1 MAGNET S FALL THROUGH REELS Lesson Plan Prof.Rossana Leandro Students from the 11th grade E.E José Chaluppe Bianca de Souza Jhéssica Carvalho Yasmyn Shantala Supervision: Prof. Marisa Almeida Cavalcante(PUC/SP) and Cristiane R.C.Tavolaro (PUC/SP) August 2009
2 Objectives Study the magnetic induction. Examine the behavior of an induced current in a reel during the movement of the magnet in its interior. To understand the characteristics of accelerated motion. Measure the acceleration s value of a falling body. Determine graphically and analytically the acceleration of a falling magnet in reels. Content The content approached in this activity allows us to treat the following structuring themes of the 12th grade: Fields and electromagnetic forces. Electromagnetic induction. The study of electromagnetism. Fall of bodies under gravitational action and in the presence of magnetic friction. Kinematics characteristics of a body in accelerated motion: variation of its position over time and graph, variation of its speed over time and graph. Competences and abilities Representation and communication Use and understand tables, graphs and graphic mathematical relationships to express physics knowledge. Express oneself correctly using the appropriate physics language and elements of its symbolic representation. Investigation and understanding To know and use physics concepts. Relate magnitudes, quantify and identify relevant parameters. Understand and use physics laws and theories. Build and investigate problems, identify the physics situation, use physics models, generalize from another situation, foresee, evaluate, and analyze prediction. Socio-cultural and historical perception To relate through written language experiments and questions relative to the identification of the relationship between magnetic field and electrical field; identify electromagnetic phenomena, establish relationships and identify regularities; use procedures and observation instruments, represent experimental results, elaborate hypotheses and interpret results in situations that involve electromagnetic phenomena. Material PC Classmate Intel
3 1 magnet (from earth is preferred) Ruler A plastic tube with 4 surbases (10 cm apart approximately) Connectors wires for the reels (with pins banana-banana) Enameled copper wires no. 26 (90 cm of wire for each surbase) Audacity (free software for sound edition and analysis) Wires with banana pins and a plug connector for the microphone entrance of the sound board of the classmate PC. Activity Amount of classes necessary for the activity: 4 classes First class Study of Faraday's Law - Theoretical Introduction When the magnetic flow through the surface of a surbase varies, an induced electric current appears in the surbase. In the presence of an induced electric current in the circuit we conclude that there should be an induced electromotive force that produces it. Experimentally Faraday observed that the faster the variation of the magnetic flow, the larger is the induced eletromotive force. It is possible to show using the simulator INTERACTIVE SIMULATIONS UNIVERSITY OF COLORADO. (http://phet.colorado.edu/simulations/sims.php?sim=faradays_electromagnetic_lab) Click on "electromagnetism laboratory". Verify what happens to the circuit when the magnet gets closer or away from the reel. Second class To assemble the equipment and give details about its operation. The reels can be built on their own tube, with a single wire, so that they will be connected in series. The terminals of that wire should be connected to the microphone entrance of the sound board of the classmate PC through a plug in connector. The magnet will be abandoned on the top of the tube and the signal will be observed in the software Audacity of sound analysis.
4 Illustration 1: Assembly of the solenoid with two surbases connected to the Classmate PC. Illustration 2: To the left, detail of the connection of the wires with the reel and to the right, detail of the position of where to leave the magnet so that it falls passing through the reels.
5 Pontifícia Universidade Católica de São Paulo Illustration 3: Fall of the small Magnet. (Plastic Tube) Illustration 4: Solenoid connected to the Classmate. Illustration 5: Terminals of the reels with the connectors.
6 Illustration 6: Screen of the Audacity software showing the variation of the induced electromotive force in the reel during the time of fall of the magnet. Third class The collection and data processing will be done through the microphone entrance in the Classmate PC. We began the experiment connecting the reels in series and the connector wire to the microphone entrance of the Classmate PC. We pressed the key "record" in the Audacity software and we left the magnet on the extremity of the tube. The software will register the variation of the electromotive force every time the magnet passes through a reel. Fourth class Analyze the fall of the magnet. To get to the result of the motion we need to measure the distances between two consecutive reels (distance that can be chosen before the experiment, during the assembly of the reels), and of the time interval of the passage of the magnet between the two reels. To measure the time, we just need to select with the mouse the time interval of the electromotive force variation in the Audacity software. Make a two column data table, one for displacement and the other for time passed. Table 1: Displacement over time t s (cm) , , ,5
7 , The magnet's fall depends on the local acceleration of gravity, that is, it executes an accelerated motion and its function is s=s 0 +v 0 t+1/2.at 2, Where s 0 =0 e v 0 =0 (s=1/2.at 2 ). We want to determine a, the acceleration of the fall. For that, we made a s x t graph, where y equals to s and x equals to t. This way, the coefficient that accompanies the term x 2 is 1/2.a. Results Analysis Determine the acceleration of the fall starting from the function obtained. In the example above we obtained: a= 493,79 x 2 = 987cm/s 2 a = 9,87 m/s 2 The obtained result is below the acceleration of the gravity (g=9,78 m/s 2 ). Despite the apparent free fall of the magnet inside the tube, we cannot forget about the magnetic friction exerted by the electromagnetic induction force created by the reels. Therefore the result is coherent with the preliminary study carried out about electromagnetic induction.
8 Links Blog with more information about the experiment (accessed on 17/08/2009) Magnet s fall through the reel simulator (accessed on 17/08/2009) Reference Audacity Software (accessed on 17/08/2009) Text about analogical digital signal conversion Cavalcante, Marisa Almeida; TAVOLARO,Cristiane.R.C.;BONIZZIA,Amanda ; e PIFER,Anderson. Novas Tecnologias no Ensino de Física GOPEF/PUC-SP