it needs to make a journey through the dielectric before you will admit to a current flowing. This is why we don't talk about charges in terms of electrons. If you stick a microCoulomb of charge into one terminal of a box with the label 'Unknown' on it and a microCoulomb of charge leaves the other terminal than the convention is that one microCoulomb of charge has flowed. Whether the current has flowed in the form of mobile charges or by the atoms of the dielectric having polarised - or even by the imbalanced charges on the two plates with a vacuum between, it has still flowed.
How could you have a special form of current or charge for going into and out of a Capacitor that is somehow different from the sort of current that flows through a resistor or a piece of wire?
I think you must be still thinking in terms of electrons actually moving at a significant rate inside conductors instead of the general concept of 'Current'.
Do you not subscribe to Kirchoff's First Law? How can you reconcile what you say with K1?

Your choice of which is the 'internal' and which is the 'external' circuit is a bit arbitrary. What if both wires are the same length?

Emilyjoint

I did not mention electrons so there can be no implications about the identity of electrons!
I refer only to 'charge'.
By definition (?) a dielectric is an insulator so I would not imply that charge can pass 'through' a dielectric.
I am also aware of the common misconception held by some students that electrons have a significant speed.... I know that they do not. I made no reference to the speed of charge carriers.
I used the word 'internal' within quotes to emphasise that it was a term that I was using....not necessarily an accepted term. It refers to the wire between plates 2 and3 in my description of 2 parallel plates in series.

sophiecentaur

Why? If you insist on some sort of difference between current in different components in a circuit then how are you going to analyse it? Can you justify your approach in as far as it seems to contravene the very useful Kirchoff's Laws?

So why do charges have to pass "through" a dielectric"?

You seem to be falling into the trap of asking the "what is really happening" question yet still talking 'electronics'. Whilst there is a 'current flowing' through the dielectric in a capacitor, charges are actually being displaced in there. How is that any different from charges moving 'through' a resistor - except that it only happens until the molecular restoring forces equal the the force due to the field between plates? When AC is concerned, the physical displacement of charges can easily be the same distance in both cases.

Emilyjoint

In post 6 mfb used a phrase....charge flowing through the capacitor.
In post 9 I suggested that use of the word 'through' could be 'misleading' because it implies that charge passes across the gap between the plates.
In post 13 mfb stated that 'no electrons fly across the gap between plates'
That is all there is to it.
Nowhere will you find a description of the charging and discharging behaviour of capacitors that requires charge to cross the gap.
To suggest that there is essentially no difference between current in resistors and current in capacitor circuits is 'misleading'.
None of this is in contradiction of Kirchoffs laws, K1 is a statement regarding conservation of charge.
AC does not flow in capacitor circuits BECAUSE they have impedance (I think you mean reactance). Reactance is a quantity calculated from the voltage across a capacitor and the current in the circuit. It is special because, under normal measurement techniques, no account is taken of the phase difference between the AC current waveform and the AC voltage waveform.
We need to know what is really happening to fully understand.

sophiecentaur

I know exactly what you are getting at and I appreciate that the dielectric is an 'insulator' because its resistivity is very high. If you connect a capacitor to a DC source, then, of course, the current will soon go to zero (once the capacitor is charged) because the internal fields will balance the applied PD.
We aren't talking about the nuts and bolts of what goes on inside the package of a component. That is not really relevant. What I refer to is what happens as far as the measurer is concerned. If you apply AC to a capacitor and measure the current by looking at the volts across a series resistor, for instance. You will see that there is a current - in just the same way as if you put a resistor or an inductor in its place. You seem to be implying that a capacitor must be treated as being fundamentally different from other components. That's crazy. We use current and volts as the appropriate quantities to measure because the sums are suitable for calculating the behaviour of all combinations of RL and C.
You insist that you want to know what's really going on. Have you no comments about my point that the amount of charge movement with AC inside a dielectric can be the same or even more than the charge movement inside a resistor. If you define current as movement of charge, how can you say one is 'different' from the other - except by sticking to a very elementary appreciation of the situation. It might help if you were to describe exactly what you 'mean' when you use the word 'current' if it's not movement of charges.
Also, where does the 'displacement current', for EM waves in a vacuum (where there are no actual charges), fit in with your view?
PS 'Reactance' is a part of 'Impedance' so either word fits and real capacitors have finite resistance - either series or parallel, depending how you like it.

Emilyjoint

Posts6,9and13 have cleared up the main issue.
Your last comment is very revealing!!!!! If you are putting forward that there is no real difference between Reactance and Impedance then it explains most of your ramblings.
You do not fully understand the meanings of terms used to describe physical processes.
You are plane wrong and should check your text books.

sophiecentaur

Z=R+jX
I believe.

Now tell me about displacement current in a vacuum.

Emilyjoint

This thread has run its course for me.
Can I offer Z = R + j(Xl ~Xc) for completeness where the textbook definition of the terms is
Z = impedance
R = resistance
Xl = inductive reactance
Xc = capacitative reactance

I will browse for other areas of physics that interest me now

sophiecentaur

So, no comment on displacement current?
You might consider the implications when you're in the mood.