The aim of this research was to evaluate and analyse second-year industrial engineering and chemical engineering students prior knowledge of conceptual aspects of "circuit theory". Specifically, we focused on the basic concepts of electric potential and current intensity and on the fundamental relationship between them as expressed by Ohm's law. In order to find out what the students' alternative conceptions were, we gave them a questionnaire containing nine questions dealing with the most basic concepts of circuit theory. These results will be extremely useful for designing and drawing up a teaching unit based on a constructivist approach to teaching and learning processes. It should be emphasized that there is little information available about the alternative conceptions of the population type investigated in this research (university students). (Contains 1 table.)