Q 12-03-207JEE MainJEE Main 2021 (26 Feb, Shift 2)Easy
A wire of $1\ \Omega$ has a length of $1$ m. It is stretched till its length increases by $25\%$. The percentage change in resistance to the nearest integer is:
Answer: (D) $56\%$
When a wire is stretched its volume stays constant, so $A \propto 1/l$ and $R = \rho l/A \propto l^2$.
$$\frac{R'}{R} = (1.25)^2 = 1.5625$$
Percentage change $= 56.25\% \approx 56\%$.
Solution by Sreeraj P, M.Sc Physics