Work done by electric force: dA=q,E·dl=q,Ecos0dl E= 4πe,r2 a dA= 9o9 cos0 dl= 99 _dr 4π6，r1 4π6.1Work done by electric force: q b a r  r   dr  dE   dA q E dl q E dl  o   o cos   2 4 r q E   o  dr r q q dl r q q dA o o o o 2 2 4 cos 4       