The theoretical framework of the Extended Mathematical Working Space (MWS) and the Blum and Leiss modelling cycle allow the analysis of a problem-solving activity within a multidisciplinary approach (contribution of physics and mathematics). This problem-solving activity explores the possibility of producing an intense magnetic field using a wire winding for a medical imaging device. The different fields used are those of electromagnetism and calorimetry from physics and algebra from mathematics. A group of volunteer 12th grade students in France completed the online activity between May and June 2020. The extended MWS framework makes it possible to analyse school tasks by considering the relationships between the cognitive plane of students, the epistemological plane of mathematics and that of physics, according to the stage of the modelling cycle.