def potential_energy(fa, voltages, position, charge):
    """ The potential energy of a charged particle in an electric field.

        Parameters
        ----------
        fa       :: FastAdjust
        voltages :: np.array([v0, v1, ... vn])            (V)
        position :: np.array([x, y, z])                   (m)
        charge   :: float64                               (C)
       
        Returns
        -------
        float64
    """
    return charge * fa.potential_r(position, voltages)