Help understand neutral / common

Discussion in 'Shock Brother's DIY Amps' started by koolaide, Jul 7, 2020.

  1. koolaide

    koolaide Tele-Holic

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    Can someone help me understand how neutral /common/ground has 0 voltage, but is a return path for current?
    Here is my current understanding: ( no pun intended)
    Based on Kirchoff's laws-Voltage and Current into a circuit are equal to Voltage and Current out.
    You can not have current without voltage. You can have voltage without current.
    A circuit (loop if you prefer) is required in order to have a working device.
    AC-Neutral DC Common/Ground (Not chassis ground-Earthing) is a return path for current to it's origin.
    Please keep it simple.

    Please educate me. All replies are appreciated.

    Peace,
    Jim
     
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  2. Peegoo

    Peegoo Friend of Leo's

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    Voltage is measured as electrical potential between two points in an AC circuit. We say neutral is zero volts because AC voltage is always measured as a reading between between any point in an AC circuit and ground. Since neutral and ground are bonded at the power distribution panel, neutral always has the same voltage potential as ground, which is zero volts.
     
  3. TeleTucson

    TeleTucson Tele-Afflicted

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    The question is a reasonable one, even if Kirchoff's law is mis-stated. And the answer given above won't satisfy the conundrum for the OP.

    The reason current flows in the neutral return path is because - where the current enters the return path - the voltage actually is just a shade above the ground at the panel where it is headed. As the OP points out, it has to be to have the current flow - but the small voltage, in conjunction with the very small resistance of a decent gauge copper wire, means a lot of current can flow. So when current is flowing, the neutral out in the circuit is not at ground, only close to ground. In fact, the voltage of the neutral out in the circuit is given exactly by the small resistance of that wire connecting it to ground back at the box, multiplied by the current that is flowing in the circuit - the "IR" Ohm's law voltage drop.

    And that's why grounding the neutral out at the branch circuit outlet is a really bad thing to do, because it needs to be above ground to make that current flow in the neutral return path. If you connect that with the ground in that branch circuit outlet, you've compromised the ground, forcing it to deviate from true ground, and you've created a real electrical hazard.
     
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  4. Steve 78

    Steve 78 Friend of Leo's

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    There is no such thing as voltage at a point. Voltage is the difference between two points. If we say the voltage is X here, we mean the difference between that point and ground. So ground is zero by definition.
     
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  5. koolaide

    koolaide Tele-Holic

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    @TeleTucson, I realize I did not quote Kirchoffs law, but... is my understanding incorrect? I am asking to gain understanding not to disagree or be a wise guy. Thanks for your input.
    Is your explanation also applicable to the common/grnd in a DC circuit?
    Maybe a simplification of my question would be- In a DC circuit- Why doesn't the circuit common/ground have any voltage? And... Yes I understand voltage is measured as differential between 2 points. My limited experience is that when I measure voltage to ground - ground is 0V
    Peace,
    Jim
     
  6. wabashslim

    wabashslim Tele-Afflicted

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    They're two different things. You're measuring voltage across two points. Period. The fact that one of the points is referenced to ground has nothing to do with it. It could be referenced to a lightning bolt, makes no difference to the measured voltage between those two points. And Kirchoff's Law, or Ohm's Law, has absolutely nothing to do with it.
     
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  7. TeleTucson

    TeleTucson Tele-Afflicted

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    I tried to frame my comments to avoid misunderstanding, so hopefully you can read them and make sure you follow them. However, they were indeed framed w/regard to the difference between a neutral and ground out at the outlet in a branch circuit as you might have in your home, stimulated in part by the correct reference from @Peegoo that these are bonded at the panel. Let's give an example: If you have a 14 gauge wire with a 100 foot run back to your panel, that has a resistance of 0.25 Ohms. If you're running 10 Amps through that circuit, the neutral out at the outlet will be 2.5 Volts - not a lot, but it can still be enough to bleed a lot of current if it was accidentally in contact with ground out there. That's why ground fault circuits check to make sure all the current flows back in the neutral and does NOT bleed out somewhere else - like through your body in a bathtub.

    But yes, the same logic applies for DC. Because of small voltage drops across circuits resulting from current flow even in low-resistance paths, you should try to respect the distinction between a neutral and a true ground - sometimes it can matter. For homes, this results in carefully adhering to only one ground point where neutral and ground are the same - at the panel. Out in the branch circuit there should be no current flow in the ground, so - at least until something goes really wrong - that means that the ground out there has no IR voltage drop back to the panel and thus retains its value as the same as the ground at the panel (which we usually reference as "O Volts").

    And specifically to your question, "Why doesn't the circuit common/ground have any voltage?", if the common has "no voltage" relative to ground then either there is no current flowing, or the product of the resistance of the path from that common to ground (the point that your referencing to) and the current flowing is so small that you're just not seeing it on your meter.

    Wrong. This reflects a rather sad misunderstanding of the relevant physics. The difference between neutral and ground has everything to do with Ohm's law, even when the resistance is very, very small.
     
