AC Current Flow Question

[charlie b]

Regarding your sketches in post #53:

Comments welcomed.

Well done. You showed the nature of a capacitor very clearly. The idea that electrons build up on one plate, and that fact causes electrons to be repelled from the opposite plate, is the essence of a capacitor.

Let me add, by way of emphasis, that electrons do not jump the gap; they do not fly through the air from the conductive body to the dirt. But it is the build up of electrons on one side and the build up of a net positive charge (i.e., when the electrons leave) on the other side that calls into existence the electric field shown in the sketches.

 

[Lxnxjxhx]

AC Current Flow Question

The current in all parts of a series circuit is the same, so both ammeters should read the same if this is really a series circuit.
However, if there are parasitic capacitances that provide other current paths then it gets a bit messy.

 

[charlie b]

What I would like to add is - that the current could flow even without the interaction between the conductive body and the earth described by the electric field lines in frame 6.

Not true. Suppose you build a box around the conductive body, and set it on the ground, such that you cannot see either the body or the dirt below it. All you can see is the wire leading to the body. If there is an electron or two flowing into the box along that wire, then there must be an electron leaving the box on the other side. Nothing stores electrons, not even a capacitor (when you look at the device as a whole, and not just at one plate). The only reason current can flow into the box and out of the box, given the large air gap between the body and the dirt, is that charges build up on the body and this causes electrons in the dirt to be pushed away. That interaction between the two parallel plates (i.e., body and dirt) is what allows current to flow.

Here?s my logic- just by virtue of having mass both the earth and the conductive body can receive charges. . .

Mass has nothing to do with it. What is required is the presence of free electrons (i.e., a material made of metal or of other material that allows its electrons to move freely through the material).

- current in a separate circuit will also flow due to parasitic capacitance from the windings. I don't believe this flow will necessarily be tied to the xfmr secondary terminals.

I have no idea what you mean by ?parasitic capacitance.?

. . . both nicely described on the Wikipedia site.

I steadfastly refuse to consult that site, and I decline all invitations to go there for information. It is not controlled in any way that would give it any level of authority.

 

[charlie b]

However, if there are parasitic capacitances that provide other current paths then it gets a bit messy.

There's that phrase again. Did I miss something by not reading 'that other thread,' or by not reading every post in this thread? :-?

 

[weressl]

I steadfastly refuse to consult that site, and I decline all invitations to go there for information. It is not controlled in any way that would give it any level of authority.

Charlie,(commenting on Wikipedia)

I don't think you really mean that. Although in principle you are right, but there is a good body of information that is valid or verifiable and some of the posters are experts in their field. Like everything it needs to be used discriminatively and discerningly.

The same can be said of this forum. There is much nonsense defended adamantly here and people not knowing better will believe those to be valid and true.

 

[weressl]

There's that phrase again. Did I miss something by not reading 'that other thread,' or by not reading every post in this thread? :-?

The "parasitic" can be also understood to be "inherent" or "incidental" and generally can be thought of components that not siginificantly effect the final results.

There is no pure resistance cirucit or circuit component without an insignificant inductive and capacitive component and vice versa.

 

[crossman]

What I would like to add is
- that the current could flow even without the interaction between the conductive body and the earth described by the electric field lines in frame 6.
Heres my logic- just by virtue of having mass both the earth and the conductive body can receive charges- this is the origin of capacitance, the capacity to hold a charge. Since this is AC, the charge/reverse charge cycles will cause a current flow by literally moving electrons on and off the earth and the conductive body. Think old fashioned leyden jar or electrometer, both nicely described on the Wikipedia site.
- current in a separate circuit will also flow due to parasitic capacitance from the windings. I don't believe this flow will necessarily be tied to the xfmr secondary terminals.

Ray: Thanks for your interest and commentary. Your discription of the way current could flow onto the conductive body without the benefit of the earth (or other conductive body in place of the earth), is, at first thought, feasible.

