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Question on electricity. . . .

  1. Aug 25, 2006 #1
    Imagine the following conductive structure built outside:


    Basically, a T-shaped piece of metal, with one end stuck in the ground, one end pointed into the air (A), and one end horizontal, parallel to the ground (C).

    Let's say lightening strikes the structure at point A. Current flows down until it arrives at point B. Here it has to make a 'choice', and my understanding is that all of the current proceeds to the ground. I.e. a measurement at point C will never show any electric current there (assume we're in a vacuum so the ambient 'air' is not conductive).

    How does the electricity 'know'--without ever investigating the horizontal element--the way to ground? When the first element of current arrives at point B, why does it automatically proceed to ground? How does it 'know' this is the 'right' way to go?

    Thanks in advance.
  2. jcsd
  3. Aug 25, 2006 #2
    The electric field will also push electrons in part C of your structure, resulting in a pile up off charge (measurable as a voltage). Charge being pushed in part C just can't go anywhere. Moreover, don't think of the current as just a fast train of electrons: the electrons might not be moving fast at all, but the field pushes ALL electrons almost instantaneously, resulting in the current. There's also a finite probability that the electrons WILL go from part C to the ground (static discharge). It's just highly unlikely.

    Edit: Let me clarify my usage of 'electric field' here: when the first electron starts to move this would cause a nonzero electric field wich has to be compensated, resulting in the aforementioned current.

    I hope I got this all right.
    Last edited: Aug 25, 2006
  4. Aug 27, 2006 #3
    Let's start with the lightning first. Why does a lightning strike the conductor in the first place? Because there exists a height dependent electric potential. Since the potential depends only on height, points B and C would have the same potential and so no charge would flow between them.
  5. Aug 30, 2006 #4
    Let's forget the issue of lightning totally: at the top of the structure (A) we have a car battery that, when a switch is thrown, discharges electricity into the structure.

    Here are the questions:
    (1) Will a measurement of current at (C) be zero? Remember that this contraption is in a vacuum so there is no conductivity between (C) and the ground.
    (2) Regardless of how we want to parse it, when the switch is thrown, current will begin to flow from the battery towards the ground. When current reaches (B), how does it "know" to proceed to the ground, and not to point (C)? Is it not the case that locally there is no difference between the little piece of metal heading to ground and the little piece of metal heading to (C)?
  6. Aug 30, 2006 #5
    1) Yes, nil current at C.
    2) Imagine a flood of electrons from A. When they reach the junction B they do begin to pour in both directions but the ones that go towards C quickly start to just pile up, since they can't get any further, whereas the rest (those going towards the bottom) freely flow out into the ground. Compared to the bottom of the T, this leaves the C arm more negatively charged (repelling any further electrons), so the vast majority of the current from A must now flow directly to the ground.
    Last edited: Aug 30, 2006
  7. Aug 31, 2006 #6
    Thanks. A couple follow-ons, if you don't mind:
    (1) How does that first electron to reach the vertex B 'choose' which way to go? The two paths are identical to the electron; what is the mechanism that determines which path is traveled?

    (2)After the charge has "pile[d] up" in the arm, how do the electrons arriving at B 'detect' that and make the 'choice' to go to ground?
  8. Aug 31, 2006 #7


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    You are using words that imply that electrons are intelligent and you simply need to drop that concept to understand what is going on. This concept is exactly the same as how water would flow in an analagous situation. "The path of least resistance" simply means water or electricity flows where there is less obstruction. How does water "know" not to flow through a dam? It doesn't - it just hits the dam and stops.
  9. Aug 31, 2006 #8
    Yes, but in my situation (1) there is no "obstruction"--both paths are locally utterly identical to the electron. Yet my understanding is that it will "choose" the one to ground each and every time.

    And in (2), the same argument exists until the charge has "pile[d] up" in C to such a degree that electrons physically prevent movement into the arm. otherwise, it's action at a distance which doesn't make any sense to me. . . .
  10. Aug 31, 2006 #9


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    Not so, the path, B - C - Ground has a larger resistance than that of B - Ground.
  11. Aug 31, 2006 #10


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    No. As already said, the instant the power is turned on, some electrons will flow into the non-grounded part. But they will quickly start to "pile up" and physically/electrically obstruct the path of other electrons.

    Imagine water flowing through a trough with a Y in it. On one side of the Y, the trough keeps going, on the other side, it is blocked after a little bit. If you start pouring water into the trough, it will flow both into the open and obstructed sections until it fills up the obstructed section and only flows through the open section.

    For your piece of metal, btw, the obstruction is the air around it.
    The way you worded it is a little confusing. I'll reframe what happens:

    When you first connect the battery, there will be an initial inrush of current into the entire object (part 1 of what happens). But remember, the object is already pretty much filled with electrons, so if you could take a photo of the object before and after you flipped the switch, they'd be pretty much indistinguishable. The inrush also takes only a tiny fraction of a second. And btw, since "c" is the end of the object, there will be no current measured there even during inrush.

    After reaching equilibrium (part 2 of what happens), electrons are physically obstructed from moving through the unconnected section of the object, and thus only move through the grounded section.

    This really is relatively straightforward and wizzart answered it quite adequately in the first reply. I'm not sure what it is you are misunderstanding.
  12. Jul 31, 2009 #11
    why dose lightning go towards the ground? how dose it "know" that that is the way to go to get to the ground? why dose it not just go which ever way has less resistance? assuming all ways around it is air why dosent it go sideways or up if the resistance there is just marginally less than down
  13. Jul 31, 2009 #12


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    Staff: Mentor

    Lightning does not always go towards the ground, it goes in the direction where there is the least resistance and most voltage. Sometimes that is a meandering path toward the ground and sometimes that is toward a cloud.
  14. Jul 31, 2009 #13


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    Sometimes it does go up also - Lightning Sprites.

    This will give you a good idea of what lightning is doing during a strike. Note the multiple attempted paths that it takes at the beginning. It's trying to find the path of least resistance - Slow Motion Video of Lightning.
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