E=dE cose de xJ014πE0 (Sing,-Sin0) 4πEna Ev=dE sine de (cos0,-cos0,) 014na 4πE0a + 讨论 (1)a>>L杆可以看成点电荷 y↑dE 1L dE tA E=0 E 4丌Ea dE (2)无限长直导线 1人6 2 0,=0 E.=0 ag 0 E 2re a Ey = Ey d  Ex = Ex d (1) a >> L 杆可以看成点电荷 = 0 Ex 2 4 0 a λ L Ey  = (sin sin ) 4 2 1 0 θ θ a −  =     = 2 1 0 cos d 4 θ θ θ θ  a  (cos cos ) 4 1 2 0 θ θ a −  =     = 2 1 0 sin d 4 θ θ θ θ  a  讨论 (2) 无限长直导线 θ 1 = 0 θ 2 =  ε a λ Ey 0 2 = Ex = 0 a P x y dq O r  2 1 E  d dEx Ey d