dR = ρ ⋅ dr 4π r 2 La corriente debe atravesar un área igual a la de un cascarón de “largo” dr. ρ dr ρ 1 1 ρ ⋅ 2 = − = 4π r 4π a b 4π a b R=∫ V i= = R dr b −a ⋅ ab r i V ρ ⋅ b −a 4 π ( ab ) r a i b v i 4ρπ ( bab− a ) Vab j= = = A 4π r 2 ρ ( b − a ) r 2 i V