magnet´s fall through reels

Pontifícia Universidade Católica de São Paulo

Junior Scientific initiation at PUC/SP and the development of experimental activities in Physics

using the Classmate PC

MMAAGGNNEETT’’SS FFAALLLL TTHHRROOUUGGHH RREEEELLSS

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




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




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.




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.




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.




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)

0 0

0.05 9,7

0.1 19,4

0.13 28,5




0.17 38,6

0.20 47.8

0.22 57

0.25 67

The magnet's fall depends on the local acceleration of gravity, that is, it executes an accelerated

motion and its function is

s=s0+v0t+1/2.at2,

Where s0=0 e v0=0 (s=1/2.at2).

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 x2 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/s2

a = 9,87 m/s2

The obtained result is below the acceleration of the gravity (g=9,78 m/s2). 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.




Links

Blog with more information about the experiment

http://picjrintelpucsp.blogspot.com/

http://www.pic.pucprofrossana.blogspot.com/ (accessed on 17/08/2009)

Audacity Software http://audacity.sourceforge.net/download/ (accessed on 17/08/2009)

Text about analogical – digital signal conversion

http://xviiisnefnovastecnologias.blogspot.com/2009/01/texto-de-orientao-para-oficina-de-

novas.html

Magnet’s fall through the reel simulator (accessed on 17/08/2009)

http://phet.colorado.edu/simulations/sims.php?sim=Faradays_Electromagnetic_Lab

http://profvalmir.blogspot.com/2009/09/roteiro-do-experiemento-do-tubo-pvc-e.html

Reference

Cavalcante, Marisa Almeida; TAVOLARO,Cristiane .R.C.;BONIZZIA,Amanda ; e PIFER,Anderson.

“Novas Tecnologias no Ensino de Física” GOPEF/PUC-SP 2008.

http://xviiisnefnovastecnologias.blogspot.com/2009/01/texto-de-orientao-para-oficina-de-

novas.html

magnet´s fall through reels

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