    Last edited: Jul 8, 2020
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  8. Paul-T

    Paul-T Tele-Meister

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    this is quite a difficult subject conceptually.

    Voltage is something we measure, what we're really talking about is potential difference . Higher PD means the electrons that make up the current are given more energy. Electrons at a higher PD want to have a lower PD - it's very like a ball at the top of a hill that wants to roll down the hill to the bottom (ground). It has more energy, and if it can it will dissipate that energy and and return to ground level.

    Neutral is a kind of ground - it's ground back at the nearest substation, but might be some amoutn away from ground potential due to the distance and other issues. We have a separate 'local' earth simply because that's safer.

    When I find it I'll add a simulation that shows how PD is very like lifting an object to a greater height, as done by some mad russian geek, it really helped my A level students. I don't think you need to know it to build an amp though!

    Here's the link. It gives, I think, a pretty good idea of what's happening, even with the hokey voiceover. His other videos are good for physics, too.




    If you think of neutral as the 'ground' level in that circuit, then you're good to go.
     
    Last edited: Jul 8, 2020
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  9. elpico

    elpico Tele-Afflicted

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    The ground outside a cabin in Denver is 5000 feet higher than the ground in LA.

    I can jump two feet above ground in either place because how high I can jump has nothing to do with a "value of ground". Ground doesn't have a value. It's just a word that means "I started counting from here".

    You'd be a lot better off forgetting you ever heard the word ground. I made a long thread explaining this once with detailed diagrams of what's really going on in an amp, how it has nothing to do with "ground", that it doesn't even need to be connected to ground, or that ground could be connected to literally any point in the circuit including the signal input and the amp would still work perfectly, but now all the diagrams have stopped showing up and I'm not sure telling people about this really helped them anyway. It's a funny topic because it's actually very simple but somehow it's hard to break through preconceived ideas about it and see the simple truths that are really there.
     
    Last edited: Jul 8, 2020
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  10. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    I’m not sure “flow” is the best way to describe current either.
     
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  11. swervinbob

    swervinbob Tele-Meister

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    It’s funny. For the last 20 years, part of my job is troubleshooting electrical. But I learned everything in the field, and can’t explain how’s and why’s. Only “if it ain’t that way it ain’t gonna work!”
     
  12. koolaide

    koolaide Tele-Holic

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    You are both correct. I too have built amps, and worked in the residential electrical field, being successful at both. All without a clear understanding of ground, common, and neutral. I used swervinbobs method ;).
    I am now at a point where the why is more important to me than simply being able to complete the task.
     
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  13. koolaide

    koolaide Tele-Holic

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    Based on all the responses here, I did a little more study, and now have a clearer understanding at least I do for now. Thanks to all who replied. It was each of your responses that allowed me to gain more understanding.
    Thanks again,
    Jim
     
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  14. tubegeek

    tubegeek Friend of Leo's

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    One analogy I like is the soda in a straw. When you apply suction at one end, the liquid at the other end is affected immediately - you don't have to wait for liquid to make it all the way through the straw.

    The electrons in a wire don't need to propagate through the entire wire - pulling them from one end exerts a force that affects all the electrons throughout the wire. And this speaks to why current travels in loops - there has to be a return path. Voltage can be static, with a force READY TO ACT once a path presents itself. But current needs to have both a force acting on the electrons, and a place to go to and to come from.

    This is an interesting thread, I like the questions it brings up.
     
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  15. andrewRneumann

    andrewRneumann Tele-Meister

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    I too like this thread, and even though @koolaide is satisfied, I kinda want to keep it going. :D

    So... how does a resistor have resistance if no current is flowing through it? :cool:
     
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  16. andrewRneumann

    andrewRneumann Tele-Meister

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    I think you might find this in a lot of high expertise fields--fields where you can only become a true professional by doing it for a long time. The expertise is gained by experience and it doesn't have to be backed up by theory in the practitioner's mind... it just works and that's all that matters. That might irk those who write books and like to theorize, but it's how we have gotten a lot done as humanity and I have to give credit where credit is due.
     
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  17. tubegeek

    tubegeek Friend of Leo's

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    How do you know it does? You certainly can't measure it without introducing a current.

    It has physical properties deriving from the materials it's made from. Some materials' properties change when current flows through them and some don't SO FAR AS WE KNOW.
     
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  18. chris m.

    chris m. Poster Extraordinaire

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    I don’t even understand why there has to be a neutral wire in a house outlet at all. I thought two wires- positive and negative- plus a third ground wire- would suffice. Good thing I just hire an electrician when stuff needs fixing.
     
  19. koolaide

    koolaide Tele-Holic

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    It is sorta like the falling tree in the forest- does it still make a sound if no one is there to hear it. But.... if you use Ohms law- No E no I and therefore no R.
     
  20. koolaide

    koolaide Tele-Holic

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    Here we go again... Oh well:lol:

    BTW the neutral carries the unbalanced load. (of the split phase) The ground-green or bare in the USA is a safety ground, and is only there in case of fault. And.... the neutral is tied to ground in the main panel.
    I know only more confusing huh?
     
    Last edited: Jul 8, 2020
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