The maximum amount of charge that can be stored by a capacitor is determined by the point at which the force of the like charges on each plate repelling each other matches the force on the charges due to the potential difference. Adding an insulator (a dielectric) between the parallel plates allows more charge to be stored, as the insulator will polarise so as to attract more electrons to the negative end of the capacitor, and repel more electrons from the positive end. This increases the capacitance of the capacitor (as capacitance is ).
