current returning to a different source

[Rick Christopherson]

The only part I disagree on is the implication you cannot have an electrical potential difference (aka voltage) without current.

You are nit picking the words that I typed, when this is not what I was attempting to say. When I typed that sentence, it was assumed that a path existed for the current to flow--and throughout this entire discussion, that has been the electrode the worker is holding.

 

[crossman]

Here is my thinking on the subject of current flow without a complete path:

Look at the diagram. To my mind it represents what Rick has been saying about current flow without a completed circuit. The conductor forms a path between the two bodies. Certainly, current can flow from the negative body to the positive body. Of course, the current flow will decrease with time as the charges equalize.

But the point is, this is absolutely a completed circuit. The two bodies seperated by distance are, by definition, a capacitor. So the conductor is simply completing the path from one side of a capacitor to the other.

Now, say we have some means to make each body alternate from negative charge to positive charge. There still exists a capacitance between the two conductive bodies, and therefore is a completed circuit.

I would speculate that there is no such thing as an open circuit, when we consider that capacitance exists everywhere.

 

[sdv]

I can understand you guys not being too thrilled with the "Earth to the Moon" analogy:smile: , so lets consider something a little more down to Earth.

Have you ever seen utility workers work on ultra-high voltage power lines? They fly to the line with a helicopter and reach out with an electrode to bring the helicopter to the same potential as the power line. This electrode begins to arc a foot or two away, and continues to arc until it makes contact. It is pretty obvious that there is a substantial electron flow in this arc, correct?

Now consider if the helicopter just hovered with the electrode 6 inches away from the line so you could see the arcing. It would continue to arc indefinitely. If you put an ammeter in the middle of the electrode, you would see a substantial AC current flowing through the electrode, yet there is no return circuit.

This isn't contradicting Kirchoff's Law, as a matter of fact, Kirchoff's Law specifically accounts for this.

Yes, the helicopter is acting a a large capacitor, but nevertheless, AC current is flowing between two voltages without a return. If you replaced the ammeter with a light bulb, it would light up.

I'm jumping in without reading all of the comments; however, in response to Rick...you absolutely have to have a completed circuit (return path) to the source for current to flow. No "reasonable" exceptions. The return path could be capacitive coupling (air), which is still a return path to the source. With any potential voltage to ground, there is capacitive coupling that results in leakage current. The higher the capacitance, the lower the capacitive reactance, then the higher the leakage current. If I have an ungrounded voltage source and touch a phase conductor, the current flow will be equal to the vector sum of capacitive coupling in parallel with the resistance (me). Sine there is no intentionally return ground path to the source, this current can be very low.

As for the helicopter, the capacitive coupling is between the power line and ground. Basically the line is on long capacitor with air as its dielectric The helicopter is introduced between these two points so that the capacitive coupling becomes the closest distance between the line to the helicopter (dielectric - air ), the helicopter itself (dielectric - metal), and the closest distance from the helicopter to ground (dielectric - air). Therefore you have three impedances between the line and earth, and you have leakage current. This creates a voltage drop or stress across each impedance. Note?a sharp rod is used to increase the stress and initiate an arc before the helicopter gets too close to the line. Rick is incorrect, the arc current is very low?and the arc energy is very low. The heat and blast of a high energy (I^2R) arc would require the electrician to wear diapers ?if he survived.

 

[zog]

You are nit picking the words that I typed, when this is not what I was attempting to say. When I typed that sentence, it was assumed that a path existed for the current to flow--and throughout this entire discussion, that has been the electrode the worker is holding.

You will find this forum excels at that, welcome to the forum Rick. BTW you are correct on this helicopter issue, maybe another example would help. Racking in a 15kV circuit breaker, no current path exists, right before the stabs touch the bus you hear a hissing noise, whats that noise??

 

[rattus]

sdv,

The metal fuselage is not a dielectric, it is the common plate of multi-section capacitor. There is capacitance to ground, to each of the phase conductors, to the towers, etc., etc, etc. This capacitance provides the return path for the arc current.

After contact is made, this capacitance appears in parallel with the line to ground capacitance, and the current continues.

Fact is, there are jillions of stray capacitors in our world, but mostly we con't care!

