Students’ reasonings in thermodynamics

Some tendencies in common reasoning aim at reducing the complexity of multi‐variable problems. Two main aspects of these tendencies are illustrated here with results of an investigation of students about thermodynamics: 1. 1. A physical quantity which depends on several others may be treated as if it were depending on only one. Such a reduction of the number of variables may also be obtained by combining two of them, as if they were two facets of the same variable. For example, mean distance between particles and mean speed are treated as equivalent aspects of thermal motion. This results in a linear shape of argument φ1 ®φ2 ®φ3 ®…, where each phenomenon φn evoked is specified with only one physical quantity2. 2. Some apparent contradictions in students’ responses may be understood if one admits that there is a chronological connotation in the argument: an arrow, ‘®’, or a ‘then’, means not only ‘therefore’, but also ‘later’ The pedagogical implications of the findings are discussed, mainly in relation to teaching goals.