Worked example: A neutral hollow metal sphere (outer radius 0.15 m) has a point charge q = +6 nC placed at its centre without touching the sphere. Describe the charge distribution and the field inside the metal and inside the cavity. Step 1: The +6 nC attracts electrons to the inner surface: −6 nC appears uniformly on the inner wall (by symmetry), leaving +6 nC on the outer surface (the sphere was neutral overall). Step 2: Inside the metal itself, E = 0 — the free charges have arranged exactly so. Step 3: Inside the cavity (r < inner radius), the field is that of the central point charge plus the inner-surface charge: by Gauss's law it equals kq/r² radially outward, as if the sphere were absent — the cavity field depends only on the charge placed inside it. Step 4: Outside the sphere, the field is k(6 nC)/r² as if +6 nC sat at the centre. The metal shields the cavity from outside influences and vice versa.