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What actually completes a circuit? Ground vs return.

  1. Feb 16, 2012 #1
    The basic concept of a circuit kind of eludes me:
    Where do neutral/ground/negative wires ultimately go to (i.e. what must the 'thing' they connect to be)? Is it always ultimately the earth or must it be connected to the actual power generation itself somehow?

    Example: Could I take a battery, connect the positive terminal to a light, then take the other terminal from the light and run it into the earth and have the light light up, or must it go to the (-) terminal on the battery?
    Similarly, could I take the 'hot' wire from a house circuit and connect it to a bulb, then run another wire out of the bulb right into a stake in the ground? Or must it run to the 'neutral' wire (which runs where, to the power production plant?)

    So a more direct question might be, what is the mechanism that actually 'completes' the circuit. If at all possible to use the water analogy here: is the water forever being recycled and the generator/battery is like a pump. If this is so, how is this water being recycled into the pump? Alternatively, if it is possible to run a load from a higher voltage potential to the earth itself (v potential = 0), would this imply that the water is not in fact being recycled but rather 'absorbed into the earth's water table'?
     
  2. jcsd
  3. Feb 16, 2012 #2
    Given a conducting path, a potential difference between two points on that path is what "completes" the circuit.
     
  4. Feb 16, 2012 #3

    vk6kro

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    A water analogy is actually not helpful here. Water does not have to complete a circle like electricity does.

    Current from a battery leaves the positive terminal and it has to return to the negative terminal before any currrent can flow.
    So connecting it to ground has no effect although it also won't do any harm if you do connect it to ground. The circuit will operate just the same if you connect one side of the battery to ground.

    Power coming into your house comes from one side of a transformer winding and it must return to the other side of the same winding for current to flow.

    One side of this circuit will normally be connected to ground but this is just a safety precaution in case of lightning strikes or power surges. No current will normally flow in this line.
     
  5. Feb 16, 2012 #4
    Water is not a good example. Circuit is a closed loop. To every source, there must be a return. Yes you can use earth ground as return path, but it is not advisable. It is very dangerous. That's the reason why all power supply in electronics are isolated from the earth ground. They connect to the earth ground at one point and there should never be current flow from the circuit to the ground. Earth ground is used for safety so the whole circuit cannot float ( charged ) up by static and give a shock to anyone standing on the ground touching it.
     
  6. Feb 16, 2012 #5
    The water analogy does work to an extent. Water doesn't require a complete circuit if you have a large water supply. Like a 55 gallon drum on your roof. Attach a hose to it and water will flow until the supply is gone. But with a fixed supply of water that needs to be recycled the analogy between gravitational potential energy acting on water molecules and electrical potential energy acting on electrons is not too bad. Another difference is that, with water, the molecules themselves are moving and doing the work. With electrical work only a very small part of the molecules, the electrons, are doing the work. And not even all the electrons in the molecule necessarily.

    In order to understand the difference between neutral and ground you have to know a bit about transformers. Neutral is just the center tap of a transformer winding which also happens to be grounded, as in actually physically connected to the earth at some point. Strictly speaking, with AC current, the kind you get from a generator, you don't really need a ground connection. With a DC voltage source, like a battery, the negative terminal is often referred to as 'ground' and it usually is in fact at 0 volts with respect to the actual ground. With the US electrical distribution system AC has a ground because the center tap of the distribution transformers is grounded. I believe that means that the other 'hot' wire will continuously vary between +120 volts and - 120 volts RMS with respect to the neutral wire. For 240 volt countries I don't think there is any need for a 'neutral' wire. You just have two hot wires and a ground for safety.

    Practically speaking an electrical circuit is a closed loop. The only electrons you get are the ones that are already in the copper of the wire or the chemical mix of the battery for DC. For AC the electrons don't actually go anywhere. The copper electrons just vibrate back and forth.

    So a water pump connected to the same hose at both ends or that recirculates filtered water to a fish tank is not so different from a DC electric circuit powered by a battery. Another problem with water analogies is they don't work at all for AC power. Although you could use the analogy of diodes as one way water valves to explain half and full wave rectifiers.

    And to answer your question about the light bulb, it wouldn't light. The neutral/ground bus on the breaker panel might connect to ground somewhere outside your house, but whatever part of the 'ground' you choose to touch with your return wire is likely to have a very high resistance path to that ground wire.
     
  7. Feb 16, 2012 #6
    Yungman, I don't think he was referring to using the actual ground wire that connects to the breaker panel as a return wire instead of the neutral wire. He was talking about connecting the neutral wire to an actual metal stake in the ground as a return. If the ground were a much better conductor I'm guessing that might actually work. Like if the entire surface of the planet were covered in copper or aluminum.
     
  8. Feb 16, 2012 #7
    I know exactly what he said about using the earth ground as a path......literally. And that's the reason I make a strong statement that he should not, even though it will work. I am more concern with the safety of the practice even to think in that route.
     
