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About battery and power grid circuits

  1. Dec 21, 2008 #1
    I'm trying to deepen my conceptual knowledge about electrical systems. I'm definitely a lay level, tradesman level user here. I'm lucky if I can do some simple algebra, so I don't want a lot of math (though I might get into that later if I want to get deeper into engineering).

    I have some basic questions that never seems to get answered by your average "plumbing analogy" descriptions about basic electricity. Literature like that always seems to start you off with a battery, tells you one side has an excess of electrons (charges) that want to move through a conductor to the other side where its electron poor, or overall positively charged, and that creates voltage (electrical pressure) and amperage and so on. However they always seems to say that the charges flow "from one point, through a load (some appliance) and back to the source" and things.

    I need to clarify: it seems to me the electrons flowing from one cell of a battery to the other are not coming back to their source but rather moving to the other "pole" (right terminology?) which is by definition separate from the source because it needs to be electron poor. then it comes into electrical balance after a while and the battery is dead. but its still got 2 cells and the whole point is that its NOT flowing back to the voltage source, but going somewhere else to enrich it with its charges, i.e. the other cell in the battery.

    If I could just clarify that then it might be easier for me to understand the power grid. I understand that ac voltage/current is generated by steam turbines whirling great magnets in induction coils and so on, leaves as 3 phase ac current (by the way, is there current in the power lines or only voltage?) and gets stepped down by transformers and things finally to end up in your house as 120 volts ac. it electrifies the hot bus bars, and then moves on out through breakers or fuses as usually 15 or 20 amp branch circuits. Then it retruns to neutral bus bars which are basically grounds.

    So my basic question which I can never seems to get answered, is - what is the other "pole" or side of this circuit? You've got current being generated on one end and going through houses (loads) but where does it end up? back at the power plant? in the ground? Or am I oversimplifying this somehow? Does the fact that its ac complicate the question and if so how exactly?

    and a related question is - does ac current just basically flow through power lines (or any wire) in a sine wave pattern but propogating forward just like dc would? i.e. basically a wavy line versus a straight one or is there more to it than that?

    Thanks a lot,

    p.s. if you're not already tired of this can you explain how a "reverse current" recharges a battery (i.e. in the case of a car battery)?
    Last edited: Dec 21, 2008
  2. jcsd
  3. Dec 21, 2008 #2


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    A simple "battery" like a AA, AAA, C or D cell consists of a single cell. A cell consists of 2 electrodes and a electrolyte material. There is a chemical reaction going on inside a cell which is transporting ions through the electrolyte and providing electrons to the cathode.
    You can make a simple battery by sticking pencil lead (graphite) and zinc into a lemon.

    A rechargeable battery is composed of chemicals whose reaction can run either way depending on the terminal voltage. When the battery is providing voltage the reaction will run until the starting chemicals have all reacted. If the terminals are then held at a high enough voltage the reaction will run "backwards" essentially reassembling the initial chemical composition. There is lots of information about this online try searching on "galvanic series". You also may want to read the Wiki article on AA cells.
  4. Dec 21, 2008 #3


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    And for AC....
    In the power grid, there are no chemical reactions, so no ions or electron sources. The grid uses only the electrons already present in the wires. The induction by the generator just gives them a push.

    Regarding their motion, I'm not sure of the actual drift velocity, but the same electrons will basically just oscillate back and forth in a piece of wire if it is long enough.
  5. Dec 21, 2008 #4
    In your home, the AC mains use the neutral as a return path in an unbalanced load. There is current flowing through the neutral wire, but it is very slow. An equation factoring in the current, charge of electrons per CC, and wire gauge can calculate the current flowing in a 100watt lightbulb to be about 3 inches per hour. It is slow like molasses.

    In North America, your house panel is fed by two 120volt conductors and 1 neutral conductor. To get 120 volts, one of the hot phases is connected through a load back to the neutral conductor. The neutral conductor comes from a center-tapped winding on the secondary side of the distribution transformer. The neutral is earthed at your panel and at the transformer to prevent any situation where the neutral could carry a voltage higher than ground potential. For a 220volt potential, the load is connected between the two 120volt phases which create a 220 volt potential. You can think of the neutral as being half-way between the two phases, thus creating half the voltage. So in a 120 volt circuit, the current flows from one of the hot conductors, through the load, and back to the transformer via the neutral wire.

    Attached Files:

    Last edited: Dec 21, 2008
  6. Dec 21, 2008 #5


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    Both,you can't have current without a voltage difference and you can't have a voltage difference without a current.
    Since power = voltage * current, you can decide what voltage you want to use to give a certain power at a certain current. Because current flowing through a wire creates heat the power grid uses a very high voltage and a low current to reduce this waste heat.

