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Where does electricity go after ground?

  1. Nov 17, 2014 #1
    Hello,
    I am sure that this question has been answered on here before, but i could not find it. Why is the earth ground, and where does the electricity go after it gets there. I understand that ground is potentially lower than that of the power company's production etc. but i am curious as to why. Isn't the earth just basically a giant resistor? So if i had a hypothetical Massive resistor the size of the earth would it be a good ground? Where do the electrons go inside of the earth, do they just stop moving?
    I have so may questions about the actual electrons behavior and also some good hypothetical questions.

    Thanks if anybody can help!

    Jeff Davis
     
  2. jcsd
  3. Nov 17, 2014 #2

    Simon Bridge

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    Because it is very big.

    ... it pretty much stays there until something separates it out again.

    Simple answer: the power company takes electricity from the ground as part of the power generation, so the power supplies must be at a higher potential than the ground - which is where it came from.

    No - the Earth is much much more complicated than that.
    The electrical component "resistor" is an idealization that only works on a small scale.

    Yes. So would a large conductor.

    ... usually they hand about near the surface and take part in chemical reactions.
    No. Look up "telluric current".

    The Earth is a very complicated system and it's charges are constantly moving around.
     
  4. Nov 17, 2014 #3

    psparky

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    In the case of a residential home, I think you could also say that current will flow through ground and then back to the grounded secondary of it's nearest transformer. I like to refer to this as trickle current. And since it's AC its actually back and forth trying to always keep ground at 0 volts.

    KCL states that the current leaving the transformer must return.

    So the electrons through the earth definitely do not stop moving, just the opposite.
     
  5. Nov 18, 2014 #4
    Grounding is also a relative zero volt point. Grounding electrical equipment stops it from having a floating voltage reference. In AC power circuits (single phase) there is a neutral conductor for the flowing of current. The grounding conductor is there to trip the fuse in the event that the "live" circuit shorts to ground. Three phase circuits also utilize a neutral conductor. If all three phases are balanced nothing flows in the neutral conductor.
     
  6. Nov 19, 2014 #5
    Personally I do not like some of the feedback you are getting on this valid question. The short answer is the Utility ( or source) is connected to the ground, (primarily for safety reasons but there are others) - if you connect a circuit to ground - it completes the circuit back to the source. So it does - in essence behave as a large resistance, but there are then many factors affecting the grounds ( Earth's) resistance back to the source. Some power systems are intentionally un-grounded, in these cases if you connect the energized conductor to ground not much typically happens.
    We use earth as ground because it is conductive - and it then serves as a common reference point. Also note the term electricity is relatively broad for the question - you should in this case think about current, and voltage.
    It is common to try to relate basic circuits - like a voltage source (utility) and earth with relation to electrons, but I believe this overcomplicates the issue, you do not need to know or think about individual electrons. Much like when we talk about water flow we do not think about molecules of H2O...
    It is fine to understand the behavior of electrons as well - but their individual behavior is determined by their immediate environment - so trying to understand what the electrons are doing under your feet, relative to how the utility is connected, possibly hundreds of miles away is again too complicated.
     
  7. Nov 21, 2014 #6
    the earth ground is the return path or low resistive path..
     
  8. Nov 23, 2014 #7
    Ground can actually mean one of two things.

    The first is just a common negative connection in a circuit, not a literal connection to the Earth. For instance, instead of having several different wires connecting to the negative terminal of the battery, you might connect the negative of the battery to a piece of metal and have all of the negative connections just connect to that metal, ie, a "chassis ground".

    The second is the case of the literal ground. I think a lot of people get this mixed up.

    It makes a little more sense to think of the ground as a reference potential than as a source of electrons. When you want electrons to do work, you need to give them a conductive path between a high potential and a low potential. Now, AC current oscillates between positive and negative, but the question is, positive and negative relative to what? Basically what we do is we define the ground's potential arbitrarily as zero. When the AC wave is positive, the potential in the circuit is higher than ground and electrons go from the line into the Earth via the neutral pin. When the wave is negative, the circuit's potential is lower than ground, and electrons are pulled out of the ground via the neutral pin and into the circuit.

    Basically, in such devices, the potential difference that provides the energy for the circuit isn't between the negative and the positive terminal, it's between the positive terminal and the ground or the negative and the ground depending on which phase of the AC cycle the circuit is in.

    You also can use it as a reference for comparison when taking measurements. For instance, you might have a ground pin for a safety feature called ground fault interrupt even in a device that doesn't depend on a constant reference to ground. This is basically a simple comparator that activates if it senses an abrupt change in potential between the circuit and the ground, usually, it triggers the breaking of the circuit in response thought it might also have other functions.
     
