Q.Explain why the electric field inside a conductor, in electrostatic equilibrium, is always zero, and why the entire conductor must therefore be an equipotential body.

A conductor contains abundant free charge carriers (free electrons in a metal) that redistribute themselves until electrostatic equilibrium is reached; an insulator's electrons stay tightly bound to their own atoms, unable to move freely through the bulk material. Under electrostatic equilibrium, every conductor obeys five properties: zero field in its interior, constant potential throughout (interior and surface), no net charge in its interior (all excess charge sits on the surface), field immediately outside always perpendicular to the surface, and surface charge density that can vary from point to point (highest at sharp points or edges).

A HOLLOW conductor placed around a sensitive region provides ELECTROSTATIC SHIELDING -- since the field inside any conductor's interior is always zero, nothing outside can influence what is inside it, the FARADAY CAGE principle. This is why a car's metal body protects its occupants during a lightning strike, and why sensitive electronics (such as an MRI room) are enclosed in conducting shields against stray external fields.