current returning to a different source

[winnie]

We discussed this video clip before, and I did some calculations. See post 18 in this thread http://forums.mikeholt.com/showthread.php?t=86536

While capacitors are generally two terminal devices, you can assign a capacitance value to a single electrode, by assuming that the other electrode is the 'rest of the universe at infinity'. The capacitance of a sphere (relative to the rest of the universe) is a well known equation, and a pretty good approximation to the capacitance of a sphere some distance from the earth.

I estimated that the helicopter would form a 330pf capacitor to the earth.

Okay, take a 500kV 60Hz source, and connect one terminal to earth, and the other terminal to a sphere big enough to form a 330pf capacitor. About 60mA will flow through this circuit.

When the helicopter is bonded to the line, something like 60mA is flowing through the bond wire. It might be 6mA, or it might be 600mA; this was not a particularly accurate estimate. But the current flowing between the line and the helicopter, charging and discharging that capacitor will be significant. Prior to connecting the bonding wire (or after disconnecting it), any arc that forms will carry nearly that same charging current. I know from experience that a 20mA 60Hz arc is quite noticeable!

I would like to note that in the circuit that I am describing, the arcing is _not_ caused by the capacitance between the line and the helicopter. Capacitive current flow does not entail electrons flowing between the capacitor electrodes. Instead electrons flow into
(or out of) the electrodes, causing an electric field between the electrodes. The capacitive current flow between line and ground does not cause an arc. Similarly the capacitive current flow between line and helicopter would not cause any arcing. But we have a series circuit; the capacitance from line to helicopter and the capacitance from helicopter to ground. When the helicopter is close to the lines, the line-helicopter capacitor breaks down, arcing over to supply the current flowing through the helicopter-ground capacitance.

-Jon

 

[winnie]

Crossman,

You mentioned something about what happens when you rub wool on glass and then separate the wool and the glass. I have a thought experiment for you:

Construct a standard parallel plate capacitor, out of two aluminium plates 30cm square, separated by a 0.5mm thick teflon sheet. One of the aluminium plates is sitting on an insulated table; the other aluminium plate has an insulated handle on it. Charge this capacitor by connecting the plates to a 12V battery, and then disconnect the battery. Now pick up the plate with the handle and move the plate to a distance of 10cm from the stationary plate. What is the voltage between these two plates?

-Jon

 

[ELA]

I had struggled with understanding this issue in another previous thread on the helicopter to line arcing during its approach. I tend towards the capacitance explanation but am still open to another idea.
I am a practical sort having moved from electrician to technician to engineer in my carrier. I often struggle with understanding when it is a purely theoretical discussion. I tend to attempt to prove things to myself when I do not understand via experimentation. If I can built it , test it and see the results, then I can more easily accept it.

Is there a way that someone can redirect this discussion to another example that could possibly be proven through experimentation? Without too much danger involved.

Could this concept be scaled down to a high voltage transformer output and a small transmission line example? In the picture I have attempted to suggest a possible 20:1 or so scale down.


Click to see image

Based on the “non capacitive” theory would it be correct to say that as the floating metal piece came close to the wire (and reached the air breakdown point) that we could establish and sustain a low current arc? (Between the energized wire and the floating metal piece.)
Is it suggested that it would not matter how far the plate was above the ground plane? Or for that matter that the ground plane did not matter or that -neither did the return connection of the transformer to the plane? (C1 is not relevant?)

If this is the supposition then it would be interesting to see the results of such a test.

I can accept that the electric field could alternately repel and attract free electrons in a metal piece near the field. I intuitively have a hard time envisioning a current flow without a return path though.

Edit by iwire, image to large, changed to link

 

[crossman]

I would like to note that in the circuit that I am describing, the arcing is _not_ caused by the capacitance between the line and the helicopter. Capacitive current flow does not entail electrons flowing between the capacitor electrodes. Instead electrons flow into
(or out of) the electrodes, causing an electric field between the electrodes. The capacitive current flow between line and ground does not cause an arc. Similarly the capacitive current flow between line and helicopter would not cause any arcing. But we have a series circuit; the capacitance from line to helicopter and the capacitance from helicopter to ground. When the helicopter is close to the lines, the line-helicopter capacitor breaks down, arcing over to supply the current flowing through the helicopter-ground capacitance.

Thanks Winnie for the information and description of what is happening. That is perfectly reasonable and logical and your commentary ties everything together for me..... capacitance.

 

[rattus]

I had struggled with understanding this issue in another previous thread on the helicopter to line arcing during its approach. I tend towards the capacitance explanation but am still open to another idea.
I am a practical sort having moved from electrician to technician to engineer in my carrier. I often struggle with understanding when it is a purely theoretical discussion. I tend to attempt to prove things to myself when I do not understand via experimentation. If I can built it , test it and see the results, then I can more easily accept it.

