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# A charge q is placed at O in the cavity in a spherical uncharged conducter . Point S is outside the conducter . If q is displaced from O towards S (still remaining with in the cavity )

$(a)\;electric\;field\;at\;S\;will\;increase\qquad(b)\;electric\;field\;at\;S\;will\;decrease\;\qquad(c)\;electric\;field\;at\;S\;will\;not\;change\qquad(d)\;can't\;say$

Answer : (c) electric field at S will not change
Explanation :
-q charge will induce on the inner surface of cavity and that will induce in such a way that out of cavity , there should be no electric field due to inner charges .
Now since the sphere is neutral +q charge will induce on the outer surface of the sphere and gets distribute uniformly so that the electric field inside the conducter is zero . It will distribute uniformly on the outer surface of sphere independent of the position of q charge .
Therefore , electric field on S due to charges on outer surface is same for all positions of q charge and due to inner charges electric field is always zero . So the electric field on S will not change on displacing q towards S .

edited Aug 16, 2014