Case: Read the passage and answer the question.
When an excess charge is placed on an isolated conductor, it distributes itself on the surface of that conductor so that all points of the conductor, whether on the surface or inside the conductor, come to the same potential. This is true even if there is a cavity inside the conductor.
If an isolated conductor is placed in an external electric field, all points of the conductor still come to a single potential, regardless of whether the conductor has an excess charge. The free electrons distribute themselves on the surface in such a way that the electric field they produce at interior points cancels the external field that would otherwise be there.
The electric field just outside a charged conductor is found by using Gauss’s law. A small pill box (a short cylinder) is drawn across the surface, partly inside and partly outside the conductor. Through which part of the pill box does electric flux pass?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The flat face outside the conductor only
Flux passes through a small area only when the field there is not zero and has a part normal to that area. Inside the conductor the field is zero. Just outside, the field is normal to the surface of the conductor.
Option (1) The flat face inside the conductor only — Incorrect. The inner face lies inside the conductor, where the electrostatic field is zero. No flux passes through it.
Option (2) The flat face outside the conductor only — Correct. Just outside, the field is normal to the surface, so it is normal to the outer face too. The flux through this face is \( E\,\delta S \), and Gauss’s law then gives \( E=\sigma/\varepsilon_0 \).
Option (3) Both flat faces — Incorrect. Only the outer face has a field passing through it. The inner face is in the region of zero field.
Option (4) The curved surface and the outer flat face — Incorrect. The field just outside runs along the curved surface of the pill box, so it gives no flux there. The part of the curved surface inside the conductor has zero field.
NCERT (2026-27):
Electrostatics of Conductors – “Just inside the surface, the electrostatic field is zero; just outside, the field is normal to the surface with magnitude E. Thus, the contribution to the total flux through the pill box comes only from the outside (circular) cross-section of the pill box.”