Final answer:
The electric field inside a conductor is zero, regardless of its net charge. For a conductor with a net negative charge, like -2nC, this excess charge distributes on the surface, and if a -2nC charge is placed inside a cavity, an equal +2nC is induced on the cavity's surface.
Step-by-step explanation:
The electric field inside a conductor is always zero, regardless of the net charge on the conductor. When a conductor has a net negative charge, such as -2nC, this excess charge resides on the surface of the conductor. If a cavity is present within the conductor and a charge of -2nC is placed inside this cavity, an equal and opposite charge will be induced on the inner surface of that cavity to ensure that the electric field inside the conductor remains zero. Concurrently, the originally excess charge (2nC) will redistribute on the external surface of the conductor. Hence, the net charge on the surface of the cavity would be +2nC and on the outer surface would be the total external charge minus the induced charge, if no additional charges are present.