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TEACHING THERMODYNAMIC USING THE PARTICULATE MODEL OF MATTER Ma-Naim Chana Kibbutzim College of Education, Tel-Aviv, Israel. [email protected] Bar Varda Science Teaching Center, Hebrew University, Jerusalem, Israel Finkental Michael Rachah Institute of Physics, Hebrew University, Jerusalem, Israel ABSTRACT Traditional teaching of thermodynamics still leaves many misconceptions amongst students and teachers. In this study, with pre service and in service teachers we used a different approach of teaching thermodynamics: teaching started from the particulate model of matter. Concepts and phenomena concerning heat and temperature changes were explained through the processes that occur among particles. It was hypothesized that using this model facilitates the assimilation of the formal scientific concepts of thermodynamics by making them more visual and concrete. In the following article, we will explain the teaching method that was employed. INTRODUCTION Research on concept development reveals difficulties with basic concepts of thermodynamics. For example, difficulty in differentiating between “heat”, an extensive parameter, and temperature-an intensive parameter; students identify certain materials as “hot” and others as “cold”. These typical misconceptions were found among students and among varied groups of adults even after being exposed to a course in thermodynamics. A survey of textbooks at different levels shows that most present “heat” and “temperature” through macroscopic phenomena, using them later to present the concept of internal energy. The microscopic explanation usually appears as an after thought. In the following study we conducted an experiment where the teaching sequence was changed. We did not follow the historic sequence where formal thermodynamics concepts were introduced before the “modern” particulate model of matter as defined towards the end of the 19th century. Instead, we presented extensively the particulate model of matter first, thus definitions of concepts of thermodynamics like heat, temperature and internal energy were based on the model. Less elaborate explanations based on the notions of the particulate model of matter were given even before Newton’s time. Explanations based on the particulate model of matter were preferred since it is a mechanical model that appeals to common sense. We believe these explanations make more sense to students than explanations based on the formal thermodynamic concepts. Macroscopic phenomena which students encounter, can all be explained by using the particulate model of matter. 1 Phenomena were discussed and explained on three levels: 1. Description of the macroscopic phenomena; 2. Explanation of the process through the particulate model of matter; 3. Statement of energy changes that occurred in the system. THEORETICAL BACKGROUND This study follows the traditional studies of conceptual development, (Driver, 1985; Novak, 1977), which have the following common assumptions: 1. Every person, regardless of age, holds a system of explanations for natural phenomena and definitions of scientific concepts. 2. These explanations based on everyday experience and common sense are usually different from the scientific ones. 3. Understanding and applying new knowledge are influenced by the former system of definitions and explanations. 4. Alternative concepts are resistant to change. These assumptions have been supported since then by a vast body of research (Pfundt & Duit, 1994). Studies based on the traditional sequence of teaching where macro descriptions come before the microscopic model, report that students show difficulties in understanding phenomena concerning heat. These difficulties were found among elementary school students, university students and university researchers (Linn & Songer, 1991). Differentiating between heat (mass dependent) as an extensive parameter, and temperature (mass independent) as an intensive parameter. Stavy & Berkovitz (1980) reported wrong answers given to questions of mixing water at different temperatures. Many of the students think that the temperature of the mixed quantity is the sum of the temperatures of the ingredients. The confusion between heat and temperature was also exposed by Shayer & Wylam, (1981); Wiser &Cary, (1984); Finley, (1985); Linn & Songer, (1991). Rozier & Viennot (1991) found this confusion among teachers and university students, as well. Accepting the fact that all inanimate objects in a room have the same temperature in spite of the fact that metals feel “cold” while plastic or wood feel “tepid”. This difficulty derives from the different feeling while touching objects and from identifying objects/materials as associated with typically “hot” (wool for example) or typically “cold” (metals or aluminum foil). This characterization of materials influences students’ alternative explanations concerning which material is needed in order to keep hot or cold body temperature constant. Certain materials are assumed to keep “heat” (keeping a comparatively high body temperature) and other materials keep “cold” (keeping a comparatively low body temperature). Thus, in order to keep the body temperature, it should be covered