An innovative experiment on superconductivity, based on video analysis and non-expensive data acquisition

Abstract

In this paper we present a new experiment on superconductivity, designed for university undergraduate students, based on the high-speed video analysis of a magnet falling through a ceramic superconducting cylinder (T c = 110 K). The use of an Atwood's machine allows us to vary the magnet's speed and acceleration during its interaction with the superconductor. In this way, we highlight the existence of two interaction regimes: for low crossing energy, the magnet is levitated by the superconductor after a transient oscillatory damping; for higher crossing energy, the magnet passes through the superconducting cylinder. The use of a commercial-grade high speed imaging system, together with video analysis performed using the Tracker software, allows us to attain a good precision in space and time measurements. Four sensing coils, mounted inside and outside the superconducting cylinder, allow us to study the magnetic flux variations in connection with the magnet's passage through the superconductor, permitting us to shed light on a didactically relevant topic as the behaviour of magnetic field lines in the presence of a superconductor. The critical discussion of experimental data allows undergraduate university students to grasp useful insights on the basic phenomenology of superconductivity as well as on relevant conceptual topics such as the difference between the Meissner effect and the Faraday-like 'perfect' induction.

An innovative experimental sequence on electromagnetic induction and eddy currents based on video analysis and cheap data acquisition

“…This is particularly true in the case of electromagnetic (EM) induction, for which the main difficulties encountered by learners in dealing with this fundamental concept are clearly identified in the literature [4,5,[12][13][14]. Furthermore, as we argued in previous papers [15,16], several didactical difficulties arise, especially when students face the role of magnetic field flux and its time variation [14][15][16][17][18][19][20][21] when the idea of 'circuit' corresponds not just to a well-defined actual closed path (e.g. a conducting wire loop) but rather to the various possible paths of electrical currents inside the bulk of an extended conductor.…”

“…The core of the experimental setup is an Atwood's machine, bearing at one end a cylindrical bar magnet, and at the other end an adjustable weight that can be filmed in order to track its motion (figures 2 and 3). This setup, successfully employed in a previous work [16], is implemented in the present experimental sequence in two different versions, respectively aimed at (1) studying the space-time pattern of the induced EMF around the fall axis of a cylindrical magnet; and (2) studying the Foucault dissipation when a conducting body surrounds the axis. In the next paragraphs, we will describe the two experimental arrangements, which from now on will be called (1) induced EMF measurements and (2) kinematic measurements, respectively.…”

“…Due to its importance, this topic has been widely treated in didactical literature [22][23][24][25][26][27][28][29][30][31][32][33], but existing experimental demonstrations present, to our knowledge, some critical issues from a didactical point of view, namely: (i) the underlying physical concept (i.e. the EM induction that is at the origin of the observed dissipative phenomena) tends to be obscured if these experimental demonstrations are mainly focused on the eddy currents (which are only a 'consequence' of EM induction); (ii) the proposed experiments rarely allow learners to perform quantitative determinations [15,16,22,23].…”

“…The learning sequence has been inspired by some questions posed by undergraduate students at the University of Calabria, after they performed some experiments on EM induction and eddy currents, described in previous works [14][15][16][17]. The questions are:…”

“…Figure 1.According to the PEC methodology, experiments in the learning path are preceded by stimulus questions guiding students through the exploration. We obtained such questions from the analysis of students' ideas that they proposed during other experimental activities[15][16][17]. As an example, the figure shows the questions that introduce activities with the experimental setup for the induced EMF measurements shown in figure 2.…”