In a Van de Graaff type generator a spherical metal shell is to be a $15 × 10^6\; V$ electrode. The dielectric strength of the gas surrounding the electrode is $5 × 10^7 \;Vm^{−1}$. What is the minimum radius of the spherical shell required?

Question

$\begin{array}{1 1} 300\;cm \\ 30\;cm \\ 67\;cm \\ 60\;cm \end{array} $