The drift speed of electrons in a conductor is estimated to be only a few mm \( \mathrm{s^{-1}} \), but on closing a circuit, how current is established almost instantly in an electric circuit?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Electric field is established throughout the circuit with the speed of an electromagnetic wave, causing at every point local electron drift causing current.
A conductor is full of free electrons everywhere along the circuit. The current does not need an electron to travel from the cell round the circuit; it needs only the electric field, which sets the electrons already present at every point drifting. The field spreads through the circuit at nearly the speed of light, so the current starts almost at once.
Option (1) Electrons are accelerated towards the positive end of the conductor while their velocity increases towards the other end — Incorrect. The electrons do not keep gaining speed along the conductor; collisions with the ions keep their average velocity at the small drift value.
Option (2) Drift velocity of electrons increases — Incorrect. The drift velocity stays a few \( \mathrm{mm\,s^{-1}} \); a larger drift velocity is not what makes the current start at once.
Option (4) The electrons drift towards the negative end — Incorrect. Electrons drift opposite to the field, towards the positive end.
NCERT (2026-27):
Drift of Electrons and the Origin of Resistivity – “… Electric field is established throughout the circuit, almost instantly (with the speed of light) causing at every point a local electron drift. Establishment of a current does not have to wait for electrons from one end of the conductor travelling to the other end. However, it does take a little while for the current to reach its steady value.”