The transfer of knowledge within and across disciplines remains a compelling challenge for modern STEM education and further research is needed to expand on the student-exhibited cognitive and affective gains achieved by innovative cross-disciplinary STEM instructional techniques. This study seeks to support cross-disciplinary STEM instruction and learning by investigating how students use the first law of thermodynamics, a crucial principle to the crosscutting concept of energy and matter, to bridge across disciplinary boundaries. An interview study was undertaken wherein chemistry-, engineering-, and physics-major students addressed a common set of conceptual prompts written with different field-specific conventions. This report focuses on students' interpretations of the provided forms of the first law and work equations between prompts. Emergent findings demonstrate field-specific interpretations of arbitrary differences in convention and strong barriers to transfer. The derived implications inform suggestions for scaffolding across such disciplinary differences and for future work in this area.