[Tex/LaTex] Beamer keeping line position constant on itemize

Question

I want to have a slide in beamer with itemize exposing new line on each click while at the bottom part of the slide there are some equations. This is my attempt

\begin{frame}
  \begin{overprint}{\textwidth}
    \leavevmode
    \begin{itemize}[<+->]
    \item <1-> Poisson Boltzman equation (PB)
    \item <2-> Debye-H\"uckle equation  (DH)
    \item <3-> The naive approach - applying (DH) equation to PE
    \end{itemize}
  \end{overprint}
  \begin{tikzpicture}
    \node at (0,0) {};
        \node<1,2> [text width = 20cm]at (10.5,-1) {$ \nabla^2 \phi =\sum_i \frac{n_i q_i\text{e}^{-q_i \phi/kT}}{4 \pi \epsilon}$};
        \node <2> [text width = 20cm]at (10.5,-2) {$q_i \phi/kT \ll 1 \Rightarrow \nabla^2 \phi \simeq \kappa^2 \phi$};
        \node <2> [text width = 20cm, font=\footnotesize]at (10.5,-2.5) {$\kappa^2 = \lambda^{-1/2} =  \sum_i \frac{n_i q_i}{4 \pi \epsilon}$};
        \node <3> [text width = 20cm] at(10.5,-1) {$\frac{F_{el}}{VkT} = - \left ( \frac{ 4 \pi e^2}{\epsilon kT}
    \sum_{i=1} n_i z_i^2 \right )^{3/2} \left ( 12 \pi \right )^{-1}$};
  \end{tikzpicture}
\end{frame}

However, the itemized line jump up and down between slides. I've tried to use itemize without any environment surrounding it. I've also tried to use overlayarea instead of overprint. How can I keep the line position constant?
I've uploaded a gif animation to display this. On my machine it run really fast and I can't fix this. Hope the problem is understood.

When I run this, I get

Answer 1

One possibility with overlayarea:

\documentclass{beamer}
\usepackage{tikz}

\begin{document}

\begin{frame}
  \begin{overlayarea}{\textwidth}{.6\textheight}
    \begin{itemize}[<+->]
    \item <1-> Poisson Boltzman equation (PB)
    \item <2-> Debye-H\"uckle equation  (DH)
    \item <3-> The naive approach - applying (DH) equation to PE
    \end{itemize}
  \begin{tikzpicture}
    \node at (0,0) {};
        \node<1,2> [text width = 20cm]at (10.5,-1) {$ \nabla^2 \phi =\sum_i \frac{n_i q_i\text{e}^{-q_i \phi/kT}}{4 \pi \epsilon}$};
        \node <2> [text width = 20cm]at (10.5,-2) {$q_i \phi/kT \ll 1 \Rightarrow \nabla^2 \phi \simeq \kappa^2 \phi$};
        \node <2> [text width = 20cm, font=\footnotesize]at (10.5,-2.5) {$\kappa^2 = \lambda^{-1/2} =  \sum_i \frac{n_i q_i}{4 \pi \epsilon}$};
        \node <3> [text width = 20cm] at(10.5,-1) {$\frac{F_{el}}{VkT} = - \left ( \frac{ 4 \pi e^2}{\epsilon kT}
    \sum_{i=1} n_i z_i^2 \right )^{3/2} \left ( 12 \pi \right )^{-1}$};
  \end{tikzpicture}
  \end{overlayarea}

\end{frame}

\end{document}

By the way, your syntax for overprint is wrong; the width should be declared as an optional argument:

\begin{overprint}[<width>]
...
\end{overprint}