current returning to a different source

Status
Not open for further replies.
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Dave,
IMO Kirchhoff's Current Law applies to the OP, but I'd consider the source as the utility generator, not the many transformers along the line.
Click to expand...
It doesn't work that way...each transformer is a new source for the purpose of current flow. Now given the fact that the secondary of many transformers is grounded, there will be a current path, but for the current to flow it must return to the transformer that it came from.
Don
 
Rick Christopherson

Rick Christopherson

Senior Member
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.
 
dereckbc

dereckbc

Moderator
Staff member
Location
Plano, TX
Rick Christopherson said:
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.
Click to expand...
Yep it is a free-air staic discharge developed between the rotor blades on the chopper and over-head lines. Once equalized it is gone.
 
Last edited:
W

wasasparky

Senior Member
Bait & Switch

Bait & Switch

Rick Christopherson said:
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.
Click to expand...

How about we discuss the OP? Of course that seems to be answered...
 
Rick Christopherson

Rick Christopherson

Senior Member
dereckbc said:
Yep it is a free-air staic discharge developed between the rotor blades on the chopper and over-head lines. Once equalized it is gone.
Click to expand...
I agree that rotor blades develop a static charge, but that is not the primary mechanism in this case. You are correct, once the rotor charge is dissipated, it will go away, but in this example above, the arcing and current flow will continue long after any charge on the rotors is neutralized.

If it was a DC transmission line, then the arcing would stop when the helicopter reached its full charge.
 
R

rattus

Senior Member
Capacitive reactance:

Capacitive reactance:

The helicopter fuselage forms one plate of a capacitor to ground. I guesstimate this to be in the order of 10pF on the low side. Then with 230KV across this cap, the current would be in the order of 1mA. Natheless this scenario has no bearing on the OP's question. He is speaking of a conductive path.

Indeed, as Crossman has said, the AC circuit is completed through a capacitor.
 
76nemo

76nemo

Senior Member
Location
Ogdensburg, NY
Rick Christopherson said:
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.
Click to expand...

Maintenance like this Chris???

http://www.youtube.com/watch?v=9tzga6qAaBA&mode=related&search=
 
iaov

iaov

Senior Member
Location
Rhinelander WI
Rick Christopherson said:
This is probably going to raise a few hackles, but any time you have a difference in voltage, you will have current flowing. It does not matter if there is a completed circuit or not. The monkey wrench in this statement is that in order to have a difference in voltage, there will generally need to be a common reference point, and in most cases, this will end up being a return current path.

I once got into this discussion a long time ago on a different forum, but the participants were the same as this forum. Frankly, it is too complex of a discussion to get into, and I don't want to try to defend it again. I just put this out there as "food for thought". It is possible, but I don't want to get into the drawn out discussion of defending it.

I registered for the forum because I wanted to ask a question, but this topic caught my eye. Sorry.:)
Click to expand...
You've raised my hackles a bit. When ever you have two different potentials you have the potential (no pun intended) for current flow but without a completed path there is no current flow!
 
iaov

iaov

Senior Member
Location
Rhinelander WI
Pierre C Belarge said:
let met start by saying I am not an engineer. Also, I am not an expert in the mathematics of this issue.

Here is how I see this based on the limited knowledge I have developed over my career.

Maybe you were mistaken about the statement you made above??



Current flowing from source-1 can travel along a conductive path through another conductor from a different source-(2), if there is some connection between the conductors of source-1 and source-2.

But...
I do not believe source-1 will push current along any conductive path to return to source-2. The current does not want to go from source-1 to source-2 regardless...it would only flow from source-1 THROUGH source-2 if it was flowing along a path between both sources.

If source-1 current were to flow from source-1 through a conductive path to source-2, there would be some kind of problem developing during this process.


"Say I have a 120V single phase transformer-1 which is sending current out via the hot leg of transformer-1 to a device, however the the nuetral coming off of the device does not return to the neutral of transformer-1, but rather the neutral of a different transformer I'll call transformer-2"


This is not a difference of potential creating this current flow, it would be current flow somehow finding a path through the source-2 as it heads back to source-1.



Now, if you could prove this to be otherwise, I would be very interested to see how.


Remember, the world was once thought to be flat, until someone was able to circumavigate around the globe without falling off. :wink:
Click to expand...
You are right on the money on this one Pierre.:smile:
 
Rick Christopherson

Rick Christopherson

Senior Member
rattus said:
The helicopter fuselage forms one plate of a capacitor to ground. I guesstimate this to be in the order of 10pF on the low side. Then with 230KV across this cap, the current would be in the order of 1mA. Natheless this scenario has no bearing on the OP's question. He is speaking of a conductive path.

Indeed, as Crossman has said, the AC circuit is completed through a capacitor.
Click to expand...
No, it is not a capacitor to ground. If the helicopter acted as a capacitor to ground, the wire would do the same thing, and the line would be constantly discharging to ground on its own, and would be very inefficient in transmitting power. This is why the higher voltage lines are strung at greater heights above the ground.

Watch that video that the other posted linked to. It is exactly what I am referring to.
 
R

ronaldrc

Senior Member
Location
Tennessee
This conversation has just blown my belief in how loads switches and everything else I thought I knew about basic electricity works.

And I'm too old to go back to school.:D
 
roger

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
ronaldrc said:
This conversation has just blown my belief in how loads switches and everything else I thought I knew about basic electricity works.

And I'm too old to go back to school.:D
Click to expand...

Ronald, what if you and I car pooled to classes? :grin:

Roger
 
Status
Not open for further replies.
Top