# Why is a grounded plate needed when applying the method of images?

In the classic image charge problem of a charge $$q$$ above an infinite grounded plane, it is well known that the field lines essentially behave as though there were a negative charge behind it, and the total charge on the plate is $$-q$$.

However, since the plate is grounded wouldn't the extra negative charge go back to the earth? How can the plate remain negatively charged even when connected to ground?

Further, how would the situation change if the plate were not grounded? I assume the positive charge would induce a negative charge on one side the plane, but the total charge would still be zero. Would the field lines terminate in this case?

• The negative charge on the grounded plate came from the earth. The act of connecting the plate to ground is what allowed the negative charge to relocate to the plate's surface by the attraction of the positive charge above it. Grounding doesn't equate with "there is zero charge." Commented Apr 3, 2016 at 23:26
• @user55515 Ah that makes sense. And then am I correct in assuming that if the plane were not grounded, it would induce a negative charge on one side and a positive charge on the other side, and while the field inside the conductor would be zero it would re-appear on the other side? Commented Apr 4, 2016 at 5:50
• Indeed. That sounds correct to me. Commented Apr 6, 2016 at 3:34
• What if the plate is not grounded? Will we still be able to use method of image charge? Commented May 5, 2016 at 5:08