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1.
Investigation of the conformational properties of a SAW polymer chain near an impenetrable, non-interacting flat surface showed that the chain at first contracted and orientated itself to slightly parallel when it pulled close to the surface and at last elongated and reoriented itself to slightly perpendicular to the surface at very small distance from the surface. Simulation showed that most of the disappeared configurations were of large size at moderate distance from the surface while they were of small size at very small distance from the surface; and that the mean-square end-to-end distance was much more prone to be influenced by the surface than the mean-square radius of gyration . The orientational correlation between the direction of the longest principal axis of the moment of inertia and end-to-end vector was also discussed. 相似文献
2.
Michael Grahame 《The American journal of distance education》2013,27(3):127-129
A comprehensive meta-analysis of the research following Glass, McGraw, and Smith's (1981) technique integrated findings from twenty-five comparative studies from 1990 to 2003 targeting student achievement and distance education in allied health professions. Student achievement was assessed through course grades and resulted in an overall effect size of 0.15, a very small positive effect for distance learners when compared to traditional students. Several dependent variables—education levels, models of distance education, and instructional design components—related to the effect size. In the distance education setting, working professional students significantly outperformed graduate and undergraduate students. Open learning and synchronous instruction were the most effective distance education models of instruction. Instructional design components implemented into a distance education course resulted in greater student achievement scores. 相似文献
3.
The study explored the effects of two kindergarten program organization factors—length of school day and class size—on kindergartners' reading, math and general knowledge achievement at the end of the kindergarten year. Two waves of data were drawn from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K) with an analytic sample of 15,575 children. A slight positive relationship was found between small class size and children's achievement in reading and math, particularly for children from minority and lower SES backgrounds. No relationship was linked with class size and general knowledge achievement. The relationship between full-day program and the three early academic skills was positive and statistically significant: almost all children made slightly higher gains in full-day programs compared with their counterparts in part-day programs. The findings suggest that policy makers may consider reducing very large class size and making full-day programs available to young children, particularly to poor minority children, at the same time aligning class size and length of kindergarten day with a child's characteristics and school curriculum. 相似文献
4.
Objectives: Polymerization shrinkage of dental composites remains a major concern in restorative dentistry because it can lead to micro-cracking of the tooth and debonding at the tooth-restoration interface. The aim of this study was to measure the full-field polymerization shrinkage of dental composites using the optical digital image correlation (DIC) method and to evaluate how the measurement is influenced by the factors in experiment setup and image analysis. Methods: Four commercial dental composites, Premise Dentine, Z100, Z250 and Tetric EvoCeram, were tested. Composite was first placed into a slot mould to form a bar specimen with rectangular-section of 4 mm mm, followed by the surface painting to create irregular speckles. Curing was then applied at one end of the specimen while the other part were covered against curing light for simulating the clinical curing condition of composite in dental cavity. The painted surface was recorded by a charge-coupled device (CCD) camera before and after curing. Subsequently, the volumetric shrinkage of the specimen was calculated with specialist DIC software based on image cross correlation. In addition, a few factors that may influence the measuring accuracy, including the subset window size, speckle size, illumination light and specimen length, were also evaluated. Results: The volumetric shrinkage of the specimen generally decreases with increasing distance from the irradiated surface with a conspicuous exception being the composite Premise Dentine as its maximum shrinkage occurred at a subsurface distance of about 1 mm instead of the irradiated surface. Z100 had the greatest maximum shrinkage strain, followed by Z250, Tetric EvoCeram and then Premise Dentine. Larger subset window size made the shrinkage strain contour smoother. But the cost was that some details in the heterogeneity of the material were lost. Very small subset window size resulted in a lot of noise in the data, making it difficult to discern the general pattern in the strain distribution. Speckle size did not seem to have obvious effect on the volumetric shrinkage strain along specimen length; however, larger speckles resulted in higher level of noise or heterogeneity in the shrinkage distribution. Compared with bright illumination, dimmer lighting produced larger standard deviations in the measured shrinkage, indicating a higher level of noise. The longer the specimen, the greater was the rate of reduction with distance from the irradiated surface, especially for the longitudinal strain. Significance: The image correlation method is capable of producing full-field polymerization shrinkage of dental composites. The accuracy of the measurements relies on selection of optimal parameters in experimental setup and DIC analysis. 相似文献
5.
