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Questions  

If a copper wire is stretched to make its radius decrease by 0.1%, then percentage increase in resistance is approximately 

a
0.35%
b
0.4%
c
0.6%
d
0.2%

detailed solution

Correct option is B

In case of stretching volume of wire is assumed to remain unchanged. ∴lA=constant and L=cA∴R=ρ.lA=ρ.CA2=ρ.C(Ar2)2=R∝ 1r4∴ΔRR=−4Δrr×100%=(−4)×(−0.1%)=+0.4%

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