I was going through the method of images using 2 positive point charges placed along a line perpendicular to an infinite grounded conducting plate. I had found the surface charge density induced on the surface by these charges and also know that the electric field lines from either of the charges must fall within a circle (since the field lines diverge radially outward from the positive charge trying to end up in the negative image charge. However, I wish to derive the radius of the circle within which the electric field lines will always fall. How am I supposed to attack the problem?


1 Answer 1


All of the field lines don’t lie within a circle of any finite radius. Think about the field lines that start out heading almost perpendicularly away from the plate.

  • $\begingroup$ But in an image problem, a negative image charge is present whose field lines can terminate on the negative charge right? There will be a maximum radius then. I was wondering how to find it? $\endgroup$
    – steve
    Jan 20, 2020 at 1:40
  • 1
    $\begingroup$ There will be a maximum radius then. That doesn’t follow from the existence of the negative image charge. $\endgroup$
    – G. Smith
    Jan 20, 2020 at 1:42

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