You've got a conducting hollow sphere of rradius 0.2 m and a charge of 20 micro culumbs. What is the potential of the sphere at distance r from the center if :
r=0.1 m?
r=0.2 m?
r=0.3 m?
if it is inside,V=0 (Gauss law, enclosed charge)
if on the surface, assume all charge is inside at r=.2, or V=kQ/.2
if outside, same as on surface, but r=.3