Arcelio Hernández Fereira, Michael Thabane Magama

Arcelio Hernández Fereira, Michael Thabane Magama,

Bindura University of Science Education, Zimbabwe

Abstract

Physics, Biology and Chemistry are considered the “experimental sciences”. For that reason it is necessary to put the required emphasis in the experimental component of the disciplines in order to develop the experimental skills and to provide the support for the better understanding of the theoretical framework of the physical laws. This statement is even more important in programmes dedicated to the training of future Physics teachers. Often universities in underdeveloped countries lack the necessary equipment for Physics Labs as a result of funding constraints. Hence access to standard laboratory equipment is limited due to the relative high prices demanded by the manufacturing firms. In this case the solution lies in the development of experimental set ups using alternative low cost materials, electronic devices, and technical solutions from creative staff. In this work, a series of alternative set ups for University Physics Labs and its assessment is presented.A comparison of their academic quality and cost is made with those of standard laboratory set ups. The assessment included such quality criteria as possibilities of satisfying the objectives of the practice in the lab, accuracy, sensitivity and reproducibility over time. These results are part of a wider project to address the shortage of qualified science teachers in Zimbabwe. This alternative allows the most disadvantaged countries to develop high quality human resources and in this way narrow the knowledge gap between underdeveloped and developed countries.

Keywords: alternative set ups, experimental component, Physics lab

Introduction

Lab work is essential for the learner in experimental sciences because some specific learning goals can only be achieved in this context. The goals of labwork are available in official curricula, in laboratory instructions or from actual practice studies. Among these goals are ‘physical manipulations of real world substances or systems, interactions with simulations, interactions with data drawn from the real world, access to databases, or remote access to scientific instruments and observations’ (Committee on High School Science Laboratories, 2006, pp. 31–32). According to this committee,labwork should help students develop an understanding of the complexity and ambiguity of empirical work, as well as the skills to calibrate and troubleshoot equipment used to make observations.

The Physics labsoccupy anincreasingly importantplaceinthe teachingof the courses,motivatedprimarily bythe active character that confer to thelearning process and because they contribute toobjectifyknowledge, tomake it moreconsolidatedand durable.Second,of all the forms ofteaching, labs direct students most naturallyto scientific-researchwork, in this way, contribute to the development ofskills for research work..Finally, performing laboratory practical developsuniqueexperimental skillswhichcannot be achievedbyany of the other(Hernández, 2001, 2002).

Recent reports from the European Commission (2007) as well as fromOECD (2006) highlights the need to change the pedagogy in science education andemphasize the importance of a positive experience with science, especially at a timeof decline in interest in sciences and technologies.

An important aim of inquiry in thelaboratory is to help students understand the link between theory and experimentalactivities. This comes back to the difficulties students often have in making the linkbetween scientific concepts and the experiment to be done, or later between theexperimental data and the conclusion to be drawn.

The lab work is an important part of the Physics courses. It provides the students with a medium to practice theirexperimental and analytical skills and helps them understand thebasis of knowledge and the relation between theoretical and empiricalwork in physics (American Association of Physics Teachers,132 • accessible elements1998).

It is very important for students tounderstand that theory and experiment are interlocked and cannotbe separated. An observation can lead to a theory, which mayor may not stand experimental testing. Therefore, properly constructedlab experiments become essential components of a Physics course.

It is important at this point to differentiate between qualitativedemonstrations which are normally used as teaching aids in classroomsand the more genuine highly quantitative experiments thatare typically conducted in physics labs. The common belief is thatsuch experiments are costly and require special support and supervision.This also leads to the perception that low-cost home lab experimentsare inferior and cannot be considered genuine. This, inour opinion, is a premature judgment. It is not fair at this time tocompare a practice that has been developing for more than a centurywith the alternative that started to develop, in a serious manner,only about a decade ago. Isolating the physical phenomenon fromthe laboratory apparatus is a first step toward finding cheaper alternativesfor observing the same physics.Therefore, we propose that with enough research and imagination,low-cost, high-quality experiments can be designed for theintroductory physics courses.

