🐌 Drift velocity — the current is fast, the electrons are not

Inside a metal the free electrons already move at ~10⁵ m/s, but randomly, so no charge is transported. Switch on a field E and that storm of motion acquires a tiny bias — a drift of v_d = I/(neA), a fraction of a millimetre per second. Yet the current is amperes.

Walkthrough 1 / 3

Field off. The free electrons (blue) already fly about between the fixed ions at about 10⁵ m/s — but randomly, so no charge is carried: the current is zero.

Current I 3.0 A

Cross-section A 1.00 mm²

v_d = I/(n·e·A) = — mm/s (n = 8.5×10²⁸ m^-3, e = 1.6×10^-19 C)
E = ρ·I/A = — V/m · one metre of wire takes — h to cross

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