△ ∑△U(q)= Q 1Q2 Cy 0C2C2 l()=606nE(r U=urdrQ C V =  0 r A C d =   2 2 0 1 1 ( ) 2 2 Q i q Q U U q dq CV C C =  = = =   2 0 1 ( ) ( ) 2 ( ) r u r E r U u r dr =   = 