The drift velocity v_d of conduction electrons is related to current density J (n = number density, e = electron charge) by:
AJ = n e v_d
BJ = e v_d / n
CJ = n / (e v_d)
DJ = n e²·v_d
Answer & Solution
Correct answer: A. J = n e v_d
1. Number of charges crossing unit area per second = n v_d.
2. Each contributes a charge e, so current density J = n e v_d.
3. Typical drift speed in copper wire is ~10⁻⁴ m s⁻¹ — surprisingly slow.
_Source: NCERT Class 12 Physics Ch 3 §3.4 Drift Velocity and Current_
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