Plate Capacitor

Parallel-plate capacitor filled with a dielectric. The free charge $\pm Q_\text{C}$ on the plates drives the field $\vec E$, which lines the molecular dipoles up with it. Alignment leaves uncompensated bound charge $\mp Q_\text{surf}$ on the two faces of the dielectric, pointing against the free charge, so the net field is weaker and the capacitance higher than in vacuum.