Cavities and Electrostatic Shielding

A conductor in equilibrium has zero electric field inside its own material, always. Free electrons are mobile, so they rearrange themselves until that condition holds. If you place a point charge +q inside a cavity carved out of a conducting shell, the field from that charge would poke into the metal unless something cancels it. Electrons in the metal shift toward the cavity wall nearest the charge, piling up until the inner surface carries exactly -q. That induced layer cancels the point charge's field everywhere inside the conducting material, which is the only region required to be field-free.

Charge conservation then takes over. The shell started neutral, so if the inner wall has -q, the outer surface must hold +q to keep the total at zero. If the shell already carried some extra charge Q, that extra charge joins the +q on the outside, since the inner surface's charge is pinned by the cavity charge alone and never changes. This is why the outer surface charge and field depend only on q and any net charge given to the shell, never on where the cavity charge sits or how the cavity is shaped. That independence is the real reason a car shell protects you in a lightning strike: current redistributes on the metal skin, and the field inside the passenger cavity stays zero regardless of the strike's exact path.

This separation of inner and outer surface charge sets up the next idea, grounding. Connect the shell to the ground and the outer charge can drain away or get replenished, decoupling the outside field from the cavity's contents entirely, while the inner surface still faithfully mirrors whatever sits inside.