Thus, for a capacitor to
store a large amount of charge without  leaking, its capacitance should
be high enough so that the potential difference and hence the electric
field do not exceed the break-down limits Put differently, there is a limit
to the amount of charge that can be stored on a given capacitor without
significant leaking In practice, a farad is a very big unit; the most common
units are its sub-multiples 1 mF = 10–6 F, 1 nF = 10–9 F, 1 pF = 10–12 F,
etc Besides its use in storing charge, a capacitor is a key element of most
ac circuits with important functions, as described in Chapter 7