 

[Smart $]

You are nit picking the words that I typed, when this is not what I was attempting to say. When I typed that sentence, it was assumed that a path existed for the current to flow--and throughout this entire discussion, that has been the electrode the worker is holding.

Just as long as we're clear on it...

"...any time you have a difference in voltage, you will have current flowing", provided there is a conductive path.

As far as the nit picking, you are correct... I am. Nothing out of the ordinary here, as zog attests.

 

[Rick Christopherson]

While I was writing this, there were several new postings that came in, and now I am not sure if this is applicable or not. It has been a long day and my brain has already shut down for the evening, so without any forethought as to whether this is still applicable, I am going to post it anyway. If necessary, I'll make my apologies after I've had a cocktail and relaxed a bit. :grin:

The two bodies seperated by distance are, by definition, a capacitor. So the conductor is simply completing the path from one side of a capacitor to the other.

It is human nature to equate an unknown topic to one we are more familiar with, and capacitors are something we can see, feel, and understand. However, the primary mechanism in this situation is not capacitance between the helicopter and the line. This is a basic charge transfer problem that we all learned in freshmen physics. I know it is a lot to ask, but blow the cobwebs off your old text books and look up the sections on charge transfer. Everything I am saying is right there in the text. There is nothing exotic about this.

I don?t remember all of the mathematical equations that go along with this topic, and that is why I stated in the beginning that I am not willing to formulate a mathematically defensible proof. However, I do recall all of the basic concepts. All you would have to do is flip through a few pages, and look at a couple of diagrams, and all of this information should come rushing back to you. You?ve seen it before, but you just don?t use it on a daily basis--nor do I. But that doesn?t mean that is doesn?t exist.

This discussion is getting to the point of being cyclical, where people are reposting the same ideas without reading the rest of the thread. I have addressed these same questions more than once. There is no point in repeating myself any further. Please, read the whole thread, get your old text books out, and then postulate an explanation that hasn?t already been discussed.

 

[crossman]

I will make a point to check out my Physics texts tonight. Rick, regardless of which side of the debate each of us is on, I thank you for getting the ball rolling. For me, it is certainly enlightening and entertaining!:smile:

 

[Rick Christopherson]

Just as long as we're clear on it...

"...any time you have a difference in voltage, you will have current flowing", provided there is a conductive path.

As far as the nit picking, you are correct... I am. Nothing out of the ordinary here, as zog attests.

Yes, we are clear, and yes, you did correctly nail me on this. But it was more like a "typo", than an actual mis-statement. We are on the same page for this particular issue, and no, I did not take it personally. I appreciate your response--it was very professional.

Now I gotta go figure out what to make for dinner before I pass out from hunger. All this thinking takes a lot of energy.:grin:

 

[crossman]

I know it is a lot to ask, but blow the cobwebs off your old text books and look up the sections on charge transfer.

Actually, it isn't asking that much. I routinely search the reference material and texts, and derive joy from the exercise and knowledge thereby gained. And now, since I did what you asked and have come to some conclusions, I trust that you will stick around a bit longer and suffer through my lengthy assertations, and perhaps offer some suggestions and corrections if my thinking is wrong.

the primary mechanism in this situation is not capacitance between the helicopter and the line. This is a basic charge transfer problem that we all learned in freshmen physics.

Concerning the helicopter: After reading the texts, I am going to disagree with the above quote. The basic transfer of charges that you mention above is not what is happening here. For charges to transfer between two bodies using this mechanism, there must be an excess of charge in at least one of the bodies. (Having said that, I agree with your original premise that current can flow without a completed circuit when we speak of "excess charge". Excess charge can flow from one body to another, but the following commentary concerns the helicopter.)

Disregarding any "outside sources" of static charge from the rotating blades or wind or whatever, I am still saying that capacitance is responsible for the current flow between the helicopter and the powerlines.

In our case, there is no excess charge in the power line or in the helicopter (ignoring any static from other sources such as the rotating blades, wind movement across the surface of the helicopter, etc etc).

To support the above hypothesis, let us consider the powerline as a long series circuit including the source (assume an AC generator) and the load. If the generator is turned off and is not creating EMF, then the powerlines, the generator, and the load are all neutral in charge. In other words, there is no excess charge in any part of the system (again ignoring static from outside sources). All components of the system are electrically neutral.

