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NCERT Class XII Chapter
Current Electricity
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Question : 13 of 24
Marks: +1, -0
The number density of free electrons in a copper conductor estimated is 8.5 × 1028 m−310^{28}\,\text{m}^{-3}. How long does an electron take to drift from one end of a wire 3.0 m long to its other end? The area of cross-section of the wire is 2.0 × 10−6 m210^{-6}\,\text{m}^2 and it is carrying a current of 3.0 A.
Solution:  
We can first calculate drift velocity of the electrons from the given data
I = AnevdAnev_d
3 = 2 × 10−610^{-6} × 8.5 × 102810^{28} × 1.6 × 10−19 vd10^{-19}\,v_d
vdv_d = 0.11 × 10−3 m s−110^{-3}\,\text{m\,s}^{-1}
Now time taken by an electron to drift a length of 3 m with drift speed is
t = 1vd\frac{1}{v_d} = 27.27 × 10310^{3} s
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