I think what's getting you confused here lies in the basic differences in how we use magnetism and electricity.
objects are almost non-magnetic. Magnetic objects are surrounded by
magnetic fields. When one magnet id in the field of another, there's a
force on it. These fields are not affected by non-magnetic objects, so
the forces between magnetic objects are not changed much by most things
that might happen to be between them.
Electrically charged objects also are surrounded by fields-
electrical fields. These fields also can go right through many objects.
If you make some big electric fields on one side of a sheet of some
typical non-conducting plastic, those fields can be measured on the
other side of the plastic, just like magnetic fields could.
When you write that electricity can't pass through plastic, you're
probably thinking of electric currents. Those consist not of electric
fields but of actual moving electrically charged particles. In many
materials the charged particles are pretty much stuck to atoms and
can't flow. That includes many plastics. Other materials, like metals
and some plastics, have electrons that are free to move so they do
support electrical currents. These same materials interfere with
electrical fields, because the charges move to partly cancel the fields
from other charges.
-Tamara (and mike)
(republished on 08/02/06)