However, your description is only looking at one of the forces applied to the conductive body. This is the way I see it:

Just as we can say the xfmr secondary is "pushing" electrons onto the conductive body, we can also say that the other side of the secondary is pulling electrons out of the earth, and as the earth becomes positive, that is what pulls the electrons onto the conductive body. In actuality, with a source such as a transformer secondary, both of the above mechanisms is needed to have a current flow.

In the diagrams: Let us take the experiment into outer space and remove the earth from the circuit and just have ammeter 2 with a short piece of wire dangling from the bottom terminal of the xfmr, with the conductive body still connected to the upper secondary terminal and ammeter 1. The current flow to the conductive body would still rely on the capacitance between the conductive body and the short piece of wire at ammeter 2.

Now, this current flow in the circuit would be way less than with the earth involved because we have significantly changed the characteristics of one plate of the capacitor. The short piece of wire in conjunction with the conductive body is going to have much less capacitance than the earth/body arrangement.

We simply cannot ignore the affects of both forces on the conductive body. One force is like repelling like, the other force is opposites attracting. It goes hand in hand. You can't have one without the other.

Thanks again for your thoughts.

 

[zog]

Because it is one half of the capacitor, and because planet Earth is a very good conductor.

Electrons are not going to jump off the "conducting body," fly through the air, and land on the transformer. Unlike planet Earth, air is a very poor conductor. There is no parallel path.

The current leaving the transformer (and passing through AM#1) is, shall be, and must be the same as the current entering it (and passing through AM#2).

Remember we are talking (Or his drawing s refering to) a transmission line, actually an experiment I conducted for the transmission line discussion.

So riddle me this, why do you accept that the electron jump through air to ground and then through earth back to the source but you dont accept that the electrons just jump through air back to the source???? Is the earth necessary for the electrons to jump through air??

 

[charlie b]

So riddle me this, why do you accept that the electron jump through air to ground and then through earth back to the source but you dont accept that the electrons just jump through air back to the source???? Is the earth necessary for the electrons to jump through air??

Electrons do not jump through air! I think I might not have made that clear enough. It does not happen. (What never? No never. What Never? Well, hardly ever.) And if it did happen, the resulting current would be lower than a billionth of an amp, not anywhere near the 60 ma we are discussing here.

In any capacitor, when an electron is pushed to one of the plates, it stays there (at least until the next half cycle). This tends to build up electrons on that plate, and the plate gets an excess negative charge. Now apply the "like charges repel" rule on the opposite plate. An electron on the opposite plate sees the excess of electrons across the gap. It is repelled by their presence, and moves along the wire back towards the source. Therefore, although you do see current apparently going "through" the capacitor, the electrons entering on one side are never, never, the same as the electrons leaving from the other side.

All this works because there is a voltage source pushing electrons first onto one plate, then (on the other half cycle) pushing electrons onto the other plate. In the sketch provided by crossman, the voltage source will not be strong enough to push electrons from the conductive body back to the transformer, and even if it could push that hard, it would also have to push them through the insulation system to get to the transformer's secondary winding conductors.

 

[LarryFine]

The one thing I haven't seen directly said yet (unless I got cross-eyed during a catch-up reading session), but effectively described above by Charlie, is that capacitors must be charged.

They cannot be looked at as simple resistors. A real current flows during the charging of a cap (twice per cycle on AC) and, while it also occurs during discharge, caps are also not sources.

Now, how this all fits into the discussion, I don't know. I just wanted to throw something that at least seems profound into the fray. (BTW, I'm a 60ma voter.)

 

[zog]

Electrons do not jump through air! I think I might not have made that clear enough. It does not happen. (What never? No never. What Never? Well, hardly ever.) And if it did happen, the resulting current would be lower than a billionth of an amp, not anywhere near the 60 ma we are discussing here.

