The number density of free electrons in a copper wire is $8.5 \times 10^{28}\ \text{m}^{-3}$. The wire is 3.0 m long, has cross-section $2.0 \times 10^{-6}\ \text{m}^2$, and carries 3.0 A ($e = 1.6 \times 10^{-19}\ \text{C}$). The time for an electron to drift the full length is about

1. Drift speed: $v_d = I/(neA)$. 2. $neA = 8.5\times10^{28}\times1.6\times10^{-19}\times2.0\times10^{-6} = 2.72\times10^{4}$. 3. $v_d = 3.0/2.72\times10^{4} = 1.1\times10^{-4}\ \text{m/s}$. 4. Time $t = l/v_d = 3.0/1.1\times10^{-4} \approx 2.7\times10^{4}\ \text{s}$ (about 7.5 hours); the small values ignore the tiny drift speed. _Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.25_