    Literally the ground! At the house end of the circuit the other side of your load is connected to a big copper rod into the ground ( actually ussually at the last substation/transformer) similairly at the power station another rod is in the ground.
    Although the Earth isn't very conductive it does have a pretty large cross-section area - so overall makes a very low resistance path back to the power station.
    AC makes this a little easier - the voltage is constantly changing and so there isn't a constant strong field in the ground. On high power DC systems like subway trains you can't use the ground as a return because the constant electric field and high current would cause chemical reactions - like corrosion of any nearby metal pipes and steelwork.
  7. Dec 21, 2008 #6
    The ground or earth if you prefer, is NOT the return conductor for the power grid. The grounding stakes are there to keep the power lines at a known potential wrt ground. Also, a grounded wire, usually smaller gauge than the power lines, is often placed above the 3 phase power lines. The idea here is that the ground wire protects the 3 phase wires by absorbing a lightning stroke.

    The load current path is always wires, and not the earth. For a residence, the hot and neutral wires carry the load current for 120V ac (USA), and the 2 hot wires carry load current for 240V ac. The earth does not carry load current.

    Your 1st paragraph is precisely correct. Cheers.

  8. Dec 21, 2008 #7
    really? i thought the only reason the earth wasn't really a return path is because in a balanced multi-phase AC system the return current is zero.
  9. Dec 21, 2008 #8
    You're right, in a balanced system, there is no return current. In an unbalanced system, the return current is sent down the neutral conductor back to the distribution transformer in your neighbourhood...not to the power plant. The only reason that the neutral conductor is grounded is to prevent it from going above earth-ground potential (voltage) - for safety and equipment reliablilty reasons.
  10. Dec 21, 2008 #9


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    There shouldn't be a significant return current for a properly balanced 3phase, but if you are using the ground as a reference then some current must flow back through the ground. You can't have two points at the same potential without a current flow.

    Does the unbalanced return current flow in the top wire? I thought is was just lightning protection and signaling.
  11. Dec 21, 2008 #10
    Do you mean "You can have two points at the same potential without a current flow" ? Two wires connected both at 0 volts are at the same potential - and no current flows.
  12. Dec 21, 2008 #11


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    Sorry that was badly phrased, what I meant was you can't say two points are connected so that they are at the same potential and then claim that no current flows between them.
    Yes - once they are at the same potential there is no longer current flow, but for them to be compared there must be a current path. You can't take two isolated points and say anything about the voltage difference.
  13. Dec 22, 2008 #12


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    Some systems actually *do* use the earth as a return path (though it's only in places with lots of remote users):
  14. Dec 22, 2008 #13
    wow thanks everybody, a lot of cool stuff to mull over there. If I understand things right, a circuit is not really circular except in the conceptually spatial sense. It could just be a line (like a wire) between two points of differing potential.
    But it is a circuit in the sense that its always EXCHANGE of charges. The flow of positive ions IS the flow of electrons in the other direction. Therefore if I understand it right, the turbine powered induction at the power station is the cathode and the the ground is the anode in the circuit as a whole, alowing for ac in the form of a sine wave pattern of charge motion (in 3 phases) to go on in an ongoing pattern. Do I have this basically right?

  15. Dec 22, 2008 #14


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    The water in pipes analogy only goes so far. There isn't really much exchange of charges - the drift velocity of electrons in household wiring is about 0.1mm/s.
    What really happens is the field pushes the first electron, which pushes the second and so on down the wire - it is the field that travels from the power station to you.
    A better model is probably to think of pressure in a pipe, like in a hydraulic system. The oil doesn't flow from the peddle to the brake cylinder - only the pressure moves through the fluid.

    The same is also true in a battery powered circuit - it's only inside the battery that electrons really move from cathode to anode.
  16. Dec 22, 2008 #15


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    No, circuits must be complete for current to flow.

    Current only has a tendency to flow back to the source from which it came and not into the earth. If it flows through the earth, it is a fault current.

  17. Dec 22, 2008 #16
    Regarding unbalanced 3 phase circuits, there is still no earth current, even with unbalanced loads.

    There are many combinations of 3 phase xfmr connections, but the most common are Y-Y, Y-delta, delta-Y, & delta-delta. The most problematic of the 4 is the Y-Y. If a Y-Y connection is used with either a 3-phase shell type core, or with 3 individual 1-phase xfmrs, then a neutral connection is needed to maintain balanced phases when the load is unbalanced. The neutral would carry current when the 3 phase currents are not balanced.

    With the other 3 configurations, NO neutral is needed, and the 3 phases remain balanced (line to line and line to neutral voltages) even when the load is unbalanced. Thus with Y-D, D-Y, & D-D systems, only 3 wires are needed under all circumstances. Having 1 delta assures balance since the delta winding can establish currents circulating inside the closed delta path. Also, if a 3 phase core type xfmr is used, it can sustain a balanced Y-Y connection WITHOUT A NEUTRAL. The 3 magnetic fluxes are coupled via the "E" core, hence only 3 wires are needed for all conditions, bal or unbal.

