For a simple homogeneous medium, is constant and . Therefore,

................(2.100)

This equation is known as Poisson’s equation. Here we have introduced a new operator, ( del square), called the Laplacian operator. In Cartesian coordinates,

...............(2.101)

Therefore, in Cartesian coordinates, Poisson equation can be written as:

...............(2.102)

In cylindrical coordinates,

...............(2.103)