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Grounded/ Earthed conducter

  1. Nov 10, 2014 #1
    What exactly does it mean when we say that a conductor is grounded? I know it generally means that its potential is zero, but shouldn't the description of potential also give the location of a reference potential?
    For instance, if a conducting spherical shell is grounded, if its potential is zero, is at the same time the potential at infinity also zero?
    I get even more confused with flat plates. For eg Consider this prob: "3 large conducting plates are placed parallel to each other. The 2 plates at the sides are grounded. If the charge on the central conductor is Q, how much charge flows from the ground to the 2 extreme end conductors?"
    Here, where do we take the reference potential? How can we make use of the fact that 2 of the plates are grounded?
     
  2. jcsd
  3. Nov 10, 2014 #2

    phinds

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    It's possible that you are being misled by the two somewhat different uses of the term "ground". In a circuit, some point is taken as the reference point and called ground. That point is usually the negative terminal of the power source for the circuit. The other use is a reference to a point that is literally connected to the Earth and is called ground or said to be grounded. The first is an arbitrary (but generally reasonable) point in a circuit and the second is more specific. A circuit ground may or may not be connected to Earth ground.
     
  4. Nov 10, 2014 #3
    But i keep finding these kinds of problems in many textbooks and exams. For instance, there are probs like 'consider 3 concentric metallic spherical shells. The inner shell carries a charge q, the outer shell carries a charge 2q, the middle shell is grounded. How much charge flows from the ground to the grounded conductor?" I know we'll get the answer if we assume the potential at infinity is zero and the potential of the middle shell is also zero. But this doesn't seem to be so obvious to me. Is the statement "the middle shell is grounded" equivalent to "the potential of the middle shell is the same as the potential at infinity "?
    For parallel plates, if i'm not wrong, u can't take the potential at infinity to be 0, so i'm completely lost there. How would one, for instance, approach the problem that i posted i the original question.
    Note that there are no electronic circuits anywhere and nothing whatsoever mentioned about the earthin these.
     
  5. Nov 10, 2014 #4
    That question doesn't make sense to me. All grounds may as well be perfect electric conductors that are all connected to one another, so no charge would ever flow from one to another.

    For charge to flow you need a current which means you need an electric field which is defined as a voltage gradient. If two conductors are both grounded then their voltage is the same so no current between them.
     
  6. Nov 10, 2014 #5
    And you can't always assume that the voltage is 0 at infinity. It's a mathematically convenient assumption for a problem in which the voltage is under constrained. If the voltage is constrained somewhere then it's unnecessary and wrong to constrain it elsewhere.
     
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