# The radius of the circular plates of the parallel plate condenser is r. Air is there as dielectric. What should be the distance between the plates if its capacitance is to be equal to that of an isolated sphere of radius r’?

$4 \pi \in_0 r '= \large\frac{\in_0\pi r^2}{d}$
$d= \large\frac{r^2}{4r'}$