Particle tracks in a cloud chamber: historical photographs as a context for studying magnetic force

We present a sequence of experiments aimed at exploring magnetic force. The activity sequence was organized into three main phases, each starting from an experiment. The first phase aimed to help students understand the direction and magnitude of the magnetic force experienced by current-carrying wires located in a homogeneous magnetic field; the second referred to the study of magnetic force acting on electrons emitted by a cathode and moving through a homogeneous magnetic field; finally, students were asked to analyse the sub-nuclear particle tracks in cloud and streamer chambers in real experiments, based on the experience they acquired during previous work with digital photographs. The activity sequence was designed for students on introductory physics courses or in advanced high-school classes and was implemented in five high-school classes (approximately 100 students). Our results compared with those reported in the literature indicate that students' understanding of the direction and magnitude of magnetic force improved markedly and that some typical difficulties were overcome. (paper)