What is happening in those wires? (Take 2 - Macro/AC example)

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
Here is a picture of some electrical transmission lines, many miles from both the generating plant and the end users:

electric transmission lines.jpg

If I could hook a scope between one of those three wires and the ground, I would see that the electrical potential difference between the wire and the ground is varying, dramatically, many times each second.

Surely something in and/or around that wire, at the atomic level or above, must be changing to account for this effect. What is it?
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
You Would see, Or DID see? o_O
I expect that I WOULD see, on the scope, a sine wave of greater magnitude but otherwise similar character as the wave I DO see when I connect the probes between the "hot' side of a socket in my home and a metal water pipe partly buried in the ground. I don't see how this is critical to the question.

It is AC after all!
I know. That's why I said that the electrical potential difference would vary, many times each second.
 
Last edited:

kubeek

Joined Sep 20, 2005
5,718
Surely something in and/or around that wire, at the atomic level or above, must be changing to account for this effect.
Around the wire. The electric field around the wire is changing with the voltage relative to ground. Nothing inside the wire changes, the wire has no clue as to at what potential relative to anything else it is.
 

MaxHeadRoom

Joined Jul 18, 2013
20,435
AFAIK the HV transmission wires are not referenced to earth ground at the source or destination.
Only the LV distribution end.
My neighbour is a engineer in the transmission dept, I will check with him.
BTW, the HV transmission versions we have here are 2 wire H.V D.C. ;)
Max.
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
Around the wire. The electric field around the wire is changing with the voltage relative to ground. Nothing inside the wire changes, the wire has no clue as to at what potential relative to anything else it is.
Thanks. Three questions:

Where does this "electric field around the wire" come from, if "nothing inside the wire changes"? (The electric field around a radio transmitter antenna, for example, comes from the movement of electrons in the antenna; and the magnetic field around a permanent magnet comes from the combined spins of a lot of aligned electrons.)

If the field around the wire is the cause of the potential differences, why does the probe have to actually touch the wire to get an accurate reading?

Why doesn't this field have some kind of effect on the electrons in the wire?
 
Last edited:

kubeek

Joined Sep 20, 2005
5,718
The electric field comes from the source, i.e. a capacitor or other EMF source. An ideal conductor has no electric field within itself (therefore the electrons inside are not affected), it only conducts the field that was placed on it by the source, and basically elongates the boundary where this field is present.
Say you have a battery, its electric field around the two contacts would look like this https://i.ytimg.com/vi/H6lb46AiXHQ/maxresdefault.jpg
If you connect two wires to it, it would look like this since the wires are at the same potential throughout https://www.physicsforums.com/attachments/electric-field-lines-between-two-plates-jpg.105526/

You can measure electric field without being in contact, but that only gives you the direction and intensity at that point in space. To get the complete potential difference, you would either need to measure all the points from one conductor to the other with an electric field meter and do a sum, or you just connect voltage meter to the two conductors.
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
An ideal conductor has no electric field within itself (therefore the electrons inside are not affected), it only conducts the field that was placed on it by the source, and basically elongates the boundary where this field is present.
Say you have a battery, its electric field around the two contacts would look like this https://i.ytimg.com/vi/H6lb46AiXHQ/maxresdefault.jpg
If you connect two wires to it, it would look like this since the wires are at the same potential throughout https://www.physicsforums.com/attachments/electric-field-lines-between-two-plates-jpg.105526/
Okay, let's talk about the second of those two diagrams:

Electric field lines between two plates.jpg
It appears that the field is related to the opposing net charges in the wires (indicated by the + and - symbols in the diagram). Since this field must reverse itself many times each second (we're talking about AC power transmission lines) don't those net charge concentrations have to reverse as well? How, then, can it be that "the electrons inside [the conductor] are not affected"?
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
Four years and one life-threatening heart attack ago, I raised similar questions on this and other forums. These questions have bothered me throughout the whole interim, and are still unresolved in my mind. So now, fully recovered, I thought I would "poke the bear" again in the hope that different people would be frequenting the forums (four years later) and that we might make progress, together, in settling these unresolved issues.

