Going from cartesian to cylindrical coordinates - how to handle division with $0$ I have three point charges with the cartesian coordinates:
$q_1(a,0,0) \: \: \: q_2(0,a,0) \: \: \: q_3(0,0,a) $,
I want to convert these into both cylindrical and spherical coordinates.
The cartesian coordinates are written like this: $(x,y,z)$
The cylindrical coordinates are written like this: $(r,\theta,z)$
The spheircal coordinates are written like this: $(\rho,\theta,\phi)$
From https://en.wikipedia.org/wiki/List_of_common_coordinate_transformations I found these conversion formulas going form cartesian to cylindrical:
$r=\sqrt{x^2+y^2}$
$\theta=\arctan(\frac{x}{y})$
$z=z$
My Problem
Now, I want to convert the cartesian coordinates $(a,0,0)$ into cylindrical. We go like this
$$r=\sqrt{a^2+0^2}=a $$
$$\theta=\arctan(\frac{a}{0})=??? $$
$$z=z $$
My problem is that I don't how to handle the $\theta$ calculation when the y-coordinate is $0$. Can somebody help me here, or maybe I'm using a wrong formula?

You need to use the arctan2 function, which takes all 4 quadrants into account.
