Addendum: Considerations about the incompleteness of the Ehrenfest’s theorem in quantum mechanics (2021 Eur. J. Phys. 42 065405)

We describe the analytical solution of the eigenvalue problem introduced in our article mentioned in the title and relative to a punctiform electric charge confined in an one-dimensional box in the presence of an electric field. We also derive and discuss the analytical expressions of the external forces acting on the punctiform charge and associated with the boundaries of the one-dimensional box in the presence of the electric field.