the electric field must be zero inside a conductor in electrostatic equilibrium, but not inside an insulator. it turns out that we can still apply gauss's law to a gaussian surface that is entirely within an insulator by replacing the right-hand side of gauss's law, qin/ε0, with qin/ε, where ε is the permittivity of the material. (technically, ε0 is called the vacuum permittivity.) suppose that a 60 nc point charge is surrounded by a thin, 32-cm-diameter spherical rubber shell and that the electric field strength inside the rubber shell is 2500 n/c.