Question

An unchanged aluminium block has a cavity within it. The block is placed in a region where a uniform electric field is directed upward. Which of the conditions in the interior of the block’s cavity?
A. The electric field in the cavity is directed upward.
B. The electric field in the cavity is directed downward.
C. There is no electric field in the cavity.
D. The electric field in the cavity is of varying magnitude and is zero at the exact centre.

Answer 1

Hint: Electric field is zero inside a metallic cavity if no charge is present in the cavity, whatever be the field outside. Zero fields exist inside the cavity of a semiconductor therefore the electric field everywhere in the conductor must be zero.

Complete step-by-step answer:
The reason the electric field everywhere in the conductor must be zero means that the electric field is zero everywhere on the wall of the cavity which implies there can be no field in the cavity. This follows that the distribution of charge on the surface is independent to the shape of internal cavities.
Therefore the option (c) There is no electric field in the cavity is the right answer.

Note: If we consider a skin of conductor let us say a thin layer right outside the cavity, then the electric field at the point can be considered as the boundary condition of the problem where there is a charge. Now we can analyse that boundary condition is unchanged whereas the cavity is filled with more conductors, or with nothing.