Since I am claiming capacitance as the mechanism allowing the current flow to the helicopter, and you are claiming it is the flow of excess charge, let's first assume there is no capacitance in the powerlines at all. When we turn on the generator, certainly electrons start flowing, but there are no "excess" electrons created or introduced into the system. The electrons which were present under no voltage are the same exact electrons which are present when the generator produces voltage. The electrons are certainly moving, but they are not bunching up anywhere in the system creating an excess charge. The electrons are simply flowing from the generator, through the powerlines, through the load, and back to the generator through the powerlines. There is no excess charge created in any of the components. The amperage in any part of the powerlines is the same as in any other part of the powerlines.

Take any given sample of the above circuit at any given time and measure the number of electrons contained in the sample. There would be the same amount of electrons in the sample whether the generator was pushing current through the system or whether the generator was turned off. Remember, we are ignoring capacitance at the moment.

As a specific example, take a five foot long section of the powerline and isolate it by putting a switch at each end in series with the rest of the circuit. With the generator off, count the number of electrons in the wire. Then turn on the generator and get some current flowing. Now open both switches simulataneously to isolate the 5 foot section. Count the number of electrons present. Do this at several different points in the sine wave. You will discover the exact same number of electrons in the isolated wire for each of the above situations.

For every electron that enters one end of the 5 foot piece of wire, there is one electron which leaves the other end of the wire. No excess charge is created by the generator.

Based on the above, there can be no excess charge produced by the generator to flow to the helicopter. Rick, if I am wrong, then I would like to hear a theory of where this excess charge is coming from. Where are the excess electrons coming from?

Now, consider the same scenario with capacitance. Is it possible for isolated portions of the circuit to have an excess of electrons? Yes. But only if some other portion of the circuit has an absence of electrons. And that is where capacitance comes in. We are dealing with a finite number of electrons in the system. To build a negative charge in one portion of the circuit requires that some other portion of the circuit becomes positive. Again, that is capacitance.

If we assume that our powerline/helicopter system is a closed system, that the only electrons in the system are those in the circuit and the helicopter, then the only excess charge which could possibly flow to the helicopter is through capacitance. If there are any other electrons which could transfer to the helicopter, they would have to come from some other "static electricity" source which is outside of the helicopter/powerline system.

Rick, I would like to ask the following question: During the half-cycle where electrons are flowing from the helicopter to the powerlines, what is the source of those electrons?

 

[jghrist]

Rick, I would like to ask the following question: During the half-cycle where electrons are flowing from the helicopter to the powerlines, what is the source of those electrons?

When the powerline is in the half cycle that has a voltage higher than ground, aren't there excess electrons in the wire?

 

[jghrist]

One thing to consider in this discussion is that actual electron flow in a wire is pretty slow. One source I found http://www.newton.dep.anl.gov/askasci/phy00/phy00989.htm says about 2 cm/sec. My Electrical Engineering Science textbook says 1.14 x 10^-4 m/sec at a current density of 1000 amp/in?. The voltage of the powerline only stays positive for 0.00833 second (1/2 cycle) before it goes negative.

This means that the electrons only get pushed down the wire toward the helicopter (or the wire hanging in free air in my post #75) a very short distance before they start getting pulled the other way by the voltage. Depending on which flow velocity you use, this distance is either 0.0167 cm (0.00656 inches) or 9.5 x 10^-5 cm (3.7 x 10^-5 inches). Not very far in either case.

 

[qcroanoke]

One thing to consider in this discussion is that actual electron flow in a wire is pretty slow. One source I found http://www.newton.dep.anl.gov/askasci/phy00/phy00989.htm says about 2 cm/sec. My Electrical Engineering Science textbook says 1.14 x 10^-4 m/sec at a current density of 1000 amp/in?. The voltage of the powerline only stays positive for 0.00833 second (1/2 cycle) before it goes negative.

This means that the electrons only get pushed down the wire toward the helicopter (or the wire hanging in free air in my post #75) a very short distance before they start getting pulled the other way by the voltage. Depending on which flow velocity you use, this distance is either 0.0167 cm (0.00656 inches) or 9.5 x 10^-5 cm (3.7 x 10^-5 inches). Not very far in either case.

