Since the potential difference of the faces of the plates does not change (remaining equal to the e.m.f of the source), the field strength between the faces does not change either. But the capacitor's capacitance is increased when it is filled with a dielectric. Additional electric charges will flow from the source to the capacitor in such a quantity that the potential difference on the faces of the plates remains the same throughout the filling of the capacitor with the dielectric. And the potential difference will also be at this level after filling.