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Back to basics! Current return path.

  1. Apr 23, 2012 #1
    Hello all,

    I have a fundamental question regarding batteries/power supplies that I've been wrestling in my hand. Essentially, what I'm stuck on is this-

    Why can't you create a simple circuit going from the + terminal of a power supply, through a lightbulb, and straight to ground (Earth)? As I understand, current flows when there is a difference in potential between one end of the circuit and the other. So if the power supply is outputting + 10V from its positive terminal and the other end is at ~0V (earth ground), what is stopping current from flowing?

    I understand that you need a closed loop for current. i.e. a break or "open" circuit will not allow any energy flow through the medium. But in my above example, there isntt technically a break in the loop.

    Also, I'm assuming its different for batteries as there is a chemical reaction element to the process that requires both ends of the battery to be used.

    Thanks for the clarification.
     
  2. jcsd
  3. Apr 23, 2012 #2
    There's nothing except resistance that will prevent you from using earth ground as a return path. You must close the circuit by connecting both the negative terminal of the supply (or battery) and the lightbulb to earth ground for this to work. Try this with a torch bulb and a battery in your backyard and report back?
     
  4. Apr 23, 2012 #3
    We need something on the - side that's ready to "receive" charge, I think, that isn't already oversaturated. Matter's an example of something that's fully saturated. I think. Chances are, I made a rubbish post here.
     
  5. Apr 23, 2012 #4
    If you took a short PVC pipe section, filled it with earth from your back yard, put wires in and out of both ends, and applied a potential difference across this arrangement, what do you think would happen?
     
  6. Apr 23, 2012 #5
    Ah. That made me think a bit. I think charge is unlikely to flow out of the wire, because of air's relatively high resistivity, so it takes a strong voltage to push the charge out of the wire into the air.

    Probably more rubbish, but hopefully my post makes sense.
     
  7. Apr 23, 2012 #6
    I'm still getting help up on the differences in potential portion of this post. Forget my theoretical circuit going straight to earth. What if the end wire landed in a bowl of dirt?

    Why can't you simply drain all your current there?

    What is it about the (-) terminal that I'm not understanding? Why must this ALWAYS be part of the circuit even it its tied directly to ground itself?
     
  8. Apr 23, 2012 #7
    I meant to connect the wires to the positive and negative terminals of a voltage source, so the circuit is closed. It does not involve the surrounding air--unlike the situation here:
     
    Last edited by a moderator: Sep 25, 2014
  9. Apr 23, 2012 #8
    What material is your bowl made of?

    Because current always flows in loops.

    The only thing you should worry about is to make sure your circuit forms a closed loop. Forget all this stuff about "ground". It's just a common reference for the rest of your circuit. What you really need is a current return path. It could be earth ground, a ground plane, or a conductive cow for all you care--it's not important. Anyway, if your negative terminal is connected to your "ground" (whatever that might be), and one end of your bulb is also connected to this ground, then it's part of the circuit and connected to ground. What's the problem?
     
  10. Apr 23, 2012 #9

    FOIWATER

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    Gold Member

    Current does not flow to ground, rather it flows back to the source.

    Ground is just more often than not, the lowest resistance path back to the source.

    In your house, if a hot wire touches a junction box behind one of your lights say, and that box is bonded and your ground strip grounded to your water main, current does not flow out to ground. The secondary of your distribution transformer is grounded. since power came from that coil, ground current flows back to that coil... through GROUND.

    Ground impedance is typically taken to be 4 ohms. At least that what we use when installing neutral grounding resistors on grounded wye transformers to solve for resistor impedance and mining code regulations.

    You must always have a reference as well. Let's suppose that you DO have your battery feeding a light, and the light fillament is grounded, and the battery is grounded, too.

    Did you know if you take a voltmeter, go from one side of your headlight (assume its on a car) and go to the chassis of ANOTHER car, you won't get 24 volts? Because if you jumpered the +24volts from the battery to another batteries ground, no current flows. It must flow in a loop, back to the source.
     
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