Physics experiment is the heart of physics education. It opens the doors for students to understand andmaster physical theories. It also provides students with the essential training to use their knowledge and skills tosolve scientific problems. Lack of resources in developing countries hinders the advances in science educationespecially in physics. In this paper we will shed some light on our effort to solve this problem by building “lowcost experiments”.

Quality education is the core of innovation and economic development. The 21stcentury calls for well trained professionals to improve the practical capability of the students and hence the quality of the education. Comprehensive physics experiments can provide suchcapability. In underdeveloped countries, lack of resources hinders the whole educational process.The advantage of low-cost experiments is apparent in this situation. The benefits of hands-onexperiments thatare both low-cost and high-tech are desired for both universities’administration and students.

The hands-on experiments supply the students with the ability to activelyinvestigate the connection between the theoretical content of the course and its application onreal physical systems, often with the additional benefit of active learning in a small groupenvironment(Heller, Keith,and Anderson, 1992). Unfortunately, many commercially existing experimental systems arecomplex and too costly, thus not all institutions have the funds to supply each student withsatisfactory opportunity to experiment with such equipment.To serve this purpose in Egypt, for example, these experiments and demonstrations have to beaffordable as much as possible. Thus a number of low-cost physics experiments have beendeveloped and used in some local universities and they proved to be very successful.

The experiments fall into two categories; those qualitative experiments that enhance the skills ofobservation and reflection, and those quantitative experiments which require the collection and manipulation ofdata. Those experiments can be used either in a laboratory or in a classroom as a demonstration to engage thestudents more in their learning process as well as elevate their understanding level. A special emphasis is givenon easy to assemble experiments that can fit within the budget of any physics department in a developingcountry.

The lab is an essential constituent of anyPhysics course.Students should understand the important relation between theoryand experiment in physics. However, the tradition has been to perform Physics experiments in especially equipped laboratories andunder the direct supervision of a lab instructor. Many of these labs involve standard experiments and use special apparatus purchasedfrom certain lab equipment providers. As a result, there is a generalimpression that in order to do an experiment you must go tothe lab and use the special equipment found there.We argue that this does not have to be the case. We believe that,with good imagination and adequate research, many high-qualityphysics experiments can be designed and performed safely by thestudents at their homes using low-cost materials and devices.

We were able to design home lab experimentsfor the physics courses using inexpensive equipment, commonhousehold items and recycled material. The quality of the resultsexceeded expectations and is comparable to what is achievedin traditional labs.

Several authors, for example, Shulman and Tamir (1973) (as cited in Hofstein &

Lunetta, 1982), Hofstein and Lunetta (1982), Bernstein (2002) have identified the goals oflaboratory instruction. They involve the development of practical skills and knowledgeand provide an opportunity to make science ‘real’. The American Associationof Physics Teachers (AAPT, 1997) has also issued a list of goalspertaining to the introductoryphysics laboratory (AAPT, 1998). According to all these authors, a good lab isone which promotes effective learning and meets the objectives while making thelaboratory experience interesting and enjoyable. The challenge for educators is todecide which concepts must be learned and which skills must be developed andthen to design a laboratory experience consistent with the identified objectives. Weidentify the following five goals for our laboratories which are consistent with theobservations of these authors:

(1) Increase knowledge of physics

(2) Develop practical abilities

(3) Arouse and maintain interest, attitude satisfaction, and open-mindedness in

Physics

(4) Develop creative thinking and problem-solving ability

(5) Promote scientific thinking and provide practice in the experimental methods

An alternative approach is described by von Aufschnaiter and von Aufschnaiter

(2007) who state that the purpose of a laboratory is to provide structured practicalactivities which promote the development of conceptual understanding, rather thanconnecting pre-existing theory to practice. Rather than searching for good experimentsthat demonstrates a specific concept, these researchers promote laboratoryinstruction that focuses on good learning experiences, where students can discoverthe concepts from their activities. Giving students the opportunity to discover ruleson their own enables them to develop an understanding of what the scientificapproach is about.

