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Differences in grounding and returning a circuit?

  1. Jan 3, 2016 #1
    So I just got finished with my second physics course (electricity and magnetism) and from what I learned about circuits, to complete a circuit there must be a potential difference to create a current. I just wired up some new off road lights on my vehicle, and when wiring, I ran a wire from the positive terminal of my battery to each light, but had a ground wire on each that I just bolted up to the frame (no return wire to the battery). Can someone explain to me how this works? Do the electrons flow through the battery, through the lights to the frame, and eventually to earth where the potential is 0, thus creating a potential difference? Why don't I have to run a return wire? What exactly is the difference between a ground and a negative terminal on a battery?
     
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  3. Jan 3, 2016 #2

    cnh1995

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    Homework Helper

    In circuits where there is a ground symbol, that point is considered to be at reference 0 potential. It need not be actual "ground" or "earth". As per my understanding, negative terminal of the battery should also be connected to the metal frame in order to draw current. Metal frame reduces wiring for return path of the current. Whatever you described, if it is working without the -ve terminal of the battery, then I'm afraid I haven't understood the situation and something else is happening here.
     
  4. Jan 3, 2016 #3

    berkeman

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    Staff: Mentor

    In vehicles, the frame is generally used for the return current path for convenience and to lower the amount of wiring. The vehicle is "floating" on insulating tires, so there is no Earth ground involved in the circuit.

    BTW, remember that the direction of the flow of electrons is opposite of the direction of what we refer to as the "current". :smile:
     
  5. Jan 3, 2016 #4
    An earth ground is added or required when there is a possibility of a short circuit that may energize some part of the device that can expose the operator to a dangerously high voltage. In such a situation the current is shunted directly to ground through a low resistance path as opposed to through the operator.
     
  6. Jan 3, 2016 #5

    sophiecentaur

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    When a vehicle gets old, there can be corrosion in the metal seams. This can produce resistance in the Earth return path. Devices that take heavy current will suffer because of the consequent voltage drops. In particular, the starter motor nearly always has its own return path, via the engine block and a heavy cable, directly to the battery negative terminal. When the starter takes hundreds of Amps (say 500A), even a comparatively low path resistance (0.01Ω) can reduce the volts delivered by 5V. Earth return is not always appropriate.
     
  7. Jan 4, 2016 #6
    Thanks guys, that definitely makes sense. I actually talked to my dad, who is an electrician, after I posted this, and he told me that the negative terminal was actually connected to the frame. I didn't know that, so I was confused as to how the circuit was completed. It seemed pretty unlikely to me that pretty much any bolt on the car was part of the circuit, but I guess everything is connected in one way or another.
     
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