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Q.

If a copper wire is stretched to make it 0.1 % longer. The percentage change in its resistance is


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a

increase by 0.2%

b

decrease by 0.2%

c

decrease by 0.05%

d

increase by 0.05% 

answer is A.

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Detailed Solution

The resistance will increase by 0.2%
Let the length of the wire be 'L' and area of cross-section be 'A' and resistivity of the material be ρ
Therefore R = ρLA     On stretching a wire its length increases but its area of cross section decreases, and volume remains constant.
V = AL
R = ρL2V
R ∝ L2
Hence   ΔRR = 2ΔLL
Hence, for  ΔLL = 0.1%
ΔRR = 0.2%
 
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