带隔离变压器的Cuk型有源功率因数校正的研究与设计

随着电力电子技术的发展,各种电力电子设备应用于电网,给电网带来谐波问题,各种谐波滤波器也应运而生,但这只是一种先污染后治理的被动办法,APFC是一种主动的办法,它用于开关电源前级整流器,输入电流为正弦波,不对电网产生污染。全文围绕一种新颖的基于隔离型Cuk变换器的有源功率因数校正电路的设计,从Cuk变换器的基本单元入手,分析了Cuk变换器的工作原理,在此基础上,确定了基于隔离型Cuk变换器APFC电路的基本拓扑。     

论康拉德小说的孤独主题:以《进步前哨》为例

孤独是康拉德小说深度刻画与反复表现的主题之一。康拉德短篇小说《进步前哨》对孤独主题作了深入思考和深度表述,孤独是人类生活、思考和书写关注的永恒主题,有其不同性质、状态和形态。     

一种高效的P2P系统

目前非结构P2P系统所采用的两层聚类结构HCPON存在的问题是超级节点仅采用单一的备份节点作为冗余备份,使得系统健壮性的提高很有限。由此提出基于三层聚类结构的高效P2P系统（PTLCA）,根据节点的拓扑优先级等对节点进行分层处理和聚类,同HCPON相比,PTLCA在性能上有显著的提高,能够满足大规模流媒体服务的需求。     

Extraction of DNA suitable for PCR applications from mature leaves of Mangifera indica L.

Good quality deoxyribonucleic acid (DNA) is the pre-requisite for its downstream applications. The presence of high concentrations of polysaccharides, polyphenols, proteins, and other secondary me- tabolites in mango leaves poses problem in getting good quality DNA fit for polymerase chain reaction (PCR) applications. The problem is exacerbated when DNA is extracted from mature mango leaves. A reliable and modified protocol based on the cetyl- trimethylammonium bromide (CTAB) method for DNA extraction from mature mango leaves is described here. High concentrations of inert salt were used to remove polysaccharides; Polyvinylpyrrolidone (PVP) and β-mercaptoethanol were employed to manage phenolic compounds. Extended chloroform-isoamyl alcohol treatment followed by RNase treatment yielded 950?1050 μg of good quality DNA, free of protein and RNA. The problems of DNA degradation,contamination, and low yield due to irreversible binding of phenolic compounds and coprecipitation of polysaccharides with DNA were avoided by this method. The DNA isolated by the modified method showed good PCR amplification using simple se- quence repeat (SSR) primers. This modified protocol can also be used to extract DNA from other woody plants having similar problems.     

一个逐渐“成熟”的声音——论严歌苓小说叙事艺术的嬗变

严歌苓小说中的叙事者常常在小说发展的过程中,对情节的发展进行主观化的阐释和干预,使小说呈现出奇妙的文学景观。纵观其自新时期到新移民的作品可以发现,这个＂叙事者＂是在创作的过程中逐渐显形并慢慢成熟的,而这个成熟的过程正是严歌苓对中西方叙事思想不断选择、吸纳、发展的过程,是对叙事艺术的不断探索和实验的结果。     

Child and family-level correlates of direct and indirect peer victimization among children ages 6–9

The purpose of this study was to examine the prevalence and child and family-level correlates of direct and indirect victimization by peers among children ages 6–9. Four hundred and twenty-five children were included in the final sample. Data for this study were drawn from the first wave of the Developmental Victimization Survey. Logistic regression models were used to examine associations between children's demographics, anxiety, depression, anger, parent–child relationship, and exposure to family violence and children's experience of direct or indirect victimization by peers. The results showed that increased depression scores and exposure to family violence were associated with increased risk for direct and indirect victimization by peers. Black children were more likely to experience direct victimization and less likely to experience indirect victimization compared to White children. Child's race significantly moderated the association between parental criticism and indirect victimization. Child's gender did not significantly moderate these associations. Implications for developmentally specific prevention and intervention approaches that are grounded in a social-ecological framework are discussed.     

MOOC系统中同伴互评过程的优化和应用

目前的MOOC虽然能支持视频课程、论坛、测试评价等功能,但其测试评价仅停留在对客观题的评测上。为了进一步优化MOOC系统的评价体系,强化其对于主观性试题评价的客观性和准确度,在对MOOC本土化的现状和发展方向研究的基础上,构建三种同伴互评过程模型。同时利用Cousera提供的HCI课程的相关数据对各模型依次进行评测。其评测结果与利用Cousera平台的同伴互评系统所得到的相应结果进行比较,准确度有显著提高。     

Group peer assessment for summative evaluation in a graduate-level statistics course for ecologists

Peer assessment is often used for formative learning, but few studies have examined the validity of group-based peer assessment for the summative evaluation of course assignments. The present study contributes to the literature by using online technology (the course management system Moodle^?) to implement structured, summative peer review based on an anchored rubric in an ecological statistics course taught to graduate students. We found that grade discrepancies between students and the instructor were fairly common (60% of assignments), relatively low in value (mean = 3.3 ± 2.5% on assignments that had discrepancies) and proportionally higher for criteria related to interpretation of statistical results and code quality and organisation than for criteria related to the successful completion of analysis or instructional tasks (e.g. fitting particular statistical methods, de-identification of one’s submission). Students reported that the peer assessment process increased their exposure to alternative ways of approaching statistical and computational problem-solving, but there were concerns raised about the fairness of the process and the effectiveness of the group component. We conclude with some recommendations for implementing peer assessment to maximise student learning and satisfaction.     

The effects of communication medium on argumentation and the development of critical thinking

The aim of this paper is to compare the effects of different communication media on the quality of arguments and the development of argumentative reasoning. The study involved 73 undergraduate students. It consisted of a pre-test, an intervention session consisting of either an asynchronous online discussion or a face-to-face discussion, and a post-test. The pre-test consisted of a mini essay to ascertain participants’ opinion of smacking, and a 21-point smacking scale, where participants indicated how much they agreed or disagreed with smacking. The online group participated in a seminar discussion over a period of 2 weeks. The face-to-face group participated in the seminar discussion for a 1-h period. The seminar discussion was about the detrimental effects of smacking on children's development. One week after the seminar discussion, the post-test was administered. No statistically significant differences were found between the two conditions in terms of improvement in the quality of the students’ arguments. However, the quality of the argumentation used in face-to-face was higher than that used in the online discussions. The implications of these findings for theoretical models of collaborative activity are discussed.     

Introducing MOOC-like methodologies in a face-to-face undergraduate course: a detailed case study

During the last years online education, in particular Massive Open Online Courses (MOOCs), has contributed to spread and popularize educational methodologies such as peer-review, automatic assessment, self-paced courses, self-evaluation, etc. Although these techniques can benefit face-to-face courses, most of them are not yet widely used in these courses. In this paper we present our experience integrating MOOC-like techniques in a face-to-face first year undergraduate course. We describe the academic and institutional context of the project, how we designed and adapted the methodologies to our face-to-face course, the software tools that we have used and developed to support and integrate such methodologies and, finally, we discuss some lessons learned from the experience that can be useful to potential users of these methodologies in face-to-face courses.