Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Electric Field Question



Michael Edmiston wrote:
...
Although our convention in physics class is
that capacitor plates are charged equal-opposite,

That conventional assumption is codified as one
of the Kerchhoff "laws".

there is no reason they
can't be charged unequal-same. In fact, this is actually common in
electronics. We have many circuits in which the capacitor is storing energy
(has an electric field in its gap) yet both plates have positive voltage
with respect to earth ground, which would typically be taken to mean both
plates have net positive charge.

Well, its common, but it's not commonly of any
importance. Specifically, the net positive charge
is verrrry small compared to the balanced charge,
because the self-capacitance of the capacitor-body
is so small compared to the ordinary two-terminal
mutual capacitance between the capacitor plates.

This gets back to our previous discussions of
the capacitance matrix. In general, the two
plates of the capacitor are described by a 2x2
capacitance matrix. If the two self-capacitance
entries are negligible, we are left with only
the off-diagonal elements (which are always
equal) and we call their shared value the ordinary
two-terminal capacitance.

If the self-capacitance terms are not negligible,
you just have to write out the whole capacitance
matrix. It's not tricky. It's just work. Not
even very much work.