




We prove this by noticing that g gives the potential produced by a point charge
at r':
This integral merely sums up the contributions from small volumes dV around
the point r', with the charge dV
in that volume contributing as if it was a point charge at r'.
We can recover v from f by taking its divergence: v = grad
f = (r')(r
 r')r
 r'^{3
}dV'.