Re: Purpose of ground rod?
Originally posted by bthielen: I'm not so sure that I agree with the statement "current is trying to get back to its source." If this were true, why would it leave its source in the first place?
The more deeply you attempt to delve into the precise nature of Nature, the less firmly you must hold to your favorite clich?s. We would be well served if we could take the clich?, ?electricity takes the shortest path to ground,? and forever obliterate it from our language. It conveys very little truth, and opens the door to many misunderstandings. However, I would hesitate to cast doubts upon the clich?, ?electricity is seeking its own source.? It is, in fact,
untrue! But we can safely ignore that fact. For the purposes of our profession, it holds
enough truth, and presents very few opportunities for misunderstanding. What follows is a deeper delving into the truth of electron flow than is needed for a member of the electrical profession. Feel free to stop reading, if you fear becoming confused.
In the presence of an electric field (as may be imposed by a battery), an electron will feel a force. [Aside: So too with a proton that sits in the nucleus of a copper atom along the length of a copper conductor. But the protons are not as free to move about as are the electrons, so we?ll speak of them no more.] In the presence of a magnetic field that is ?moving? (i.e., varying in strength or direction, as may be imposed by the stator of a generator), an electron will feel a force. If either force is strong enough to overcome the attraction that the electron has to its host atom, the electron will move. That creates two unstable entities: One is the electron, which would be happier (i.e., stable) if it were to be attached to an atom. The other is the atom from which the electron moved away. The atom would be happier if it were to gain back an electron. Nature doesn?t like such instabilities, and quickly acts to eliminate them. It does so by having the electron attach itself to the next atom in line, and by having some other electron (from further back along the wire) attach itself to the atom that lost the first electron. Since the electric field (or the moving magnetic field) is still present, electrons will continue to be knocked free of their host atom, and will continue to jump from atom to atom along the line.
So is any given electron trying to get back to its source? Well, the answer is ?No,? if by the question we mean, ?Is the electron trying to get back to the atom to which it was attached before we threw the ON switch?? No, once any electron has found another atom that had been missing an electron, whether that atom is part of the copper wire, or part of the battery, or part of a human, or part of planet Earth, then it will be perfectly happy to remain there forever. But it might not remain there, if the electric field (or moving magnetic field) is still present.
So when we say that ?current is trying to get back to its source,? the truth is that the source is capable of imposing the electric field (or moving magnetic field) along the entire closed path of the circuit for the one any only one reason that the path is, in fact closed. If you break the path (e.g., by opening the switch), then the source cannot impose its field along the entire path. Therefore, electrons do not feel the force, and remain happily attached to their host atoms. But so long as the path is complete, so long as the field is present along the entire path, so long as electrons are kicked out of their happy homes (i.e., their host atoms), then the most readily available place an electron can find to re-attach itself to a different host atom is along the path. In the due course of time, moving along the path will bring the electron back to the source.
