R 0 lo 0 12 R14兀Er 40R14丌R2 C 0 U-U C4丌1R R 0 R-R R 特别是当 R,→>∞ C=4Ter0 2 0 0 1 0 2 0 0 1 2 4 4 4 2 1 R q R q dr r q U R R       = = −  2 1 4 0 1 2 R R R R C −  =   1 2 0 U U q C −  = 特别是当 R2 →  C = 4 0 R1 R1 R2 o q o − q 讨论