A Figure 4-2 Parallel-plate capacitor. The lower end of the small cylindrical figure is situated inside the lower plate where E=0
Example 2. Concentric-spherical-shell capacitor One has a radius ra and carries a charge Q, the le other Bb and -Q. (rb> ra So the field btwn the shells is e 丌e0T and the potential difference bwtn the shells is BbE·dl=Ra4 Qdr Q 1 1 a 2 trEo Ra rb le capacitance is Q4丌0B2aB v Rb- Ra As Rb Is approaches to the isolated conduc- tor Taking Ra rb=60M, rb- Ra=0. 1mM, then C is as large as that of the earth