gdr 2√r2+Z Z 80(r+Z R2+22 R2+22-1|2 28g (2+R)2=z(+(2yz+2(2 I R R V(Z) 28 2 Z2 4re z2 4re Z R>>Z (R2+z)%=R(+(2y7≤R+121R+2R 2R 2e(R-1ZD ≈ (6)电势→电场2 2 2 1 2 2 2 0 0 1 2 4 4 ( ) R dr r Z r Z σ σ σ ε ε = = + ∫ + 22 2 0 [ ] 2 R Z Z σ ε = +− r z 2 2 0 [ | 2 R Z Z |] σ ε = +− Z >> R 1 1 22 2 2 2 2 1 2 ( ) [ (1 ( ) ] | |[1 ( 2 R R ZR Z Z ) ] Z Z + =+ ≈+ 2 2 2 2 0 0 1 ( ) 22 4 4 RR Q V Z 2 Z Z Z0 σ πσ ε πε π = == ε R >> Z 2 2 1 1 2 2 2 2 2 2 1 ( ) [(1 ( ) ] [1 ] 2 2 Z Z RZ R R R R R R Z R + = + ≈ + ≈+ ≈ 0 () ( | |) 2 V Z σ R Z ε ≈ − ~z ~z -2 z V （6）电势 电场 ⇒