Question

The length of a certain conductor of resistance 100 Ω is doubled and its cross-sectional area is halved. Its new resistance is

Answer 1

The resistance of a condacting wire

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

Hence,

$$R'=\rho\frac{l'}{A'}=\rho\frac{2l}{A/2}=4\rho\frac{l}{A}=4R\\ =4*200\:\Omega=800\:\Omega$$