Following Young Students’ Understanding of Three Phenomena in which Transformations of Matter Occur     

Lena Löfgren  Gustav Helldén 《International Journal of Science and Mathematics Education》2008,6(3):481-504

In order to develop successful teaching approaches to transformations of matter, we need to know more about how young students develop an understanding of these processes. In this longitudinal study, we followed 25 students from 7 to 13 years of age in their reasoning about transformations of matter. The questions addressed included how the students’ understanding of transformations of matter changed and how we can make sense of individual learning pathways. In interviews performed once or twice every year the students described and explained three situations: fading leaves left on the ground, a burning candle, and a glass of water covered with a glass plate on which some mist had formed. When analysing the interviews, we found a common pathway of how the students’ ideas changed over the years in each one of the situations. When analysing individual student’s interviews with Ausubel’s assimilation theory we could discern subordinate, superordinate and combinatorial learning. How these findings can contribute to an improvement of teaching about transformations of matter is discussed.  相似文献   

Editorial: Globalization and educational challenges of tertiary,vocational and community sectors in the Asia-Pacific     

Oon Seng Tan 《Educational Research for Policy and Practice》2008,7(2):71-72

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A study on the effects of education welfare action zone policy in Korea     

Hae-young Lee 《Educational Research for Policy and Practice》2008,7(1):35-45

This study was designed to analyze the effects of the support programs provided in education welfare action zones. Education welfare action zone policy came into effect by government from 2003 to guarantee actual educational opportunity to disadvantaged children in urban areas by providing educational, cultural, and welfare service. Education welfare action zones were selected in low-income group area in the metropolis. In this regard, both quantitative and qualitative analytical methods were used to ascertain the effects of the program. The qualitative analysis included a preliminary examination of conditions prior to the actual commencement of the program, which was then followed by an investigation of specific schools used, as case studies in order to identify the changes that had taken place during the program’s implementation process, and the program’s effect. Meanwhile, a quantitative analysis was employed to verify the actual changes that took place in several different aspects, changes that were revealed by the case studies.  相似文献   

Differentials in Mathematics Achievement among Eighth-Grade Students in Malaysia     

Noor Azina Ismail  Halimah Awang 《International Journal of Science and Mathematics Education》2008,6(3):559-571

This paper examines differences in mathematics achievement among eighth-grade students using a secondary analysis of the data. The students, who are from Malaysia, had participated in the Trend International Mathematics and Science Study (TIMSS) 1999. A series of school, home, demographic and socio-economic variables were used to investigate the differences in the mean student mathematics scores. Evidence from the data shows that gender, the language spoken at home, expected educational level, family background, and home educational resources and aids have a significant influence on the students’ level of mathematics achievement.  相似文献   

Research in non-equalization machining method for spatial cam   总被引：1，自引：0，他引：1  

Jun-hua Chen  Yi-jie Wu 《浙江大学学报(A卷英文版)》2008,9(9):1258-1263

Many kinds of devices with cam have been widely used in various mechanical equipments. However, non-equalization machining for spatial cam trough remains to be a difficult problem. This paper focuses on the analysis of running conditions and machining processes of spatial cam with oscillating follower. We point out the common errors in the biased distance cutting. By analyzing the motion of oscillating follower of spatial cam, we present a new 3D curve expansion model of spatial cam trough-outline. Based on this model, we have proposed a machining method for trochoidal milling with non-equalization diameter cutter. This new method has led to a creative and effective way for non-equalization diameter machining for spatial cam with oscillating follower.  相似文献   

Input-output approach to robust stability and stabilization for uncertain singular systems with time-varying discrete and distributed delays   总被引：3，自引：0，他引：3  

Hui-jiao Wang  An-ke Xue  Yun-fei Guo  Ren-quan Lu 《浙江大学学报(A卷英文版)》2008,9(4):546-551

Based on input-output approach, the robust stability and stabilization problems for uncertain singular systemswith time-varying delays are investigated. The parameter uncertainties are assumed to be norm-bounded and the time-varying delays include both discrete delay and distributed delay. By introducing a new input-output model, the time-delay system is embedded in a family of systems with a forward system without time delay and a dynamical feedback uncertainty. A sufficient and necessary condition, which guarantees the system regular, impulse-free and stable for all admissible uncertainties, is obtained. Based on the strict linear matrix inequality, the desired robust state feedback controller is also obtained. Finally, a numerical example is provided to demonstrate the application of the proposed method.  相似文献   

