This discussion of capacitors reminds me of the interminable
philosophical discussions about how many angles can dance on the head of a
pin. If the process of charging a capacitor is discussed with reasonable
care, most - if not all students - understand that one plate ends up with
an EXCESS of positive charge, while the other ends up with an EXCESS of
negative charge. In most - but not all - case the capacitor remains
neutral.
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.....
Mark
from Fullerton where we try not to mistake our ignorance for
understanding.
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866