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 a long, straight wire with linear charge density 250 nC/m is covered with insulation whose permittivity is 2.5є₀. What is the electric field strength at a point inside the insulation that is 1.5 mm from the axis of the wire?