Is there a way that someone can redirect this discussion to another example that could possibly be proven through experimentation? Without too much danger involved.

Could this concept be scaled down to a high voltage transformer output and a small transmission line example? In the picture I have attempted to suggest a possible 20:1 or so scale down.

Click to see image


Based on the “non capacitive” theory would it be correct to say that as the floating metal piece came close to the wire (and reached the air breakdown point) that we could establish and sustain a low current arc? (Between the energized wire and the floating metal piece.)
Is it suggested that it would not matter how far the plate was above the ground plane? Or for that matter that the ground plane did not matter or that -neither did the return connection of the transformer to the plane? (C1 is not relevant?)

If this is the supposition then it would be interesting to see the results of such a test.

I can accept that the electric field could alternately repel and attract free electrons in a metal piece near the field. I intuitively have a hard time envisioning a current flow without a return path though.

ELA,

Place this sheet in a steady electric field, and you will see a negative charge on one side and a positive charge on the other. The field terminates on the negative charge and picks up again on the positive. Equal and opposite forces act on the plate so it doesn't move. In an alternating field, the charges swap sides 60 times a second.

Now, distributed capacitance exists between the line and the ground, and capacitive current flows. But, as Winnie says, electrons do not flow through the air. Only the electric field changes.

Now, suspend this plate horizontally between the line and ground. There is capacitance from line to plate and from plate to ground. That is, we have have two caps in series, and that is the mechanism providing the complete loop.

There is capacitance everywhere, we cannot avoid it. You can forget any notion that the arc can be explained by science fiction.

Edit, changed image to link

 

[rattus]

Winnie,

Thank you for this ray of light in this sea of darkness.

I would argue though that your one plate capacitor has zero value since the dielectric thickness is infinite. As you know,

C = e0 x A/d

 

[crossman]

You mentioned something about what happens when you rub wool on glass and then separate the wool and the glass. I have a thought experiment for you:

I have actually already been thinking of similar experiments while I was thinking about this thread. Not sure as to the conclusion.

Construct a standard parallel plate capacitor, out of two aluminium plates 30cm square, separated by a 0.5mm thick teflon sheet. One of the aluminium plates is sitting on an insulated table; the other aluminium plate has an insulated handle on it. Charge this capacitor by connecting the plates to a 12V battery, and then disconnect the battery. Now pick up the plate with the handle and move the plate to a distance of 10cm from the stationary plate. What is the voltage between these two plates?

My first thought is that there will still be 12 volts between the plates. There is still the same negative charge on one plate and the same positive charge on the other plate.

Let's assume that the movable plate will receive the negative charge and have excess electrons and the stationary plate will be positive. It doesn't seem that distance would affect the voltage once the capacitor is charged.

However, I am thinking a bit further, let's say we seperated the plates by 4,000 miles. At some point, the electrostatic attraction of the negative charges on the movable plate to the positive charges on the stationary plate will be so weak that the electrons on the movable negative plate will be free to wander about "as they please", and the positive charges on the stationary plate would attract some electrons out of the insulating table and the air to "equalize" the charges in that localized area.

In other words the electrons on the negative plate would no longer be concentrated on the side of the plate closest to the positive plate. These "excess electrons" would actually equalize throughout the plate and possibly even into the insulator handle and human body and maybe even into the air around the plate. I say this because even though the handle is an insulator, it can still have excess electrons on it just like the glass rod with excess electrons after being rubbed with wool. The same thing can happen to the positive plate. It would begin acquiring electrons out of the air and the insulated table. So, the charges would spread to larger areas.

Now, if this experiment took place on the earth, I think the charges would actually equalize on the whole earth... the excess electrons would migrate through the human body into the earth, and the positive charges would end up taking electrons from the earth via the table, and the whole thing would equalize leaving no voltage.

The above could occur only when the electrostatic attraction between the charges on the plates is weaker than the electrostatic attraction from each plate to localalized objects.

On the other hand, if this took place in outer space with no matter in the vicinity of the plates, I suppose we may have the 12 volts regardless of distance?

So my answer is going to be a big "hmmmmmmm?"

Your thoughts?

 

[jghrist]

I had struggled with understanding this issue in another previous thread on the helicopter to line arcing during its approach. I tend towards the capacitance explanation but am still open to another idea.
I am a practical sort having moved from electrician to technician to engineer in my carrier. I often struggle with understanding when it is a purely theoretical discussion. I tend to attempt to prove things to myself when I do not understand via experimentation. If I can built it , test it and see the results, then I can more easily accept it.

Is there a way that someone can redirect this discussion to another example that could possibly be proven through experimentation? Without too much danger involved.

Could this concept be scaled down to a high voltage transformer output and a small transmission line example? In the picture I have attempted to suggest a possible 20:1 or so scale down.

