Aws policy evaluation logic

Foucault dissipation of a magnet falling through a copper pipe studied by means of a PC audio card and webcamAssunta Bonanno1, Giacomo Bozzo1,3, Michele Camarca1, Marisa Michelini2, Peppino Sapia11.Physics Education Research Group, Physics Department, University of Calabria – Rende (CS) – Italy2.Physics Education Research Group (URDF), University of Udine –Udine – Italy3.bozzo@fis.unical.itAbstract. In this paper we describe an experimental learning path on the electromagnetic induction based on the original use of an Atwood machine to obtain a controlled fall of a cylindrical magnet. Two different experimental setup are used: i) magnet falling across a coil (allowing to quantitatively study the Faraday-Neumann-Lenz law directly); ii) magnet falling across a copper pipe. This last configuration allows to investigate complex induction phenomena, by quantitatively determining power dissipation due to Foucault eddy currents, arising in the copper as magnet travels through it. Both mechanical and electromagnetic aspects of the phenomenon are continuously and quantitatively monitored by a common personal computer (PC) equipped with a webcam, and a freely available specific software allowing to employ PC as an oscilloscope through the audio card. Measurements carried out when the various experimental parameters are changed provide a useful framework for a thorough didactic discussion of the conceptual knots related to electromagnetic induction. The proposed learning path is under evaluation in some high school, within the Project “Lauree Scientifiche” promoted by the Italian Department of Education.1. Introduction Experimental activity plays a crucial role in Physics Education research. Many researchers stressed the importance of laboratory activities in learning process to increase student interest towards Physics and to create connections between science knowledge and everyday world experience (Bosio et al 2001, Bosio et al. 1997). Physics, in fact, is one of most difficult and boring school subject in spite of its many everyday applications (Bonanno et al. 2009b). Hands-on and minds-on activities can boost reasoning skills of students, through problem solving (Watts, 1983) and small group collaborative work (Heron 2008 Meltzer and Manivannan 2002, Coletta et al. 2007), as well as can effectively address the main student’s misconceptions if supported by appropriate learning/teaching strategies. Student interest is enhanced by on-line measurements, who offer a great support for experimental activity, since they allows to follow in real time the evolution of phenomena and to perform quantitative investigations (Gervasio et al. 2009, Bonanno et al. 2010). On-line acquisition systems, cheap and easy to use, capture student attention, allowing them to understand contents and to improve their own knowledge. However, although many not expensive on-line acquisition devices are commercially available, many schools had great difficulties to buy them. Furthermore, students cannot repeat these experiences at home, although they are easily made at school, because acquisition systems and data analysis software are not accessible for free.On the other side, international literature has clearly identified the main difficulties encountered by learners in dealing with conceptual challenges of electromagnetic induction (Michelini and Viola 2007, Stefanel 2008, Bonanno et al. 2010), especially in facing the role of magnetic field flux and its time variation (Maloney 2001, Thong 2008, Galili 1997, Michelini 2008, Michelini 2009, Bonanno et al. 2010). In this context we proposed, in MPTL 2009 Conference (Bonanno et al.
2010), an on-line experiment in which real time graphs allow to face the conceptual knots relative to induced currents in electromagnetic phenomena, stressing the role of magnetic field flux variation. Anyway, during our didactic experimentation, we encountered several difficulties to introduce data acquisition system in schools because of their limited economic resources. In this context, to overcome described troubles, we propose here an improved version of the experimental set up presented at the MPTL 2009 Conference, specifically designed to address some conceptual knots related to energy conversion (from mechanical to electrical), in addition to those concerning the role of magnetic flux variation in electromagnetic induction. The need for such an improvement has been suggested to us by our didactical experimentation, that has shown as conceptual knots related to energy conversion remained not fully clarified when the activity is specifically and exclusively focused on the magnetic flux variation (Bonanno et al. 2009a). 2. Experimental set up with Atwood MachineProposed experimental learning path is based on an improved Atwood machine holding at one end a cylindrical magnet falling through a Plexiglas guide (Figure 1). This guide acts as a mechanical support for either a coil (Figure 2) or a conducting copper pipe through which the magnet can fall. In this way, two experimental configurations are possible, each aimed to address complementary conceptual knots concerning electromagnetic induction phenomena. A first configuration, featured by the guide surrounded with an induction coil, lets the magnet falling through the coil with controlled acceleration, so that a quantitatively study of the induction law is directly allowed. In the other experimental arrangement the guide is surrounded by a copper pipe, whose length may be determined by the experimenter. In this way, kinematic study on the falling magnet permits the quantitative determination of power dissipation due to Foucault eddy currents (arising in the copper as magnet travels through it) and a consequent deeper insight into the complex induction phenomena. In both configurations, magnet acceleration may be set at wish by adjusting Atwood machine’s counterweight. One of the strengths of proposed experimental set up resides in the fact that both aspects of the phenomenon under investigation (mechanical and electromagnetic, as regards respectively, the motionand the induction) are continuously and quantitatively monitored through a common home PC, used as a low cost acquisition system. In fact, a freely available specific software11 “Visual Analyzer Project”, Tor Vergata University, Italy. On line at the URL: , permits to http://www.sillanumsoft.org/Figure 1: Experimental set up: 1) Atwood machine; 2) falling magnet; 3) counterweight; 4) inextensible string, 5) home-made coil, 6) support pole. The coil is connected to the microphone plug-in of the PC audio card through a clipper circuit (employing 4 diodes 1N4007 connected as in the figure), to protect the card from possible voltage spikes. The clipper threshold is about 1.3 V.