Conductors and Insulators: Free and Bound Charges

Every material is built from atoms and molecules, each with its own positively charged nucleus and surrounding negatively charged electrons -- but materials differ sharply in how easily those electrons can move through the bulk of the material, and this single distinction is what separates a conductor from an insulator.

Conductors. In a typical metallic conductor, the outermost electrons of each atom are only loosely bound to their parent atom and are free to wander throughout the entire body of the material, forming what is often pictured as a mobile "sea" of delocalised free charges (free electrons), moving freely past the fixed, positively charged ion cores left behind. When an external electrostatic field is applied to such a conductor, these free charges respond almost instantly: they redistribute themselves over the conductor's surface until the field they themselves produce exactly cancels the external field everywhere WITHIN the bulk of the conductor. This redistribution process, called electrostatic induction, involves no net transfer of charge into or out of the conductor -- charge merely rearranges itself internally, appearing as an excess of one sign on one part of the surface and the opposite sign elsewhere.

Once this equilibrium redistribution is complete, several important consequences follow directly. The net electric field inside a conductor, in electrostatic equilibrium, is exactly zero. Since $E=0$ throughout the interior, and $E=-dV/dl$ (Section 2.3), the potential $V$ must be exactly CONSTANT throughout the interior -- a conductor is always an equipotential body, its entire volume (and its surface) sitting at one single value of potential. The field immediately outside a charged conductor's surface is always perpendicular to that surface (any tangential component would drive the free surface charges to keep moving, contradicting equilibrium). Any charge placed on an isolated conductor ultimately resides entirely on its outer surface, with none left in the interior. A further, technologically important consequence is that an empty CAVITY inside a conductor experiences exactly zero electric field, regardless of however strong a field is applied outside the conductor -- the principle behind electrostatic shielding (a Faraday cage), used to protect sensitive equipment from external electric fields. …