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A question about electrical ground (with diagrams!)

  1. Apr 3, 2012 #1
    Ok, so I've been wrestling with the concept of ground for the past two weeks. I'm in an electrical engineering program at college, and let me preface this by saying I have no difficulty dealing with ground in a practical sense and understand its general function very well. That being said, I've posed a few hypothetical scenarios about ground to a few of my professors and none of them can really give me a straight answer.

    So the crux of the issue is whether ground acts as an infinite source/sink of electrons, or whether it acts as a return path for current. Based on my musings, the two seem to be mutually exclusive.

    Take hypothetical scenario A. So we have a DC power source, let's say it's a battery, but it could really be any power supply, not necessarily a battery. If you were to hook one end of the supply up to ground, and the other end to a long wire up to the moon with a light bulb on the end, and grounded it on the moon, would current not flow? For the purpose of this hypothetical, let's assume the moon is at 0v, or earth ground potential. My thoughts are that it would work because electricity flows from a higher potential to a lower potential, but if electricity needs a complete circuit, then this would seem to be impossible. Here's a crappy little diagram:
    http://dl.dropbox.com/u/3372365/earthtomoon.jpg [Broken]



    Ok, so if scenario A doesn't work, than does scenario B work? In this case the power supply is using two points on earth ground to complete the circuit.
    http://dl.dropbox.com/u/3372365/scenB.png [Broken]

    If neither of these two would work, than it seems like earth ground has absolutely no use, so one of them must work. If current doesn't in fact flow THROUGH ground, then ground must be an infinite source/sink for current. But if scenario A doesn't work because it needs a return path, than scenario B must work because it's using earth as a ground return.

    If scenario B would work, then it would seem to follow that scenario C would not work, but convention indicates that it would, because otherwise how would you ever get electrocuted by making a connection to ground?
    http://dl.dropbox.com/u/3372365/scenC.png [Broken]

    General feedback has said the first and third options are incorrect, yet isn't the whole point of grounding in the first place to prevent shock hazard? If ground needs a return path in order to conduct, then what's the point of grounding circuits in the first place? It would seem like it only allows people to be electrocuted more easily since essentially the surface of the planet becomes a return point for any electric current, whereas without grounding at all electricity can never escape the circuit except to bridge to another point in the same circuit.

    Furthermore, if the ground DOES act as a return path, how does it know where to travel to? If nearly all points connected to ground anywhere are at 0V, how does the current see a return path. Assuming both actual earth ground wires would measure 0V with reference to ground, so the potential difference between the respective grounding wires would be 0V, or some negligible voltage across some amount of resistance so no current would flow.


    If after reading everything so far, you're about to say "well earth does not act as a ground return, it just acts as a sink/source" then why wouldn't this last scenario actually turn the bulb off?
    http://dl.dropbox.com/u/3372365/scenD.png [Broken]

    I'm almost 100% positive the last scenario would not work, because you can't drain a battery by hooking one terminal up to ground.


    Someone clarify this for me if you can. I've run these past several people including two analog electronic instructors, an electrical physics instructor, several classmates and my Dad who knows quite a bit about electricity and everyone is giving me conflicting answers or is just plain confused by it altogether.


    tldr: Does current travel through the actual ground or does the ground simply sink and source current when appropriate?
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Apr 3, 2012 #2

    berkeman

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    Welcome to the PF.

    The correct answer is (b). The ground is a moderate conductor of electricity.
     
  4. Apr 3, 2012 #3
    In that case why is it that you can't drain a battery like in example D?

    edit: it would also seem that if A would work, than B would work also
     
  5. Apr 3, 2012 #4

    berkeman

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    Example D does not show a completed circuit. If the wire on the right went into the ground too, that would complete the circuit and light the bulb until the battery drained.
     
  6. Apr 3, 2012 #5
    Completed circuit seems like a loose term then, because example A does not 'complete' a circuit, but rather a single path with a starting point and a destination.
     
  7. Apr 3, 2012 #6

    berkeman

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    No. The bulb does not light in example A, and there is no shock in example C.
     
  8. Apr 3, 2012 #7
    In that case, what's the point of grounding in the first place? If you need to complete a circuit then it seems to me that grounding the circuit at any point just creates the potential for electrocution that wouldn't exist otherwise.

    (I do appreciate your advice, I'm just trying to wrestle my mind around this)
     
  9. Apr 3, 2012 #8

    berkeman

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    You are right that if you have a floating circuit, there is no danger of electrocution when a person forms a "ground fault" with that circuit. However, if the high voltage is accessible, then you can still get electrocuted if you grab two different parts of the circuit that are at different voltages.

    For example, if you are using an isolation transformer (1:1) to isolate power to a test circuit from the AC Mains, you are able to connect the ground clip of your oscilloscope to any single part of the circuit as a reference point, and no current flows in that ground connection. But if you try to connect your 'scope ground clips to two different points in the isolated circuit, you will likely fry your 'scope leads.

    In regular household AC Mains distribution (in the US), the Neutral wire is grounded to Earth Ground at the breaker panel. You can still get a little voltage developed on the Neutral wire as it goes farther into the house, since current is flowing through the finite resistance of the Neutral wire.

    One reason for grounding some devices and circuits is to provide a return path for transient currents like ESD and Surge hits. After you walk across the carpet and build up a static charge on yourself, when you touch metal objects, that charge tries to return to "ground" most of the time (either in a DC sense or an AC transient sense). Having a grounded metal enclosure on a product helps to protect the circuitry inside from having that transient current go through it.
     
