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.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): When an uncharged metal slab is placed in a uniform external electric field, its two faces get equal and opposite charges.
Reason (R): In the external field, the free electrons of the metal drift along the direction of the field.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is correct but (R) is not correct.
Assertion (A) When an uncharged metal slab is placed in a uniform external electric field, its two faces get equal and opposite charges. — True. The free electrons move and gather on one face. The other face loses the same number of electrons, so it is left with an equal positive charge. The slab stays neutral as a whole.
Reason (R) In the external field, the free electrons of the metal drift along the direction of the field. — False. An electron carries a negative charge, so the force of the field on it is opposite to the field. The free electrons therefore drift against the direction of the field.
NCERT (2026-27):
Dielectrics and Polarisation – “… The free charge carriers move and charge distribution in the conductor adjusts itself in such a way that the electric field due to induced charges opposes the external field within the conductor. …”
Electrostatics of Conductors – “In a metal, the outer (valence) electrons part away from their atoms and are free to move. … In an external electric field, they drift against the direction of the field.”