Simulations as a tool for visualization of the phenomenon Magnetic hysteresis
N. Nancheva, A. Aleksandrova
University of Rousse
Introduction
Experiments based on computer simulations have already found widespread use as a supplementary material in the teaching process. The simulation exercises sufficiently complement traditional method of education and follow the present trends of cheap education (especially expensive laboratory education) provide to obtain measured data alike as in classical laboratory. Visualization of physics phenomena through such techniques as demonstrations, simulations, models, video clips and movies can contribute to students` understanding of physics concepts by attaching mental images to these concepts [1]. The use of virtual instrumentation and related Computer Based Learning techniques produce a reduced workload for teaching staff, a more user friendly interactive environment to study in and allow students to study remotely if desired. With the use of visual aids within the laboratory, such as Virtual Instrument, animations and narration students can not only gain a better understanding of what they are doing but why they are doing it [2, 3].
The major advantage of computer simulations is an excellent visualization effect [2-4] that is of special interest in the description of phenomenon “Magnetic hysteresis” since the effects of magnetic field on ferromagnetic materials through action of abstract field is hard to imagine. In this paper we present the development of a novel concept of teaching, allowing students to explore theoretical and experimental aspects of act of magnetic field on ferromagnetic material through simulation. Focussing on the essentials, it is particularly suited for increasing students` insight into the physics of magnetic hysteresis. For engineering students this phenomenon is of big interest according its practical applications. The magnetic memory aspects of iron and chromium oxides make them useful in audiotape recording and for the magnetic storage of data on computer disks.
A brief introduction into the physics of magnetic hysteresis
One of the most typical characteristic of ferromagnetic materials is magnetic hysteresis that is due to a long-range ordering phenomenon at the atomic level which causes the unpaired electron spins to line up parallel with each other in a region called a domain. The long range order which creates magnetic domains in ferromagnetic materials arises from a quantum mechanical interaction at the atomic level. This interaction is remarkable in that it locks the magnetic moments of neighboring atoms into a rigid parallel order over a large number of atoms in spite of the thermal agitation which tends to randomize any atomic-level order [5]. Within the domain, the magnetic field is intense, but in a bulk sample the material will usually be unmagnetized because the many domains will themselves be randomly oriented with respect to one another. Ferromagnetism manifests itself in the fact that a small externally imposed magnetic field can cause the magnetic domains to line up with each other and the material is said to be magnetized. Ferromagnets will tend to stay magnetized to some extent after being subjected to an external magnetic field. This tendency to "remember their magnetic history" is called hysteresis. The fraction of the saturation magnetization which is retained when the driving field is removed is called the remanence of the material, and is an important factor in permanent magnets. When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed. It must be driven back to zero by a field in the opposite direction. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. Once the magnetic domains are reoriented, it takes some energy to turn them back again.
Magnetic hysteresis in simulation laboratory experiment
Every simulation is to a certain extent only a simplified model of the real experiment. Core of presented simulation laboratory exercise “Magnetic hysteresis” is Java applet [6]. For creation of simulation experiment Web based technology has been used. Web pages were created by Microsoft FrontPage, ArcSoft Photostudio 5.5 and Microsoft Photo Editor. Because most standard applets are written with English controls and instructions, students with limited knowledge of the English language might have problems understanding the basic concepts governing the applet behaviour. For our purpose some of the text in the applet has been translated in Bulgarian. The experiment has been created for students from engineering courses of University of Rousse and can be used for distant learning or in addition of real laboratory experiment. This simulation experiment has analog in real laboratory experiments in University of Rousse, but in real experiment it is impossible to receive notion for domains and their behaviour.
The simulation laboratory experiment “Magnetic hysteresis” contains typical parts or pages:
· An introduction theoretical part informing the student about the physical phenomena, which is demonstrated in simulation lab exercise (Fig.1).
/ Fig.1. “Magnetic hysteresis” – theoretical part “· A part where the students may be prompted for the answers to three questions (entry test). The student can only proceed with the exercise after giving the correct answers (Fig.2).
· A part giving the practical instructions describing how to carry out the actual activities and measurements of the lab exercise.
· Detailed measuring process description, the applet used in simulation and measuring process (Fig.3).
· A part giving an overview of the results of the exercise allowing the student to print these results. It is based on these data that the student will prepare a lab report.
Theoretical introduction and analysis of physical phenomenon is a very important part. When student do not know the theory of physical phenomenon, shown in simulation experiment, it will be only a computer game. Students must be very quickly introduced to the bases of physical phenomenon, because they usually do not hold enough concentration and are not patient when working at Internet.
