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Earthing charged bodies

  1. Dec 21, 2015 #1
    I understand earthing as - if we earth a charged body, its charge distribution changes such that, its potential becomes zero.

    So now I have a doubt that if we have 2 charged spheres(Q1 , Q2) ,separated by some distance and then if we earth one of them , then how will the charge distribution change ?
    On similar lines, what will happen when 3 charged spheres kept in the same way and then if I earth
    1 the middle one
    2 the one at extreme
    ??
     
  2. jcsd
  3. Dec 21, 2015 #2

    haruspex

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    Are you asking what the charge distribution looks like on two conducting spheres near each other? I don't think there's a closed form solution for that. You can get successively better approximations using the method of images.
    Or are you simply asking how earthing one affects it? Before earthing, each is at a uniform potential. After earthing, each is at a uniform potential. So the charge distributions are essentially similar. It might be possible to figure out the charge ratio (as a function of the radii and the distance beteween the spheres) to get one of the potentials to be zero.
     
  4. Dec 22, 2015 #3
    What is method of images ??
     
  5. Dec 22, 2015 #4

    haruspex

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  6. Dec 22, 2015 #5
    ???
    there will be only one solution this !

    I agree that there are many possible charge distributions allowing potential of the earthed body zero , but only one of that is naturally feasible , obeying Uniqueness theorem !! (this is all i know about uniqueness theorem)

    So what I exactly want to know is - which will be the right charge distribution for the situation I mentioned ??
     
  7. Dec 22, 2015 #6

    haruspex

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    I didn't say there would not be a unique solution, I said it would not be expressible in 'closed form'. https://en.m.wikipedia.org/wiki/Closed-form_expression
     
  8. Dec 23, 2015 #7
    Ok

    But now have you understood what exactly my doubt is ?
     
  9. Dec 23, 2015 #8

    haruspex

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    I'm not sure what sort of answer you are looking for to the original question. Do you want to find a specific equation for the distribution, or just a verbal description of how earthing one will change it?

    Does this work: Suppose that before earthing the charges are -Q and +Q, and the potentials -V and +V. Zero potential is just a convention, so if after earthing the -Q one it is at ground and the other at V', just reset the 'zero' to V'/2. Then you have potentials -V'/2 and +V'/2. That's the same as you would have had if you had started with charges -+QV'/2V. Then add back equal uniformly distributed charge to each to bring the unearthed one back to +Q. Does that make sense?
     
  10. Dec 23, 2015 #9
    Check out the attached file,

    there , after grounding 3 work done to get a test charge from infinity to 3 will be zero, but in for moving the test charge -from infinity to 1 ; from 1 to 2 ; from 2 to 3 , if work done is W1 , W2 , W3 respectively , the what will be relation between W 1, W2, W3
    OR
    Will W1=W2=W3= 0 ??

    I want to know the general idea of nature for deciding charge distribution in such kind of systems !!
     

    Attached Files:

  11. Dec 23, 2015 #10

    haruspex

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    Try this: http://rspa.royalsocietypublishing.org/content/468/2145/2829
     
  12. Dec 23, 2015 #11
    Can you put that qualitatively !
     
  13. Dec 24, 2015 #12

    ehild

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    The charge distributions on the spheres will change if you ground one of them, but the charge remains the same on the isolated spheres, and all points of a sphere are at the same potential. If these potentials are V1, V2 and V3=0, the work done when you bring in a test charge from infinity to sphere 1 is V1=W1.
    When moving the test charge from infinity to sphere 2 the work is V2, so the work when moving the test charge from sphere 1 to sphere 2, is W2=V2-V1.
    Sphere 3 is at zero potential so the same work is done when the test charge is moved from sphere 2 to sphere 3, as from sphere 2 to infinity, that is -V2. Your W3=-V2.
    The total work between infinity and the grounded sphere is zero, but the individual works are not..
     
  14. Dec 24, 2015 #13
    Ok
    But now if we have n infinitely large plates placed parallel to each other at equal distances, with charges Q1, Q2. . . . . . .
    And then if we ground r th plate then what will be subsequent charge distribution? ?
     
