I have this problem that I dont understand the answer to at all. Could you please help me with my misunderstanding?

So as far as I know, when image charges are used you do superposition and take one charge then find its mirror charge with respect to the grounded plane with potential 0V. Then you place it and after that repeat for the second charge. If I would follow my own method I would basically end up with the same image but with charge -2q below and +2q above at the same location as previously. What am I doing wrong and what should be done here instead?

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Answer: enter image description here


1 Answer 1


You're only solving for the field intensity above the plate. The "effects" of $-q$ there will be felt inside the region of $\epsilon$. From the perspective of that region of interest, $-q$ might as well be sitting in a region of $\epsilon$ aswell, the form of the magnetic field above the plate will not change.

  • $\begingroup$ Thank you for your answer! So in other words the $-q$ below does groud plane does not matter at all? Would the answer still be the same if we had same dielectrics $\epsilon$ above and below the grounded plane in the problem? $\endgroup$
    – Clone
    Apr 3, 2021 at 21:34
  • 1
    $\begingroup$ The answer should be the same for the area above the plate, yes. That area doesn't really care what's going on below the plate, except for charge distribution. $\endgroup$
    – V.L. Proud
    Apr 4, 2021 at 3:11

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