Identify the correct statements among the following.
(A) Inside a conductor, electrostatic field is non-zero
(B) Electrostatic potential is constant throughout the volume of the conductor and has the same value (as inside) on its surface
(C) In a conductor with a cavity, with no charges inside the cavity, the electric field inside the cavity is non-zero, whatever be the size and shape of the cavity
(D) At the surface of a charged conductor, electrostatic field must be normal to the surface at every point
Choose correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
In electrostatic equilibrium the free charges in a conductor are at rest, so there can be no field inside the material to push them. All the properties below follow from this.
(A) Inside a conductor, electrostatic field is non-zero — Incorrect. If there were a field, the free electrons would move; in equilibrium the field inside a conductor is zero.
(B) Electrostatic potential is constant throughout the volume of the conductor and has the same value (as inside) on its surface — Correct. With \( E = 0 \) inside, \( dV = -E\,dr = 0 \), so the potential is the same everywhere in the conductor and on its surface.
(C) In a conductor with a cavity, with no charges inside the cavity, the electric field inside the cavity is non-zero, whatever be the size and shape of the cavity — Incorrect. This is electrostatic shielding: the field inside a charge-free cavity is zero, whatever its size and shape.
(D) At the surface of a charged conductor, electrostatic field must be normal to the surface at every point — Correct. A tangential component would move the surface charges, so in equilibrium only the normal component remains.
NCERT (2026-27):
Electrostatics of Conductors – “… 1. Inside a conductor, electrostatic field is zero”
Electrostatics of Conductors – “… 2. At the surface of a charged conductor, electrostatic field must be normal to the surface at every point”
Electrostatics of Conductors – “… 4. Electrostatic potential is constant throughout the volume of the conductor and has the same value (as inside) on its surface”
Electrostatic Shielding – “… A remarkable result is that the electric field inside the cavity is zero, whatever be the size and shape of the cavity and whatever be the charge on the conductor and the external fields in which it might be placed.”