Another intuitive approach is to consider a negatively
charged wire. Electrons, as William wrote, are
mobile and self-repelling, therefor the charged particles adjust
themselves
to minimize potential energy. This can be done quantitatively if
students know how to calculate potential energy of a system of
n point charges. Select two or three hypothetical distributions
and calculate the potential energy for each. Some will be lower
than others. Try these cases:
Q/2 and Q/2 at each end.
Q/3, Q/3 at the ends and another Q/3 in the middle.
Q/4, Q/4 at the ends and another Q/4, Q/4 pair between them.
Computing potential energies by hand would be tedious but
those who know Basic can write short codes for a system of
n point charges distributed in an arbitrary way among n cells.
For example 50 cells uniformly distributed over the length of
0.1 m, some empty, some containing known charges. The
sum of all charges must be the same.
Bringing your own (or commercial) working program, and
using it in class, would not promote learning as much as
programming. If it was up to me I would insist on teaching
very simple programming as early as possible, and on using
it in all math and science classes.