The problem with actually trying to experiment with this is that the capacitances are going to be so small that measuring any voltages, even with a high impedance digital meter, will alter the circuit significantly. The small charge will be drained off by the voltmeter and you will measure nothing. Trying to measure the extremely small currents involved will be even more difficult.

 

[crossman]

Click to see image

Very nice diagram. I think it would make a great experiment. Just be careful!

Edit: Jghrist is probably correct. The metering may be difficult. Still, with proper equipment, I think it could be done.

 

[ELA]

I never said anything about measuring voltages (meter loading understood). This test would just note if a sustained arc could be held regardless of the capacitance.

Yeah I think I will just accept the capacitance idea. 25KV is still a bit high for me:smile:

With regard to Winnies test:

V=Q/C with a fixed amount of charge on the plates if you increase the distance between the plates the capacitance decreases. This causes the voltage to increase.

I had done lots of static electricity testing.
Charge your body up up, measure the voltage with an electrostatic voltmeter and now step up on a ladder (note the increase in voltage).

 

[winnie]

I would argue though that your one plate capacitor has zero value since the dielectric thickness is infinite. As you know,

C = e0 x A/d

Correct, except that the above formula is an approximation that ignores 'fringe effects'. Fringe effects are significant extending outward from the edges of the plates by about the plate separation. When the plates are infinitely far apart, then all you have are fringe effects.

Seriously, one way of thinking of capacitance is to consider the change in potential between the electrodes when a unit charge is moved from one electrode to the other. You can perform this exact analysis if you define potential in terms of the work necessary to bring a charge in from 'infinity', and you look at the change in potential of the single electrode as unit charges are brought in from infinity.

-Jon

 

[winnie]

I never said anything about measuring voltages (meter loading understood). This test would just note if a sustained arc could be held regardless of the capacitance.

Just a thought: wouldn't those 'plasma globe' things be an example of this sort of capacitively coupled arcing? When you touch the outside of the globe, you get a discharge between the center electrode and the glass. Electrons are not flowing through the glass, and you don't get a shock, but I think that the person is completing the circuit via capacitive coupling. The frequency is rather higher than 60Hz, so that very small capacitances result in large currents.

V=Q/C with a fixed amount of charge on the plates if you increase the distance between the plates the capacitance decreases. This causes the voltage to increase.

Exactly. The plates are attracted together by electrostatic forces, and the mechanical work of moving these plates against the electrostatic force is stored as potential energy. The process of mechanically moving charged objects apart is the basis of various high voltage electrostatic generators. The 'Van de Graff' generator works by using moving belts which mechanically carry charge around.

-Jon

 

[iwire]

Just a thought: wouldn't those 'plasma globe' things be an example of this sort of capacitively coupled arcing?

This site agrees with you.

 

[crossman]

The plates are attracted together by electrostatic forces, and the mechanical work of moving these plates against the electrostatic force is stored as potential energy. The process of mechanically moving charged objects apart is the basis of various high voltage electrostatic generators. The 'Van de Graff' generator works by using moving belts which mechanically carry charge around.

That makes perfect sense concerning the thought experiment you gave me.

So as the plates move apart, the voltage increases. And in a perfectly empty universe, the voltage would increase indefinitely.

But considering a real universe populated with planets and all sorts of diverse matter, the charges in the seperated plates would actually become "coupled" to other bodies in the space and the situation would become rather different?

 

[winnie]

So as the plates move apart, the voltage increases. And in a perfectly empty universe, the voltage would increase indefinitely.

But considering a real universe [...] become "coupled" to other bodies in the space and the situation would become rather different?

Yup. Mechanically pull apart charges, and eventually _something_ will break

For example, there is a device known as a Pelletron http://www.pelletron.com/charging.htm which has a 'charging chain'. This chain is something like a ball chain, consisting of little metal cylinders separated by insulating links. The chain runs in a continuous loop around a set of pulleys. After going around the 'base' pulley, the chain passes through a device which makes each link in turn part of a charging circuit, and the link gets charged to 50kV relative to the surrounding 'ground'. The link is then pulled along to the other pulley.

As the link is mechanically pulled away from 'ground', carrying it is little bit of charge, up to the other pulley. At the other pulley there is a device which discharges this little capacitor to the local electrode.

This entire process can charge the insulated electrode up to 25MV. While we are talking about 'static' electricity, that chain is carrying charge and continuously moving...so you actually see a current flow, of perhaps 0.1mA. Quite impressive, delivering 2500W at 0.1mA DC

But as you note, interaction with the rest of the universe makes things more complex. If that insulated electrode were in air, then various forms of discharge would prevent the voltage from ever getting close to 25MV. So the entire system is mounted in a large vacuum chamber. But 25MV makes spooky stuff happen, such as ripping electrons right off the surface of your insulators. These electrons get accelerated by the electric field, and hammer into other insulating surfaces, knocking electrons free. At these voltages, surfaces need very careful design, and insulators still fail quite impressively.

