Examining physics teacher understanding of systems and the role it plays in supporting student energy reasoning

In this paper, we argue that the definition of what constitutes a system differs in physics from other sciences, and in particular, biology, and that these differences matter for learning. Furthermore, even within physics, what textbooks (and instructors) mean by the phrase "energy is conserved" is not unambiguous, often giving the impression that whether or not energy is conserved is contingent on the type of system one chooses for analysis, inappropriately, thereby, interweaving conservation with constancy. These discrepancies and ambiguities in the canonical approach to systems as a tool for energy reasoning may, in turn, undermine the knowledge that teachers need to support energy learning among their students. We present data from a validated assessment of the specialized physics knowledge that teachers use to help students make progress in energy learning, which we administered to hundreds of high school physics teachers and senior physics majors. Assessment results support the following claims: (a) Both high school teachers of physics and senior physics majors manifest significant difficulties in applying a consistent systems approach to energy analysis; (b) Teachers who demonstrate a strong understanding of a systems approach to energy analysis are also better equipped to respond productively to student reasoning about a system approach to energy; and (c) Teachers with a strong understanding of a systems approach to energy analysis are also significantly better equipped to respond productively to student reasoning than senior physics majors who demonstrate a similarly strong understanding. Our results have implications for the professional preparation of teachers, graduate Teaching Assistants, Learning Assistants, and physics faculty in their role as instructors.