We present a sequence of activities aimed at promoting both learning about magnetic forces and students' reflection about the conceptual bridge between magnetic forces on a moving charge and on a current-carrying wire in a magnetic field. The activity sequence, designed for students in high school or on introductory physics courses, has been tested with about 150 high school students and 50 pre-service teachers. Results indicate that students' understanding of the direction and magnitude of the magnetic force improved markedly and that some typical difficulties were overcome. Comparison of the two types of forces and of their effects allowed focusing on their crucial difference with respect to the possibility of doing work and encouraged students to search for models that are able to link one force to another.