A cross-layer design approach on spectrum allocation and resource scheduling in cognitive PMP networks     

Jie Chen  Min-jian Zhao  Qiao Zhou  Shi-ju Li 《浙江大学学报(A卷英文版)》2008,9(4):435-444

We propose the spectrum allocation and resource scheduling algorithms in cognitive point to multipoint （PMP） networks with rapid changes of spectrum opportunities and present a media access control （MAC） protocol based on these algorithms. The objective of spectrum allocation is to make efficient use of the spectrum while maintaining the transceiver synchronization on frequency and time in the network. The objective of resource scheduling is to guarantee the quality of service （QoS） requirements of different kinds of connections and to minimize the total energy consumption in the network as well. By sensing only a small set of possible channels in each slot based on the state transition probability of each channel, our spectrum allocation algorithm achieves high spectrum efficiency in the network. The resource scheduling problem is divided into three sub problems and we derive optimal solutions to these problems by greedy algorithm and convex optimization. The simulation results show that our algorithm can make efficient use of the spectrum and the network resources at a cost of low computational complexity.  相似文献   

Eccentricity and thermoviscous effects on ultrasonic scattering from a liquid-coated fluid cylinder     

Seyyed M. Hasheminejad  M. A. Alibakhshi 《浙江大学学报(A卷英文版)》2008,9(1):65-78

Calculation of the scattered field of the eccentric scatterers is an old problem with numerous applications. This study considers the interaction of a plane compressional sound wave with a liquid-encapsulated thermoviscous fluid cylinder submerged in an unbounded viscous thermally conducting medium. The translational addition theorem for cylindrical wave functions, the appropriate wave field expansions and the pertinent boundary conditions are employed to develop a closed-form solution in the form of infinite series. The analytical results are illustrated with a numerical example in which the compound cylinder is insonified by a plane sound wave at selected angles of incidence in a wide range of dimensionless frequencies. The backscattered far-field acoustic pressure amplitude and the spatial distribution of the total acoustic pressure in the vicinity of the cylinder are evaluated and discussed for representative values of the parameters characterizing the system. The effects of incident wave frequency, angle of incidence, fluid thermoviscosity, core eccentricity and size are thoroughly examined. Limiting case involving an ideal compressible liquid-coated cylinder is considered and fair agreement with a well-known solution is established.  相似文献   

Zero-voltage switching converter absorbing parasitic parameters for super high frequency induction heating     

Zheng-shi Wang  Hui-ming Chen 《浙江大学学报(A卷英文版)》2008,9(4):564-571

This paper presents a novel mega-Hz-level super high frequency zero-voltage soft-switching converter for induction heating power supplies. The prominent advantage of this topology is that it can absorb both inductive and capacitive parasitic components in the converter. The switch devices operate in a zero-voltage soft-switching mode. Consequently, the high voltage and high current spikes caused by parasitic inductors or capacitors oscillation do not occur in this circuit, and the high power loss caused by high frequency switching can be greatly reduced. A large value inductor is adopted between the input capacitor and the switches, thus, this novel converter shares the benefits of both voltage-type and current-type circuits simultaneously, and there are no needs of dead time between two switches. The working principles in different modes are introduced. Results of simulation and experiments operated at around 1 MHz frequency verify the validity of parasitic components absorption and show that this convener is competent for super high frequency applications.  相似文献   

Bubble counter based on photoelectric technique for leakage detection of cryogenic valves     

Tao Jin  Bin Xu  Ke Tang  Jian-ping Hong 《浙江大学学报(A卷英文版)》2008,9(1):88-92

In order to overcome the inconvenience of manual bubble counting, a bubble counter based on photoelectric technique aiming for automatically detecting and measuring minute gas leakage of cryogenic valves is proposed. Experiments have been conducted on a self-built apparatus, testing the performance with different gas inlet strategies （bottom gas-inlet strategy and side gas-inlet strategy） and the influence of gas pipe length （0, 1, 2, 4, 6, 8, 10 m） and leakage rate （around 10, 20, 30, 40 bubbles/min） on first bubble time and bubble rate. A buffer of 110 cm^3 is inserted between leakage source and gas pipe to simulate the downstream cavum adjacent to the valve clack. Based on analyzing the experimental data, experiential parameters have also been summarized to guide leakage detection and measurement for engineering applications. A practical system has already been successfully applied in a cryogenic testing apparatus for cryogenic valves.  相似文献