SabyE

So my teacher, as we made the multipole expansion of Vector Potential (A) decided to proof that the monopole term is zero doing something like this: ∫∇'⋅ (J.r'i) dV' = ∮ r'i J ndS' = 0 The first integral, "opening" the nabla: J⋅(∇r'i) + r'i(∇⋅J) this must be equals 0 J = current density vector n = unit vector My question: I didn't get why the last thing (the nabla opened thing) was zero... He couldn't explain quite well and got confused, then the next day he added one condition to that be zero: [*] J is defined only in a certain region of space and that's why it's zero. I didn't get at all... *Sorry for my bad english