Problem $1$:

$4\mathrm{.}33$ Show that the electric potential difference $V_{12}$ between two points in air at radial distances $r_1$ and $r_2$ from an infinite line of charge with density ${}_l$ along the $z$ axis is $V_{12}=(\frac{{}_l}{2}lo{n}_0)ln(\frac{r_2}{r_1}).$