Drift speed of electrons in a copper wire of cross-section A carrying current I is approximately:
AFew cm s⁻¹
BFew m s⁻¹
CFew km s⁻¹
DFew mm s⁻¹
Answer & Solution
Correct answer: D. Few mm s⁻¹
1. Typical free-electron number density in Cu ≈ 8.5 × 10²⁸ m⁻³.
2. For I = 1 A in a 1 mm² wire, v_d = I/(n e A) ≈ 7 × 10⁻⁵ m s⁻¹ ≈ a few mm s⁻¹ at best, often much less.
3. Even though drift speed is tiny, electric signals (waves) travel at near light-speed through the wire.
_Source: NCERT Class 12 Physics Ch 3 §3.4 (drift-speed estimates)_
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