TEACHING SCIENTIFIC RESEARCH AND PRACTICAL APPLICATION OF
what happens during body charging. In the new model, it is necessary to know how the charges are distributed on the body surface. To correctly use a formula (for example, C = Q / U (1), where U is the difference of potential), it is vital to switch to causal reasoning based on the phenomena taking place and on the changes that are necessary during body charging (Kohlmyer et al., 2009). After that, the interaction between different parts of the general scheme (the battery, the environment and the bodies that interact) can be discussed, taking into account the changes happening when the current passes through.
This is what allows for explaining the mechanism of acquiring the new equilibrium (Mulhall, Mckittrick,
& Gunstone, 2001). The energy model (Guisasola et al., 2010) provides a more common and less fragmented vision of the current passing through the circuit having a capacity. Here the students should be taught this directly so that they could use this phenomenon in practical applications. To this end, the distribution of the potential of each charge acquired by the bodies should be discussed, taking into account that it is connected with the work performed by the environment. State standards of many countries envisage the students being able to explain the movement of charges in conductors and the processes of body charging, basing their explanation on the concept of electric capacity. This implies that students should be familiar with body charging processes and with devices like capacitors. Thus, at physics courses taught at universities, students use quality models based on charge attraction and repulsion and explain charging and discharging of bodies. A lot of works in different countries have been devoted to the problem of body charging efficiency and to the explaining models that include the concepts of charge, difference of potential and electric capacity [(Guisasola et al., 2010), Irodov I.E.
(1986). This means that among the first principles to be offered by the sequence of content delivery in use is not a model based around electric forces but another one that requires contemplating the concept of the difference of potential and the electric capacity. Thus, the explanatory model gets increasingly complicated. Several models can be suggested that provide for the understanding of this fact and for the clarification of its implementation in different capacitor constructions. However, the model based on the use of standard formula (1) poses a series of difficulties for the students, because the model of forces is a more direct causative model than the energy model (Guisasola et al., 2002). The experience in discussing this problem with students shows that the mosaic structure can be implemented as a specific configuration of heterogeneous substances that fill the space between the capacitor planes. The capacity of a capacitor changes because of charge distribution alterations, both on the material surface and inside its volume (Fig.
1., a, b, c), taking place because of the use of different materials.
Figure 1. The scheme of dielectric materials filling flat capacitors
Another problem that is important for understanding is the time over which the charge is distributed on the surface of a body and throughout its volume. This ensues from the structure of formula
(2)
t
c C Idt
U (1/ ) (2). If the system has a capacity, then under the current impulse application, the capacity is charged to reach a certain potential Uc (formula (2). Students should understand that the capacity can alter during the current impulse application (formula (2). Thus, for practical exercises, project type tasks Hake (1998), Dancy & Henderson (2007), Zelichenko & Larionov (2009) should be introduced, in particular, those for designing capacitors (e.g. like those shown in Fig. 1). As a result, the mechanism of transforming instructional tasks into project tasks (those of implementational nature) is actuated. The model of problem-oriented teaching of physics (Zelichenko & Larionov, 2009) holds true for any types of activities at seminars and laboratory workshops and includes a mechanism for managing the project performance. The model includes the following: task administering; task solution, the detection and structuring of the problem and the problem situation; searching for the problem solution, creating computational software, formulating the idea at the project level; review of devices available in cited literature (also using the Internet); project creation, with some elements of technical implementation; project presentation and defense.
At each stage and in every element of the system, a target and motivational component is involved. This happens because individual parts of the teaching system under consideration contribute differently to the achievement of goals. In this vision, the technical creativity of the students Eylon, Ganiel (1990), (Zelichenko & Larionov, 2009), is regarded as an exploratory activity aimed at the final result, which is not limited to the knowledge acquisition and retention but also envisages the elaboration of the project implementation proposal. The management of students' project activities can be illustrated
through the example of studying the electrization phenomenon via friction. This simple and widely known phenomenon can have a project- and implementation-driven future.