马海珠 《浙江教育学院学报》2006,(4):55-60
利用构象．构型统计方法，推导了均方回转半径和温度系数公式，并对同为单侧基二态模型的聚苯乙烯和聚丙烯酸甲酯链均方回转半径特征比作了计算比较，发现构象能对链无扰尺寸和温度系数的依赖关系是不同的，说明侧基对聚合物性质有重要影响． 相似文献
6.
Pulse electro-coagulation flotation was used to treat oil wastewater of high oil content. Different operational conditions were examined, including current density, reactive time, electrode distance, pH and pole switching time. Orthogonal tests were carried out to identify the optimal operational conditions for this technique. Considering the treatment cost and efficiency together the optimal operational conditions were an electrode distance of 3.3 cm, pH of 4, current density of 49.38 mA/cm2, reaction time of 15 min and pole switching time of 10 s. The removal efficiency of oil wastewater under normal conditions reached up to 96.21%. The influences of different factors on removal efficiency were in the following decreasing sequence： pH〉 current density 〉 pole switching time 〉 reactive time 〉 board distance. 相似文献
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8.
Nest-site selection, an ecologically relevant behavior, was studied in the golden hamster in a model environment where the animals could choose between nestboxes differing in distance from resource or in structural features (size and illumination). Experiment 1 showed that hamsters can decrease foraging costs by setting their nests in the nestbox nearest to the food and/or nestmaterial sources, and that hoarding costs, as distinct from simple procurement costs, are taken into account in the choice process. Preferences for darker and larger nestboxes were obtained in Experiment 2. Experiment 3 revealed an internally coherent hierarchical pattern in preferences, with illumination being more important than size and resource distance, and size more important than resource distance. The results suggest that nest-site selection and foraging behaviors are parts of an integrated causal system. 相似文献
9.
William Pelech Dan Wulff Ellen Perrault Jessica Ayala Myra Baynton Margaret Williams 《Journal of Teaching in Social Work》2013,33(4-5):393-407
One of the first tasks of the Social Work Distance Education Network at the Faculty of Social Work at the University of Calgary was to review the literature and address three research questions to inform policy making and planning relating to distance social work education programming. This paper is intended to disseminate responses to these questions from a small group of dedicated and experienced distance educators who have adopted the moniker Elluminati. (The Elluminati adapted their name from a popular online audio-conferencing program.) These questions include: (1) What is the optimal class size for a distance delivery offering? (2) What supports does an online course require for development and effective delivery? (3) What types of courses are suitable/unsuitable for online learning? 相似文献
10.
以DMF、丙酮、二氯甲烷和丙酮／二氯甲烷为溶剂,通过静电纺丝方法制备PCL亚微米纤维．通过SEM观察纤维形貌及其孔径大小,通过N2吸附一脱附曲线计算纤维的比表面积,通过拉伸实验测试纤维的力学性能．实验结果表明：以单-DMF、丙酮为溶剂制备得到表面光滑无孔亚微米纤维．而以二氯甲烷为溶剂无法制备连续的纤维．以丙酮／二氯甲烷为溶剂制备得到直径为0．86±0．14／μm、孔径大小为65．0±17．5nm,比表面积为21．15m2／g的亚微米纤维．相比较无孔纤维,多孔PCL亚微米纤维的拉伸强度和杨氏模量略有下降,而断裂伸长率有所增加,主要是因为拉伸过程中,多孔纤维发生一定程度的收缩．PCL亚微米纤维表面的多孔结构有利于细胞的黏附与生长,因而有望作为组织工程支架材料． 相似文献
11.
