A) \[640\,\Omega \]
B) \[64\,\Omega \]
C) \[10\,\Omega \]
D) \[6400\,\Omega \]
Correct Answer: A
Solution :
(a) \[640\,\Omega \] We know that. Resistance (R) \[=\rho \frac{l}{A}=10\] ...(1) When it is stretched, length (l) increases 8 times and cross-sectional area (A') decreases eight times of previous. Hence. \[l'=8l\,\,and\,\,A'=\frac{A}{8}\] New resistance, \[R'=\frac{\rho (8l)}{A/8}=64\rho \left( \frac{l}{A} \right)\] \[=64\times 10\] (from eq.(1)) \[=640\Omega \]
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