“The basis of what scientists believe and why they believe it is not the result of mere thinking or reading in a textbook. The basis of what scientists believe is the result of the careful collection and analysis of laboratory evidence. In any physics class, the differentness of science will be most evident when it comes time for lab.In the physics class, lab work is central, integral and sacred. More than a mere place in the back of the classroom, the laboratory is the place where physics students do physics. It is in the laboratory that physics students learn to practice the activities of scientists - asking questions, performing procedures, collecting data, analyzing data, answering questions, and thinking of new questions to explore”. (The Physics classroom, 2013).

Practices in Electromagnetism courses aredominated by those dedicated to circuits and to a lesser extent those related with measurements of magnetic field and finally only a few associated with the electrostatic field. That is why the traditional lab obtaining the equipotential lines of the electrostatic field in a tank should not be abandoned because it can give a direct image and its operation is simple. As a resource to make it more attractive we decided to combine the experimental construction of equipotential lines with their calculation using QuickField, specialized software, easy to use and available for free (Quickfield, 2012). By the way, we also contribute, to the development of professional skills related with modelling, simulation and comparison of the results with experimental measurements.

Materials and Methods

Set up for the study of the Electric Field.

Obtainingexperimentalequipotential lines was done inrectangular acrylic tray dimensions19.3x30.3x4.5cm. In the same,water was pouredandthe metal electrodes were locatedto generatethe electrostatic field when some voltage was applied. Threedifferentgeometrieswere usedfor the electrodes, two sharpthin rods, two flatand twoconcentriccircular rings. In this way, situationscorresponding to two point chargestwoinfinite parallelflatplatesand twocoaxialinfinite cylinders were modelled. In this situation the obtained equipotential lines were the result of the interception of the equipotential surfacesof the abovesystemswith the horizontal planeof the tray.

Theelectrodes wereconnected to theterminals of adirect current sourceandapplying apotential difference of10V.Figure1 showsa schematic of theinstallation.For the construction ofthe equipotential linespotentialswere measuredat differentpoints locatedbetween the electrodes andtheir coordinateswere recorded. The values​​were obtained by adigitalvoltmeterand the coordinates ofeach pointwere taken froma graph paperin the transparent bottomof the tray for the Cartesian coordinatesandpolarroleforpolar coordinates.Excel was used to represent graphically the tabulatedvalues​​of Cartesian or polarcoordinates(depending onthe geometry) of the points withthe same value ofpotentialand plottedinscatter plotsand radial.

Thecomputer calculationofthe equipotential lines was done ​​usingfree softwareQuickFieldstudent version5.3. This softwareis veryeasy to usebystudents.Initiallywhen drawingthe geometric modelplanedata corresponding to the dimensionsof the tray were usedandthe location and dimensionsof the electrodes.Thenthe properties of water were introducedas a mediumthat fillsthe tray andthe values ​​ofthe potential differencebetween the electrodes.In this waythe calculations were done tothe same conditions aswhenexperimentalmeasurements of the potential were made.

Figure 1. Schematic of the installation for equipotential lines electric field

Set ups for the study of the Magnetic Field:

Tangencies galvanometer. Determination of the local horizontal component of the earth magnetic field using the interaction of magnetic dipole (a needle of a compass) with magnetic field generated using coils with different geometries triangle, square, rectangle, circular, etc.)

Figure 2. Schematic representation of the tangencies galvanometer

The two vectors B0 and B are perpendicular; the resulting field Bres will form an angle  with the field B0. The simple relations are:

  (1)

Changing the values of the current circulating in the coil we modify the angle  and in the plot of B versus tan  the gradient is equal to the horizontal component of Earth’s magnetic field. In this set up when the needle of the compass is deviated by 45 degrees the horizontal component Earth’s magnetic field is equal to the applied magnetic field of the used coil that can be calculated with Biot-Savart´s law.

