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B About Voltage, Current and Ground

  1. Aug 23, 2016 #1
    First of all , I'm a completely new to this current - voltage concept , I have problems understanding Grounding and the current flow in circuits . 1) Which path do currents take when there are multiple grounds in a circuit .
    2) How and where should a circuit be grounded and why ?
    3) Why is earth considered as an Infinite source /sink ?
    4 ) If we use a "'metal plate "" in a circuit ( Would it get charged after the electrons flow to it from the battery ( Please bear with me , it might be a stupid question ))
    4) If a circuit( consider a basic circuit , where there is a battery and a bulb ) When the grounding is done to this circuit ,Why wont the electrons from the Battery flow to the Ground ? Since Ground is considered as an infinite source/sink .
    5) I have trouble understanding How battery eliminator works ( comparing it to chemical cells) .
    Thank you ! and Please help me .
     
  2. jcsd
  3. Aug 23, 2016 #2

    cnh1995

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    Homework Helper

    This discussion might help.
    https://www.physicsforums.com/posts/5393297/.
    It will not be charged. Current will flow through the metal plate.
    They do, but through the bulb, if the bulb is glowing. All the currents in the circuit flow to the ground. Ground is a single reference node in the circuit. In many single-source circuits, it is assumed at the negative terminal of the source.
     
  4. Aug 23, 2016 #3

    Dale

    Staff: Mentor

    All of the grounds in a circuit should be connected with a very low resistance connection. So it shouldn't matter which path is taken.

    Generally current will take all possible paths. The amount of current will be greatest on paths with low resistance and high voltage, but there will be some current even through high resistance and low voltage paths.

    Because it is physically large enough to take a large amount of charge without changing voltage too much. Plus, it is convenient for safety also.
     
  5. Aug 23, 2016 #4

    Drakkith

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    Staff: Mentor

    Assuming you have a circuit similar to the one below, the simple answer is that they charges don't have the choice of flowing to ground. The potential difference across the terminals forces charges off of one terminal and into the other. The number of charges leaving the negative terminal must equal the number of charges entering the positive terminal, otherwise a charge imbalance develops inside the battery and the resultant electric field prevents charges from leaving the negative terminal in the first place, stopping the flow of current. If charges moved from the negative terminal to ground in the following circuit then just such a charge imbalance would develop.


    battery.gif
     
  6. Aug 23, 2016 #5
    there is a subtle difference between 'grounding' and 'earthing'
    A 'ground' is usually also an 'earth' ...but not always.
    Earthing is concerned with safety !!!
    Under normal circumstances no current should flow to earth !!!
    Are you aware of these differences?
     
  7. Aug 26, 2016 #6

    David Lewis

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    Gold Member

    No matter what shape of conductors, the circuit will possess some capacitance. In a well designed circuit, however, stray capacitance is usually negligible, so the charge stored will be small.
     
  8. Aug 30, 2016 #7

    CWatters

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    In an ideal world all grounds are connected together by a conductor with zero resistance and as Dale said it shouldn't matter which path the current takes. In the real world not all ground connections have zero resistance so you may have to take that resistance into account.

    This is a very big subject. Not all circuits even need to be grounded. One common reason for grounding mains powered equipment is safety... Consider what happens if a wire becomes loose inside a piece of equipment and comes into contact with it's metal case. That could put a high voltage onto the metal case. A person might touch the case and electricity could flow through their body to ground giving them an electric shock. If the case is grounded then when the loose wire comes into contact with the case it should cause a fuse or circuit breaker to open preventing someone getting an electric shock.

    See other replies.

    Yes and no. The plate will act like a conductor so electrons will flow through it. However the plate may have some capacitance with respect to other parts of the circuit and/or earth. It's possible for this capacitance to become charged. The significance of this will depend on the circuit. It might be insignificant in a typical low frequency, low voltage circuit or very significant in a higher voltage or high frequency circuit.

    For a current to flow you need a closed circuit and a voltage. With only one connection to earth that part of the circuit isn't closed so current doesn't flow in it. You can get current to flow to (and from) earth like this.

    Earth return.png

    A battery eliminator turns the AC voltage provided by the grid into a DC voltage similar to that produced by a battery. A typical battery eliminator does this in several stages/steps....

    A Transformer converts the high voltage AC into low voltage AC.
    A rectifier and capacitor that together convert the low voltage AC into DC.
    A regulator that converts the DC voltage produced by the rectifier to the DC voltage that is actually required at the output. The regulator usually improves the accuracy and stability of the output voltage. eg it might turn 15V +/- 3V into 9V +/- 0.5V. Not all battery eliminators have or need a regulator stage.

    There are other ways to build a battery eliminator.
     
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