Conductors

When a conductor is placed in an electric field, charge carriers from throughout the volume will flow to the surface, polarizing the material and creating a counterfield. While real conductors have a finite number of charge carriers, in practice we can assume that conductors contain an inexhaustible source of charge to create the counterfield.

So when do these charges stop flowing? Well, the reason the charges are moving is because an electric field exists inside the conductor. As the counterfield gets stronger, the net electric field inside the conductor gets weaker. Eventually the counterfield will be exactly as strong as the external field, and the net electric field inside the conductor is zero. No net electric field means that the charges no longer feel a force, and the charges stop moving (besides the random motions due to thermal fluctuations). We call this electrostatic equilibrium, when the charge carriers stop moving, and we note:
Inside a conductor in electrostatic equilibrium, the electric field is zero.

Note that this is different from the case of the charged spherical shell, which created no electric field inside itself, but which did not prevent other electric fields from existing there. The charge carriers in a conductor actively arrange themselves to ensure that the electric field inside is zero, and do so relatively quickly. The only times they are thwarted are when the electric field is changing rapidly over time, or if charge is continuously being added to the conductor.

When a conductor is in electrostatic equilibrium, it exhibits several other properties as well.