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971.
972.
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
973.
974.
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
975.
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
976.
Research in non-equalization machining method for spatial cam 总被引:1,自引:0,他引:1
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
977.
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
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
978.
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
979.
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
980.
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