  15. Dec 24, 2015 #14

    ehild

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    The charge distribution on the infinite parallel plates has to be uniform. In case the charges are finite, the surface charge densities are zero.
     
  16. Dec 24, 2015 #15
    What will be the charge on the grounded plate ?
     
  17. Dec 24, 2015 #16

    haruspex

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    Perhaps I misunderstood (Shreyas had not made the question precise) but I thought the problem here was to find the potentials given the charges, radii and separations. Or, failing that, to make some qualitative statement about how the potentials change when one of the spheres is grounded. I see from post #11 that such a qualitative description may be what is sought, but even that is not straightforward.
    If the grounded sphere had been at a positive potential then clearly it will go to a lower potential. Seems also clear that the potentials will become lower at the other spheres. But will they drop by as much?
     
  18. Dec 24, 2015 #17

    haruspex

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    Infinite parallel plates are much easier. What is the potential at distance x from a single charged infinite plate?
     
  19. Dec 24, 2015 #18

    ehild

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    Potential with respect to what? What would be the potential of a single charged infinite plate at infinity?
    Instead of infinite plates we should say that the extension of the plates are very large with respect to the distance between them. In that case is it possible that the plates are at non-zero potentials with respect to infinity.
     
    Last edited: Dec 24, 2015
  20. Dec 25, 2015 #19

    haruspex

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    For infinite uniformly charged plates, it is usually taken as relative to the potential at the plate. The potential at infinity becomes +/-infinity.
     
  21. Dec 25, 2015 #20

    ehild

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    What is the situation if the infinite plate gets finite charge q?
     
  22. Dec 25, 2015 #21

    haruspex

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    It doesn't mean anything to say a finite but nonzero charge is uniformly spread on an infinite surface. It's like trying to pick an integer at random, all equally likely.
     
  23. Dec 25, 2015 #22

    ehild

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    Well, a finite charge uniformly distributed over an infinite area results in zero surface charge density. Don't you agree?
    In Classical Electrodynamics, charge is not quantised.

    All points of a conductive plate are at the same potential. If the plate is infinite, its potential is the same at infinity as it is at a given point. P, as you do not need to do work when moving a test charge along the conductive plate.
     
  24. Dec 25, 2015 #23
    What made me start the thread was - that problem of n infinitely large plates placed parallel to each other at equal distances.

    So now let me be very clear!
    Before I tell you what problem I exactly have with that infinitely large plates problem, let me tell you one thing that I observed for infinitely large plates -
    If we have 2 infinitely large plates(they are truly infinite no approximation) placed parallel to each other, and if we ground one of them, then potential of only one plate can become zero ie we approach the grounded plate from two sides say we are approaching from right(where we encounter the grounded plate directly when we approach from infinity, not the non grounded plate first and then the grounded one) so then if the potential of the grounded plate is to be zero then electric field to the right of the grounded plate has to be zero. But charge distribution favouring this won't make work done zero if we approach the grounded plate from left.
    I hope I'm clear !!!
     
  25. Dec 25, 2015 #24
    So now my doubt
    As of the problem where n infinitely large plates placed parallel to each other at equal distances
    There if we choose some r th plate having charge Q r and ground it then what will be subsequent charge distribution?

    Answer given is Σ Q i =(Q1 + .....+Qn)
    This much charge will go into ground , from that r th plate.

    Why so ?
     
  26. Dec 25, 2015 #25
    Of course the work is not zero. If we try to add charge to the left plate (the one that is not grounded if I understood you correctly) and we come from infinity and approach from the left, then there is some work required to do this. The larger the plates are, the smaller the field strength on the left side becomes but at the same time the field strength drops off more slowly so assuming a constant charge density on the plate the work required will stay the same. If the size of the plate approaches infinity the field strength will approach zero but to calculate the required energy per charge (i.e. the voltage) you have to integrate that field strength over an infinite distance. So you end up with infinity times zero which (in this case) is not equal to zero.
     
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