  9. Nov 23, 2014 #8

    davenn

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    in an AC circuit, the electrons don't really travel any significant distance. They are just oscillating back and forward about a given point at whatever frequency is being used

    and again as in your response in another thread ... be careful describing where electrons are originating and which way they are moving

    Dave
     
  10. Nov 25, 2014 #9
    Thank all of you for your with my question, and i am sorry that i have not been more interactive in this post. I got very busy for a while. What i am gathering from you guys then is that, and please guide these thoughts if they are wrong:
    1.) The electrons come from the ground. (I had not really thought of this until i recalled that + is really -)
    I have some questions about this as well. If you have a copper wire and magnet, and you do your business making electricity, are you just moving electrons around in the wire? Do you ever actually move electricity out of the wire, or is it just a loop?
    2.) There is nothing special about the earth that makes it ground other than the fact that is it electrically neutral? I am still fuzzy on this. Why does a pile of dirt on your desk not act like an earth ground? Or any other neutral material for that matter. But i do understand what was said about it being a reference point, that makes some sense.

    Another question that is brought to mind now is to question that, if when you use an earth ground it just completes the circuit. How is it possible that power is running in a loop then? If you had a very long wire for example, plugged to china for ground, and your other end was in USA; how would it be logical for that to be a closed loop? It seems to make more sense that you are taking electrons from one spot, and depositing them in another.... But, can an electron travel thru the earth in any short period of time? or would you just in essence create a positive flow of electrons thru the middle of the earth?

    (This is just my inner mind thought and not trying to contradict anybody's post. I am just trying to understand and work this out.)
    Thanks,

    Jeff
     
    Last edited: Nov 25, 2014
  11. Nov 25, 2014 #10

    davenn

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    HI Jeff
    consider a battery connected to a light globe by a couple of wires. This is a loop, a complete circuit ( note no ground involved)
    electrons glow out of the negative terminal of the battery through the wire to the globe, through the globe, and then the other wire back to the battery

    The electrons move very slowly ... google electron drift, but the energy in the electric field moves quickly ... near light speed.
    the electrons just stay in that loop, unless it comes into contact with something else

    its no different to a pile of anything else, your arm, some clothes, whatever sitting in the desk



    Often power or a telephone line will just use a single wire strung up on poles from source to destination
    the earth is used as a return path ... it saves the cost of running 2 wires

    Swer.gif

    so follow the loop .... out of the top of the 19kV transformer, through the kilometres of wire, down through the fuse and local transformer to earth
    through the earth for many kilometres again and back up into the transformer

    cheers
    Dave
     
  12. Nov 25, 2014 #11

    davenn

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    Note again what I said in post #8 .....

    its the oscillating electromagnetic field that carries the energy through the circuit

    EDIT ... I will expand on that....
    Its highly probably that the electrons in the AC generator at the power station will never be seen at your house. In the light globe in your house, its the same electrons just oscillating back and forward within the filament of the globe, or the heating element in your oven or room heater.
     
  13. Nov 25, 2014 #12

    I was talking about the voltage, not the charge. But thank you for the correction with conventional/vs physical current though, that always gets me :)
     
  14. Nov 26, 2014 #13

    davenn

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    no, you were talking about the movement of electrons

    There's a lot of mis-information in those statements :)

    As has been said several times in this thread, the electrons in an AC system are not really going anywhere

    cheers
    Dave
     
  15. Nov 26, 2014 #14
    Hello davenn, and thanks for the information. I hope that you can clarify a few things even further for me now if you have a moment.

    When you posted:
    Were you saying that the pile of dirt on the desk does not work because it is just like a pile of some other stuff sitting on the desk? Why is a pile of something on the desk different than the earth? The only reason the earth works as a ground is because it is a massive pile of dirt? If i had a massive pile of clothes (or arms :D) the size of the earth would it do the same thing? I wonder what the limit to the size of this hypothetical pile is before it is just a pile?

    What you are saying about the electrons in AC not really traveling any distance but just oscillating back and forth makes sense and clears things up a bit for me, but then i get confused about another aspect.

    If the electrons are not traveling thru the earth but oscillating, why does it need to go thru the earth and back to the transformer? Can you please clarify this for me? I am gathering that it is just like a bicycle chain (only oscillating) and so when one electron moves forward it pulls another behind it. But is this really a chain connecting that that particular circuits start and end point in a loop, or is it just shifting from one side into the other? Pulling from the earth on one side and then pushing back into the earth on the other, back and forth. I am having trouble trying to word this question, but i hope that you can understand what i am asking.

