Case: Read the passage and answer the question.
Conductors contain mobile charge carriers. In metallic conductors, these charge carriers are electrons. In a metal, the outer (valence) electrons part away from their atoms and are free to move. These electrons are free within the metal but not free to leave the metal. The free electrons form a kind of ‘gas’; they collide with each other and with the ions and move randomly in different directions. In an external electric field, they drift against the direction of the field. The positive ions made up of the nuclei and the bound electrons remain held in their fixed positions.
Consider the following statements about conductors:
(A) In electrolytic conductors, both positive and negative ions carry the charge.
(B) In the static situation, any excess charge on a conductor resides only on its surface.
(C) In a system of several conductors, all the conductors must be at the same potential.
(D) If a conductor has no surface charge, the electric field at its surface is not zero.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (B) only
In a metal the free electrons carry the charge. In the static situation they settle so that there is no field inside the metal.
(A) In electrolytic conductors, both positive and negative ions carry the charge. — True. In an electrolyte the charge carriers are ions of both signs. Their motion also depends on chemical forces.
(B) In the static situation, any excess charge on a conductor resides only on its surface. — True. Take any small closed surface inside the conductor. The field on it is zero, so by Gauss’s law it encloses no charge. Any excess charge must therefore sit on the surface.
(C) In a system of several conductors, all the conductors must be at the same potential. — False. Each conductor has one constant potential throughout its volume. This constant can be different for different conductors.
(D) If a conductor has no surface charge, the electric field at its surface is not zero. — False. The field just outside a conductor has magnitude \( \sigma/\varepsilon_0 \). With no surface charge, \( \sigma=0 \), so the field at the surface is also zero.
NCERT (2026-27):
Electrostatics of Conductors – “In electrolytic conductors, the charge carriers are both positive and negative ions; but the situation in this case is more involved …”
Electrostatics of Conductors – “When the conductor is charged, the excess charge can reside only on the surface in the static situation.”
Electrostatics of Conductors – “In a system of conductors of arbitrary size, shape and charge configuration, each conductor is characterised by a constant value of potential, but this constant may differ from one conductor to the other.”
Electrostatics of Conductors – “(For a conductor without any surface charge density, field is zero even at the surface.)”