In this work we show an approach based on models, for an usual subject in an introductory physics
course, in order to foster discussions on the nature of physical knowledge. The introduction of
elements of the nature of knowledge in physics lessons has been emphasised by many educators and one
uses the case of metals to show the theoretical and phenomenological dimensions of physics. The
discussion is made by means of four questions whose answers cannot be reached neither for
theoretical elements nor experimental measurements. Between these two dimensions it is necessary to
realise a series of reasoning steps to deepen the comprehension of microscopic concepts, such as
electrical resistivity, drift velocity and free electrons. When this approach is highlighted, beyond
the physical content, aspects of its nature become explicit and may improve the structuring of
knowledge for learners on this subject.