(N/A) Consider a conductor with a cavity as shown in the figure.
There are no charges inside the cavity.
Regardless of the size and shape of the cavity, if a conductor with a cavity is placed in an external electric field, the electric field inside the cavity remains zero.
If the conductor is charged or charges are induced on a neutral conductor by an external field, all charges reside only on the outer surface of the conductor.
Any cavity in a conductor remains shielded from external electric influence. This phenomenon is known as electrostatic shielding.
For example, when we are in a car during a thunderstorm with lightning, we should close all the doors of the car. If lightning strikes the car (or a live current-carrying wire falls on the car), electrostatic shielding develops on the outer surface of the car, and we remain safe inside.