Solution:
The current I is calculated by:
$$I = \frac{N \cdot e \cdot v_d}{L} = \frac{(2 \times 10^{21}) \times (1.6 \times 10^{-19}) \times (0.25 \times 10^{-3})}{0.1} = 0.8\text{ A}$$
Thus, the correct answer is 0.8 A (Option 4)
The number of free electrons per 100 mm of ordinary copper wire is /[ 2 \times 10^{21} /] . Average drift speed of electrons is 0.25 mm/s. The current flowing is :
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