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NCERT Class XII Chapter
Current Electricity
Questions With Solutions

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Question : 13
Total: 24
The number density of free electrons in a copper conductor estimated is 8.5 × 1028m–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 × 10–6m2 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 = Anevd
3 = 2 × 10−6 × 8.5 × 1028 × 1.6 × 10−19vd
vd = 0.11 × 10−3ms−1
Now time taken by an electron to drift a length of 3 m with drift speed is
t =
1
vd
 = 27.27 × 103 s
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