Graphical Representations of Speed: Obstacles Preservice K‐8 Teachers Experience

This study examines the difficulties college students experience when creating and interpreting graphs in which speed is one of the variables. Nineteen students, all preservice elementary or middle school teachers, completed an upper‐level course exploring algebraic concepts. Although all of these preservice teachers had previously completed several mathematics courses, including calculus, they demonstrated widespread misconceptions about the variable speed. This study identifies four cognitive obstacles held by the students, provides excerpts of their graphical constructions and verbal interpretations, and discusses potential causes for the confusion. In particular, misconceptions arose when students interpreted the behavior and nature of speed within a graphical context, as well as in situations where they were required to construct a graph involving speed as a variable. The study concludes by offering implications for the teaching and learning of speed and its interpretation within a graphical setting.     

Synthesis of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> by molten salt technique

Lithium manganese oxide powders have been successfully prepared by a molten salt synthesis using eutectic mixture of LiCl and MnO₂ salt at 900 °C. The synthesis was performed in open atmosphere. The crystalline powders were characterized for their phase identification using X-ray diffraction analysis. The physicochemical properties of the lithium manganese oxide powders are investigated by thermal analysis (thermo gravimetric analysis/ differential thermal analysis), Fourier transform infrared spectroscopy, Raman spectroscopy, atomic absorption spectroscopy, electron spin resonance spectroscopy, and scanning electron microscopy. This work shows the feasibility for obtaining lithium manganese oxide at low-temperature molten salt flux method.     

The busy period of an M/M/1 queue with balking and reneging

It has been shown by (R.O. Al-Seedy, A.A. El-Sherbiny, S.A. El-Shehawy, S.I. Ammar, Transient solution of the M/M/c queue with balking and reneging, Comput. Math. Appl. 57 (2009) 1280–1285) that a generating function technique can be successfully applied to derive the transient solution for an M/M/c queueing system. In this paper, we further illustrate how this technique can be used to obtain the busy period density function of an M/M/1 queue with balking and reneging. Finally, numerical calculations are presented.     

Copper and iron interactions with angiotensin-converting enzyme inhibitors. A study by fast-atom bombardment tandem mass spectrometry

Fast atom bombardment, combined with high-energy collision-induced tandem mass spectrometry, has been used to investigate gas-phase metal-ion interactions with captopril, enalaprilat and lisinopril, all angiotensin-converting enzyme inhibitors.Suggestions for the location of metal-binding sites are presented. For captopril, metal binding occurs most likely at both the sulphur and the nitrogen atom. For enalaprilat and lisinopril, binding preferably occurs at the amine nitrogen. Copyright 1999 John Wiley & Sons, Ltd.     

One-Pot Synthesis of Pd@PtnL Core-Shell Icosahedral Nanocrystals in High Throughput through a Quantitative Analysis of the Reduction Kinetics

The rational design and implementation of a one-pot method is reported for the facile synthesis of Pd@Pt_nL (nL denotes the number of Pt atomic layers) core-shell icosahedral nanocrystals in a single step. The success of this method relies on the use of Na₂PdCl₄ and Pt(acac)₂ as the precursors to Pd and Pt atoms, respectively. Our quantitative analysis of the reduction kinetics indicates that the Pd^II and Pt^II precursors are sequentially reduced with a major gap between the two events. Specifically, the Pd^II precursor is reduced first, leading to the formation of Pd-based icosahedral seeds with a multiply-twinned structure. In contrast, the Pt^II precursor prefers to take a surface reduction pathway on the just-formed icosahedral seeds. As such, the otherwise extremely slow reduction of the Pt^II precursor can be dramatically accelerated through an autocatalytic process for the deposition of Pt atoms as a conformal shell on each Pd icosahedral core. Compared to the conventional approach of seed-mediated growth, the throughput for the one-pot synthesis of Pd@Pt_nL core-shell nanocrystals can be increased by more than 30-fold. When used as catalysts, the Pd@Pt_4.5L core-shell icosahedral nanocrystals show specific and mass activities of 0.83 mA cm⁻² and 0.39 A mg_Pt⁻¹, respectively, at 0.9 V toward oxygen reduction. The Pt-based nanocages derived from the core-shell nanocrystals also show enhanced specific (1.45 mA cm⁻²) and mass activities (0.75 A mg_Pt⁻¹) at 0.9 V, which are 3.8 and 3.3 times greater than those of the commercial Pt/C, respectively.     

大学物理实验中的不确定度分析

在大学物理实验中,用不确定度来评定实验结果是非常必要的。本文介绍了不确定度的有关概念.并结合“用单摆测重力加速度”实验的例子,进行了分析。     

A rapid technique to determine the internal area function of finite-length ducts using maximum length sequence analysis

This paper describes a rapid technique for reconstruction of the internal area function of a duct using blockage-induced eigenvalue shifts determined from eigenfrequencies measured under two sets of duct termination boundary conditions. A single broad band maximum length sequence (MLS) measurement of short duration is utilized to obtain the transfer function of the duct, which in turn can be utilized to determine its eigenvalue shifts and subsequently its internal area function using an inverse perturbation technique. The reconstruction results display the same order of accuracy as those obtained previously using swept sine measurements of extended duration. An expression for the determination of the area function is presented utilizing resonant frequency information alone, thus rendering duct length determination unnecessary. A computational routine further simplifies the process such that the accuracy of the technique could be ascertained for a range of configurations including longer ducts and ducts that initially have nonuniform internal cross section over their length. Development of a relationship between obstacle length and wavelength of the lowest eigenfrequency required for successful reconstruction is also described. This is an important result for longer ducts where measurement of lower eigenfrequencies may present problems using standard measurement equipment.     

Beiträge zur Stickstoffbestimmung in organischen Substanzen durch katalytische Hydrogenolyse in der Gasphase. I

Zusammenfassung Die Anwendbarkeit der Methode nach ter Meulen zur Bestimmung des Stickstoffes in organischen Verbindungen wurde mit Rücksicht auf die Stickstoffbindung überprüft. Eine Reihe von Testsubstanzen aus verschiedenen Körperklassen wurde auf drei Nickelkatalysatoren (Nickelasbest, Nickel + Thoriumoxid + Asbest, granuliertes Nickel + Magnesiumoxid) hydriert. Substanzen, welche den Stickstoff in Form von Amino-, Azo-, Azoxy-, Isonitroso-, Mononitro-, N-N oder heterocyklischem Stickstoff enthielten, lieferten richtige Stickstoffwerte. Substanzen mit zwei oder drei Nitrogruppen oder einer Nitrosogruppe im Molekül gaben wesentlich zu niedrige Stickstoff werte im Vergleich zur Theorie. Versuche, die Hydrierungsbedingungen durch Abänderung des Standardverfahrens besser zu gestalten, schlugen fehl.

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Contributions to nitrogen assay in organic substances by catalytic hydrogenolysis in the gas phase. I

Summary The applicability of the method for determination of nitrogen in organic compounds according to ter Meulen was checked with regard to nitrogen binding. A series of test substances from various organic groups was hydrogenated on three nickel catalysts (nickel asbestos, nickel + thorium oxide + asbestos, granulated nickel + magnesium oxide). Substances which contained nitrogen in the form of amino, azo, azoxy, isonitroso, mononitro, N-N or heterocyclic nitrogen provided correct nitrogen values. Substances with two or three nitro groups or a nitroso group in the molecule gave nitrogen values which were far too low compared to theory. Attempts to improve the hydrogenation conditions by altering the standard procedure failed.