It is normally assumed that during the electrization of two different dielectrics by friction, their surfaces acquire approximately homogeneous distribution of charges of the opposite signs. However, in a number of occasions body electrization leads to the mosaic distribution of charges (Baytekin et al., 2011) on the surface of a body, with randomly interspersed oppositely charged areas (Fig.2). The following problems should be isolated and discussed here: the duration of dielectrics' friction, the pressure applied, the way of friction and the heterogeneity of the surfaces. It is necessary to highlight that those factors do not have a considerable effect on the emergence of a mosaic. In addition, no mosaic distribution of the opposite charges occurs on the surface made of elementary substances (e.g., silicon and aluminum) exposed to identical electrization.
Figure 2. Uniform (a) and mosaic (b) charge distribution after the contact and separation of two surfaces 1 and 2 (the light part of the sample marks the positive charge, the dark one, the negative charge) (Baytekin et al., 2011)
Figure 3. Polycarbonate surface after the contact with polydimethylsiloxane surface (Baytekin et al., 2011). Positive charge obtained. Area density is 0.16 nC/cm2. Mosaic cell size is about 100 nm. Potential is 1 V
This physical problem should be divided into three parts: the electrization of metal by metal, of dielectric by metal and dielectric by dielectric. For the first combination, the electrization process is well- studied: charge distribution is taking place until the Fermi levels of the two metals get even, when the substance with a higher (lower) Fermi level is charging positively (negatively). The discussion of this phenomenon makes it easier for a working subgroup of students to move forward to the concept of the emergence of contact potential difference Zelichenko, Larionov and Pak (2012). Students can create a model of mosaic distribution of the irregularities on the surface of a dielectric, as well as its alteration and calculation method. The project implementation plan emerges during the analysis of all the projects formulated by all the students of the group on the basis of the full problem analysis. Problem books, for example Irodov (1986), suggest tasks for calculating charges with non-uniform distribution. Proceeding to the problem solution after conducting the problem analysis with students allows for creating a device for studying such a charge distribution on the surface of a dielectric.
2. Method of a project-type class implementation
Let us consider the solution to the problems outlined in the introduction through the example of studying discharge distribution on the surface of a body. Discharge distribution on the surface of a body is of paramount importance for the generation of materials with targeted properties, generation of charged particle focusing systems, formation of variable capacitors. Moreover, it allows gaining a better understanding of the notion of electrical capacitance (Guisasola et al., 2002, 2010 and teaching students to measure distributed charges (Baytekin et al.,2011), Irodov (1986) with a predetermined manner of their distribution on material surfaces. The demonstration of this phenomenon as well as the formation of systems with distributed charges is an important practical task in terms of preparing prospective students to implementational engineering activity and is interesting as part of the teaching process, as teaching requires creativity driven experiments. As a rule, after students successfully solve a problem, they are
asked to implement a device and test the controlled quantitative values of discharge distribution on the surface of a body as well as to use them for demonstration and assessment purposes.
The project task is to make a device that would allow producing a distributed electrical charge on the surface of a dielectric with a predetermined manner of distribution on the dielectric being charged.
