  We examine the difficulties that introductory physics students, undergraduate
physics majors, and physics graduate students have with concepts related to
conductors and insulators covered in introductory physics by giving written
tests and interviewing a subset of students. We find that even graduate
students have serious difficulties with these concepts. We develop tutorials
related to these topics and evaluate their effectiveness by comparing the
performance on written pre-/post-tests and interviews of students who received
traditional instruction vs. those who learned using tutorials.