M. Tim Mainhard Mieke BrekelmansPerry den Brok Theo Wubbels 《Contemporary educational psychology》2011,36(3):190-200
In this study the mean stability of classroom social climates during the first months of the school year and the deviation of individual classrooms (N = 48) and students (N = 1208) from this general trend were investigated by taping students’ interpersonal perceptions of their teachers. Multilevel growth modeling was used to identify the average development of the classroom social climate and between classroom and student differences in terms of this development. While significant differences between classrooms and students were found, these differences were rather stable over time. Most classrooms showed a slightly u-shaped trajectory. In only very few classrooms the quality of the social climate had increased at the end of the research period as compared to the very first lesson. Differences in the development between classrooms were associated to the emotional distance between teacher and students during the first lesson of the school year. 相似文献
12.
Two experiments investigated the whole number bias in the representation of fraction magnitudes with adults. A fraction magnitude comparison task was used where half of the comparisons were consistent with whole number ordering and the other half were not. Distance effects were found in Experiment 1 indicating that participants were comparing the magnitude of the whole fraction rather than just the parts. However, accuracy and response time also depended on the comparisons' consistency with whole number ordering. Experiment 2 manipulated the distance between the fraction pairs and showed that the whole number effect was strongest when the distance between the fraction pairs was very small. The results suggest that even skilled adults do not always have direct access to a fraction's magnitude on the number line. When the magnitudes are especially close together, adults may rely on alternative implicit or explicit strategies, such as examining the whole number parts, to evaluate the comparison. 相似文献
13.
One hundred and sixty-eight genotypes of cotton from the same growing region were used as a germplasm group to study the validity of different genetic distances in constructing cotton core subset.Mixed linear model approach was employed to unbiasedly predict genotypic values of 20 traits for eliminating the environmental effect.Six commonly used genetic distances (Euclidean,standardized Euclidean,Mahalanobis,city block,cosine and correlation distances) combining four commonly used hierarchical cluster methods (single distance,complete distance,unweighted pair-group average and Ward's methods) were used in the least distance stepwise sampling (LDSS) method for constructing different core subsets.The analyses of variance (ANOVA) of different evaluating parameters showed that the validities of cosine and correlation distances were inferior to those of Euclidean,standardized Euclidean,Mahalanobis and city block distances.Standardized Euclidean distance was slightly more effective than Euclidean,Mahalanobis and city block distances.The principal analysis validated standardized Euclidean distance in the course of constructing practical core subsets.The covariance matrix of accessions might be ill-conditioned when Mahalanobis distance was used to calculate genetic distance at low sampling percentages,which led to bias in small-sized core subset construction.The standardized Euclidean distance is recommended in core subset construction with LDSS method. 相似文献
14.
This study investigated students' with visual impairment concepts about linear size and scale. Specifically the study examined the accuracy of students' concepts over many orders of magnitude as well as experiences students have had in and out‐of‐school learning about size and scale. The results of assessments of 17 students with visual impairment were compared to those of students with normal sight. The study showed that students with visual impairment were most accurate for measurements in the human scale and were least accurate with very large and very small scales that cannot be directly experienced. However, when compared to students with normal sight, students with visual impairment were more accurate at large and small scales than their normally sighted peers. © 2009 Wiley Periodicals, Inc. J Res Sci Teach 46: 506–519, 2009 相似文献
15.
远程教育的运行机制,指的是远程教育系统中各个构成要素之间,以及与远程教育紧密相关的社会经济因素之间相互联系、相互作用的方式。远程教育运行机制包括了主管部门、远程教育院校、社会和人才市场等要素。这些要素之间的关系及其作用方式,决定了远程教育的发展趋势。目前远程教育运行机制的主要问题：1．动力与决策机制。专业开设未能适应市场经济,对于需求的变化反应迟缓,学校自主权不够充分;2．竞争与合作机制。各院校之间竞争有不合理之处,存在合作不足、资源浪费现象;3．监控、评估、反馈和改进机制。缺少用人单位和师生参与的评价机制,反馈不够及时,改进措施滞后;4．激励机制与约束机制。主管部门对下属各院校的工作缺少激励和约束;另外,缺乏鼓励创新的制度。远程教育运行机制的改进,有待于教育体制和远程教育院校的深入持续改革。 相似文献
16.