    Last edited: Feb 16, 2012
  9. Feb 16, 2012 #8
    It will work? So you're saying that if I take the neutral wire from a lamp cord and stick it in the dirt outside with the hot wire connected to the hot bus in the breaker panel, that the bulb will light up? Now I may actually have to test this theory. I don't see why it would work. Doesn't the ground have a pretty high resistance? And I'm not even sure where the nearest ground connection is. If it's down at the telephone pole at the street that current would have to travel a pretty long way to get back to that metal stake in the ground that connects to the center tap on the secondary winding.
     
  10. Feb 16, 2012 #9
    Okay it is the transformer specifically (in household electrics) that must be completed?

    This potential difference cannot simply be created between one single live wire and the physical earth ground, regardless of its conductivity? How is it that one can be electrocuted by grabbing a live wire and standing on the ground. (or would they have to also be touching a return wire?) This is what's confusing me...
     
  11. Feb 16, 2012 #10

    jim hardy

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    VK6 answered your conceptual question.
    To have a circuit, all charge must find a path back to its source, whether that source be a battery in orbit around Pluto or a transformer on the pole in your back yard.

    IF that path back to source happens to include a few feet of Mother Earth, well, it'll probably work okay provided the soil is damp there.

    Yungman pointed out that it is DANGEROUS to rely on Mother Earth for completing a circuit. You should always provide a wire instead.

    Since the transformer on your power pole is connected to earth (look at the big copper wire on side of pole, it goes to bottom of pole where earth is moist),, a lamp connected as Metiman describes will light if he gets his neutral wire into conductive dirt.. The current flows through the soil , leaves ground in that copper wire that's alonside pole and gets back up to the transformer thaty way.
    I have a neighbor who wired a barn light that way (mentioned on similar thread months ago), i told him it was an electrocution hazard.

    There's nothing special about ground. It's just another wire that goes most places and we tap into it frequently. Electric charge has no particular affinity or need for it.

    Observe that the Voyager spacecraft is unlikely to ever touch "ground" again and its circuits are working fine.

    In your mind get your circuit working in mid-air and then set it on the ground.
    As if it were a humble flashlight.
     
  12. Feb 16, 2012 #11
    If you stick a stick connect to neutral long enough and the soil as part of the loop, yes!!!! The minerals and moisture in soil will conduct. When I redo the wiring of the house, the contractor hammer a six feet copper stick into the ground and use is as ground. this is required by law. Of cause you stick it a few inches in dry soil, it will not conduct, you stick few ft, soil is wet under a few feet and minerals will conduct. You just need a lot of surface area on the rod. Is it as good as a real heavy gauge conductor, I don't think so, but to light up a light bulb, it is doable. Of cause, you don't stick it 3 blocks away unless you are willing to stick a big long stick.

    It is doable, not a good way, particularly not a safe way.
     
  13. Feb 16, 2012 #12
    Don't quote me on this, I heard the neutral is bury into the ground in the power plan to make the neutral wire neutral ( 0V). Earth as a whole conduct current. Remember it only takes a few mA to kill, so even if there is resistance on the ground, current will flow.
     
  14. Feb 17, 2012 #13
    Thanks for the reply's, it' making a bit more sense.

    It seems like maybe I need to look more into power generation itself. Because as I'm understanding it, the power source itself (coil and spinning magnets, or battery, etc) must have an in and an out. The ground cannot be the end of the line for power. I'm still a bit hazy on this though, such as how lightning finds the ground to be satisfactory.
    I understand that it is the fact that the ground has less potential than the cloud, just as the negative terminal on a battery does compared to its positive, but why then can't the earth ground be that same less potential voltage sink for something like a turbine? To me the lightning example seems almost like a linear flow of electricity (not a circuit at all) but I feel I'm probably missing some major difference here.
     
  15. Feb 17, 2012 #14

    jim hardy

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    wow now it's getting into interpretations...

    I tell my technicians this, when discussing Kirchoff's current law and lightning and ground...

    Charge must get back to where it came from.
    That is what Kirchoff's current law is telling us.

    In the case of static electricity, Kirchoff will accept a temporary delay and storage of charge.
    The most commonlly encountered real world example of this is on a cold winter day you walk across the carpet and touch a doorknob and get a spark.
    As you walked your shoesoles picked up charge from the carpet, it flowed out onto your body, and when you touched the doorknob that charge began its journey back to the carpet fibers from whence it came, via door material, hinges and doorjamb....

    Lightning is similar. That charge was somehow swept aloft, perhaps on water molecules and it is returning to the earth from whence it came. Water moecules are quite polar and evaporation is thought by some researchers to impart charge as they're ripped apart from one another. (Question - is its polar nature related to water's remarkble surface tension?)

    Intersting aside - i once had reason to look into anti-static carpet.
    The industry standard test was to don a pair of shoes with chrome-tanned leather soles, walk so many steps across carpet lifting feet so many inches, then touch an electrostatic voltmeter.
    All i could think was - What a practical test !!

    pardon the digressions, and i hope this simplistic explanation isn't at odds with scientiific fact.
    It works for me.
     
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