Students make up working groups under the teacher’s guidance to implement the project. For these purposes, a survey matrix is used that reflects students’ requests and is adjusted by the teacher. An example of the device offered after solving the task is given in Fig. 4. The device consists of a dielectric disk with same-size (radius R) metal balls fixed around its large perimeter (circle), a direct current supply, an adjustable resistor to vary the voltage on the balls, a voltmeter and an electric metal probe that can move from one ball to another. The metal probe with a pointed end is brought close to a ball. The probe is connected to a voltmeter in series, which, in its turn, is connected to the direct current supply through the adjustable resistor. The said resistor varies the voltage applied to the electric probe and is used to set the target voltage on that probe. Its value is controlled by means of a voltmeter. The disk is turned so that the probe could touch the next ball, the turning angle θ is measured and the adjustable resistor is used to set a new voltage on the probe. With each turn, the turning angle is measured and the voltage on the probe is changed. The actions are repeated until all the balls of the disk are charged. For the device to operate, we determine the electric capacity C for the ball according to the following formula: C4
0R(2), where ε0 is an electrical constant, and R is the ball radius. The charge Q of each ball is determined as Q = Cφ (3), where φ is the voltage applied to the ball, which equals the probe voltage. The probe voltage is set using the adjustable resistor. Consequently, the ball charge Q = 4π ε0R φ. Since the potential φ for both the probe and the ball is changed through the adjustable resistor at every turn of the disk by the angle θ, the charge of the disk is changed according to the required principle. For instance, if φ = φ0cos θ, then Q = 4π ε0Rφ0cos θ (4), where φ0 is the initial voltage (potential) of the probe and the first ball. The physical basis of the device is the dependence of a metal ball charge on the size of the ball. For the charge to be distributed smoothly along the disk perimeter, the size balls must be far smaller than that of the disk, i.e.more than 10 times smaller.
Example of implementation in experimental workshops of the university business center.
The device is made of a circle with the radius of about 60 mm cut from a 1-mm-thick sheet of dielectric material (Teflon). One drills orifices therein with the diameter of 1 mm at the distance of 0.5 mm from each other along the circle with the radius of 50 mm. Then one glues metal balls 1 mm in diameter into the orifices. The disk is mounted on a firmly fixed dielectric stand so it could rotate around the axis that goes through its geometrical center. The probe is fixed firmly on an isolated holder and brought close to the balls of the disk. The probe is a metal rod with the diameter of 0.5-0.8 mm and the length of 15 mm.
A way to determine the potential in the center of the disk is yet to be technically implemented.
Figure 4. The scheme of a device to form a distributed charge on the surface of a body
Legend to Fig.4: (1) dielectric disk (students choose the material for it by themselves) that is mounted on the stand (7); (2) metal ball; (3) metal probe, which is a 10-mm needle with 2-mm diameter;
(4) voltmeter; (5) adjustable resistor connected according to the scheme of a potentiometer; (6) power supply. The following additional issues are to be discussed: how to charge the balls positively and negatively, how to connect an adjustable resistor as a voltage divider, what voltmeter to choose, etc. By using the Internet to search for the devices and their possible schemes one can minimize the time needed for that search.
One of the ways to organize students’ joint activities and to form physical ideas at the project level can be simple tasks with an implementational follow-up. A collection of problems for university students Irodov (1986) invites students to determine the difference in refractive indices of an ordinary and an extraordinary beam in response to an electric field applied on an isotropic material (Kerr effect).
This problem can easily be turned into an implementational project. One can design Venetian blinds with
each slat being an isotropic material. Students stick conductive strips on the slat edges and apply DC voltage on them.
3. Conclusion
While accomplishing the projects aimed at studying the phenomena of electrization and electric capacity, students offered about ten options to take advantage of this phenomenon in practice. Among the options is the utilization of contact potential difference, piezoelectrics and the devices for creating and researching the charges with random distribution of their density throughout the dielectric's surface. As a result, we observe an increase in the efficiency of managing the performance of project activities by students. At the project level that involves students’ teamwork, the said organizational and procedural aspects of professionally oriented physics teaching allow for changing the physics teaching system at universities so that the implementational nature of the future professional activities (i.e. the activities of a certain purport) could be reflected in the training of a future engineer in physics. The suggested method and model do not only teach subexperiments and objectives but also leads to formulating physical ideas at the project level, as well as amplifies their pre-professional preparation and subsequent motivation for studying future professional disciplines. The tasks under consideration bear the property of structural comprehensiveness. It rules out same-type tasks for inserting numeric expressions, it envisages element- wise analysis and the performance of design and graphics rendering with the help of IT technologies that allow for demonstrating that the tasks intrinsically contain the elements of the past and futureknowledge.
The study of a physical phenomenon using the project method creates synergistic, structure-forming effect that unites series of phenomena for their new practical applications.
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