For this space-charge-filled parallel-plate capacitor, place the first plate at and the second at , ignoring fringing field of a capacitor. With vacuum permittivity between the plates and fixed charge density , Poisson's equation and the prescribed boundary electric potentials give
Integrating twice and imposing both values yields
The electric field is minus the gradient of the electric potential, hence
Thus the electric potential is a parabola, concave downward for and upward for . The signed electric field is a straight line. Its magnitude has a V-shaped corner if its zero
lies inside the gap; otherwise its magnitude is a single straight segment. An interior zero requires . At that point the electric potential has its interior extremum. For , these reduce to a linear electric potential and a uniform electric field.
Figure 1.
Potential and electric-field magnitude in a parallel-plate capacitor with fixed space charge
. Illustrative dimensionless curves for positive charge density, using , and . The two choices show an interior field reversal and a field with one sign throughout. They do not assume numerical source data. Details.

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