平行板电容器 E 0 d b=Ed==d C s Es b 同心球电容器 E 4丌Enr R-R U,- B∫A edl 4 4丌ER,R R A A B 4兀ER,R AB U-U. R-R平行板电容器 a b q C U U = − q S  = 0 E   = 0 0 a b qd U U Ed d S    − = = = 0 a b q S C U U d  = = − 同心球电容器 a b q C U U = − 2 0 1 4 q E  r = 2 0 0 1 4 4 B A B R B A A B A R A B q q R R U U Edl dr   r R R − − = = =  0 4 A B a b B A q R R C U U R R  = = − −