一-y-6d =-fE.d7-fE.d =-IR-Ir+8 =1心dr-6=-￡ ()E.d7=R ∫ ⋅−=− ba ab lEVV r r d = −IR − Ir + ε ∫∫ ⋅−⋅−= bc ca lElEr r r r dd = Ir − ε ∫ = bc drI IRlE c a =⋅ ∫ r r d )1( −ε