  The estimation of the electric field in simple situations provides an
opportunity to develop intuition about the models used in physics. We propose
an activity aimed at university students of General Physics where the electric
field of a finite line of charge is compared, analytically or numerically, with
the fields of an infinite line and of a point charge. Contrary to intuition, it
is not necessary to get very close for the line charge to be considered
infinite, nor to move very far away for the finite line field to resemble that
of a point charge. We conducted this activity with a group of students and
found that many of them have not yet developed an adequate intuition about the
approximations used in electromagnetism.
