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

Discussion in 'General Electronics Chat' started by Gerry Rzeppa, Sep 10, 2019.

  1. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
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    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?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You Would see, Or DID see? o_O
    It is AC after all!
    Max.
     
  3. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
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    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.

    I know. That's why I said that the electrical potential difference would vary, many times each second.
     
    Last edited: Sep 10, 2019
  4. kubeek

    Expert

    Sep 20, 2005
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    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.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    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.
     
  6. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
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    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: Sep 10, 2019
  7. kubeek

    Expert

    Sep 20, 2005
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    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.
     
  8. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
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    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"?
     
  9. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
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    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: Sep 14, 2019
  10. BobTPH

    Senior Member

    Jun 5, 2013
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    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
     
  11. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    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.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    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.
     
  13. nsaspook

    Expert

    Aug 27, 2009
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    There are special probes for that.
    https://www.rossengineeringcorp.com/hv-measurement/hv-probes-multimeters/hv-probes.html
     
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  14. kubeek

    Expert

    Sep 20, 2005
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    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: Sep 14, 2019
  15. BobaMosfet

    AAC Fanatic!

    Jul 1, 2009
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    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.
     
  16. BobaMosfet

    AAC Fanatic!

    Jul 1, 2009
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    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).
     
  17. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
    170
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    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.
     
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