-Jon

 

[iwire]

there is a device known as a Pelletron http://www.pelletron.com/charging.htm which has a 'charging chain'. This chain is something like a ball chain, consisting of little metal cylinders separated by insulating links. The chain runs in a continuous loop around a set of pulleys. After going around the 'base' pulley, the chain passes through a device which makes each link in turn part of a charging circuit, and the link gets charged to 50kV relative to the surrounding 'ground'. The link is then pulled along to the other pulley.

Isn't that just a variation of a Van de Graaff generator?

 

[winnie]

Isn't that just a variation of a Van de Graaff generator?

Exactly.

You are using a metal-insulator-metal chain, rather than an insulating belt.

This lets you get to higher voltage gradients, and is cleaner for operation in a vacuum chamber.

But it is the exact same principal of operation.

-Jon

 

[crossman]

But as you note, interaction with the rest of the universe makes things more complex. If that insulated electrode were in air, then various forms of discharge would prevent the voltage from ever getting close to 25MV. At these voltages, surfaces need very careful design, and insulators still fail quite impressively.

As Iwire notes, it seems similar to the Van De Graf generator. I was looking at that in the physics text last night. Thank you for confirming my suspicions on how the charges would interact with other matter.

Hey, I have a good feeling about all this discussion, it seems we have actually come to a satisfactory conclusion of the mechanism at work with the helicopter.

 

[crossman]

And now, my apologies, but I would like to post a "finalized" thought on what is going on with the helicopter, just to confirm my thought process. Hopefully this will be of benefit to others. Of course, as always, if my thinking is incorrect, I welcome any commentary. Rick?

In the following, we will view only one phase wire and the earth. We assume a high voltage AC 60Hz source at point on the sine wave when the powerline is connected to the negative terminal of the source and the earth is connected to the positive terminal of the source. (We will ignore the current in the powerline which is flowing to the load because that current does not contribute to the phenomenon at hand. That current simply flows to the load and then back on the other circuit conductors.)



In Diagram 1, the positive terminal of the source has pulled some electrons out of the earth and has sent some electrons into the powerline. This is current flow due to the capacitance between the earth and the wire and this current could not exist without the electrostatic field that is set up between the earth and the wire. This is capacitance.

So, we have a positive charge (holes) on the earth and a negative charge (excess electrons) on the wire.

Diagram 2:

The helicopter approaches. When it is far away from the powerlines, it is essentially neutral in charge. It contains the same number of holes as electrons.

Diagram 3: As the chopper approaches the negative wire, the helicopter electrons which are closest to the wire are repelled by the excess electrons in the wire. Like charges repel.

This leaves the left hand side of the chopper with extra positive charge (holes) and the bottom and right hand side end up with extra electrons which were repelled out of the holes.

Now, it isn't just the negative powerline which is repeling the electrons. There is another force at work here. The positive charge in the earth is also attracting the helicopter electrons to the bottom of the chopper.

Diagram 4: As the chopper gets closer to the wire, the effect becomes more pronounced. More and more electrons are pushed-pulled to the right/bottom of the chopper. More holes are vacated on the left of the chopper.

Again, this redistribution of charge in the helicopter is entirely due to both the repulsion of the like charges in the chooper and wire, and the attraction of the opposite charges in the chopper and earth.

Finally, in diagram 5: As the distance between the chopper and wire diminish, there comes a point when the resistant of the air can no longer prevent the excess electrons in the wire from jumping into the holes in the chopper. Current flows from wire to chopper, and the chopper becomes the same potential as the wire.

In the video clip of the helicopter doing maintenance, the electrode probe serves to focus the electric potential of the charges to a small point and facilitates the breaking down of the air resistance in a small area.

So there you have it. All explained by capacitance.

 

[winnie]

Okay, I went back and reviewed the earlier part of the thread, in particular the point that Rick Christopherson was making about closed paths not being required for current flow. I am going to play with this a bit.

I think that most of us agree that a capacitor can be part of a closed circuit. A capacitor does not have actual flow of charge between its electrodes; instead it has a changing electrostatic field. So rather than saying 'current can only flow in a closed circuit', it is probably more accurate to say 'when current flows in an open circuit, a changing electric potential develops'. This changing electric potential acts to block the flow of current; the capacitor charges up and the current flow drops.

But if you had an _infinite_ capacitance, then current could flow forever through this 'open' circuit.

*queue the sci-fi music here*

I bet that one could build an infinite capacitance with a couple of black holes and a couple of space stations

If someone else brings the black holes, I'll bring the beer. Then we can set up an infinite capacitance and run a TV with the open circuit and watch the game.

-Jon