We developed and applied a three hour based on models educational unit, in which 12 university pre-service primary education student teachers in Thessaloniki were engaged in the predictive use of two simulated models concerning the different way of charging insulators and conductors. The aim of the present study is to investigate the effect of the two simulated models in the capability of students to predict the phenomenon of the rotation of a simple balloon and the non rotation of a balloon with a tinfoil when we rub them with a piece of wool on their front side and next to them there is a positive charged rod. Data that were obtained by the written predictions and the analysis of the transcribed interviews in specific phases of the educational unit have shown that the students succeeded in predicting the phenomenon with the use of the simulated models and to work in a scientific way.