When the macroscopic neutrality of a plasma is instantaneously perturbed by external means, the electrons react in a such a way as to give rise to oscillations at the electron plasma frequency wpe=(n092/meco)1/2. consider these oscillations, but including the motion of the ions. show that in this case the natural frequency of oscillation of the net charge density is given by w = (we + %)/2 where wpi=(no92/m.€o)/2. use the linearized equations of continuity and of momen- tum for each species, and poisson equation, considering only the electric force due to the internal charge separation.