Another set up for magnetic field “Determination of the local horizontal component of Earth’s magnetic field using the interaction of magnetic dipole (a permanent magnet) with magnetic field generated with a solenoid coil” allows us obtain the value of the horizontal component of Earth’s magnetic field and the ratio inertia moment and magnetic dipolar moment of the permanent magnet.

The deviceis shown schematically in Figure3. It consists of asolenoid coil and a permanent magnet placedin its centre and suspended from avery long thread. The threadexertsvirtually norestoringtorquetorotatethrough small angles. This magnetinteracts, by itsmagnetic dipole moment, withthe resulting magnetic fieldof the solenoidand thehorizontal component of Earth's magnetic fieldand executesharmonic oscillations, whose periodis determinedusing atimer.

Figure 3. Schematic of installation for the magnet-magnetic field interaction

For thisassemblythe following the relationship may be deduced:

(2)

Thisshowsthe linear relationship betweenthe square of theangularfrequencyandthe current iflowing through thesolenoid coilwithNturns andlengthL.From the slopecan be extracted the ratio. HereBextis thehorizontal component of thelocalEarth magnetic field and can be determinedfrom theintercept. Withthe mass anddimensionsof the magnet moment of inertiaI may be calculatedand from the relationshipthemagnetic dipole momentmof the magnet can be obtained.If thepermanentmagnet materialis knownand thereforeit’s mass, it is possible to estimatethe magnetic moment ofthe atomsunder the assumptionoftotal alignmentofatomic dipoles.

Construction of a Teslameter using a Hall Effect sensor

The main characteristics of the Standard Miniature Ratiometric Linear Sensors are:

• Small size
• Low power consumption
• Single current sinking or current sourcing linear output
• Built-in thin-film resistors - laser trimmed for precise sensitivity and temperature compensation
• Rail-to-rail operation provides more useable signal for higher accuracy
• Responds to either positive or negative gauss
• Quad Hall sensing element for stable output

Miniature Ratiometric Linear sensors have a ratiometric output voltage, set by the supply voltage. It varies in proportion to the strength of the magnetic field.Figure 4 illustrates a ratiometric analog sensor that accepts a 4.5 to 10.5 V supply. This sensor has a sensitivity (mV/Gauss) and offset (V) proportional (ratiometric) to the supply voltage. This device has “rail-to-rail” operation. That is, its output varies from almost zero (0.2 V typical) to almost the supply voltage (Vs - 0.2 V typical).

Figure 4. Ratiometric linear output sensor

The transfer function of a device describes its output in terms of its input. The transfer function can be expressed in terms of either an equation or a graph. For analog output Hall Effect sensors, the transfer function expresses the relationship between a magnetic field input (gauss) and a voltage output. The transfer function for a typical analog output sensor is illustrated in Figure 5.

Figure 5. Transfer function analog output sensor.

Equation 3 is an analog approximation of the transfer function for the sensor.

Vout (Volts) = (6.25 x 10-4 x Vs)B + (0.5 x Vs) (3)

-640 < B(Gauss) < +640

From this equation may be deduced the expression for determination of magnetic field starting from the measurement of the output voltage (Vout) and the applied voltage from DC source (VS):

(4)

Figure 6. Physical aspect of the Standard Miniature Ratiometric Linear Hall Effect Sensors
With a 5V Dc source, a voltmeter and two Hall Effect sensors we are able to measure the magnetic field transversal and axial. The sensors should be placed in plastic tube and in a pad in order to build the axial and transversal probes.

Figure 7. Schema of the disposition of the Hall Effect sensor in the axial and transversal probes

The following is a relation of some possible experiments about magnetic field that can be done using a Hall Effect sensor.

1)Magnetic field in a solenoid coil. Dependence of the intensity of the magnetic field in a solenoid coil with its parameters (length/diameter ratio and linear density of turns) and the position respect to the centre.