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A current is established almost the instant a circuit is switched on, even though the electron drift speed is only a few mm/s. The best explanation is that
Aelectrons travel from the switch to the bulb at the speed of light
Bthe drift speed momentarily rises to the speed of light
Cthe thermal speed of electrons carries the current
Dthe electric field is set up throughout the circuit almost instantly
Answer & Solution
Correct answer: D. the electric field is set up throughout the circuit almost instantly
1. The electric field propagates through the circuit at nearly the speed of light.
2. This field causes a local electron drift everywhere almost at once.
3. Current need not wait for one electron to travel end to end.
4. So D is correct; electrons themselves move slowly (A, B wrong) and random thermal motion gives no net current (C wrong).
_Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.7_
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