Conductors, Shielding and Surface Charge

Copper contains conduction electrons that can move through the solid metal. An applied electric field gives them a net drift, producing current. The copper atoms do not travel along the wire with that current. In an insulator, charge moves much less easily, so deposited charge can remain near its starting place for a long time.

Electrostatic equilibrium means no net flow of charge, not motionless electrons. A nonzero field in the metal would cause drift, so charges rearrange until that field is zero. Excess charge is on surfaces, not in the bulk. A field parallel to a smooth surface would also move charge. Any nonzero field just outside must be perpendicular to that surface.

Use Gauss's law with a closed imaginary surface inside the metal wall, surrounding its cavity. Zero field gives zero flux and zero enclosed charge. If a separate cavity charge is q, the total inner-surface charge is -q. For an isolated shell whose net charge is Q, the outer surface carries Q + q. A neutral shell has Q = 0, so its outer charge is +q. The cavity charge must not touch the shell.

During lightning, a hard-topped vehicle with a continuous metal frame routes much of the current around the passenger space. This is current flow, not electrostatic equilibrium. Rubber tires are not the main protection; convertibles do not offer the same protection. Stay inside with windows closed and avoid touching metal parts or connected wiring. A strike can still damage the vehicle.

For an ideal closed conductor in electrostatic equilibrium, a cavity containing no charge has zero electric field, even with charges outside. If a charge is placed in the cavity without touching the metal, the cavity generally has a field while the metal wall remains field-free. This static result does not promise shielding from every changing electromagnetic field.