A global shortage of qualified physics teachers requires some teachers to teach physics out-of-field (OOF). A concern for OOF physics teaching is that without appropriate physics content knowledge (CK), these teachers tend to focus on rote learning and may find it difficult to support students who require additional assistance or extension; they tend to teach in ways that do not foster conceptual understanding. This paper reports on research that investigated how content representations (CoRes) could be used to support OOF physics teachers' knowledge for teaching electricity and magnetism. CoRes were used to elicit discussions between OOF and in-field physics teachers about important teaching and learning considerations (e.g. appropriate CK for students and appropriate instructional strategies for promoting conceptual understanding). Prior to engaging with CoRes, three OOF teachers were interviewed about teaching electricity and magnetism and their lessons during this topic were video recorded. Later, as these OOF teachers worked with their in-field colleagues using the CoRe framework, all discussions were audio recorded. To explore changes to the OOF teachers' knowledge and practice from the CoRe process, they were interviewed and their follow-up electricity and magnetism lessons (for a different class) were video recorded. Findings showed that the CoRe process-stimulated important discussions about teaching electricity and magnetism and considerations for students learning this topic. As a result, the OOF teachers developed their professional knowledge, including refinement of their CK, acquirement of new topic-specific instructional strategies, and more awareness of students' understanding during lessons. This study concludes that CoRes are an effective strategy for supporting the professional learning of OOF physics teachers.