Question

If a copper wire is stretched to make it $0.1$% longer, what is the percentage change in its resistance?

• A. $0.1$%
• B. $0.2$%
• C. $0.4$%
• D. $0.02$%

Answer 1

The correct option is B $0.2$%

$R=\rho \frac{L}{A}$

On stretching a wire its length increases but at the same time its area of cross section , but volume remains constant.

$V=AL$

$⟹R=\rho \frac{L^2}{V}$

Hence $\frac{\Delta R}{R}=2\times \frac{\Delta L}{L}=2\times 0.1\%$

$⟹$ percentage change in resistance is $0.2\%.$

Answer-(B)