The continuum theory is used to study a polar electrooptic effect in a nematic insulator which is caused by applying an electric field normal to the sample planes having different director anchoring strengths. In the low field limit the magnitude of the linear electrooptical effect is studied as a function of anchoring strengths, director tilt at the boundaries, magnitude of flexoelectric constants and an applied magnetic field. The effect becomes stronger with increasing flexoelectricity or a stabilizing magnetic field and becomes weaker with a destabilizing magnetic field, pretilt of the initial director away from the homogeneous orientation or increase in sample thickness. As in the case of the Fréedericksz threshold, the magnitude of the applied voltage at which the director field orients along the sample normal is found to depend on the sign of the voltage as a consequence of unequal anchoring strengths and flexoelectricity. Under the rigid anchoring hypothesis the mathematical model is extended to study magnetic field induced bistability. It is found that the bistability width as well as the associated optical properties of the nematic sample can be profoundly influenced by the application of an electric field.