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AC question

  1. Feb 17, 2009 #1
    This is a pitifully simplistic interrogatory, but here goes.

    Is this roughly how an AC circuit works?:

    An AC generator supplies an electron flow to start the circuit. These electrons then flow down a single wire to say a household fusebox/breaker. From there they flow to a wall plug and then which has two prongs, one 'hot' and one 'neutral'. Then the electrons pass through the 'hot' wire and through a device and return on a separate wire (although bundled in a cord with the hot wire) to the 'neutral' plug. The neutral plug wire then completes the circuit by going down a separate wire and eventually back to the AC generator. Parenthetically, I realize that the current switches polarity 60 times per second and that I have ignored transformers, capacitors, etc. But is this crudely correct?

    Edit: it appears, after viewing this: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html that I may have displayed further ignorance...

    So, it's two 'hots' and a neutral that come into the home, with two 'hots' going to the breaker and the neutral going to a 'neutral tie in box'. It still appear from the diagram that the THREE wires go back to the AC supply generator? So, are the three wires not to be thought of as having electrons flowing to and from but more properly thought of as all with a back and forth electron flow? Or is it just the 'neutral' wire that completes the circuit? Convoluted, but I am confused.
     
    Last edited: Feb 17, 2009
  2. jcsd
  3. Feb 17, 2009 #2

    mgb_phys

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    Firstly electrons don't really flow down the wire, it is the electric field that flows from the power station to you. Think of it as more like 'pressure' in the wire that is supplied, or like water waves - the waves travel for 100s of miles but an individual bit of water just goes up and down locally.

    The neutral wire doesn't go back to the generator it is normally connected to the ground either at your house or at the last transformer. The power station is referenced to the same ground. Again you can think of it as changes in pressure.

    Finally the split 110V two hot is a peculiarity of US wiring to allow you to also get 220V for heavy appliances, it just complicates the understanding.
     
  4. Feb 17, 2009 #3

    Dale

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    I agree with mgb_phys. The two-hot wiring is probably not essential to learn as a first pass. But basically you are correct, current flows from the transformer, to one prong on the plug, through the device, to the other prong on the plug, and back to the transformer. It is generally more useful to think of "current" rather than specifically electrons. The drift velocity of the electrons is really small so the electrons themselves barely move.
     
  5. Feb 17, 2009 #4
    First: Thank you both.

    My issue is with visualizing a complete circuit. There must be some form of connection back to the generator, no matter how many transformers are involved? Correct? Or am I reading that electricity is generated at the generator and (simplified for thought) simply goes out and comes back with the 'switching' back and forth. I get the electric field but I don't get the notion of a complete circuit if there is none?
     
  6. Feb 17, 2009 #5

    berkeman

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    Yes, the transformers are really irrelevant. The power generated at the generator flows through the load. The step-up and step-down transformers in the power distribution grid are mainly there just for power efficiency reasons.
     
  7. Feb 17, 2009 #6
    A "complete circuit" does not necessarily mean that it is "physically" complete.

    This is especially true with some AC circuits, as well as with some pulsed-DC circuits.
     
  8. Feb 17, 2009 #7

    mgb_phys

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    It's a little confusing - there isn't the same complete circuit in an AC grid as a DC circuit.
    The power is transmitted relative to earth, so at the power station one side of the generator is grounded and the other side connects to the high voltage lines (through some tranformers) at your house the neutral wire is connected to gorund and the live wire varies in a sine wave to +/-315V (Europe) or +/-155V (US).
     
  9. Feb 17, 2009 #8

    Dale

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    Yes.
     
  10. Feb 18, 2009 #9
    And to elaborate...

    There is no universal requirement that the specific electrons moving as a result of a "generator" be returned to that generator.
    Magnetic and capacitive coupling comes to mind.
     
  11. Feb 18, 2009 #10

    mgb_phys

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  12. Feb 18, 2009 #11

    Dale

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    I hadn't seen that thread, but you are correct as is the thread: there is no appreciable current through the ground back to the power station. The current back to the power station goes through the wires. The ground is just used to fix the reference potential and for various safety reasons.
     
  13. Feb 18, 2009 #12

    mgb_phys

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    So you really have to think about all 3phases to understand where the return current goes?
     
  14. Feb 18, 2009 #13
    Hmm...

    Whether there is current or no current is there a physically extant line that returns to the original source? I thought I was finally clear but after reading the last post, I am not. To not think in terms of a wire that tangibly exists is difficult. I have a grasp of Potential (house) and potential differences (the transformer). However if each step-off or group of stepoffs involves a grounded transformer then there is no need for a physical wire? I have read through the above thread and still am murky.

    Simplified, of course: I thought that current goes out and has many transformers that do have complete loops for each user (current going out to an individual then looping back to a ground at the transformer). I thought that the current simply switches polarity back and forth thus alleviating the need for a return line... I was just assuming this occurred x three... :)
     
    Last edited: Feb 18, 2009
  15. Feb 18, 2009 #14

    Dale

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    Yes, but I was hoping to kind of gloss over it for the sake of simplicity and just talk about a single phase.

    Yes. Basically, power lines always come in sets of 3, the return current for each line is split between the other two lines. The Wikipedia page on http://en.wikipedia.org/wiki/Three-phase_electric_power#Generation_and_distribution" has a very nice animation that really shows how the current flows in a simplified three-phase transmission scheme.
     
    Last edited by a moderator: Apr 24, 2017
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