  10. Apr 3, 2012 #9
    I like your drawings.
     
  11. Apr 3, 2012 #10
    So what sort of range is electricity capable of moving through ground? In the case of major power surges due to shorts to ground it would seem like the current would have to travel all the way through the ground to the power station or transformer terminal. In either case, what dictates that each current flow back to their respective transformers?

    I guess what I'm having trouble with is how can ground form a complete circuit if multiple independent circuits use ground as a return? It would seem like current could just as easily flow to my neighbors transformer ground as it could to my transformer ground. If that is the case, then I don't see how it's considered a complete circuit and my scenario A should theoretically work.
     
  12. Apr 3, 2012 #11

    berkeman

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    It's just V=IR Ohms Law at work.

    Picture a circuit that has a voltage source on the left and a voltage source on the right, and a network of resistors in the middle. The currents and voltages in the network of resistors are defined by the connections and the resistor values, and you can solve for them in a number of mathematical ways (have you learned KCL equations yet?). The currents flow in complete circuits.

    Now, whether a particular electron that was pumped out by one voltage source returns to that same voltage source eventually -- we don't know that. But the same number of electrons will return to the source as were pumped out by it (it's all a continuous flow out and in).
     
  13. Apr 3, 2012 #12
    Well, I understand all of that but how does current flow through the ground if both the grounding wires are at 0V potential?

    A better question might be how does the grounding wire at my transformer know to absorb X amount of electrons when my house has a power surge to ground of X number of electrons? I'm having trouble understanding the "complete circuit" part of it.

    I'm very familiar with Ohm's law and all the other circuit staple formulas. I'm well into complex analog components at this point like op-amps and transistors so it shouldn't be a problem in that sense.

    Again I just don't get how it's a complete circuit if the current doesn't flow back to its respective transformer ground, and if it does flow back to its respective transformer ground then I don't get how it knows to. Wouldn't both ground wires be held at virtual ground?
     
  14. Apr 3, 2012 #13

    berkeman

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    I think you are getting tripped up on the idea of "same potential". Think of a short circuit wire. The wire has a finite resistance, so even though we call it a "short circuit", there is a finite voltage drop across it. If there were no voltage drop, there would be no current (Ohm's Law again).

    And in fact, Earth Ground has a resistance, so there will be voltage drops when there are currents flowing.

    Also, remember that with AC currents like with the AC Mains, the electrons are just moving back and forth in place, not actually flowing anywhere. And for most current levels, they hardly move at all (not very far). It is the AC potential around the complete circuit that couples all the electron movements together.
     
  15. Apr 3, 2012 #14
    I think the issue I'm having is separating scenario A from scenario B. The method of action you're describing seems to indicate that there is no direct path from one ground to another, rather all overflow currents get assimilated in ground and any wires drawing from ground just pull electrons out of it. If that's the case, then I wouldn't really call it a complete 'circuit' and I would think scenario A would work.

    Sorry if I'm seeming stubborn. It's not that I am trying to argue a point, I'm trying to wrap my mind around this because the logic seems to be in conflict.
     
  16. Apr 3, 2012 #15

    berkeman

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    No worries. The Earth and Moon are electrically isolated from each other by the vacuum of space. If you connected a battery between them with a very long wire, the Earth and Moon would be at different voltages (different by the battery voltage).

    The dirt and groundwater of the Earth are what provide the paths for ground currents.
     
  17. Apr 3, 2012 #16
    That still doesn't answer how they find their way to their respective circuit origins. Like if what I'm understanding is true, then you could stick a ground peg in your neighbor's lawn and apply a really negative voltage and draw all of his return current out of the ground, which would break the complete circuit to his power transformer and his power would shut off.
     
  18. Apr 3, 2012 #17

    davenn

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    forget about individual electrons its seems to be what is tripping you up. we are not dealing with an electron labelled A another labelled B etc etc
    we are talking about the mass general movement of electrons .... countless numbers of them

    no cuz to "BREAK" the circuit you would have to pull his ground peg out of the ground
    then you would break his circuit, otherwise his circuit is still intact

    Dave
     
    Last edited: Apr 4, 2012
  19. Apr 3, 2012 #18

    davenn

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    100's of kms when you are dealing with the hi voltage potentials of transmission lines

    Even the lower voltage potential used in a telephone circuit ~48 - 50VDC will happily go 10's of kms. single wire above ground and using a ground return

    Dave
     
  20. Apr 3, 2012 #19

    davenn

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    As Berkman said in Diag-A the earth and the moon are electrically isolated
    the only 2 ways it could work would be
    1) if you brought the surfaces of the earth and moon into contact
    (I suggest there would be a lot of people unhappy about that plan :eek: )
    2) if you pegged a very long wire into the moon's surface and pegged the other end of it into the earths surface. Then you would have a return path for your circuit

    Dave
     
  21. Apr 4, 2012 #20
    Well first of all it's my understanding that ground is never used as a return path except in situations where there is an extreme amount of voltage where there shouldn't be, like if lightning strikes a house or some wires cross or something.

    But I still have difficulty with this return path thing. If it's not an electron-by-electron movement then it would seem irrelevant whether the two grounds were electrically isolated. In this case, current flowing into ground is dissipating to everywhere an since you can't pull earth ground up high it's not like the return ground wire would notice whether it's flowing into a current sink here with no explicit return path, or an isolated current sink on the moon or wherever.

    In both cases you have a high potential grounded to a low potential sink that cannot be pulled up above 0V so we're back to the scenario A confusion again.
     
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