Hello again,
Here i try to summarize the process of the transfer of charge and compare it to the so called displacement current by giving clear examples of how these processes really work in the real world. There should be absolutely no guesswork if you read this carefully and dont skip over any details. We may not have developed a TOE (yet ha ha) but at least we can explain the way some things work. In this way we can also bring this thread to a head.
I wish to first reply to the two previous posts and then i'll get on with it...
Mr RB:
The term "current" itself is the problem. "Displacement" isnt necessarily.
A "current" is something moves continuously while a "displacement" isnt quite the same. I'll explain more about how different these two things really are below after my other reply.
Yes, in the non physical sense if we have 1 amp in and 1 amp out that could be 'said' to constitute a 'current'.
But what about in the case where we have 1 amp in and 1 amp out and we dont actually have a 'current'? More about this below in full detail.
We can 'say' we have a current because it looks like we do in the leads of the capacitor. But just because it looks like an ordinary current that doesnt mean that it is. More below.
ljcox:
The thread is going around in circles, but we are talking about current flow and that goes around in circles too
You are right. We dont know everything but we can know the difference between conduction current and an interpretation of Maxwells "displacement current". The problem is knowing when we can apply either one, and apparently there is a difference of opinion when we can apply the displacement current and when we cant, other then in pure theory. In pure theory we can use Maxwells equation, but that's just an equation, an abstraction of reality, and even Maxwell did not like that, not at all. He sought to attach a visualization to every phenomena he tackled. One of these interpretations is known as the "displacement current".
We'll see how this is a displacement of charge and not a flow of charge.
Ok, done with the replies. Much thanks to those that replied.
If you followed this thread, you'll see that i was agreeing that something different was happening, other than the normal conduction current. You'll also see that i quoted an MIT professor that seems to deny that displacement current is real at all, and quoting again: "We now know that this is not the case". Note the use of the word 'now', as if something new has been discovered since the great Maxwell.
So what could possibly be going on here? One guy says this, one guy says that. But one thing has been common to all the arguments:
the displacement current is not named "conduction current", and conduction current is not named "displacement current". If the cap was simply conducting current Maxwell would not have had to add a new term to the equation. So we clearly have two different things happening here, but only happening when they truly apply to the case, and we must approach the these problems on a case by case basis. That's an integral part of being able to explain this phenomenon. Lose that, and pay the price with confusion.
The leaves in the water tells us that there is something there that is actually happening and something else there that is not only going along for the ride, it's masking the truth behind what is really happening physically. The difference is, in a circuits theory class you might be able to teach that "The leaves are moving and we can see that is true, so that's the answer to what is happening. The leaves move somehow because of their own power".
But in a physics class, you'd want to dig deeper and show that the water is what is really moving the leaves. That's like the water and lock example, where you can say that the boat flows through to make it simpler. In other words, the boat flows right through the lock and gets to the other side without any impediment. But that's not the way it really works. Again, in a circuits class you can say that the boat flows right through the lock, but in a physics class you would want to show how the lock actually operates. The idea is to dig as deep as you possibly can and see what you find, then try to describe it.
Now what we need is a good analogy that properly addresses the difference between conduction current and an interpretation of Maxwells displacement current. We know that these two things are different, in some way, however small, so we need at least two different examples to show how each one works, and that will reveal how they are truely different.
I also was pointing out that the application must be specified beforehand too, so that we may apply the correct example. For example, we would not apply the example of conduction current for the displacement current, and vice versa.
We also have to keep in mind that it is virtually impossible to show how something so fundamental to nature works by using items that are fundamentally made out of fundamental nature. This is a big problem, but we are nonetheless going to try very hard. In the end, we end up with an approximation at best, but it should give us much more intuition as to what is actually happening.
And with that good gentlemen, we now get to the examples of conduction current vs displacement current...
(Refer to attached diagram)
Case 1: The wire. Conduction current dominates and tiny tiny displacement current explained.
The wire conducts current by the process of continuous electron motion. One electron moves to the right, another moves in to take its place. The entire chain of electrons moves to the right continuously. But these electrons are not alone, they are part of the atoms that make up the wire. Those atoms also have other electrons, but there is not enough energy to move them out of their 'positions' so only the one or two electrons from each atom are free to move. These are sometimes called "free electrons", or "free charge". The other electrons are bound to the atoms, so they are called "bound electrons". Thus we have two kinds of electrons in a sense, the free ones and the bound ones. The bound ones can not move too far, but the free ones move and create the thing we call 'current'. We also note that the bound ones can move a small distance under the presence of a changing electric flux. It's interesting that if we could look at the end of the wire, we might detect an electron sticking out of the end, although still bound to it's atom. The other free electrons would be moving past that point, but that one bound one would be sticking out of the end at least partly.
Now we have 100 egg cartons lined up end to end and arranged in a circle so that the last egg carton meets the first at it's starting end. These egg cartons of course all have 12 pockets for eggs to rest inside of. Each of these 100 egg cartons has 12 eggs in two rows filling the entire set of cartons, except for the first one. That one has only 11 eggs in it. The other 99 cartons each have 11 eggs glued in place and one free to move in or out. The first one has all 11 eggs glued in place.