    Thus, the power grid always uses an xfmr configuration that has at least 1 delta connected winding to maintain balance without a 4th wire, or if the primary & secondary must both be Y-connected, then a tertiary delta winding is used, or a 3-legged E core.

    Regardless of how unbalanced the load is, the power company never relies on the earth/soil to conduct load current. A small amount does conduct by virtue of current division, but too small to be concerned about. The power grid relies on 3 wires to carry load current. For unbalanced loads, the 3 currents are unequal, but the voltages remain balanced by virtue of the delta windings or the 3 legged E core.

    Make sense?

    Last edited: Dec 22, 2008
  18. Dec 23, 2008 #17
    No Claude, that makes very little sense. It's a bunch of shop talk that doesn't help me much at my current level of engineering background.

    and since Stewartcs made the oft repeated remark that "the circuit must be complete" then perhaps he can, in simple terms, just describe the "circuit" of a power grid which was my orginial question actually.


    It's my understanding that for charge to flow from point a to point b there must be a difference in electrical potential, and thats basically the "polarity" that makes the whole process occur. So as charge is flowing from point a to point b, opposite charge is also flowing from point b back to point a. Cations in one direction are anions going in the other direction.thats a circuit, no?

    So what are the two points of differing potential in the power grid, thats all I want to know. Is the turbine/inductuion coil both the beginning and end of that circuit, just with a bunch of transformers and loads in between?
  19. Dec 23, 2008 #18
    The turbine is the active device driving the entire grid. Of course, other turbines are interconnected into that same grid and they also are actively driving said grid.

    To understand conceptually, it is best to start with just 1 generator. Fuel is burned, and that energy is translated into mechanical power, which then gets translated into electric power. The work done moving charges around the circuit ultimately begins with burning of fuel. Energy conversion is what it is called in the engineering world.

    Is that easier to understand? BR.

  20. Dec 23, 2008 #19
    ac power became a standard largely because it could be transmitted long distances economically. Thomas Edison liked dc better, but lost.

    By stepping up voltages to very high levels, power (IE) can be transmitted long distances: line losses of i^2(R) where R is the transmission line resistance, can be reduced by keeping current small...that means we want voltage high...say 300,000 volts or more for distance transmissions even though customers may want 120 or 220 volts. That means it musty be stepped up and down, very expensive if dc, much more efficient with transformers and ac.

    You should get a basic book on electricity as learning a variety of basics all at once is difficult to do in forums like this where responses, explanations are necessarily limited....
    and you are more likely to get accurate explanations as well...
    Last edited: Dec 23, 2008
  21. Dec 23, 2008 #20


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    You're asking for deeper understanding, but now it feels like you're trying to drink from the firehose? Well, it depends on how much deeper you actually want to go.

    This was addressed in one of the earlier posts in this thread by russ_watters. No single electron will go all the way around the circuit, but at any given point, there is a net movement of electrons that flow from higher potential to lower potential. But there is a net flow of electrons at any given point due to the potential difference (and not polarity) from point a to point b. That's a somewhat DC view of things, but we still say that various points in an AC circuit are at higher (or lower) potential than others (which is true if you're looking at the RMS voltage--don't worry about that for now, if ever).

    It's much easier if you think in terms of potential (and current--conventionally, we use the net 'flow' of positive charge rather than negative--blame Ben Franklin for this) rather than the actual charges. And perhaps it's best to just think of the potential rather than current because in AC systems, even the net flow just oscillates back and forth. I think that would help to simplify the picture.

    The most simplistic picture has one side (the hot side) of the generator at the high potential. This potential is transmitted along the transmission line to your house (along perfect zero resistance cables) where your appliance (the load) drops all the potential. A return path (the neutral transmission line) back to the other side of the generator completes the circuit, and allows the current to flow.

    Got that? Good. That's not a bad picture. In reality, transmission lines have some resistance, and the higher the transmission voltage, the lower the losses in the line, which is still up to 40% (or somewhere around this ballpark) of the power, depending on the distance between the plant and end user. So the power plant (which has many generators) feeds a high amount of power into a transformer station which steps up the potential. Some of the power (and not just potential) is lost along the lines, but the majority makes it to the substation, which transforms this very high potential to high potential (actually, this is usually only an intermediary as there's a transformer at your house or in your block which does the final conversion to whatever mains voltage you use in your country). You use most of the remaining power, and the rest of the power is used up in the return leg (through the various transformers again) back to the power plant.

    If that didn't make any sense (but the previous paragraph did), well, take heart in the fact that you've got a high level picture of how things work. That and visit the Wikipedia page on power transmission:
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