A two-step method was developed to quantitatively assess the infection rate of the entomophthoraceous fungus, Zoophthora anhuiensis (Li) Humber, on the green peach aphid, Myzus persicae (Sulzer). Firstly, a standard time-dose-mortality relationship, established by modeling data from bioassay 1 at varying conidial dosages (0.4-10.4 conidia/mm2) of Z. anhuiensis F97028, was used to yield an estimate of expected mortality probability at a given dosage. Secondly, bioassay 2 was conducted by simultaneously exposing six ≤4-day-old nymphal colonies to a shower of Z. anhuiensis conidia at each of four dosages (resulting from exposures of 0.3-8.0 min). Subsequently, the colonies were separately immersed in a 0.1% chlorothalonil solution for 0.5 min to disinfect all surviving conidia on the host integument from 1-12 h after exposure under temperature treatments of 15 and 20℃, respectively. The infection rate during a specific period from the end of the exposure to the immersion was then estimated as the ratio of the observed mortality over the expected mortality probability at a particular dosage. The results showed that the infection of M. persicae from Z. anhuiensis was highly rapid with little difference between aphid colonies maintained at 15 and 20℃ before being immersed in the fungicidal solution after exposure. The first 6-hour period after exposure was most crucial to successful infection of the fungus with the infection rate greatly depending on conidial dosages. It took ≤1 h to infect>50% of the aphids at a dosage of >1.5 conida/mm2 and >90% at >50 conidia/mm2. 相似文献
17.
To optimize the hydrolysis conditions to prepare hydrolysates of jellyfish umbrella collagen with the highest hydroxyl radical scavenging activity, collagen extracted from jellyfish umbrella was hydrolyzed with trypsin, and response surface methodology (RSM) was applied. The optimum conditions obtained from experiments were pH 7.75, temperature (7) 48.77 ℃, and enzyme-to-substrate ratio ([E]/[S]) 3.50%. The analysis of variance in RSM showed that pH and [E]/[S] were important factors that significantly affected the process (P<0.05 and P<0.01, respectively). The hydrolysates of jellyfish umbrella collagen were fractionated by high performance liquid chromatography (HPLC), and three fractions (HF-1>3000 Da, 1000 Da相似文献
18.
Mesoporous SnO2 was synthesized using cetyltrimethyl ammonium bromide （CTAB） as supermolecule-template by hydrothermal method followed by calcining under different temperature in air. X-ray diffraction analysis （XRD） and transmission electron microscopy （TEM） techniques were used to characterize the structure of mesoporous SnO2. The results indicated that the gas sensors prepared by using mesoporous SnO2 after calcination at 400 ℃ showed quick response and recovery to ethanol at 200 ℃. It was also found that the mesostructure SnO2 with small particle size had higher sensitivity and selectivity to C2HhOH than the SnO2 nanoparticles the particle size of which is 20 nm synthesized by sol-gel method. 相似文献
19.
Little research has examined factors influencing statistical power to detect the correct number of latent classes using latent profile analysis (LPA). This simulation study examined power related to interclass distance between latent classes given true number of classes, sample size, and number of indicators. Seven model selection methods were evaluated. None had adequate power to select the correct number of classes with a small (Cohen's d = .2) or medium (d = .5) degree of separation. With a very large degree of separation (d = 1.5), the Lo–Mendell–Rubin test (LMR), adjusted LMR, bootstrap likelihood ratio test, Bayesian Information Criterion (BIC), and sample-size-adjusted BIC were good at selecting the correct number of classes. However, with a large degree of separation (d = .8), power depended on number of indicators and sample size. Akaike's Information Criterion and entropy poorly selected the correct number of classes, regardless of degree of separation, number of indicators, or sample size. 相似文献
20.
蒋家卉 《中学英语之友(教育研究与实践)》2011,(11):28-28
新认识一个老外,该和对方聊什么来breaktheice（打破沉默）,而又不显得唐突呢？聊电影是个不错的选择,今天我们就和大家来学习几句吧!
特技太棒了。我喜欢电影里的美妙背景、以及充满外星风味的服饰,武器和生物。
这部影片的整体情节很有趣,但是我觉得角色的发展不是很好。 相似文献