Conductor

The CED files the definition under electric permittivity: conductors are made from electrically conducting materials in which charge carriers move easily. Metals are the standard case and the carriers are electrons.

That one property forces the electrostatic results. Once the charges stop moving, the conductor is in electrostatic equilibrium and three things hold.

• The field inside the material is zero. Any leftover interior field would keep pushing free charge, and while charge is still being pushed, equilibrium has not been reached.
• Excess charge sits on the surface. Like charges repel and nothing holds them in place, so they spread as far apart as the object allows.
• The field at the surface is perpendicular to it. A parallel component would drag surface charge sideways.

Two consequences follow. Outside an isolated sphere with a spherically symmetric charge distribution, the field matches that of a point charge of the same net charge at the center, so the $r$ in $k|q|/r^2$ is measured from the center, not the surface. And conductors in electrical contact end at the same surface potential, which makes a conductor's surface an equipotential surface.

Read the zero-field result precisely. It is the field within the conducting material that vanishes, not the field in a hollow cavity that itself contains charge.