Most didactic studies in the field of electrokinetics take the electric diagram as a starting point (Durey-Mesmin 1975). In the situation reported, a somewhat different method is used. The author wanted to consider the diagram as a structural representation of the reality which, on the one hand, has particular rules of construction and use and, on the other hand, maintains a close relationship with physical concepts. The latter do not necessarily develop in the mind of pupils at different levels in the same order in which they appear in the physical theory of electrokinetics (Vergnaud 1978). Two types of problems tend to arise, as follows: Graphical problems: a diagram is first a picture. What are the effects on learning of a graphically different presentation of identical physical problems? Apart from higher or lower degrees of 'simplicity' of a diagram (this notion itself is not easy to define), is it possible that a particular form of presentation induces certain kinds of reasoning? And, if so, by what kind of mechanism? Physical problems: a diagram is not an ordinary picture, but the result of a process of abstraction which remains implicit most of the time. One can suppose that the rules which allow one to elaborate and to use an electrical diagram are based on particular concepts (here, the notions of intensity and potential) (5 refs.)