In a Van de Graaff type generator a spherical metal shell is to be a `15 xx 10^6 V` electrode. The dielectric strength of the gas surrounding the electrode is `5 xx 10^7 Vm^-1`. 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.)