In any capacitor, when an electron is pushed to one of the plates, it stays there (at least until the next half cycle). This tends to build up electrons on that plate, and the plate gets an excess negative charge. Now apply the "like charges repel" rule on the opposite plate. An electron on the opposite plate sees the excess of electrons across the gap. It is repelled by their presence, and moves along the wire back towards the source. Therefore, although you do see current apparently going "through" the capacitor, the electrons entering on one side are never, never, the same as the electrons leaving from the other side.

All this works because there is a voltage source pushing electrons first onto one plate, then (on the other half cycle) pushing electrons onto the other plate. In the sketch provided by crossman, the voltage source will not be strong enough to push electrons from the conductive body back to the transformer, and even if it could push that hard, it would also have to push them through the insulation system to get to the transformer's secondary winding conductors.

Electrons dont jump through air huh? What is an arc? How do you know what the voltage is in Crossmans drawing? Ib elieve it is around 500,000V because this whole discussion started from a 500kV transmission line. Also the transformer secondary is not insulated, I know this because he is reproducing a experiment I did with a 100kV hipot.

You sure do make alot of assumptions and turn them into facts. I am not saying your theory is wrong at all, just your assumptions.

 

[RayS]

OK. Now I am going to go way out on a limb, and posit that in this new example, the ammeter connected to the conductive body would read higher, as it (the body) is now a much larger "sink" than the dangling wire.

Charlie- parasitic capacitance (in this instance) is the capacitance that occurs between the coil and the core/case of the xfmr- not designed in, but it is there due to the physical arrangement. I believe the current flow from this is not bound or limited to being "within" the main circuit.

 

[crossman]

the ammeter connected to the conductive body would read higher, as it (the body) is now a much larger "sink" than the dangling wire.

Hey Ray, :smile:

Your specualtion above is the same speculation we were arguing over in the "current returning to a different source" thread. Please allow me to give my thoughts and I certainly welcome your return comments. (note to moderators: I am being civil and am in no way demeaning Ray for his thoughts)

Your proposed principle above leads to some absurdities. For example, I am taking that you say "the larger the sink" the larger the current flow.

Absurdity 1) Given: Experiment in outer space, Conductive body connected to one side of AC source, dangling wire on the other side of AC source: If current increases for the larger conductive body or "sink", then if we start using larger and larger conductive bodies, then we will have larger and larger current flows to the conductive body. What if the conductive body is the earth itself? How much current would flow to and from the earth? it should be quite a large amount, right? Thousands of amps possibly? Well, we know that isn't true because there are thousands of grounded phase deltas working right now as we speak. And there is no large current flow from the phase to the huge "sink" of the earth.

Absurdity 2) Given: Experiment in outer space, Conductive body connected to one side of AC source, dangling wire on the other side of AC source: Say we have the means to connect a conductive body to the AC source only for the first quarter cycle. Current will be flowing to the conductive body and it will be gaining electrons.

At the peak of the first quarter cycle, we remove the conductive body from the circuit. We have trapped the excess electrons on the body. Then we take a new conductive body and do the same thing in the first quarter cycle. More electrons removed from the AC source. Now repeat over and over with new conductive bodies. Pretty soon, we will have deleted ALL of the electrons in the source. I am pretty sure that this is impossible.
----------------------------------------------------------------------

One thing we can't forget. When we speak of voltage sources, however many electrons are flowing out of the negative terminal of the source, there must be the same number of electrons entering the positive terminal of the source. This is basic physics. (Of course, something wild like a lightning strike to one terminal of the source may violate the rule, but that is way beyond and is definitely not what we are talking about)

So if we are moving electrons to the conductive body during the first quarter cycle, we are also pulling electrons out of the dangling wire. The amount of electrons moving to the conductive body will be the same exact amount that we pull out of the dangling wire. The dangling wire will become positive. And since the conductive body is negative, there is an electric field between the conductive body and the dangling wire. This electric field created by the source voltage is what allows electrons to flow onto the conductive body. It all works hand in hand. The conductive body can only accept as many electrons as the positive side of the circuit will allow. And the ammeters will always read the same. (again, we are talking theoretical experiments here, so things like resistance of the air, electrons flowing through the air around the meters and such are not what we are talking about. All of those things can be accounted for by basic electricity and resistors and ohm's law)

It is the capacitance of the dangling wire to the conductive body that determines the amount of current flow to the conductive body. Again, it is the capacitance of one body to the other that is what matters.

