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, Q_in/є₀, with Q_in/є, where є is the permittivity of the material. (Technically, є₀ is called the vacuum permittivity.) Suppose that a 50 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. What is the permittivity of rubber?