  In this paper the response of ionic systems subjected to a spatially varying
electric field is studied. Following the Nernst-Planck equation, two forces
driving the mass flux are present, namely, the concentration gradient and the
electric potential gradient. The mass flux due to the concentration gradient is
modelled through Fick's law, and a new constitutive relation for the mass flux
due to the potential gradient is proposed. In the regime of low screening the
response function due to the potential gradient is closely related to the ionic
conductivity. In the large screening regime, on the other hand, the response
function is governed by the charge-charge structure. Molecular dynamics
simulations are conducted and the two wave vector dependent response functions
are evaluated for models of a molten salt and an ionic liquid. In the low
screening regime the response functions show same wave vector dependency,
indicating that it is the same underlying physical processes that govern the
response. In the screening regime the wave vector dependency is very different
and, thus, the overall response is determined by different processes. This is
in agreement with the observed failure of the Nernst-Einstein relation.
