# Is it necessary that the potential difference between any two equipotential surfaces in a system be constant?

Is keeping $$dV$$ constant a necessary condition in the formula $$E=-\frac{dV}{dr}$$ for equipotential surfaces ?

The equation you give $$\mathbf E=-\frac{\text dV}{\text dr}\hat r$$ is true in general for potentials with only a radial component.