I'm not sure I agree with John's statement that the earth deviates
"substantially" from neutrality. It is true that the fair weather
electric field near the surface of the earth is of the order of 100 V/m.
However, a charge density of slightly less than 1 nC/m^2 on the surface of
the earth is all that is needed to support a field of this magnitude. In
comparison a small parallel plate capacitor (plate area 1 cm^2 and plate
separation of 0.1 cm) charged to 12 V will store a charge of about 120 nC,
which corresponds to a charge density of 1,200,000 nC/m^2. Thus, any
slight charging of the battery-capacitor system by atmospheric effects
would be negligible in comparison to the amount of charge being separated
by the action of the battery.
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866