The 3-D Gauss's law is obtained after a volume integration of the 3-D Poisson's equation and takes the form
![2236_a8af0bd9-b3ec-4fda-90ae-da7e355abedc.png](https://secure.tutorsglobe.com/CMSImages/2236_a8af0bd9-b3ec-4fda-90ae-da7e355abedc.png)
Where the left-hand side is an integral of the normal electric field over a closed surface S, and Q is the net charge enclosed within S. Use it to derive the electric field at a distance r from a point charge Q(Coulomb's law). What is the electric potential in this case?