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Charging an earthed conductor?

  1. Oct 26, 2014 #1
    1. The problem statement, all variables and given/known data
    I never quite understood the principles of earthing, but I think that the general idea is that if you have an object with excess charge (whether positive or negative), then connecting it to the earth with a conductor will mean that either the excess electrons flow to the earth (if object is negatively charged) or electrons flow from the earth to the object (if the earth is positively charged) and that this happens until both the earth and object have the same charge density, but because the earth is so large this means that all of the electrons from the object end up flowing to the charge, or as many electrons as are needed to make the positive object neutral flow to it. I have two questions about this:
    1. Is the effect of earthing difference if your object is an insulator with charge gathered on it, or is it is a charged conductor?
    2. In my text book, it says that 'an earthed conductor will become charged is a charged object is placed near it'. Should it not be that the charge then flows to the earth so neither the object, nor the conductor next to which the object is placed, end up with a charge?
    2. Relevant equations
    n/a

    3. The attempt at a solution
    My guess for the first question is that for an insulator it depends on whether the insulator is earthed at the point on its surface where charge is stored, because here the excess electrons may be able to just move enough to get to the earthing lead, but it depends on how good an insulator it is. But I'm not sure...
    For the second question, I have no idea :(

    Thank you in advance!
     
  2. jcsd
  3. Oct 26, 2014 #2

    gneill

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    Staff: Mentor

    At this course level you should assume that an insulator, unless otherwise specified, is a perfect insulator and that any charges on it are fixed in place. If the charges cannot move then bringing a charged object near will not cause the charges to move or rearrange in any way despite any attraction or repulsion they may feel from the object's charges. Likewise, charges from the vast neutral pool of charges that we associate with "earth" cannot flow onto or into an insulator if you earth a point on it.

    For 2, consider that opposite charges attract and like charges repel. So if you bring a charged object near an earthed conductor then it can attract charges of the opposite polarity up from the neutral pool (or, if you like, repel like charges from the conductor and force them down into the earth). These charges will move toward the charged object and pool at the edge of the conductor nearest the attracting charge. This pool of charge will eventually repel any further charges from moving onto the conductor from the earth --- the electric field from this collected pool of charges cancels the electric field from the charged object --- a balance is achieved. The net result is an net "induced charge" on the conductor.
     
  4. Oct 26, 2014 #3
    So then if you did this and removed the earthing wire, you would end up having a charged conductor. I think I get that now!

    Regarding this part, how then do you charge an insulator? I think I have read somewhere that you can gather charge onto an insulator...

    Thank you for your reply!
     
  5. Oct 26, 2014 #4

    gneill

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    Staff: Mentor

    With an ideal insulator in a thought experiment you do it by simply declaring it so :smile: In real life, a real insulator has an atomic structure and electrons that can be stripped off (even if the work function to do so is very high), or electrons can be "sprayed" onto the surface where they will stick (if loosely) since they've no where else to go. The transfer is often done by wiping the surface with a good donator or attractor of charges, such as rabbit fur or silk. Look up "Triboelectric effect".
     
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