Ic=C x dv/dt

[Laplace]

For the most part the question as to whether electrons are flowing through the dielectric is one of semantics.

FYI: Ratch's schtick is semantics.

 

[Ratch]

="Electrical Engineer David, post: 1733691, member: 48873"]My apologies. I misunderstood to whom it was directed.

Apology accepted.

As for current flowing through the dielectric I might be able to clarify. Current is defined as the net flow rate of electrons through a component. It might surprise some to find out when you are measuring the current flowing through a wire the electrons do not really flow through the wire. Much like when I turn on a hose the water immediately begins flowing out the end even though the water flowing out of the spigot has yet to traverse the entire hose. It is the force which propagates through to the outlet not necessarily a specific droplet of water. Some of the water molecules actually traverse back into the spigot. It is the net flow of water we care about.

Yes, in electrical terms, current is the movement of charge carriers. Electrons do move in a random manner in a inert wire. When a voltage is applied, the electrons move by displacement, like a hose filled with marbles. One marble inserted at one end causes a different marble to pop out the other end. This displacement has a very slow net directional movement called the drift velocity. This is unlike the movement of electrons from the cathode of a CRT to the phosphor screen, where the electrons travel is a direct manner.

Similarly when we look at a capacitor with current "flowing through it" we are looking at the system as a whole. We don't normally track and aren't normally concerned with the movement of a single electron. The electrons drift from one collision to the next in a haphazard manner. Some even flow back towards the source. Also some positive ions flow towards the source and even towards the load. The ion flow is much lower due to their larger mass and the crystalline structure of the metal. In fact almost half of the free-electrons in a wire flow in a direction opposite the flow of current. If 1 amp of current is flowing through a wire this amounts to 1 Coulomb of charge per second tending to flow more towards the negative terminal of a battery. Yet the number of free electrons moving in the wire is many millions of Coulombs. In other words if you have 5 million Coulombs of electrons flowing in the wire only a small percentage above 50 percent tend to flow in the direction of current. It is this small imbalance of electron flow we care about on the macroscopic level.

The drift velocity of a charge carrier is a statistical variable depending largely upon the voltage applied to the wire. The ionic core of the conductor is fixed place and does not move. Current measurement and calculation is concerned with current present in the branch where the component is located, not through the component itself. If that fact were emphasized, less confusion would occur.

It is entirely correct that if you were to follow the movement of a particular electron it is unlikely to flow through the dielectric (some small amount do however). If the voltage rating of the capacitor is exceeded you will get enough electrons tunneling through the dielectric to destroy the component due to heating. Under normal operating conditions the electrons tend to build up at the plates until the "back-voltage" matches the source voltage.

For the most part, we ignore surface leakages and other secondary effects.

If you charge the plates of a capacitor then disconnect the source from the capacitor charge will remain on the plates. However, this charge will not remain indefinitely. It will slowly drain over time. Not all of which occurs through the gases surrounding the terminals. Even in a vacuum, some charge will slowly leak through the capacitor dielectric. So a real capacitor is modeled as an ideal capacitor with a leakage resistor in parallel.

True, no real world capacitor is perfect.

For the most part the question as to whether electrons are flowing through the dielectric is one of semantics. It doesn't matter in terms of what we are interested in (normally). We are concerned with the system as a whole. The times we are concerned with dielectric current mainly involve understanding capacitors at the atomic level. In order to understand capacitors it does help to pretend they are ideal capacitors which have no leakage initially. Especially since it is the build-up of charge at the plates and the electric field which results which is so critical to the functioning of a capacitor. Once the physics are understood at the atomic level we must abandon the ideal model in some situations if leakage current is of concern.

I don't think we have to get involved in quantum physics to understand the basics of capacitors. They store energy from charge separation, thereby producing an electric field where the energy is stored. Leakage can be represented by a high value resistor in parallel and lost energy by a low value resistor in series. Overall, it is essential that no significant current exists through the dielectric for proper operation.

Ratch

 

[Ratch]

FYI: Ratch's schtick is semantics.

Semantics is the meaning of word and descriptions. Especially in science, correct descriptions and meaning are of primary importance. That is why I dislike to observe folks using slang descriptions like [current flow = charge flow flow] . That kind of talk might be OK in a bathroom stall, but has no place where precise meaning is important. Similarly, averring that current exists through a dielectric is talking technical trash (TTT).

Ratch

 

[(*steve*)]

The drift velocity of a charge carrier is a statistical variable depending largely upon the voltage applied to the wire.

More precisely, to the electric field within the wire.

(I hate loose and imprecise statements ;-))

 

[Ratch]

More precisely, to the electric field within the wire.

(I hate loose and imprecise statements ;-))

Yes, and to be even more precise, the electric field is the negative gradient of the voltage between the ends of the wire. (E = - del V)

Ratch

 

[(*steve*)]

Yeah, and although we all understand what you meant, "a voltage applied to a wire", if applied equally to the whole wire (i.e. there is no gradient) won't result in a net force on electrons in one direction or the other.

And of course electrons aren't current. The speed of those electrons isn't a measure of current. In fact we can have current in the absence of electrons.

 

[Electrical Engineer David]

I would again strongly stress you not make any attempt to impart your understanding on others. You just aren't cut out for it. Homework forum isn't your niche.

 

[(*steve*)]

Homework forum isn't your niche.

I assume you're talking to me.

At this point we're not talking to anyone doing homework.