If we were to increase the area of the dangling wire, then you would notice the current to the conductive body going up. But both meters would read exactly the same.

Summing up: If you have a voltage source such as a xfmr secondary, and if electrons are pushed out of one terminal and cause a particular conductive body to become negative, then I can show you another conductive body that has become positive by the same exact amount.

Even with a conductive body connected to terminal X1 of a secondary and nothing connected to the X2 terminal, the same thing will still happen. If Terminal X1 sends electrons to the conductive body, Terminal X2 itself will become equally and oppositely charged (positive) compared to the conductive body. This is capacitance and the physicists have defined capacitance to account for the very observations of the experiments. There is not other "mystery force" that allows current to flow other than capacitance, resistance, and inductance, and these items are well understood and documented.

Conclusion: The amount of current flow from a given voltage source to an isolated conductive body is not determined solely by the characteristics of the conductive body. It is also determined by the characteristics of whatever conductive body is connected to the opposite terminal of the voltage source and the distance betwen the two bodies. . And these characteristics of the conductive bodies and distance between them is capacitance.

 

[winnie]

I claim that ammeter 2 will show different current than ammeter 1, by the parallel paths which bypass one or the other ammeters.

The current flowing through the circuit is made up of galvanic current flow (actual charges moving from place to place) and 'displacement current', which is the integrated _change_ in electric field surrounding the conductive items.

I claim that there will be parasitic displacement currents all over the place. If you simply consider the pair of electrodes formed by the large conductive body and the earth to be the only 'parasitic capacitor' present, then the current through the two ammeters will be exactly the same. But since there will be parasitic capacitors formed between the conducting body and the transformer, the transformer and the earth, etc, then there are parasitic capacitors which bypass the ammeters.

I suspect that these various parasitic capacitors between the transformer and the rest of the world explain the results that zog saw.

-Jon

 

[crossman]

Winnie,

I don't think there is anyone here who will deny that, in the real world, there may be some "parasitic" capacitance or extraneous resistive paths that could make the meters read different. However, these mechanisms operate under the same well-known laws of physics of capacitance and inductance and resistance that any voltage source with current flow operates under. There is no mystery there.

On the other hand, it would be possible to construct the above experiment in the real world with proper insulation and spacings, and symetrical placement of components, and meters installed as close to the secondary windings as possible, to make the meters read within 1% of each other and within 1% of what is theoretically expected to flow in the series circuit.

Of course my entire point is that any current flow in the diagram in the original post can be explained by the well-known mechanisms of capacitance, inductance, and resistance.

There is no mechanism involving the voltage of the source which would make current flow to/from the secondary of the xfmr to the conductive body without relying on a circuit completed by a resitive path, capacitance and an electric field, or inductance, or a combination of these 3 items.

 

[crossman]

I suspect that these various parasitic capacitors between the transformer and the rest of the world explain the results that zog saw.

I agree absolutely and totally! That is what I have been saying all along.

 

[RayS]

Hey Ray, :smile:
Your specualtion above is the same speculation we were arguing over in the "current returning to a different source" thread. Please allow me to give my thoughts and I certainly welcome your return comments. (note to moderators: I am being civil and am in no way demeaning Ray for his thoughts)

well, first lemme see if I can get the quoting right so I can reply by item.
Do not worry about me taking offense- I've enjoyed the debate so far, tho I haven't yet finished the big thread. Opinions welcome, cordiality appreciated.

Your proposed principle above leads to some absurdities. For example, I am taking that you say "the larger the sink" the larger the current flow.

