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In many electronic applications you do indeed depend on the fact
that the plates of a capacitor store EXCESS charge as well as energy. For
example, the resistor divider chain setting the voltage on the dynodes of
a photomultiplier tube is bypassed with capacitors. They are put there to
make sure that there is a source of charge it the dynodes "ask" for more
charge than the power supply can provide during the short period of time
that it take a pulse of charge to transit the tube (a few tens of
nanoseconds).
Likewise electrolytic capacitors are used as "filters" in power
supply circuits. The "filter" capacitors don't "filter" anything. They
supply charge to the load during part of the a.c. cycle.
Indeed, in some circuits capacitors do store a NET amount of
charge. This is done to shift the d.c. level from one side of the
capacitor to the other.....