In a Van de Graff type generator a spherical metal shell is to be a `15 xx10^(6)V` electrode. The dielectric strength of the gas surrounding the electrode is ' 5 x 10^(7) V /m'. What is the minimum radius of the spherical shell required? (You will learn from this exercise why one cannot build an electrostatic generator using a very small shell which requires a small charge to acquire a high potential)