Students were asked to consider four problem situations dealing with electromagnetic events in an examination setting; some student volunteers were involved in semistructured follow-up interviews to clarify their examination responses. Their considerations of the problems were to be conceptual (i.e. not involving mathematical calculations) so as to arrive at an argument with explicit claims and justifications. We asked students to justify their arguments, individually and in writing. The data were collected upon completion of the second semester as an examination to guarantee that students remained interested in the task. The written answers to the different problem situations were the main data source (Rivard, 1994), which we understood to be a context with less pressure and more appropriate than an oral examination for students to explain their solution proposals with reasoning. The post hoc interviews were not viewed with any high degree of pressure by the 20 students, chosen from a group of volunteers who had achieved an average level in physics learning. The interview responses intended to clarify their approaches