User:Ehickers/sandbox

$$\nabla^2\psi$$ = $$\frac{\partial^2\psi}{\partial x^2} + \frac{\partial^2\psi}{\partial y^2} + \frac{\partial^2\psi}{\partial z^2} $$ = $$-\frac{\rho_e}{\epsilon\epsilon_0} $$