with the appropriate material. A dichotomized concept of hot and cold fighting each other was observed by Tiberghien (1985); Tomasini, & Balandi (1987); Lewis & Linn (1994). Heat is thought to be something contained in a body, something (“matter”) that flows from a hot body to a colder one (Finley 1985): “Heat is something in the body that 2 changes the temperature and velocity of its particles.” This is a crude conception of the classical caloric. A hot/cold body is made of hot/cold particles (Finley 1985), when one side of a body is heated, “hot molecules move from that side to the other”, namely, students apply macroscopic characteristic to a microscopic quantity. These findings strengthen an assumption that one has to start with the particulate model of matter; to specify first what is happening in the microscopic level, the macroscopic phenomena then become clear. METHODOLOGY Description of the new curricula unit One. A thorough instruction of the particulate model of matter preceded the instruction of temperature and “heat” Two. Within this unit the words “heating” and “cooling” were consistently used instead of the notion quantity of “heat”. Three. All the macroscopic phenomena were explained on 3 levels: description of the process using the particulate model of matter; the changes of energy involved in those processes; a detailed description of the macro changes manifested during the observation of the phenomena and experiments. Four. Three forms of energy were specifically defined and used consistently: Kinetic Energy, Potential Energy and Radiation Energy. Kinetic Energy – the energy the particles have due to their motion, it is mass dependent 1 and the amount of this energy is given by the formula E k mv 2 . 2 Potential Energy – is the energy the particles have due to the state they are located in a substance. The potential energy is actually the sum of the bond energy between the molecules. The bond energy is lower when the distance between the molecules is small and higher when the distance between the molecules is greater. We can compare the bond energy to a spring’s potential energy, when the spring is extended (similar to longer distances between molecules) the potential energy is higher. The internal energy of a system is defined as the sum of the Kinetic Energy of and Potential Energy of its particles. 3 Examples of issues treated in the unit Heating a block of ice A thermometer is inserted in a block of ice contained in a metal box. The box is laid over a Bunsen burner. Looking at the thermometer, we see an increase of temperature until the reading is 0 0C. At this point, the temperature remains constant for some time and the ice melts. When all the ice has melted, the temperature starts to increase again and some steam comes out. The temperature increases till the reading is 100 0C. At this point, an increase in steam emitted is observed and there is no change of temperature. From the energy point of view, we say that there is a change in the internal energy of the ice, a change that occurs because we heat the ice. From the particulate model of matter aspect, we should discuss the following issues: What happens to the particles while heating (in the solid, liquid and gaseous phases)? Heating changes both the kinetic energy of particles and the potential energy of the particles in the solid. The increase in the potential energy causes an increase of distance between the molecules. The increase of kinetic energy of particles causes increase of temperature. Why is there a period of time with no change of temperature when the thermometer reading is 0 0C or 100 0C? The change of temperature is observed until the distances between some particles are at the distances typical of the liquid state of matter. From this point, the change in internal energy is only due to a change in the potential energy. There is no change in kinetic energy, therefore there is no change in temperature. When the bonds between the particles weaken, a phase change is observed. Further heating causes changes in both the potential and the kinetic energy. The same happens in boiling. All the energy given to the liquid through heating leads only to a change in the potential energy. There is only a change in the potential energy of matter at 1000C, a change that causes the molecules to separate and, thus, the liquid changes into gas. There is no change in the kinetic energy, therefore, there is no temperature change. This change in potential energy is what is called “latent heat” a name that was given in the 17th century. Why does steam come out of the liquid even before boiling? The temperature of matter depends on the average kinetic energy of its particles. Among the particles, some have high kinetic energy while some have low kinetic energy. If a particle with a high kinetic energy is on the surface of the liquid, there is a possibility that it will be ejected from the liquid. These particles are the ones that form the steam that comes out of heated liquid, even before the boiling temperature. Why does the phase change liquid–gas, need a longer period of heating than the phase change solid-liquid? The change in distances between molecules from solid to liquid is very small while in the phase change of liquid-gas we have to break the inter-molecular interactions and separate the molecules. Therefore, this phase change needs a greater amount of energy and a longer heating period. What is there in the bubbles that rise from the bottom of the liquid? 