Fig.2. “Magnetic hysteresis” – entry test
Fig.3. “Magnetic hysteresis” – tasks, applet and results
The simulation lab exercise allows investigating the magnetization of ferromagnetic material at different intensities of external magnetic field - and different temperatures - . The laboratory exercise can be integrated in real laboratory experiment. It can also be profitably used by the distant learners, who have often scarce or null opportunity to access the laboratory in the university. Of course, when using simulation exercises, it is important to bear in mind that the actual reality is inevitably more complicated than the virtual one. Although the simulation exercises are very good as an educational addition, they could not recompense the real classical lab experiments.
Video clips as a tool for visualization of magnetic hysteresis
Interactive video clips and movies are logical step in the progression of creating useful visualizations for students. According to Kozma [7, 8] “the advantage of any video is in its use of dynamic, visual symbolic systems that allow scientists to view any scientific experiment or discovery from multiple or different perspectives”. In [9, 10] has been shown that:
· The digital video activities and tools can be used by students to make connections between concrete, real-life phenomena and the abstract ideas and models of physics.
· Students can create visual representations of their model and display it directly on the video scene. In this way students can make direct visual comparisons between complex events and simplified scientific models.
· Students can also combine images from different video frames and modify the presentation of motion on the screen.
In the case of lab exercise “Magnetic hysteresis” we used movie [11] and video clip [12] respectively, to visualize the phenomenon because the effect of magnetization is quantum process and it is hard to explain and imagine. Students can compare behavior of the different ferromagnetic materials; compare their hysteresis loops and connect it’s with their properties, that there is important against to their practical applications (Figs. 4-6). Students can create visual representations of different stages of magnetization, how is generating the hysteresis loop and observe behavior of the magnetic domains (Fig. 7).
Fig.4. Fig.5.
Fig.6.
Fig. 4, Fig. 5 and Fig.6 present three simulations at different values of the random field, which determines the influence of disorder on the system [11]. The panel on the left displays the lattice, spins are colored blue when down and colored green when up; clusters of spins flash red when they flip. The panel on the right is a plot of magnetic field (horizontal) versus magnetization (vertical.) The movie will run the model to the maximum and minimum magnetic fields to generate a hysteresis loop; after that, it is possible to tell the movie to run the model to a particular magnetic field by clicking on the control bar underneath the plot. Fig.4. presents mostly small avalanches and narrow hysteresis loop that implies a small amount of dissipated energy in repeatedly reversing the magnetization. Materials with narrow hysteresis loop are desirable for transformer and motor cores to minimize the energy dissipation with the alternating fields associated with AC electrical applications. Fig.5 demonstrates both very small and very large avalanches and saturation magnetization. The area of the hysteresis loop is related to the amount of energy dissipation upon reversal of the field. With the movie presented at Fig.6 students have possibility to observe very large avalanches as most of the system changes magnetization as once. Materials with wide hysteresis loop are desirable for permanent magnets and magnetic recording and memory devices.
a – No external field
b – Week applied field
c – Strong applied field
d – Curve of initial magnetization
Fig. 7.
Fig.7 shows frames from the visualization experiment [12] and presents different stages of magnetization and behavior of domains (a – no external field; b – week applied field; c – strong applied field; d – curve of initial magnetization). These illustrations of domains are conceptual only and not meant to give an accurate scale of the size or shape of domains but students get an idea about what happen in material. The microscopic evidence about magnetization indicates that the net magnetization of ferromagnetic materials in response to an external magnetic field may actually occur more by the growth of the domains parallel to the applied field at the expense of other domains rather than the reorientation of the domains themselves as implied in the sketch.
The sketches above suggest that the effect of external magnetic fields is to cause the domain boundaries to shift in favour of those domains which are parallel to the applied field. It is not clear how this applies to bulk magnetic materials which are polycrystalline. Keep in mind the fact that the internal magnetic fields which come from the long range ordering of the electron spins are much stronger, sometimes hundreds of times stronger, than the external magnetic fields required to produce these changes in domain alignment. The effective multiplication of the external field which can be achieved by the alignment of the domains is often expressed in terms of the relative permeability.
Simulation may be used by the teacher to explain and demonstrate the phenomenon before the student actually enters the real lab exercise.
Conclusions
The laboratory exercise presented in this paper can be used in addition of real laboratory experiment. It can also be profitably used by the distant learners, who have often scarce or null opportunity to access the laboratory in the university. Of course, when using simulation exercises, it is important to bear in mind that the actual reality is inevitably more complicated than the virtual one, so everybody should be invited to try, whenever possible, home made experiments or to directly look at physical phenomena in nature. Although the simulation exercises are very good as an educational addition they could not recompense the real classical experiments.
Similar experiments allowing the student to compare the results of a numerical simulation in a model and the behaviour in nature, and by that understanding how effectively a theory allows the description of the physical reality.
References
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