Question

Why electric field is generated in insulator while not in conductor?

Answer 1

In electrostatics, free charges in a good conductor reside only on the surface.
There are at least two ways to understand this:
a) The free charge inside the conductor is zero. So the field in it is caused by charges on the surface. Since charges are of the same nature and distribution is uniform, the electric fields cancel each other.
b) Consider a Gaussian surface inside the conductor. Charge enclosed by it is zero (charge resides only on the surface). Therefore electric flux = 0. Furthermore, electric flux = electric field \times area. Since area cannot be zero, an electric field is zero.
c) Gauss's law states that the electric field flux through a closed surface is equal to the quotient of the load inside the surface divided by \varepsilon _0\varepsilon _0.