4 The bubbles consisting of liquid or vapor are at a temperature higher than the temperature of the surrounding liquid. These bubbles rise due to buoyant force. Mixing liquids in different temperatures A plastic container comprises of two compartments of equal volume, separated by an insulated partition. Water at a temperature of 10oC is poured into one compartment and water at a temperature of 600C is poured into the other compartment. The partition is then removed and the water from the two compartments is mixed together. In the discussion that follows, we describe the exchange of kinetic energy between colliding molecules. Molecules with high kinetic energy (from the high temperature liquid) collide with molecules with low kinetic energy (from the low temperature liquid). After some time all the molecules of the liquid will have the same mean kinetic energy. In the case of equal quantities of the same liquid the final mean kinetic energy of the molecules is PURPLE the average of the kinetic energy of the components. 5 Temperature measuring In the language of the particulate model of matter, the collisions between the substance molecules and the thermometer molecules is changing the kinetic energy of both. After a while the “temperature” is the same. The description involves the change of temperature of the thermometer components from the outer glass cover to the liquid in the inner narrow tube. From this discussion two question rise: 1. How do we know that the thermometer is really measuring the temperature and gives the correct system temperature? 2. How can we be sure that the thermometer is not changing the system’s temperature? From our experience it is usually the first time the students deal with measuring instruments the way the are designed and the principle of their operation. A detailed discussion using the particulate model of matter follows. In every process in every question, qualitative as well as quantitative, we ask the student to give the scientific story from the three points of view we mentioned above: macroscopic; the particulate model of matter; energy changes. We stress the fact that this story is not the story of a single molecule or a single particle but the story of a cluster of a great number of particles, almost an infinite number. The teaching follows the model of Mastery Learning, where understanding and utilizing the model is more important than covering the whole unit. SUMMARY AND CONCLUSION The main assumption of this study, that the particulate model of matter should be well known and correctly applied by the learners before approaching the topic of “heat”, was supported both by observing the quantitative results of the pre/post tests and by analyzing the qualitative explanations. Explanations were clear, concrete and not declarative as we found in the control group. For instance in the case of cooling due to water evaporation the control group used the explanation “water have a tendency to cool”. The experiment group explained that some of the particles with high kinetic energy escaped leaving behind molecules with lower mean kinetic energy. REFERENCES 1. R. Driver, E. Guesne, & A. Tiberghein, Childrens’ Ideas and the Learning of Science, in R. Driver, E. Guesne & A. Tiberghien (eds.), Children’s Ideas in Science, Philadelphia Open University Press, 1985. 2. F. N. Finley, Variations in prior knowledge, Science Education, vol. 69, (5), pp. 697-705, 1985. 3. E. L. Lewis, and M. C. Linn, Heat energy and temperature concepts of adolescents, adults, and experts: implications for curricular improvements. Journal of Research in Science Teaching, vol. 31, (6), pp. 657-677, 1994 4. M. C. Linn, and N. B. Songer, Teaching thermodynamics to middle school students: What are appropriate cognitive demands? Journal of Research in Science Teaching, vol. 28, (10), pp. 885-918, 1991. 6 5. J. D. Novak, An Alternative to Piaget, Science Education, vol. 61, pp. 453-450, 1977. 6. 7. S. Rozier, and L. Viennot, Students' reasoning in thermodynamics. Int. J. Science Education, Vol.13, (2), pp. 159-170, 1991. 8. M. Shayer, and H. Wylam, The development of the concepts of heat and temperature in 10-13 year-olds, Journal of Research in Science Teaching, vol.18, (5), pp. 419-434, 1981. 9. R. Stavy, & B. Berkovitz, Cognitive Conflict as a Base for Teaching Quantitative Aspects of Temperature. Science Education vol. 64(5), pp. 679-692, 1980. 10. A Tiberghien,. "Part B: The development of ideas with teaching”, in R. Driver, E. Guesne & A. Tiberghien (eds.), Children’s Ideas in Science, Philadelphia Open University Press, 1985. 11. N. G. Tomasini, and B. P. Balandi, Teaching strategies and children's science: an experiment on teaching about "hot and cold", Proceedings of the international seminar on misconceptions in science and mathematics, Ithaca, N.Y. Cornell 12. M. Wiser, and S. Carey, When Heat and Temperature Were One, in D. Gentner, D and A. Stevens, Mental Models, pp. 267-297, Hillsdale, N.J. Erlbaum, 1986. 7