MOD: links Deleted
 
Last edited by a moderator:

BobTPH

Joined Jun 5, 2013
2,348
You start with a false premise and then blame us for not supporting it. You will never get the answer that you want because you won’t abandon your false premise. Have fun.

Bob
 

Tonyr1084

Joined Sep 24, 2015
4,381
Look up "Right Hand Rule". Basically when electrons are traveling through a wire there's a magnetic field circling the wire. Those lines of magnetic flux can induce a current in another conductor or other items, namely your scope.

By the way - what the heck are you doing touching high voltage transmission lines for? And I don't personally know of a scope that is capable of measuring AC voltages as high as those transmission lines may be operating at. I'm afraid to scope my electrical outlet using my scope. Don't want to blow it out.
 

crutschow

Joined Mar 14, 2008
24,725
How, then, can it be that "the electrons inside [the conductor] are not affected"?
The are affected only to the extent that in one wire they are slightly more closely packed and in the other slightly less packed as determined by the voltage/electric field.
You can look at the wires as the two plates of a capacitor (with a very low capacitance per unit length).
It's not that complicated.
 

nsaspook

Joined Aug 27, 2009
7,181

kubeek

Joined Sep 20, 2005
5,718
It appears that the field is related to the opposing net charges in the wires (indicated by the + and - symbols in the diagram). Since this field must reverse itself many times each second (we're talking about AC power transmission lines) don't those net charge concentrations have to reverse as well? How, then, can it be that "the electrons inside [the conductor] are not affected"?
Thinking about it, this is because the wires have some capactiance, and now it seems that there is no way to clearly explain what is happening if they are neglected, so thinking about electric fields only without taking capacitance into account makes no sense.

So, even though the conductors are perfect, and inside the conductor the charge is constant, there will be surface charge on the outermost atoms of the wire, and this charge is directly linked to the voltage and capacitance between the two wires. So in a nutshell, nothing inside the wire changes, but with AC a tiny portion of the electrons that reside at the surface are being moved in and out of the wires, and there will be an AC current flowing related to the capacitance and frequency.

Its interesting how the high school physics and first year of college still seem to be back there somewhere in the memory, more than ten years later.
 
Last edited:

BobaMosfet

Joined Jul 1, 2009
1,063
Here is a picture of some electrical transmission lines, many miles from both the generating plant and the end users:

View attachment 185866

If I could hook a scope between one of those three wires and the ground, I would see that the electrical potential difference between the wire and the ground is varying, dramatically, many times each second.

Surely something in and/or around that wire, at the atomic level or above, must be changing to account for this effect. What is it?
You're failure is ignorance of how electric generation is handled. Those lines are 'actively' controlled, between substations and generation in order to minimize losses. What you see on main transmission lines is not what you see below your local transformer.
 

BobaMosfet

Joined Jul 1, 2009
1,063
Thanks. Three questions:

Where does this "electric field around the wire" come from, if "nothing inside the wire changes"? (The electric field around a radio transmitter antenna, for example, comes from the movement of electrons in the antenna; and the magnetic field around a permanent magnet comes from the combined spins of a lot of aligned electrons.)

If the field around the wire is the cause of the potential differences, why does the probe have to actually touch the wire to get an accurate reading?

Why doesn't this field have some kind of effect on the electrons in the wire?
You're asking basic, 101 level questions, and would be better served by reading a book like this from the start:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

All conductors are inductors. All electron movement has an electromagnetic component as it is a property of electrons. What you think of as the 'field' is the electromagnetic component in terms of concentration of electrons, and is subject to reactance (inductive and capacitive).
 

Thread Starter

Gerry Rzeppa

Joined Jun 17, 2015
170
You're asking basic, 101 level questions, and would be better served by reading a book like this from the start:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

All conductors are inductors. All electron movement has an electromagnetic component as it is a property of electrons. What you think of as the 'field' is the electromagnetic component in terms of concentration of electrons, and is subject to reactance (inductive and capacitive).
Thanks for the advice. I dug through the book piles in my office and found the answers to my questions in the second volume of this:

https://www.amazon.com/dp/0470503475

Sorry for any inconvenience.
 
Top