Michael D. answered 07/15/19
U of M Math Teaching Program, MS Purdue Advanced Math Apps Physics
If I remember correctly, we have the boundary condition that the potential energy at an infinite distance is zero for a test charge moved from infinity to the distance r from the point charge. Then writing the charge distribution as a Dirac delta function in position vector r for the point charge we obtain the expression for the electric field vector for a point charge of magnitude + q upon integration
The integral version of Gauss's equation can thus be rewritten as
This yields Coulombs law for the point charge in free space.