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: Ions



Suppose that a glass cube is exposed to beams of electrons whose
ranges are slightly less than the size of the cube. There are six
such beams so that "stopped" electrons are distributed more or
less uniformly inside. The uniformity can also be assured if the
cube is rotated in the single beam during the bombardment.

If the cube were metallic than the answer to the "how much Q
can be accumulated?" would be simple. As Q (distributed on the
surfaces, predominately near corners) becomes larger the value
of Emax grows and at some E (10^5 V/m in air ?) the rate of
discharging ("corona" effect) will match the rate of charging, as
in the Van de Graaff machine. For a metallic sphere E (in V/m
at the surface) is equal to sigma/8.85*10^-12, where sigma is
the charge density in C/m^2. The field can also be calculated from
Coulomb's law E=k*Qtot/r^2, where r is the radius of the sphere.
I suppose that the same reasoning applies to a glass sphere. The
formula E=k*Qtot/r^2 is applicable, if Q is distributed uniformly.

But here is something puzzling. What happens when positive ions
from air start to accumulate of the surface of the glass sphere (or
cube)? If the insulator is ideal then the established neutrality, after
the beams are turned off, is only apparent. You would have a
more or less uniform "cloud" of net negative charge inside the
sphere and a thin layer of equal and opposite charge on the
surface. Is there any good argument against the formation of
"the negative space charge" inside the irradiated dielectric?
What can one do with "pseudo neutral" pieces of irradiated
material?
Ludwik Kowalski