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Relationship between E and 1/r

  1. Oct 12, 2008 #1
    Hi all,
    I was taught that E(field strength) is proportional to 1/r for a setup of a ring(earthed) and a positive electrode(in the centre of the ring).
    can anyone confirm this or guide me its derivation?
    thanks in advance!
  2. jcsd
  3. Oct 12, 2008 #2
    think in terms of flux lines. you can see geometrically why the field strength
    from a point is r^-2.
    from a one dimensional line is r^-1.
    from a 2 dimensional sheet is r^0.
  4. Oct 12, 2008 #3
    hi, i do not understand exactly wad you mean... is there any diagram i cud refer to?
  5. Oct 12, 2008 #4
  6. Oct 12, 2008 #5
    oh thanks, it is clearer now.
    but how about the part of point vs 1d line vs 2dimentional sheet?
  7. Oct 12, 2008 #6
    it should be obvious. visualize it. thats what flux lines are for. start with a 2 dimensional sheet extending to infinity. where are the flux lines going to go? how does their density change as you move away from the sheet?
  8. Oct 13, 2008 #7
    errr, how do i model the ring and a round electrode pressed onto a plane as?
    a point charge with a spherical shell? but that will give me E is proportional to 1/r^2. this is the part i do not understand..
  9. Oct 13, 2008 #8


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    Staff Emeritus
    Science Advisor

    No, you would model the electrode as the single point, the origin, and the ring as a circle of radius r. Using a spherical shell gives you a 3 dimensional problem so you get 1/r^2 again.
  10. Oct 14, 2008 #9
    oh okay. the theory confused me when it insisted 1/r. i think i gotta clarify with the lab lecturer. i was sure it was 1/r^2 by that understanding. thanks ppl!
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