Vacuum: its meaning and its effects throughout experimental activities

The central topic of this didactic exercise is the vacuum, its meaning and its effects. The choice of this topic was suggested by the fact that the concepts involved are often explained from the theoretical point of view without an experimental verification. This exercise has been tested with a group of high school students, both in the university laboratory and by teachers in the classroom. In the first part we present some qualitative experiences concerning the balance of forces; before the experiment we ask students to make a prediction of the result and to give a physical motivation of their response. Using a bell inside a vacuum chamber we emphasizes a difference between mechanical and electromagnetic waves: how the first one requires a medium for propagation. In the second part a quantitative measure relative to the phenomenon of the fall of a weight is treated. The experimental apparatus allows a measure of the acceleration of gravity in a vacuum and at different pressure values. These measurements allow students to quantify the buoyant force and the force of viscous friction. In the third part it is foreseen to observe phase transitions; in particular it is required to measure the temperature of a boiling point. A fundamental educational part is the calibration phase. Using a solution of water and salt it is possible to compare the boiling point of distilled water and of this solution at the same pressure. The interdisciplinarity of physics and chemistry in this measurement is relevant. The removal of internal energy, by activating the vacuum pump, decreases the temperature of water with its subsequent freezing; this observation stimulates an interesting discussion with students on the phenomena involved. This exercise enables students to understand some fundamental physical concepts and is mainly intended for high school students.