In this post, let’s derive a general solution for the Laplacian in Spherical Coordinates. In future posts, we shall look at the application of this equation in the context of Fluids and Quantum Mechanics.

where

The Laplacian in Spherical coordinates in its ultimate glory is written as follows:

To solve it we use the method of separation of variables.

Plugging in the value of into the Laplacian, we get that :

Dividing throughout by and multiplying throughout by , further simplifies into:

It can be observed that the first expression in the differential equation is merely a function of and the remaining a function of and only. Therefore, we equate the first expression to be and the second to be . The reason for choosing the peculiar value of is explained in another post.

(1)

The first expression in (1) the Euler-Cauchy equation in .

The general solution of this has been in discussed in a previous post and it can be written as:

The second expression in (1) takes the form as follows:

The following observation can be made similar to the previous analysis

(2)

The first expression in the above equation (2) is the Associated Legendre Differential equation.

The general solution to this differential equation can be given as:

The solution to the second term in the equation (2) is a trivial one:

Therefore the general solution to the Laplacian in Spherical coordinates is given by: