In a conductor, if the number of conduction electrons per unit volume is $8.5 \times 1 0^{28} m^{- 3}$ and mean free time is $25 f s$ (femto second), it’s approximate resistivity is: $(m_{e} = 9.1 \times 1 0^{- 31} k g)$

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We know i = n e A v d ​ ⇒ i = R V ​ = ρl V A ​ = ρ E A ​ ρ = i E A ​ = n e A v d ​ E A ​ Drift velocity can be written as v d ​ = m e ​ e E ​ τ ρ = n e A e E τ E A m e ​ ​ = n e 2 τ m e ​ ​ = 8.5 × 1 0 28 × ( 1.6 × 1 0 − 19 ) 2 × 25 × 1 0 − 15 9.1 × 1 0 − 31 ​ ≈ 1.65 × 1 0 − 8 Ω m

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