OW,OW,OW!!!!
My head is hurting again!! LOL!!

 

[zog]

One thing to consider in this discussion is that actual electron flow in a wire is pretty slow. One source I found http://www.newton.dep.anl.gov/askasci/phy00/phy00989.htm says about 2 cm/sec. My Electrical Engineering Science textbook says 1.14 x 10^-4 m/sec at a current density of 1000 amp/in?. The voltage of the powerline only stays positive for 0.00833 second (1/2 cycle) before it goes negative.

This means that the electrons only get pushed down the wire toward the helicopter (or the wire hanging in free air in my post #75) a very short distance before they start getting pulled the other way by the voltage. Depending on which flow velocity you use, this distance is either 0.0167 cm (0.00656 inches) or 9.5 x 10^-5 cm (3.7 x 10^-5 inches). Not very far in either case.

Most think of current as the flow of electrons when it is actually the transfer of energy between them, that energy transfer travels near the speed of light, but as you said the actual electron flow is slow.

 

[crossman]

When the powerline is in the half cycle that has a voltage higher than ground, aren't there excess electrons in the wire?

If we omit capacitance and static from outside sources, then there are no excess electrons in any of the system at any given time.

To produce "excess electrons" at a given point in any system, there are only two ways to do it. It can be done through "static electricity" such as rubbing a wool cloth on a glass rod, or it can be done through "capacitance".

Are there any other ways to create excess electrons at a point in a system so that these electrons are free to flow to other bodies?

Rick said that the electron flow to the helicopter was not through the mechanism of capacitance. That leaves the only other choice being static electricity.

 

[jghrist]

To produce "excess electrons" at a given point in any system, there are only two ways to do it. It can be done through "static electricity" such as rubbing a wool cloth on a glass rod, or it can be done through "capacitance".

Doesn't a dc generator produce a voltage between it's terminals? Or a battery? Isn't this voltage an excess of electrons in one terminal compared to the other? If you connect the two terminals, won't you have a flow of electrons?

 

[crossman]

Doesn't a dc generator produce a voltage between it's terminals? Or a battery? Isn't this voltage an excess of electrons in one terminal compared to the other?

The only excess electrons created at the terminal of a generator are those due to the capacitance between the positive terminal of the generator and the negative terminal of the generator.

Assume a universe where capacitance did not exist. Assume a 100 volt DC generator with an open circuit = infinite resistance between the positive and negative terminals. There would be no electron movement in the generator other than the normal random movement.

My point is, if the generator sends an electron out to the negative terminal, this causes an absence of an electron at the positive terminal, and this is absolutely due to the capacitance between the terminals.

If you connect the two terminals, won't you have a flow of electrons?

Of course. But nowhere will there be an "excess of electrons" except that allowed by capacitance.

I mean, we can't create electrons out of nothing. We can't suck electrons out of the air without causing that air itself to be charged. The only way a generator can cause an excess of electrons to build up at a given pont in the circuit is if another point in the circuit is building up with a positive charge. And there will be an electrostatic field between the two opposite charges, and that is capacitance.

As for the battery, it is chemical reactions that cause the electron flow, and without this flow, the chemical reaction will not take place, therefore, a battery does not build up excess electrons on one terminal and excess positive charges on the other terminal, save for a tiny bit of electron movement, again due to capacitance between the terminals.

 

[crossman]

Rick, or anyone, here is a question that deserves an answer. Look at the diagram and give me your thoughts. it should be obvious there is more to it than just "left" or "right". I already know the answer. But I need someone else to commit so I can continuee this line of thinking.

 

[jghrist]

Rick, or anyone, here is a question that deserves an answer. Look at the diagram and give me your thoughts. it should be obvious there is more to it than just "left" or "right". I already know the answer. But I need someone else to commit so I can continuee this line of thinking.

It doesn't depend on the current in the line, only on the voltage.

 

[jghrist]

The only excess electrons created at the terminal of a generator are those due to the capacitance between the positive terminal of the generator and the negative terminal of the generator.

I suppose it's a matter of semantics, but to me, the electrons are pushed to one side by the action of the generator or battery, and this results in a voltage across the capacitance between leads. I can't picture the capacitance causing the excess of electrons.