    Also, and this may be a topic for a new thread, if the electrons are oscillating then there must be a point between the wire and the earth where an electron is jumping ship and then back again right? Where is it going? To some atom in the soil? Wouldn't this be a pretty serious event if you were an electron, or even as small as an electron. I bet there is some serious forces happening at that point and in that moment that i don't understand..... But then i suppose the atoms and electrons do not differentiate dirt from wire and just hop down the line. It must all look like a never ending sea of similar particles with no start or stop.

    Thanks,
    Jeff
     
    Last edited: Nov 26, 2014
  16. Nov 26, 2014 #15

    nsaspook

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    Most problems in the area are with people not looking at the system as a whole. You can't look at the movements of electrons here and there in isolation. You must see it as a part of a propagation media for electrical energy much like air molecules being a propagation media for acoustic energy (sound). Think of the electrons in a wire or dirt as part of the guide-path for the energy to flow. The electrons are part of a system but in most simple circuits their absolute motion is much less important than understanding how charge and fields are related to the actual energy flow and storage. Once you have a basic understanding of this then you can relate it back to electron movements easily.
     
  17. Nov 26, 2014 #16

    davenn

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    Jeff,

    a great reply by nsaspook :)
    I cant really add to that
     
  18. Dec 1, 2014 #17
    Could you please explain then? That's the understanding I took away from my circuits class...

    And yes, I know there is no net movement of charge, is it not that electrons move back and forth around fixed points in the wire, and sometimes that means electrons are being pulled out of the ground and then put back? Now I'm even more confused...
     
  19. Dec 1, 2014 #18

    psparky

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    Negative on that sir. The electrons are going back and forth a short distance kind of like the piston of a car engine. 99.9% of this is going on in the hot and nuetral wires, not the ground wire!!! Could be line to line voltage as well obviously. The ground is not used to power a device. The device or load will work without a ground. However, for safety purposes of not shocking people the ground is used. It will bleed off any extra voltage, static charges or whatever.

    Does some trickle current go thru the ground (earth) back to the secondary of the transformer? Yes it does. Might these electrons interact with the earth thru their travels through it? Perhaps, but most of the current flow is based off a KCL and the releiving of extra charges from the the panel or device itself.

    Also, don't forget about the "grounds" main purpose. It does get rid of excess charges but it also has this important purpose.
    Let's use a electric dryer for example. If there is no ground and the live wire shakes lose after 30 years and is now sitting on the chassis, the breaker does not trip and the next person who walks up to dryer may get shocked to death.

    Enter the ground wire on the dryer that shakes the live wire lose. The second the live wire hits the chassis, the shorted current blasts back thru the ground wire to panel, then back thru the center tap nuetral wire to secondary of transformer and trips the breaker instantly. (and yes, during this short circuit example, some of the shorted current went from the panel thru ground to the secondary as well) Now when the next guy uses the dryer, instead of getting shocked he simply notices the dryer doesn't work....contacts electricitan, and all is well.

    Remember as well, the nuetral wire and ground are tied together in the same bus in a residential panel, then this bus is grounded in the earth.

    Same with a 480/277 or 208/120 three phase panel. The 277 or 120 volt nuetral wire will be tied togehter with ground in the same fashion.
     
    Last edited: Dec 1, 2014
  20. Dec 8, 2014 #19

    sophiecentaur

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    psparky has it spot on.
    I do not know which part of the world you guys all live in but it is extremely rare (more or less never) that the electricity supply comes to your house on just one wire. The 'return' will be along a cable, alongside the supply live cable. In fact, it is pretty well irrelevant which of those two cables is actually 'live' as they could well be isolated from the ground and everything would work fine. There are several examples of systems that use a 'balanced pair' of conductors. A ground return is not a good reliable low resistance and it could also be a safety hazard if there is a route from the live cable to all the pipes and metal bits in your home. The Earth on your home is a Safety Earth and all (most) metal structures are connected to it. If a fault in any device makes a contact with the case of an appliance, the Earth connection ensures that the voltage of the bits you might touch is (very near) the ground potential so you will not get a shock. Often, a short to ground will also cause a fuse to blow, rendering things safe.

    The movement of electrons is not really relevant to the way circuits work. Whilst there is some very slow drift of electrons with DC, (a few mm per second - so how long would it take for an electron to get back to the power station from your home at that speed???) there is no net movement of electrons at all, with AC. Someone reckoned it would be a good idea to teach 'Electric Current' in schools, as if the flow of the Electrons was of paramount importance. Seriously, they pitched it entirely wrong and most students have just been confused by it. (And don't get me started on Photons, either.)
     
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