Consider a wire of length 4m and cross-sectional areal `1 mm^(2)` carrying of 2A. If each cubic metre of the material contains `10^(29)`free electrons, find the average time taken by an electron to cross the length of the wire.

Explanation

`l =4m,
` A= 1mm^2 = 1 xx (10^-6) (m^2)`
` l=2A, n/V = 10^29, t =?`
` l=nA V-d e `
` rArr 2 = ((10^29) xx 1 xx (10^-6) xx Vd xx 1.6 xx (10^-19))`
` rArr Vd = (2/ (10^29) xx 1 xx (10^-6) xx 1.6 xx (10^-19))`
= (1/ 0.8 xx (10^4))= 1/8000`
` t = l/Vd = (4/1/8000) = 4 xx 8000 `
` 32000 = 3.2 xx (10^4)sec ` .