Example 2. Concentric-spherical-shell capacitor One has a radius ra and carries a charge Q, the other Rb and-Q(Rb> Ra) So the field btwn the shells is e=9g and the potential difference bwtn the shells is E·dl Qdr Q 1 1 Ra t 4丌∈ 4丌∈ Ra R the capacitance is Q4丌∈n Rb v Rb -Ra As Rb ->0, this approaches to the isolated conduc- tor Taking Ra n rb=60M, rb- Ra=0.1mM, then C is as large as that of the earth