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Homework Help: What happen when a plate of charged capacitor is earthed?

  1. Oct 20, 2011 #1
    1. The problem statement, all variables and given/known data

    A capacitor is charged with some potential difference. A charge will developed on both of it's plates. Now remove this capacitor from battery and ground/earth one of the plate through a wire. What will happen??

    2. Relevant equations

    gauss law

    3. The attempt at a solution

    Potential of the surface that is earthed should became zero but in this case how to manage other things like charge on the other plate so the electric field inside metal plates of the capacitor remain zero.

    [highlight] NOTE: all the things are talked about parallel plate capacitor[/highlight]
  2. jcsd
  3. Oct 21, 2011 #2
    friends please reply.
  4. Oct 21, 2011 #3


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    Staff: Mentor

    Not much happens. Before making contact the capacitor is a net neutral object (even if the charges inside are separated to some degree). After making contact, the plate in contact with the Earth then has the same potential as the Earth. But no charges flow because there's not a complete circuit, and because the charges on either plate are attracting each other and holding them to the inside plate surfaces. The capacitor is still a net neutral object (as it the Earth).
  5. Oct 22, 2011 #4
    Yeah that is what i think.
    But let us think in this way. what the potential of the plate that is connected to earth. let me say that is negative plate then potential at this plate is not zero But the potential of the earth is zero. So here two objects are connected through conducing wire one with higher potential and other with lower potential so there must something that make this potential difference zero,
  6. Oct 22, 2011 #5


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    Staff: Mentor

    Potential is a relative thing. The negative plate has a potential WITH RESPECT TO the positive plate, but the capacitor as a whole is neutral. It would take no work to move that capacitor anywhere you wish in an electric field. Connecting either plate to the Earth would only serve to establish its potential as being the same as that of the Earth.
  7. Oct 22, 2011 #6


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    Gold Member

    I used to service, repair and restore tube-driven guitar amplifiers. The method I used was to connect the caps feeding the B+ rail with ground, using jumpers and a bleed resistor to prevent sudden discharge. Keep the caps connected to ground all the time you've got your fingers in the chassis because those big caps have some "memory" and can re-develop a charge.

    Use brightly-colored (fluorescent) jumpers so that you remember to remove them before buttoning up the amp and powering up.
  8. Oct 23, 2011 #7
    I like your answer but it is still doubt, I will discuss with my physics teacher.
    thanks both the persons.
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