Schematizzazioni frettolose nell’insegnamento della fisica: il caso dei condensatori

Dealing with a new physical subject one often goes on too hastily in the initial stage, leaving some uncertainties and ambiguities in definitions and in basic features. Furthermore one performs schematizations and simplifications which may be in contradiction with fundamental principles or experimental results. This paper considers the case of capacitors and it makes a critical analysis of the definitions of capacitance and capacitor, with some didactic hints. In detail it is analyzed the plate capacitor, it is demonstrated the physical impossibility of the commonly used schematization that contradicts the general properties of the electric field and the conservation of energy and it is given another description, based on elementary reasoning, that better fits reality and that is coherent with the general physical laws. Finally the case of two conducting spheres is studied.