I live in a universe that I will never completely understand- I am not God- this is a given. Therefore some things will seem absurd to me. This one, however, seems relatively simple.
Only thing I would add re:the size of the sink, is that I think the surface area is more important than the volume- the electrons don't want to "bunch up" due to like charges repelling.

 

[RayS]

crossman- I may edit some of the quotes for the sake of brevity. My apologies in advance if I change the intent of what you stated.

Absurdity 1) Given: Experiment in outer space, Conductive body connected to one side of AC source, dangling wire on the other side of AC source: If current increases for the larger "sink", then if we start using larger and larger conductive bodies, then we will have larger and larger current flows to the conductive body.

You got my opinion 100%, the larger the sink, the greater the flow.

What if the conductive body is the earth itself? How much current would flow to and from the earth? it should be quite a large amount, right? Thousands of amps possibly? Well, we know that isn't true because there are thousands of grounded phase deltas working right now as we speak. And there is no large current flow from the phase to the huge "sink" of the earth.

thousands of amps, no. Some 10s of mA, I'd say sure. In the poll you posted, the next current value up was 20mA more, that's what I voted for. If we put an ammeter inline with the XO bond to ground on a large xfmr, what does it read? I'd love to see someone measure this. Where does that current go? I say it does not have to return to the secondary terminals. As far as a calc for the value, we are admittedly at the edge (beyond?) my analytical capability- best thing I can do is hope someone else here wants to work it out. Winnie seems to be pretty sharp at this..... HELP! :smile: Best I can do is offer my gut feeling that to drive massive current into the earth, you would need either a correspondingly massive xfmr or have it connected to another massive sink on the other side of the xfmr. Ya gotta get some "traction" to push much load. I gotta chew on this a while

Absurdity 2) Given: Experiment in outer space, Conductive body connected to one side of AC source, dangling wire on the other side of AC source: Say we have the means to connect a conductive body to the AC source only for the first quarter cycle. Current will be flowing to the conductive body and it will be gaining electrons. At the peak of the first quarter cycle, we remove the conductive body from the circuit. We have trapped the excess electrons on the body.(1) Then we take a new conductive body and do the same thing in the first quarter cycle. More electrons removed from the AC source.(2) Now repeat over and over with new conductive bodies. Pretty soon, we will have deleted ALL of the electrons in the source.(3) I am pretty sure that this is impossible.

1-yes 2-yes (1 and 2 assuming negative going 1/4 cycle- otherwise you're removing electrons from the sink) 3-Hmm. seems like you would run out of charge given such a small source. I'm guessing that there would be less and less charge transfered each time- maybe even after the first round. Assume no source at all, just an open ended xfmr. Anything happen? I'm guessing once the xfmr as a whole goes so far positive from losing electrons, flow would stop.

One thing we can't forget. When we speak of voltage sources, however many electrons are flowing out of the negative terminal of the source, there must be the same number of electrons entering the positive terminal of the source. This is basic physics. (Of course, something wild like a lightning strike to one terminal of the source may violate the rule, but that is way beyond and is definitely not what we are talking about)

agreed completely. Charges must balance. Even in the lightning example, the charge is just transferred. I guess my point being that these charges can flow without a classical circuit being set up.

So if we are moving electrons to the conductive body during the first quarter cycle, we are also pulling electrons out of the dangling wire. The amount of electrons moving to the conductive body will be the same exact amount that we pull out of the dangling wire. The dangling wire will become positive. And since the conductive body is negative, there is an electric field between the conductive body and the dangling wire. This electric field created by the source voltage is what allows electrons to flow onto the conductive body. It all works hand in hand. The conductive body can only accept as many electrons as the positive side of the circuit will allow. And the ammeters will always read the same.

I am with ya here, maybe even re: the ammeters. In this example, removed from a ground connection, there's not much room to return current elsewhere.

