  We discuss the clusters of resources that emerge when upper-division students
write about electromagnetic fields in linear materials. The data analyzed for
this paper comes from students' written tests in an upper-division electricity
and magnetism course. We examine how these clusters change with time and
context. The evidence shows that students benefit from activating resources
related to the internal structure of the atom when thinking about electric
fields and their effect on materials. We argue that facilitating activation of
certain resources by the instructor in the classroom can affect the plasticity
of those resources in the student, making them more solid and easily activated.
We find that the wording of the questions posed to students affects which
resources are activated, and that students often fill in resources to link
known phenomena to phenomena described by the question when lacking detailed
mental models.
