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Physics · 2026 · 2 marks
CBSE 2026 · Region 1 · Set 1 · Q20
What is the order of magnitude of drift velocity of electrons in a conductor? Deduce the relation between the current flowing through a conductor and…
What is the order of magnitude of drift velocity of electrons in a conductor ? Deduce the relation between the current flowing through a conductor and drift velocity of electrons in it.
Marking-scheme solution
Magnitude of drift velocity in conductor is of the order of a few mm/s
For n number of free electrons per unit volume in a conductor of cross section area A, the total charge transported across the area in time $\displaystyle \Delta \mathrm{t}$ is
$\displaystyle Q=-\mathrm{ne} \mathrm{~A} \mathrm{v}_{\mathrm{d}} \Delta \mathrm{t}$
As electron moves in a direction opposite to that of electric field
$\displaystyle \mathrm{I} \Delta \mathrm{t}=\mathrm{ne} \mathrm{~A} \mathrm{v}_{\mathrm{d}} \Delta \mathrm{t}$
$\displaystyle \mathrm{I}=\mathrm{neAv}_{\mathrm{d}}$
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CBSE Class 12 Physics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.