It is the capacitance of the dangling wire to the conductive body that determines the amount of current flow to the conductive body. Again, it is the capacitance of one body to the other that is what matters.

and here is where I don't want to get hung up in the semantics. I feel this basic charge flow is not "capacitance" but a simpler precursor. I see it as simple charge flow, not requiring any capacitance between the 2.

If we were to increase the area of the dangling wire, then you would notice the current to the conductive body going up. But both meters would read exactly the same.

yes to the 1st part, and in your space model I cant see much of an alternate path to unbalance the ammeters.

Summing up: If you have a voltage source such as a xfmr secondary, and if electrons are pushed out of one terminal and cause a particular conductive body to become negative, then I can show you another conductive body that has become positive by the same exact amount.

agreed completely

Even with a conductive body connected to terminal X1 of a secondary and nothing connected to the X2 terminal, the same thing will still happen. If Terminal X1 sends electrons to the conductive body, Terminal X2 itself will become equally and oppositely charged (positive) compared to the conductive body.

Yup, adding that it wouldn't be just the terminal, but also the winding, core and housing.

This is capacitance and the physicists have defined capacitance to account for the very observations of the experiments. There is not other "mystery force" that allows current to flow other than capacitance, resistance, and inductance, and these items are well understood and documented.

simple charge flow. I say it can happen with 0 capacitance, driven solely by electrostatic forces or voltage.

Conclusion: The amount of current flow from a given voltage source to an isolated conductive body is not determined solely by the characteristics of the conductive body. It is also determined by the characteristics of whatever conductive body is connected to the opposite terminal of the voltage source and the distance betwen the two bodies.

.
Yes!

And these characteristics of the conductive bodies and distance between them is capacitance.

... ONE of the forces between them...

well, I hope to revisit this later, it's getting late

 

[RayS]

ok, my mind won't let this go yet.

In the example of the xfmr secondary connected to ground- current flow will be limited by the ability of the secondary voltage to strip electrons from the transformer and whatever is connected to the other end.

Assume no connections except 1 terminal to ground. During the first half-cycle, (assume negative going) the transformer will drive electrons into the ground conductively from 2 mechanisms:
1.-the electrons present in the windings driven directly by the voltage.
2.-additional electrons will be supplied from the windings driven by capacitive effects from the rest of the xfmr. (as the windings become less negative/more positive, electrons will flow toward them because opposites attract). These electrons cannot actually jump onto the winding, but the interaction will make it seem to deliver them. This is the parasitic source referred to earlier.

Second half cycle, the electrons come back. Repeat as long as voltage is applied. Current will flow. Thousands of amps? No. Milliamps? Sure. How much? Depends on the voltage, with HV transmission it could be significant, but I don't know how to figure the reactance to ground. This is why I say there can be unbalanced flow in the original example. Mechanism 1 may not count as it will influence both ammeters.

Found some info on medical isolation xfmr leakage, 100uA max per UL 2601.1, and this is a small, 1kva/120 volt model. Keep in mind that these are specially built to minimize leakage, and are built with electrostatic shielding. http://www.isobox.com/
For KV and MW I think we could be dealing with a few amps.

 

[zog]

I claim that ammeter 2 will show different current than ammeter 1, by the parallel paths which bypass one or the other ammeters.

The current flowing through the circuit is made up of galvanic current flow (actual charges moving from place to place) and 'displacement current', which is the integrated _change_ in electric field surrounding the conductive items.

I claim that there will be parasitic displacement currents all over the place. If you simply consider the pair of electrodes formed by the large conductive body and the earth to be the only 'parasitic capacitor' present, then the current through the two ammeters will be exactly the same. But since there will be parasitic capacitors formed between the conducting body and the transformer, the transformer and the earth, etc, then there are parasitic capacitors which bypass the ammeters.

I suspect that these various parasitic capacitors between the transformer and the rest of the world explain the results that zog saw.

-Jon

Right. and since ammeter #1 was internal to the test set and the HV lead is insulated, ammeter #2 would read lower.