 For an isothermal fluid, the pressure force takes the form 
while the electric term is 
Integration leads to the expression given above.In many problems of plasma physics, it is not useful to calculate the electric potential on the basis of the Poisson equation because the electron and ion densities are not known "a priori", and if they were, because of quasineutrality the net charge density is the small difference of two large quantities, the electron and ion charge densities.If the electron density is known and the assumptions hold sufficiently well, the electric potential can be calculated simply from the Boltzmann relation.