Assertion (A): An electric bulb glows almost immediately when it is switched on.
Reason (R): The drift velocity of electrons in a metallic wire is very high.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is true but (R) is false.
Assertion (A) An electric bulb glows almost immediately when it is switched on. — True. When the switch is closed, the electric field is set up throughout the circuit almost instantly, travelling with the speed of light. The free electrons at every point of the wire start drifting at once, including those in the filament. So the current does not wait for electrons to travel from the switch to the bulb.
Reason (R) The drift velocity of electrons in a metallic wire is very high. — False. The drift speed is very small, of the order of \( 1\,\mathrm{mm\,s^{-1}} \) for an ordinary current in a copper wire. At this speed an electron would take many minutes to cover even a metre of wire.
NCERT (2026-27):
Example 3.2 – “(a) 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.”
Example 3.1 – “(ii) An electric field travelling along the conductor has a speed of an electromagnetic wave, namely equal to 3.0 × 108 m s–1 (You will learn about this in Chapter 8). The drift speed is, in comparison, extremely small; smaller by a factor of 10–11.”