Oh yes, these cartons are very different than any you might buy at the store, as they are made of hard rubber. Also, the eggs are hard boiled (except for the ones with little chicks inside)
Now to start the current flow, we take the last egg out of the last carton and place it into the first carton.
This means that one egg was transferred ahead one carton so egg was therefore transported. This also created an empty pocket in the last carton, so we take an egg out of the second to last carton and place it into the last carton, but then the third to last carton is missing an egg, so we go to the one before that and take the egg from there and place it into the third to last carton, and we continue this process for all the cartons.
When we are done, we've managed to transport 99 eggs from one carton to the next, so each egg moved over by one carton. Note only the free ones could move however.
Something else happened too though because we found that we could get very good at this. We can move them so fast that the rubber carton holes start to warp, to bend in the direction of travel of the eggs. This happens to all the pockets where the stationary eggs are placed, but just a very very tiny amount. The rubber is very hard and only gives just a very small amount. We view this setup from the side so we can see the eggs moving and the rubber bending.
If we look at this very carefully, what this means is that we have transported more than 99 eggs. We've transported 99 full sized eggs, but what about the bent rubber holes? At the starting point we find that one of the eggs in the first carton has moved very slightly inward in the direction of travel, while in the last carton one egg has moved very slightly outward, thus we might say that we moved 99 eggs plus a tiny fraction of another egg. Thus, the bound eggs were very very slightly displaced as well as the free eggs having been moved. Thus we can say that here was conduction egg movement and tiny displacement egg movement. Note also that the displacement eggs moved must eventually return to their original position, while the conduction eggs dont have to.
Case 2: The dielectric. Displacement 'current' dominates and explained.
Above, we found that the free electrons move continuously through the wire, so it is easily conceivable that any electron can make it all the way around the closed loop circuit. But what about the bound electrons? It's pretty certain that since none of them can move very far none of them make it around the circuit. They stay right within the atom of the material that makes up the wire. But that does not mean that they can not move just a little. Read on.
Now instead of 100 egg cartons lined up end to end in a circle, we instead have 100 cartons made of rubber lined up end to end in a straight line, with 12 eggs in each carton, but all glued in place. That's 1200 eggs and that's a lot of eggs
We are going to view this not from the side, but from the open end of the first carton. Note our viewpoint has now changed, as we are looking straight at the end of the carton and we see all the other cartons in line. We have 100 cartons lined up end to end now so we have a whole wall of eggs, two eggs thick and 600 eggs wide.
Ok, so what we do now is we try to pull or push (to the right) one or more eggs out of EACH carton at the same time, but what happens is because they are glued in place we can not push or pull any of them out, but the force causes the rubber to bend to the right. Thus, at least some of the eggs in each carton lean to the right side, but not one of them moves out of its carton.
Now at first it may appear that we have not moved any eggs, but if we look closely at the right side of this structure of egg cartons (the front of the wall) we can see that there are possibly 600 eggs sticking part way out of the side of their cartons. Now if we have pushed 600 eggs only a small distance that amounts to one-sixth of their diameter, volume wise we have accomplished moving the equivalent of 100 eggs out of their cartons. Thus, although 100 eggs have not been actually moved out of their cartons, the equivalent of 100 eggs have been displaced. If we view this from outside of the egg cartons, it looks like 100 eggs moved out even though none actually did.
Case 3: The vacuum between two plates with moving charge. No conduction current, no real displacement current, field dominates.
The vacuum contains no charge whatsoever, but we have free charge to either side of the vacuum space and the moving electrons in the two plates will abruptly change direction and emit a virtual photon.
Now we have egg cartons with eggs, but they are lined up on either side of the vacuum. But in the vacuum we dont have any egg cartons, and we dont have any eggs. Instead, we have an empty space. We can do whatever we want to do, but with no eggs to move we can not cause a continuous egg flow, and with no cartons to bend we can certainly not displace any eggs. We basically cant do anything.In the egg cartons to the left and right of the space however that contain free eggs that are already moving, the free eggs might change direction abruptly and so an egg might 'crack' and the chick inside it could fly up and away from the egg. Thus, we would see chicks fly but we would never see any eggs move or displace.
Case 4? The end.
In the end it turns out that conduction current is a thin continuous STREAM of electrons, motivated to move by a close range imbalance of charge along a thin line, while displacement current,when it exists, is a WALL of electrons, motivated to move slightly by a long or short range imbalance of a wide field of charge which in turn causes a temporary apparent conduction movement in the wire and a temporary conduction in the wires of a dielectric capacitor.
In the vacuum capacitor, we have no current whatsoever, but only the field in which to store energy. The field comes from the change in direction of the electrons such that when the momentum of an electron changes it emits a photon. The photon, on encountering another electron, is free to again join with it to return the original energy to the plate. Thus there is no transfer of charge by any means, but merely an exchange of particles both during charge and during discharge.
Summary:
1. Conduction is a thin continuous lateral stream of electrons.
2. Displacement "current" is a tiny lateral movement of bound charge through an area, which can cause a conduction current in the wire. A better terminology choice would be "charge displacement" (not the same as charge separation).
3. In a vacuum (such as a vacuum capacitor), the exchange of particles is responsible for all the behavior and there is no conduction current nor displacement current within the vacuum itself.